- 1 Organisms in the Kingdom Monera – Archaebacteria, Cyanobacteria, Mycoplasma, Rickettsia and Actinomycetes
- 1.1 I. Archaebacteria
- 1.2 II. Eubacteria
- 1.3 A. Bacteria
- 1.4 Structure of Bacteria
- 1.5 Reproduction in Bacteria
- 1.6 Classification of Bacteria
- 1.7 Utility of Bacteria
- 1.8 Economic Importance of Bacteria
- 1.9 B. Cyanobacteria (Blue Green Algae or Cyanophyceae or Myxophyceae)
- 1.10 Importance Cyanobacteria:
- 1.11 Mycoplasma (PPLO)
- 1.12 Rickettsiae
- 1.13 Actinomycetes
Microbiology is one of the Biology Topics that involves the study of microorganisms, including bacteria, viruses, and fungi.
Organisms in the Kingdom Monera – Archaebacteria, Cyanobacteria, Mycoplasma, Rickettsia and Actinomycetes
Kingdom Monera includes all prokaryotes. Therefore it is also referred to as prokaryota. The monerans include the most primitive and ancient organisms that developed from the earliest living stock or progenitor. They are adapted to all types of environments and are found everywhere. Their population is the highest among all organisms. They are unicellular motile or non-motile and can be mycelial, filamentous, or colonial. There is very little morphological differentiation. Therefore, it is very difficult to distinguish groups, subgroups, genera, and species on the basis of structural and morphological characters alone. Their classification requires biochemical, physiological, and ecological characteristics in addition to morphological features. Monerans are divided into two major groups – Archaebacteria and Eubacteria.
It includes ancient and primitive forms of bacteria. The important characteristics of Archaebacteria are
- They can live in different adverse habitats like
- Extreme acidic and hot (100°C) environment.
- Extreme saline condition.
- In anaerobic conditions.
- They are a group of primitive prokaryotes that are believed to have evolved from progenitors quite early. They flourished on primitive hostile environments of earth and continue to live under extreme conditions, hence are also referred to as living fossils.
- The cell wall is devoid of peptidoglycan and is made up of protein and non-cellulosic polysaccharides. Pseudomurein occurs in some methanogens.
- The cell membrane is characterized by the presence of a conolayer of branched chain lipids. The fatty acids are attached to glycerol by ether bonds (-OCH2-) instead of ester bonds (-G-CO-). Branched-chain lipids decrease membrane fluidity and increase tolerance to extremes of heat as well as pH.
- They have a covalently linked closed circular genome and their size is generally smaller than other prokaryotes. The size of the genome ranges between 0.8-1.1 × 109 daltons.
- Ribonucleotides of 16S rRNA are different from those of other organisms. RNA polymerase has also a different constitution.
- Archaebacteria are either obligate anaerobes or facultative anaerobes. Obligate anaerobes can live under anaerobic conditions only. They get killed in the presence of oxygen, i.e., methanogens. Facultative anaerobes are actually aerobic archaebacteria that can live under anaerobic conditions, i.e., thermoacidophiles and halophiles.
- Nutritionally, they range from chemolithoautotrophs to organotrophs.
Similarities with Eubacteria:
- About lpm in diameter.
- Lack of membrane-bound organelles.
- Nucleic acid is not bound by a nuclear membrane.
- Ribosome 70S type.
Similarities with Eukaryotes:
- Presence of cell wall but devoid of peptidoglycan.
- Mechanism of protein synthesis and structure of protein.
- Some of their pigments, and biochemical processes closely resemble those found in eukaryotic cells.
- Genes occasionally have introns.
Archaebacteria are of three major types – Methanogens, Halophiles, and Thermoacidophiles. Methanogens and halophiles are placed in division euryarchaeota while thermoacidophiles are placed in division crenarchaeota.
These archaebacteria are strict anaerobes occurring in marshy areas. They are characterized by their habit of producing methane (CH4) from carbon dioxide or fumaric acid. They are, therefore, called methanogens. Some of them live as symbionts in the rumen or first chamber of the stomach of ruminant animals and cause fermentation of cellulose. Methanogens are also responsible for the production of methane in biogas fermenters. They are widely used in sewage treatment processes, e.g., Methanobacterium, and Methanococcus. Methane gas is also produced from rice fields and marshes by methanogens.
These archaebacteria live in extremely strong brine or salt solutions, salt beds, and salt marshes. They are, therefore, known as halophiles. Some halophiles occur in deep-sea volcanic vents at 110°C, a temperature at which water remains liquid because of extreme hydrostatic pressure. Their cell membranes have red carotenoid pigment for protection against harmful solar radiation. Under anaerobic conditions, halophiles can not use external materials. At this time they develop a purple-pigmented membrane, which can absorb solar radiation. The absorbed light is utilized in the synthesis of ATP. With the help of this ATP, they carry out their metabolic processes. Halophiles growing in salt beds give an offensive smell and undesirable pigmentation to the salt, e.g., Halobacterium, Halococcus. Halophiles are able to live under high salt conditions due to the following reasons:
- Presence of special lipids in the cell membranes.
- Occurrence of mucilage covering.
- Absence of sap vacuoles.
- High internal salt content.
These archaebacteria are able to tolerate high temperatures as well as high acidity. They occur in hot sulphur springs, where the temperature may be as high as 80°C and pH as low as 2. e.g., Thermoplasma, Thermoproteus. These archaebacteria are chemoautotrophic which obtain energy for the synthesis of food by oxidizing sulphur. Under aerobic conditions, they oxidize sulphur to sulphuric acid.
2S + 2H2O + 3O2 → 2H2SO4
Some forms of archaebacteria can reduce sulphur to hydrogen sulphide under anaerobic conditions. Thermoacidophiles are able to tolerate high temperatures as well as acidity due to
- Branched-chain lipids in the cell membranes
- Presence of special resistant enzymes capable of operating under acidic conditions as well as high temperatures.
Importance of Archaebacteria:
Archaebacteria live under various extreme conditions like high temperature, high salinity, high pressure, and high acidic conditions. These properties are utilized in modern biotechnology in the field of
- Thermophilic enzymes.
- Generation of biogas.
- Restriction enzymes.
- Bioleaching of poor mines.
Eubacteria known as ‘true bacteria’, are prokaryotic (lacking nucleus) cells that are very common in human daily life, encountered many more times than the archaebacteria. Eubacteria can be found almost everywhere, but also serve as antibiotics, producers, and food digesters in our stomachs. Eubacteria is of further two types: Bacteria and Cyanobacteria. Eubacteria also includes mycoplasma, rickettsiae, actinomycetes, chlamydiae, spirochaetes, etc.
Introduction of Bacteria: Bacteria are prokaryotic, simple, unicellular, smallest, microscopic organisms that possess cell walls. The study of bacteria is called Bacteriology. Anton van Leeuwenhoek, a Dutch scientist first discovered bacteria in pond water in 1675. He named bacteria as animalcules. Louis Pasteur (1864) made a detailed study of bacteria and proposed the germ theory of disease. Robert Koch (1876) found that some diseases like Tuberculosis, Cholera in man, and Anthrax in cattle are caused by bacteria. Ehrenberg (1828) was the first who coined the term bacteria. Bacteria are unicellular, microscopic organisms. They differ from animals for their rigid cell wall, ability to synthesize vitamins, and autotrophic nature. For this reason, these are included in plant groups.
Characteristics of Bacteria:
- Bacteria are the primitive, smallest (0.1-few micron), and simplest cellular organisms.
- They are mostly unicellular, very few are colonial filamentous, or mycelial.
- They are motile or non-motile; motile bacteria swim by means of flagella.
- Flagella when present, are single-stranded and composed of a protein flagellin.
- Presence of a rigid cell wall which is made up of peptidoglycan.
- The cells lack membrane-bound organelles like Plastids, Mitochondria, ER, Golgi bodies, Lysosomes, etc.
- Only ribosomes are present which are 70S type and often clustered to form polyribosomes or polysomes.
- Bacterial cells lack organized nuclei; instead, a double-stranded, circular, supercoiled DNA lies inside the cytoplasm. It is called nucleoid or genophore.
- Vacuoles containing sap are absent. Instead, gas vacuoles may be present.
- Most of the bacteria are heterotrophs-they include obligate or facultative parasites or saprophytes.
- Some are autotrophs-they may be photoautotrophs or chemoautotrophs. Some live symbiotically with higher plants.
- Bacteria are both anaerobes and aerobes.
- Reproduction is primarily asexual by binary fission.
- The spindle apparatus does not develop at the time of cell division.
- Sexual reproduction is of a special type: gene recombination takes place by conjugation, transformation, and transduction.
- A few bacteria can fix atmospheric nitrogen.
- Photosynthetic bacteria contain pigments like bacteriochlorophyll, bacterioviridin, chromium chlorophyll, and carotenoids.
- Bacteria can survive in a wide variety of environments. Some live in acidic or alkaline conditions, some are halophilic.
- On the basis of temperature, they may be Psychrophilic (upto 0°-22°C), Mesophilic (10°-45°C), and Thermophilic (45°-90°C).
- Bacteria play an important role in maintaining the fertility of soil and nitrogen balance of the atmosphere.
- A few bacteria are important for their production of antibiotics, organic acids, enzymes, alcohols, etc.
- Bacteria cause various diseases of plants, animals, and man.
Why Bacteria are included in Prokaryotes?
- An organized nucleus is absent, the nucleus is primitive, without a nuclear membrane and nucleolus. A double-stranded DNA is present as the genetic material, called nucleoid.
- Membrane-bound cell organelles are absent.
- The ribosome is of 70S type.
- Cell division is amitotic, and binary fission is the usual method of reproduction.
- The cellular organization is simple.
Why Bacteria are included in Plant Groups?
- Presence of a definite and rigid cell wall.
- Autotrophic bacteria contain photosynthetic pigments like bacteriochlorophyll, bacterioviridin, and chlorobium chlorophyll with which they synthesize organic food from inorganic materials such as CO2 and water.
- Ability to synthesize vitamins and amino acids.
- Presence of peptidoglycan in cell wall like that of blue-green algae.
- Holophytic mode of nutrition through diffusion.
- A few of them are able to fix atmospheric nitrogen.
- The tendency of some to grow in filaments.
Why Bacteria are Considered as most Primitive Organisms?
- Bacteria were first formed living beings and appeared in the archaeozoic period.
- They can survive in extreme adverse conditions of the environment.
- Simple cellular organization.
- Many of them are anaerobic.
- Genetic material is simply double-stranded DNA.
- Reproductive methods are of primitive type.
- Ribosomes are of 70S type.
What are the Similarities between Bacteria and Blue-Green Algae?
- Mostly unicellular structure in both groups.
- Presence of slime layer outside the cell wall.
- Both are prokaryotic cells with the simple organization of genetic material.
- Presence of mucopeptides in the cell wall.
- Ribosomes are of 70S type.
- Ability to synthesize vitamins and amino acids.
- Photosynthetic and nitrogen-fixing abilities are present in both groups.
- Reproduction by vegetative and asexual means. True sexual reproduction is absent.
Structure of Bacteria
Under an electron microscope, the following structures are observed in a bacterial cell. The bacterial cell consists of a cell envelope and a protoplast. External to the cell wall there may be present a slime layer.
1. Cell Envelope
(i) Slime Layer: It is a viscous or gelatinous substance secreted by the cell protoplast. The slime layer is usually composed of polysaccharides or of polypeptides of one or two amino acids. Under certain conditions, the slime accumulates to form a thick layer called a sheath or capsule. The sheath protects the organism against desiccation and antibodies.
(ii) Cell Wall: The cell wall is a thin, tough envelope around the protoplast. Its thickness is around 0.02 µ. The cell wall is composed of basically acetyl glucosamine and acetyl muramic acid that constitute peptidoglycans. The peptide part of peptidoglycan consists of amino acids L-alanine, D-glutamic acid, diaminopimelic acid, and D-alanine connected by peptide linkages. Other compounds are polysaccharides, amino acids, and in some cases lipids. In gram-negative bacteria, a covering layer over the peptidoglycan consists of lipopolysaccharides. Other distinctive constituents are muramic acid, diaminopimelic acid, and teichoic acids. In a gram-negative wall, the region between the cytoplasmic membrane and the outer membrane is filled with a gel-like fluid called periplasm. Periplasm consists of many proteins associated with cellular functions.
(iii) Cell Membrane: It is a delicate, selective permeable fine structure enclosing the cytoplasm and lies close to the cell wall. Chemically it is composed of phospholipids with proteins and some polysaccharides. It serves as a matrix in which many respiratory and other metabolic enzymes are stored. In gram-positive bacteria the membrane forms infoldings which are called mesosomes. Mesosome contains respiratory and other enzymes. For that, they are often considered as incipient mitochondria. It also helps in the separation of replicated nucleoids and in the formation of a septum.
Protoplast is a slightly viscous substance differentiated into cytoplasm and nuclear body or Nucleoid.
(i) Cytoplasm: It is a complex mixture of proteins, carbohydrates, lipids, minerals, nucleic acids, and water. Organic matter exists in the form of a colloidal state. The non-living inclusions are the storage granules of volutin, glycogen, lipid granules, or protein crystals. The cytoplasm shows no streaming movements and contains few or no vacuoles. Ribosomes present either freely or in groups by a strand of mRNA called polyribosomes. Mitochondria, endoplasmic reticulum, chloroplasts, and definitely organized nuclei are absent. The pigments in the photosynthetic bacteria are present within chromatophores and in some cases within lamellar structures.
(ii) Mesosome: It is a circular to villiform specialisation of the cell membrane of bacteria, which develops as arf outgrowth from the plasma membrane and lies in the cytoplasm. It consists of vesicles, tubules, and lamellae. Mesosome is of two types: septal and lateral. The septal mesosome connects the nucleoid with the plasma membrane and the lateral mesosome is not connected with the nucleoid. Septal mesosome takes part in the replication of nucleoids. It helps in septum formation. The lateral mesosome contains respiratory enzymes. Hence it is also called chondrioid. It resembles the mitochondria of the eukaryotic cells.
(iii) Ribosomes: Ribosomes are small membranes, ribonucleoprotein entities having a size of 20 nm × 14-15 nm. There are two types of ribosomes in bacterial cells: Fixed-type ribosomes are attached to all membranes and Free ribosomes lie free in the cytoplasm. The ribosomes of bacteria are 70S in nature. Each 70S ribosome has two subunits, a larger 50S and a smaller 30S. Ribosome takes part in protein synthesis. Ribosomes generally occur in helical groups called Polysomes or Polyribosomes. In each polyribosome, 4-8 ribosomes are attached to a single strand of mRNA (Messenger RNA).
(iv) Chromatophore: In photosynthetic bacteria, chromatophores are present in the cytoplasm which develop as membrane-lined sacs or thylakoids. Thylakoid contains photosynthetic pigments in cyanobacteria and purple bacteria. Photosynthetic pigments include bacteriochlorophyll, bacteriophaeophytin, and carotenoids.
(v) Nucleoid or Nuclear body: It is located in the central area, containing chromatin or the genetic material of the cell. It lacks a nuclear membrane and nucleolus. The shape of the nuclear body varies from spherical, oval, elongated, dumbbell-shaped, or irregular in structure. It does not divide by mitosis. It shows tightly packed, circular fibrils of double-stranded DNA. DNA duplex is supercoiled with the help of RNA. The folding is 250-700 times. In E. coli nucleoid has an 1100 pm long DNA duplex with a 4.6 × 106 base pair. It is often referred to as a chromosome.
(vi) Plasmids and Episomes: Bacterial cells also contain some extra-chromosomal self-replicating hereditary determinants called plasmids. Some of the bacteria contain important genes like fertility factor (F), resistant factor (R), etc. Plasmids that integrate into the main chromosome are called episomes.
(vii) Gas Vacuoles: The gas-storing vacuoles found in cyanobacteria, purple bacteria, green bacteria, etc. Each gas vacuole is surrounded by a single non-unit, non-lipid protein membrane. Gas vacuole contains a variable number of hexagonal, hollow gas vesicles.
(viii) Reserve Food: Cyanobacteria have cyanophycean starch or α-granules, β-granules, lipid globules, and protein granules as reserve food. In bacteria, starch is replaced by glycogen. Other reserve foods are volutin granules, PHB(poly β hydroxybutyric acid), and elemental sulphur.
(ix) Flagella: These are thin hair-like appendages that protrude through the cell wall and are responsible for the motility of bacterial cells. They are made up of identical spherical subunits of a protein called flagellin. The flagellum is about 20 nm in diameter and 1-7 µm in length. The bacterial flagellum is made up of 3 parts-basal body, hook, and filament. The basal body is like a rod. It is inserted in the cell envelope. The basal body bears ring-like swellings in the region of the cell membrane and cell wall. There are two pairs of rings L and P rings in the cell wall and S and M rings embedded in the cell membrane. The flagellum performs rotation type of movement that brings about backward pushing of the water. It results in the bacterium moving forward.
(x) Pili and Fimbria: Pili (Singular pilus) are longer, very fine, hollow, nonhelical, filamentary appendages in gram-negative bacteria, which develop in response to F+ (fertility factor). The pilus is made up of the protein pilin. The pili of the donor bacterial cell are responsible for attaching to the recipient cell forming a conjugation tube. Fimbriae are small bristle-like fibers found on the surface of bacterium (E. coli) in large numbers. These are about 300-400 in number of bacterium cells. The length of fimbriae is 0.5-1.5 µm and the diameter is 3-10 nm. Fimbriae are involved in attaching bacteria to solid surfaces or hot tissues. Some fimbriae cause agglutination of RBC.
Differences between Pili and Fimbriae:
|1. The number of Pili is 1-4 per cell.||1. The number is 300-400 per cell.|
|2. It is developed in response to fertility factors (F+).||2. It does not require any fertility (F+) factor to develop.|
|3. Pili are longer and broader than fimbriae.||3. Fimbriae are shorter and narrower than pili.|
|4. They are tubular structures.||4. They are bristle-like solid structures.|
|5. They help in conjugation.||5. They take part in adhesion.|
Reproduction in Bacteria
Bacteria reproduce by the following three methods:
- Vegetative reproduction
- Asexual reproduction
- Sexual reproduction
(i) Vegetative Reproduction:
It takes place through three processes, namely Fission, Budding, and Fragmentation.
(a) Fission: It is a common method of vegetative reproduction when conditions of food, water, and temperature are favourable. During this process, the single circular chromosome duplicates itself. Along with DNA duplication, the cytoplasm divides into two halves, each having its own nuclear material. Later on, wall formation and separation of two daughter cells take place, e.g. E. coli, and Streptococcus.
(b) Budding: In this process, the cell wall forms a small outgrowth-like structure that gradually increases in size. In the meantime, replication of DNA takes place, one of which enters into the bud. The bud gradually matures and detaches from the mother cell, e.g., Rhodopseudomonas, Hyphomicrobium.
(c) Fragmentation: This process is observed in filamentous bacteria like Streptomyces. Cells of the filament are broken down into many fragments. Each fragment forms a separate filament.
(ii) Asexual Reproduction:
(a) Endospore: When environmental conditions are adverse, some bacteria produce resting spores called endospores. At the time of endospore formation, a part of the protoplast forms an impermeable coat around the chromosome and the remaining part of the protoplasm. Endospore assumes various shapes like oval, spherical, ellipsoidal, etc. These are resistant to heat, radiation, desiccation, and other environmental adverse conditions. Under favourable conditions, endospore germinates by rupture of the wall into a vegetative cell e.g. Bacillus, or Clostridium.
(b) Conidia: These are found in filamentous bacteria like Streptomyces. The conidia are spore-like structures formed in chains. Each conidium gives rise to a new bacterium.
(c) Cyst: Cyst formation is very rare in bacteria. A cyst is a spherical structure surrounded by a heavy wall. The cysts develop due to the shortening and rounding up of an entire vegetative cell. They germinate and give rise to a single vegetative cell, e.g., Azotobacter.
(d) Gonidia: In some bacteria the protoplasts divide into a few smaller endospore-like flagellate structures called Gonidia. After the rupture of the mother cell wall, each gonidium forms a new bacterium.
(iii) Sexual Reproduction:
Typical sexual reproduction involving the formation and fusion of gametes is absent in bacteria. However, genetic recombination takes place by three different methods. These are conjugation, transformation, and transduction.
(a) Conjugation: It was first observed in E. coli by Lederberg and Tatum (1946). Conjugation occurs between two mating types: F+, the male or donor, and F-, the female or recipient. The male or the donor cells (F+) possess sex pili and fertility factors in their plasmid. The female (F-) or recipient cells lack sex pili and fertility factors. When two different mating types of cells come in contact with each other, the pilus of the male cell grows in size and produces a conjugation tube or cytoplasmic bridge. The plasmid having fertility factor replicates and transfers a copy of it to the F- or the recipient cell through the conjugation tube. Now the recipient cell also becomes a donor or F+. After some time both the cells separate.
Sometimes F+ factor in plasmid gets associated with the bacterial chromosome. Such a male cell is called Hfr (high frequency of recombination). The attached plasmid is known as episome. Following conjugation, the progenies of recipient cells express some of the characteristics of the donor. Bacterial conjugation, though different from typical sexual reproduction, is a means of making new genetic combinations. The recipient cells with the new genetic combinations are called merozygotes (Partially diploid).
(b) Transformation: The process in which the naked DNA of one bacterium is transferred into another bacterial cell is called transformation. In this process, the genetic material (DNA) from dead and decaying donor bacterial cells enters the living bacterial cell of a different strain. Once the DNA is taken up by the recipient cell, recombination occurs. The bacterium that has inherited the DNA of the donor is said to be transformed. This phenomenon was discovered by Griffith (1928) in pneumonia-causing and Non-pneumonia-causing strains of Diplococcus bacteria.
(c) Transduction: The process of gene transfer from one bacterium to another by means of a bacteriophage, serving as a vector, is called transduction. This phenomenon was discovered by N. Zinder and J. Lederberg in 1952. The process takes place in the following steps when progeny phages are released from lysis of an infected bacterial cell, they occasionally pick up short DNA fragments from the killed donor bacterium. These DNA fragments may enter another recipient bacterium. Such DNA fragment is now incorporated into the DNA of the recipient bacterium. For this reason, this process is known as ‘phage-mediated gene transfer.
Differences between Transformation, Conjugation, and Transduction in Bacteria
|1. Transfer of a small segment of isolated DNA takes place from a donor cell to a recipient cell.||1. Gene transfer takes place between cells of two opposite mating types. A partial diploid or merozygote is formed.||1. Transfer of DNA segment from donor to recipient cell is mediated by a vector.|
|2. Naked DNA molecule enters through a cell wall. No vector is present.||2. Plasmid may be called a vector.||2. Bacteriophage acts as a vector.|
|3. Physical contact between two living cells is not essential.||3. Physical contact between two cells (F+ & F-) is required. A conjugation tube is formed.||3. Physical contact and penetration of phage DNA are required within the bacterial cell.|
|4. Sex pili have no role.||4. Sex pili play an important role in gene transfer.||4. Sex pili have no role.|
|5. First reported in Diplococcus pneumoniae by Griffith (1928).||5. First reported in E. coli by Lederberg and Tatum (1946).||5. First reported in Salmonella typhimurium by Zinder and Lederberg (1952).|
Classification of Bacteria
(a) On the basis of Morphology:
On the basis of morphology, Cohn (1872) classified bacteria into the following main types:
1. Cocci (Singular-Coccus) Spherical or Ovoid-Shaped:
The bacteria are spherical or ovoid in shape. Cocci are subdivided into six groups according to cell arrangement and cell division.
- Micrococci: The cells are arranged singly or irregularly, e.g., Micrococcus flavus.
- Diplococci: Cells divide in one plane and remain attached in pairs, e.g., Diplococcus pneumoniae.
- Streptococci: Cells divide in one plane and are arranged in chains of different lengths, e.g., Streptococcus pyrogenes.
- Tetracocci: Cells divide in two planes at right angles to one another and form groups of four cells, e.g., Tetracoccus.
- Sarcinae: Cells divide in three planes at right angles to one another and resemble packets of 8, 16 or more cells, e.g., Sarcina lutea.
- Staphylococci: Cells divide in several planes resulting in irregular bundles of cells, sometimes resembling clusters of grapes, e.g., Staphylococcus aureus.
2. Bacilli (Rod-shaped):
These are rod-shaped or cylindrical in form. They also exhibit differences in form. Some are short, others are long. These are further subdivided into
- Monobacilli: Bacteria occurring singly, e.g., E.coli.
- Diplobacilli Salmonella typhii
- Streptobacilli: Bacteria occurring in chains of different length, e.g., Lactobacillus.
3. Spirilli (Spiral-shaped):
These are spirally coiled, short or long, and usually live singly.
- Vibriones: These are comma-like in shape. Cells generally show a single curve, e.g., Vibrio comma.
- Helical: These are coiled forms of bacteria having one or more turns, e.g., Spirillum, Rho do spirillum.
They are made up of cylindrical cells attached to one another. They are similar to chains of Streptobacillus but have a larger area of contact between the adjacent cells, e.g., Beggiatoa sp.
These are flexible spiral bacteria and are able to twist along their long axis, e.g., Treponema pallidum.
6. Hyphae (Sing-Hypha):
These are long, multicellular, or branched filaments formed by some bacterial types, e.g., Streptomyces sp.
These bacteria occur in more than one form i.e. have variable shapes, e.g., Mycoplasma, Rhizobium.
8. Stalked or Prosthecate Form:
These bacteria possess stalk or prostheca one each at the place of flagellum. This stalk may be part of their cells i.e., Caulobacter, or it may be a secretion product, e.g., Gallionella.
(b) On the Basis of Nutritional Requirements:
1. Autotrophic Bacteria:
They prepare their organic food from the inorganic materials obtained from the environment with the help of solar energy or exergonic chemical reactions. Autotrophic bacteria include Photosynthetic (Photo-autotrophs) bacteria and Chemosynthetic (Chemo-autotrophs) bacteria.
- Photo-autotrophic Bacteria: These bacteria can utilize solar energy for the synthesis of food where CO2 is the principal source of carbon. They are also called photolithotrophs as they utilize inorganic carbon (CO2) for carbohydrate food synthesis. These bacteria possess photosynthetic pigments Bacteriochlorophyll and Chlorobium chlorophyll to trap solar energy, e.g., Chlorobacterium, Chlorobium, Chromatium, etc.
- Chemo-autotrophic Bacteria: These bacteria obtain energy from oxidation of inorganic compounds and their source of carbon is CO2. Therefore these bacteria are also called Chemolithotrophs. They include nitrifying bacteria like Nitrosomonas, and Nitrobacter; Sulphur oxidising bacteria like Beggiatoa, and Thiobacillus; Iron bacteria like Ferrobacillus, etc.
2. Heterotrophic or Organotrophic Bacteria:
The majority of bacteria show heterotrophic nutrition. They are unable to synthesize their own food. They depend on the supply of organic substances for their growth. Heterotrophic bacteria mostly include free-living saprophytes and symbiotic and parasitic bacteria. They are divided into chemoheterotrophs and photoheterotrophs. According to their energy source, they are further divided into two groups
- Photoheterotrophic or Photoorgonatrophic Bacteria: The bacteria that use sunlight as the source of energy and organic compounds as the source of carbon are photoheterotrophs, e.g., Rhodospirillum rubrum, R. palustris, etc.
- Chemoheterotrophic or Chemoorganotrophic Bacteria: The bacteria that utilize both carbon and energy as a source of the organic compound are called chemoheterotrophs.
Depending upon the source of organic compounds chemoheterotrophs are further divided into the following types:
- Metatrophic Bacteria: Bacteria that draw organic compounds from dead and decaying organic matter are called metatrophic bacteria. They are also known as saprotrophs, e.g., Clostridium butyricum, and Pseudomonas putida.
- Paratrophic Bacteria: Bacteria that draw organic compounds or food directly from the living host are called paratrophic bacteria. They are also known as parasitic bacteria, e.g., Salmonella typhi, Treponema pallidum, and Clostridium tetani.
- Symbiotic Bacteria: Bacteria that draw organic compounds with a mutual relationship with the other living host are called symbiotic bacteria. They are gram negative bacteria, e.g., Rhizobium leguminosarum.
- Saprophytic Bacteria: Saprophytic bacteria grow on dead and decaying organic matter. Saprophytes may be obligate saprophytes e.g., Clostridium sp. or facultative saprophytes, e.g., Vibrio cholerae.
- Symbiotic Bacteria: These are the bacteria that form mutually beneficial associations with other organisms. A common symbiotic bacterium is Rhizobium. Enteric bacterium E.coli is another example of symbiotic bacterium.
- Parasitic Bacteria: Parasitic bacteria live at the cost of a living host, which may be a plant, animal, or human being. Parasites may be obligate parasite e.g. Neisseria or Facultative parasite, e.g., Staphylococcus.
- Heterotrophic bacteria are also classified as Photoheterotrophs or Photo-organotrophs and Chemoheterotrophs or Chemo-organotrophs depending upon their source of energy or chemical substance.
(c) On the basis of Staining Property:
Scientist Christian Gram of Denmark first demonstrated the method of Gram staining in 1884. This is a kind of differential staining procedure. On the basis of the reaction of Gram stain, bacteria are classified into two groups: Gram-positive and Gram-negative forms.
- Gram-Positive Bacteria: Bacteria retaining the violet colour of crystal violet are called Gram-positive bacteria. The deep violet colour could not be washed by treatment with a solvent like acetone or alcohol. Examples of common Gram-positive bacteria are Staphylococcus, Bacillus, Streptococcus etc.
- Gram Negative Bacteria: Bacteria that cannot retain the violet colour of crystal violet after treatment with acetone or alcohol are called Gram-negative bacteria. After acetone or alcohol treatment, bacteria are counter-stained with Safranin which imparts their red colour. Examples of Gram-negative bacteria are E.coli, Salmonella, Neisseria etc.
Gram Staining Procedure:
- A small amount of bacterial culture is smeared in a clean grease-free free-slide.
- The smear is then stained with a dilute alkaline solution of crystal violet for a minute.
- The slide is rinsed in water.
- The smear is then treated with a 0.5% solution of iodine and then rinsed with water.
- The slide is then treated with 90% alcohol.
- Drops of safranin or eosin solution are added to the smear.
- The slide is washed in water and dried in air.
- Bacteria retaining the violet colour of crystal violet are Gram-positive and those that could not retain the violet colour and took the red colour of safranin is Gram-negative.
Differences between Gram Positive and Gram Negative Bacteria:
|Gram-Positive Bacteria||Gram Negative Bacteria|
|1. Retain the colour of crystal violet.||1. Do not retain the colour of crystal violet, counter-stained with safranin.|
|2. The cell wall is rigid, thick and single-layered.||2. The cell wall is thin, wavy, and two-layered.|
|3. The peptidoglycan layer is about 70-80% of the cell wall.||3. The peptidoglycan layer is 10-20% of the cell wall.|
|4. The outer membrane is absent.||4. The outer membrane is present.|
|5. The lipid and lipoprotein contents are low.||5. The lipid and lipoprotein contents are higher.|
|6. The lipopolysaccharides are absent.||6. The lipopolysaccharides are present.|
|7. Teichoic acid is present.||7. Teichoic acid is absent.|
|8. The periplasmic space is absent.||8. The periplasmic space is present.|
|9. The cell wall has fewer amino acids.||9. The cell wall has several types of amino acids.|
|10. The flagella contains two rings in the basal body.||10. The flagella contains four rings in the basal body.|
|11. The pili are absent.||11. The pili are present in a few cases.|
|12. Most of them are spore formers.||12. Most of them are non-spore formers.|
|13. They are less pathogenic and sensitive to lysozyme action.||13. They are more pathogenic and resistant to lysozyme action.|
|14. They are highly susceptible to antibiotics.||14. They are less susceptible to antibiotics.|
|15. Examples: Bacillus, Staphylococcus, Streptococcus, Lactobacillus, Clostridium etc.||15. Examples: E.coli, Rhizobium, Vibrio, Acetobacter, Azotobacter etc.|
(d) On the Basis of Thermal Sensitivity:
Temperature requirement for normal growth and function of different bacteria is widely varied. Some bacteria cannot tolerate high temperatures whereas, others can grow well even at 75°C. On the basis of tolerance to temperature, bacteria are classified into 3 main groups: Psychrophilic, Mesophilic, and Thermophilic.
Those bacteria that can grow at very low temperatures are called psychrophilic. They can grow even below 0°C and are found in lakes, rivers, and seawater of colder regions. Psychrophilic bacteria are again divided into two groups:
- Obligate psychrophilic: They grow well at 5°C temperature from below, e.g., Arthrobacter.
- Facultative psychrophilic: They may grow well at a temperature range from 0°C to 25°C, e.g., Pseudomonas.
These bacteria generally grow well at a temperature range from 25°C to 45°C, e.g., Rhizobium, Azotobacter.
These bacteria grow well and can tolerate a temperature of about 46°C to 75°C. They are found to grow in hot springs. They are again divided into two groups:
- Obligate thermophilic: These bacteria grow well above 50°C, e.g., Thermus aquaticus.
- Facultative thermophilic: These bacteria grow well below 45°C and upto 50°C temperature and above, e.g., Bacillus coagulans.
(e) On the Basis of Flagella:
Flagella are unbranched hair-like helical filaments that emerge from the cell wall. These are the locomotory organs and are usually longer than the cell length. Their length varies from 10-20 µm. Flagella are present in all spiral-shaped bacteria, most of the rod-shaped bacteria, and rare in cocci. On the basis of the presence or absence of flagella, their number and location in the cell, bacteria are classified into the following groups:
- Monotrichous: A single flagellum is present at one end, e.g., Pseudomonas aeruginosa, Vibrio cholerae.
- Amphitrichous: A single flagellum or more flagella are present at both ends, e.g., Aquaspirillum serpens.
- Lophotrichous: A tuft of flagella is present at one end, e.g., Spirillum.
- Peritrichous: Numerous flagella are present all over the bacterial surface, e.g., Salmonella typhi, and E.coli.
- Atrichous: These are non-flagellate bacteria, e.g., Corynebacterium diphtheriae.
Utility of Bacteria
1. Role in Agriculture:
Many bacteria in the soil improve soil fertility by fixing atmospheric nitrogen and the formation of humus, manure, etc. Non-symbiotic nitrogen-fixing bacteria can fix nitrogen freely in the soil. These are Azotobacter, Clostridium, Beijerinckia, Derxia, Klebsiella, Spirillum etc. Symbiotic nitrogen-fixing bacteria are mainly Rhizobium, Bradyrhizobium, Azorhizobium, Sinorhizobium etc. They are present in the nodules (root, stem, and leaf) of mainly leguminous plants. Some other bacteria like Streptomyces, Nocardia, Frankia, etc. form nodules in the roots of non-leguminous plants. Many species of Cyanobacteria ( Blue-green algae) fix atmospheric nitrogen either freely or in association with other plants. Important members are Nostoc, Anabaena, Gloeocapsa etc. Symbiotic or endobiotic associations are Anabaena-Azolla, Nostoc/Anabaena-Anthoceros, Nostoc-Cycas root etc.
Azotobacter, and Rhizobium sp., are widely used in the preparation of biofertilizers in the present days. The number of these bacteria is also increased by the application of organic matter in soil. Azotobacterin is a kind of biofertilizer prepared from the species of Azotobacter. Such preparations are widely used in different countries of Europe, Soviet Russia, etc., in crops like wheat, maize, barley, beet, carrot etc. Nitrogen-fixing bacteria also produce growth hormones, other beneficial metabolites in soil that influence the growth and yield of crop plants. Biofertilizers are also produced using Rhizobium, Azospirillum, Nostoc, Anabaena, and Azolla-Anabaena associations. The use of legume-Rhizobium association as green manure in agriculture is an old practice. Mycorrhizal association is also used in agriculture and forestry as a biofertilizer in the present day.
Advantages of using Biofertilizers:
- The cost of production is less, application is easier, and therefore acceptable to the small and marginal farmers.
- The use of biofertilizers has no pollution effect on the environment, it only increases soil fertility.
- Continuous use of biofertilizers restores their influence in the future.
- Nitrogen-fixing microbes produce IAA, IBA, NAA, amino acids, protein, vitamins, etc., in soil that enhances crop growth.
- Rhizobium application increases 50-150 kg of nitrogen per year per Hectar in soil.
- Azotobacter, Azospirillum sp., secrete antibiotic in soil that acts as a pesticide.
- Application of biofertilizer increases water holding capacity, amount of nutrients, and other physical qualities of soil.
- The decomposition of organic matter by microbial activity leads to the formation of humus. Humus improves the aeration and fertility of soil.
2. Role in Industry:
Useful activities of various bacteria are employed in the production of a number of industrial products. Large-scale productions of useful substances are possible using suitable strains of micro-organisms in factories. The products are then extracted, purified, and marketed for human beings for different purposes.
Production of Food Substance:
Food substances formed as fermentation products are mainly milk products, bakery products, vinegar, fermented meat, fermented vegetables, alcoholic beverages, single-cell protein etc. Names of a few products, raw materials used, and the micro-organisms employed for this purpose are tabulated below:
|Food Products||Raw Materials||Micro-organism Employed|
|1. Cheese||Milk Protein||Streptococcus, Leuconostoc.|
|2. Butter||Cream of Milk||Leuconostoc citrovorum, Streptococcus lactis, S.cremoris.|
|3. Yogurt||Milk, Creamed Milk||Leuconostoc bulgaricus, Streptococcus thermophilus.|
|4. Butter Milk||Creamed Milk||Lactobacillus lactis, Leuconostoc mesenteroides.|
|5. Kefir (Russia)||Fermented Milk||Streptococcus lactis, Lactobacillus bulgaricus.|
|6. Kumisa||Maren’s Milk||Lactobacillus bulgaricus.|
|7. Taette||Milk||Streptococcus lactis var. taette.|
|8. Idli||Rice, Pulses||Leuconostoc mesenteroides.|
|9. Fish Sauces||Small Fishes||Bacillus sp., Halophilic sp.|
|10. Kimchi||Cabbage, Vegetables||Lactic acid bacteria.|
Enzymes obtained from certain bacteria as fermentation products are used for the tanning of leathers. A few names such as enzymes and the concerned bacteria are mentioned in the table:
|Enzyme||Name of Bacteria||Use|
|1. Protease||Bacillus subtilis||Tanning|
|2. Keratinase||Streptomycetes fradiae||Unhairing of leather|
|3. Lipase||Pseudomonas sp.||Tanning|
Those bacteria that produce alcohol by fermentation and are used in the brewing industry are mainly species of Clostridium, Klebsiella, Leuconostoc, Sarcina, Zymomonas, and Lactobacillus etc. Some of the products, concerned bacteria, and their uses are mentioned in the table:
|Product||Name of Bacteria||Use||Industry|
|1. Alcohol||Sarcina ventriculi||Beer, Wine||Brewing Industry|
|2. Alcohol||Zymomonas mobilis||Alcohol Rum, Whisky||Brewing Industry|
|3. Butanol||Clostridium acetobuticum||Alcohol Butanol Production||Brewing and Chemical Industry|
|4. Alcohol||Lactobacillus sp.||Sake||Brewing Industry|
3. In Medicine Production:
Microbes have maximum application in medicines which include antibiotics, steroids, vitamins, and vaccines.
There are thousands of antibiotics produced by bacteria and other microorganisms. The major antibiotics used in medicines and their microbial sources are listed below:
|1. Streptomycin||Streptomyces griseus|
|2. Chloramphenicol||Streptomyces venezuelae|
|3. Tetracyclines: Chlortetracycline, Oxytetracycline||Streptomyces aureofaciens, S. rimosus|
|4. Erythromycin||S. erythreus|
|5. Neomycin||S. fradiae|
|6. Mitomycin||S. antibioticus|
|7. Bacitracin||Bacillus subtilis|
|8. Polymyxin B||B. polymyxa|
|9. Nocardins||Nocardia uniforms|
Microbial biotransformation of steroids is very important in the pharmaceutical industry. Steroids are used in the treatment of various disorders. The chemical synthesis of steroids is very complex, difficult, and costly. The use of microbial biotransformation in the formation of steroids has lowered the original cost manyfold. Some of the important steroid transformations and the microbes involved are listed below:
|1. Cortisol||Prednisolone||Corynebacterium simplex|
|2. Cortisone||Prednisone||C. simplex|
|3. 19-Nor testosterone||Estradiol + Estrone||Pseudomonas testosteroni|
Vaccines are produced from micro-organisms possessing antigenic properties. Mutant strains and inactivated virulent pathogens or cellular parts are used for the production of vaccines. A short list of principal bacterial vaccines is given below:
|Active Against Disease||Vaccine Composition|
|1. Cholera||Crude fraction of Vibrio cholerae|
|2. Tuberculosis||Attenuted Mycobacterium tuberculosis (BCG)|
|3. Plague||Killed Yersinia pestis|
|4. Haemophilus meningitis||Purified polysaccharide from Haemophilus influenzae|
|5. Pertussis (whooping cough)||Killed Bordetella pertussis|
4. In Vitamin Production:
Microbial fermentations are used for commercial production of several essential vitamins. Some important vitamins and their sources are shown below.
|1. Riboflavin (Vitamin B2)||Clostridium acetobutylicum, Mycobacterium smegmatis|
|2. Cyanocobalamin (Vitamin B12)||Propionibacterium shermanii, Pseudomonas denitrificans|
|3. Ascorbic acid (Vitamin C)||Gluconobacter oxidans|
|4. Phylloquinone (Vitamin K)||Streptococcus hemolytic|
Bacteria that produce vitamins in culture media use glucose as the source of carbon. Organic substances like sorbitol, cornsteep, glycine, collagen, etc. are also used in a few cases. It has become possible to obtain mutants of microorganisms that produce more vitamins than natural ones. Pseudomonas denitrificans is able to produce 50,000 times more vitamin B12 than its parental strain (Sasson, 1984).
Economic Importance of Bacteria
Bacteria play an important role in human life. Some of them are harmful. Others are extremely useful to man.
1. Useful Activities:
- Decomposition of Dead Plant and Animal Bodies: Saprophytic bacteria cause decay and decomposition of dead bodies of plants and animals. They release raw materials into the soil and clean the earth. Therefore, they are called nature’s scavengers.
- Sewage disposal: The organic content of sewage is decomposed by bacteria.
- Ammonlfication: Ammonifying bacteria such as Bacillus vulgaris and B. ramosus reduce amino acids, formed during putrefaction of ammonia.
- Nitrification: Nitrifying bacteria convert ammonia to nitrites and nitrates. Nitrosomonas and Nitrosococcus oxidize ammonia to nitrites. Nitrites are further oxidized to nitrates by Nitrocystis and Nitrobactor.
- Nitrogen fixation: Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrogenous compounds. Azotobacter, Beijerinckia, and Clostridium pasteurianum are free-living nitrogen-fixing bacteria. They are able to pick up free atmospheric nitrogen and fix it in some organic compounds like amino acids. Symbiotic nitrogen-fixing bacteria Rhizobium occur in the root nodules of legumes. Sesbania and some other legumes are used as green manures. Non-leguminous plants like Casurina and Alnus bear root nodules formed by species of Streptomyces, Frankia, etc., that fix atmospheric nitrogen.
- Manure and gobar gas: Saprophytic bacteria convert farm refuse dung and other organic wastes into manure. Gobar gas is obtained as a byproduct when the bacteria convert dung into manure.
- Dairy industry: Lactic acid bacteria Streptococcus lactis is extensively employed in the dairy industry. It converts milk sugar lactose into lactic acid. Lactic acid coagulates the milk protein casein. Different strains of lactic acid bacteria are used to convert milk into curd, yogurt, and cheese.
- Vinegar: Vinegar (acetic acid) is obtained by the activity of acetic acid bacteria Acetobacter aceti and Acetomonas. The bacteria oxidize ethyl alcohol obtained from molasses by fermentation to acetic acid.
- Alcohol and Acetone Production: Butyl alcohol, methyl alcohol, and acetone are prepared from molasses by the fermentation activity of the anaerobic bacteria Clostridium acetobutylicum.
- Curing of tea, tobacco, and coffee: The leaves of tea and tobacco and seeds of coffee are fermented by the activity of certain bacteria (e.g., Bacillus megatherium) to develop characteristic flavour.
- Retting of fibres: Stem and leaf fibers are separated from softer tissues by the enzymatic action of bacteria like Pseudomonas fluorescens and Clostridium sp.
- In the leather industry: The bacterial protease obtained from Bacillus subtilis is used in tanning leather.
- Amino acid production: Many amino acids are produced on a commercial basis with a bacterial fermentation process. Lysine is produced from diaminopimelic acid by E.coli, glutamic acid is produced by species of Micrococcus, Arthrobacter etc.
- Vitamins: E.coli living in the human intestine produces large quantities of vitamin K and vitamin B complex, vit B12 is obtained from Bacillus megatherium. Vit B2 was formally prepared from Clostridium acetobutylicum.
- Antibiotics: Antibiotics are produced by several species of bacteria. Common antibiotics produced by bacteria are Streptomycin, Tetracycline, Neomycin, Nystatin, Chloramphenicol, Gramicidin, Bacitracin, Polymyxin, etc.
2. Harmful Activities:
- Spoilage of food: Saprophytic bacteria cause the rotting of meat, fruits, and vegetables, the souring of milk, and spoilage of jams, jellies, and pickles.
- Food poisoning: Some bacteria produce toxins in the food and cause food poisoning. The common type of food poisoning is caused by Staphylococcus aureus. It results in diarrhea and vomiting. An anaerobic bacterium Clostridium botulinum causes botulism. Salmonellosis is produced by eating meat contaminated by the bacteria Salmonella enteritidis and Salmonella typhimurium.
- Deterioration of domestic articles: Some bacteria like Spirochaeta and Cytophaga deteriorate cotton fabrics, leather, and wooden articles.
- Denitrification of soil: Denitrifying bacteria like Thiobacillus denitrificans, Micrococcus denitrificans, etc. transform nitrates of soil into gaseous nitrogen. This change causes a neat loss of nitrogen from the soil.
- Water pollution: The most important microbial diseases transmitted through water are cholera, typhoid, jaundice, paratyphoid fever, amoebic dysentery, bacillary dysentery etc.
- Bacteria and diseases: Many parasitic bacteria cause serious diseases in man, animals, and plants.
Diseases of Man:
|1. Typhoid||Salmonella typhi|
|2. Cholera||Vibrio cholera|
|3. Plague||Pasteurella pestis|
|4. Tuberculosis||Mycobacterium tuberculosis|
|5. Tetanus||Clostridium tetani|
|6. Leprosy||Mycobacterium leprae|
|7. Syphilis||Treponema pallidum|
|8. Gonorrhoea||Neisseria gonorrhoeae|
|9. Diptheria||Corynebacterium diphtheria|
|10. Pertussis||Bordetella pertussis|
|11. Pneumonia||Diplococcus pneumonia|
|12. Botulism||Clostridium botulinum|
|13. Gas gangrene||Clostridium perfringens|
|14. Peptic ulcer||Helicobacter pylori|
Diseases of Animals:
|1. Anthrax||Bacillus anthracis|
|2. Plague||Pasteurella pestis|
|3. Leptospirosis||Leptospira interrogans|
Diseases of Plants:
|1. Bacterial blight of rice||Xanthomonas oryzae|
|2. Citrus canker||Xanthomonas citri|
|3. Black rot of cabbage||Xanthomonas campestris|
|4. Crown gall||Agrobacterium tumifaciens|
B. Cyanobacteria (Blue Green Algae or Cyanophyceae or Myxophyceae)
Introduction: Cyanobacteria also known as blue-green algae (BGA) are Gram -ve photosynthetic prokaryotes. The term “Cyanobacteria” was coined for blue-green algae under the rules of the International Code of Nomenclature of Bacteria (1978). They evolved more than 3 billion years back. Oxygenic photosynthesis also evolved with them. Cyanobacteria made the earth’s atmosphere oxygenic and provided conditions for the evolution of aerobic bacteria and the eukaryotes.
Occurrence: Cyanobacteria are one of the most successful organisms which occur in all possible types of environments. They occur in freshwater, seawater, salt marshes, moist rocks, tree trunks, moist soils, hot springs, and frozen waters. Some species occur as symbionts with different fungi and form lichen (e.g., Nostoc, Scytonema, Gloeocapsa, etc.) Species of some members like Anabaena grow as endophytes within the thallus of Anthoceros (Bryophytes), leaves of Azolla (Pteridophytes), and Nostoc in the root of Cycas (Gymnosperm).
Morphology: Cyanobacteria may be unicellular, colonial or filamentous. Each filament consists of a sheath of mucilage and one or more cellular strands called trichomes. Single trichome filaments may further be of two types, homocystous (= undifferentiated, e.g., Oscillatoria) and heterocystous (e.g., Nostoc). Spirulina has a spirally coiled filament. Colonies develop in some cases e.g., Nostoc. There is complete absence of flagellated structures. Some members show a slow gliding movement on the substratum. Oscillatoria shows pendulum-like oscillating movement by its tips.
A cyanobacterial cell is generally larger and more elaborate than bacteria. The cell has a typically prokaryotic structure. It possesses one envelope organization with a peptidoglycan cell wall, naked DNA, and 70S ribosomes. The membrane-bound structures like mitochondria, plastids, endoplasmic reticulum, Golgi bodies, lysosomes, and sap vacuoles are absent in it. The cell wall is rigid and contains peptidoglycan and muramic acid. The cell wall always possesses an external investment of a jelly-like slime sheath. A lipoprotein cell membrane lies just below the cell wall. Sometimes it becomes folded in some places and the folds are called lamellasomes. The protoplast is differentiated into peripheral pigmented chromoplast and the central cytoplasm.
Chromoplasm contains a number of free photosynthetic thylakoids. Thylakoid membranes contain chlorophyll-a, carotene, and xanthophyll. There are small granules attached to the thylakoid membranes called phycobilisomes. They possess accessory photosynthetic pigments known as phycobilins. The phycobilins are of three types- phycocyanin (blue), allophycocyanin (blue), and phycoerythrin (red). The pigments impart characteristic green to deep purple and more often green colour to the different species of Cyanobacteria. Many forms show the Gaidukov phenomenon or chromatic adaptation, where colour changes according to the wavelength of light received by the Cyanobacteria. Trichodesmium erythraeum is a reddish coloured cyanobacterium that occurs in such abundance that a sea is named after its colour – the Red Sea.
Instead of typical sap vacuoles, gas vacuoles are found. Each gas vacuole consists of a number of submicroscopic units called gas vesicles. Gas vacuoles function as light screens, and provide buoyancy-regulating mechanisms and pneumatic strength. The cell contains four types of inclusions. They are a-granules (cyanophycean starch), ^-granules (lipid droplets), volutin granules, and polyhedral protein crystals.
A naked, circular, double-stranded DNA lies coiled generally in the central part of the cytoplasm known as cytoplasm. The DNA is coiled to form a nearly compact structure called nucleoid. As in bacteria, the nucleoid of cyanobacteria is equivalent to a single chromosome of eukaryotes but without its organization. Extrachromosomal circular DNA segments called plasmids may also occur. The plasma membrane often remains attached to the nucleoid by a semicircular group of coiled membranes called lamellasome. Certain species of cyanobacteria possess some special cells called heterocysts. In filamentous species, these occur in terminal, basal, and intercalary positions. Heterocysts are the sites of nitrogen fixation. Like nitrogen-fixing bacteria, many species of cyanobacteria can fix atmospheric nitrogen (N = N) to form nitrogenous compounds for their nitrogen nutrition.
Cyanobacteria reproduce by vegetative and asexual methods. Sex organs, gametes, and flagellated zoospores are altogether absent. Vegetative reproduction takes place by fission and fragmentation. Unicellular forms multiply by binary fission. In filamentous forms, the multiplication is due to their breaking down into small pieces called hormogonia. Each hormone is capable of growing into a new individual. Asexual reproduction takes place by akinetes, exospores, endospores, nanopores, heterocysts, etc. Akinetes are thick-walled structures that survive unfavorable conditions. Typical sexual reproduction is absent in cyanobacteria, but like bacteria gene recombination by conjugation, transformation, and transduction has been recorded in some cases (e.g., Anabaena, Cylindrospermum).
1. Beneficial Role:
- Food and fodder: Spirulina, a fast-growing cyanobacterium, is an important source of protein for animals. In some regions of Africa, it forms a part of traditional food. Nostoc commune is eaten as food in China and Java. Anabaena is also an important animal fodder.
- Reclamation of usar soils: Saline and alkaline soils could be reclaimed and put under cultivation by allowing cyanobacteria like Nostoc, Anabaena, Aulosira, etc., to grow on these soils. The cyanobacteria neutralize alkalinity by their acidic secretions.
- Evolutionary role: Cyanobacteria are the earliest photosynthesisers. They have changed the primitive earth’s anaerobic atmosphere into an aerobic atmosphere.
- Nitrogen fertility: Several cyanobacteria have the ability to fix free atmospheric nitrogen into soil nitrogen. These cyanobacteria are now regularly inoculated in the rice fields in place of nitrogenous fertilizers. About twenty-two members like Nostoc, Anabaena, Aulosira, Anabaenopsis, etc., are known to fix atmospheric nitrogen.
- Early colonizers: Cyanobacteria are the early colonizers of bare and barren areas. They provide suitable conditions for the growth of other organisms even in the most hostile environments.
- Symbiotic association: Cyanobacteria successfully develop symbiotic relations with other organisms. They are photosynthetic partners in many lichens. Further, these are symbionts with protozoa, liverworts, ferns, gymnosperms, and angiosperms.
- Sewage disposal: Sewage disposal is an aerobic process and the production of oxygen from cyanobacteria like Oscillatoria is helpful. Oxidation of organic wastes is facilitated by cyanobacteria capable of growing in these habitats.
- Mosquito repellent activity: Species of Anabaena and Aulosira do not allow mosquito larvae to grow nearby. Such species can be inoculated in village ponds and rice fields to prevent the growth of mosquitoes.
- Antibiotics: A number of cyanobacteria secrete antibiotics, e.g. Microcystis, Lyngbya, Haplosiphon.
- Green manure: Anabaenopsis and Spirulina are harvested from water bodies are applied in the field as green manure. The manure increases nitrogen fertility and retains soil moisture.
- Protection from soil erosion: The growth of cyanobacteria is useful in preventing soil erosion because of mucilage holding and covering the soil particles, e.g., Tolypothrix, Anabaena.
It is found in all types of aquatic habitats, moist soils, and rocks. They also grow on polluted water or muddy banks of ponds and rivers. They are also available in water reservoirs. The plant body is a trichome. The trichome is a long, thread-like, unbranched structure and consists of numerous cells, arranged in an uniseriate fashion. The trichomes show definite polarity and the terminal cell is of different shapes in different species. Cells are small and discoid. They often possess large granules and pseudo vacuoles. Cell walls have lateral pores for the excretion of mucilage. Oscillatoria is famous for pendulum-like oscillations of its terminal half. Gliding, sliding, and rotation movements also occur. Multiplication takes place by fragmentation and hormogonia.
It is a filamentous cyanobacterium that forms rounded, oval, bulbose, foliose, or irregular colonies popularly called moon split, fallen star or star jelly. Nostoc occurs in fresh water and also in subaerial habitats. It also forms symbiotic associations with Anthoceros, Cycas roots, Gunnera stems, and Trifolium roots. A colony contains a number of flexuous intertwined filaments on the periphery, a mucilage-filled hollow interior, and a dense mucilage covering on the outside. The plant body is a trichome, it has three types of structures-vegetative cells, akinetes, and heterocysts. Heterocysts are somewhat large, cylindrical, spherical, or barrel-shaped colorless empty cells generally intercalary or terminal in position. The intercalary heterocysts have two polar nodules each and the terminal one has only one basal polar nodule. Heterocysts are specialized to perform nitrogen fixation. Akinetes are thick-walled dark-coloured resting spores. They generally develop in chains. Vegetative cells divide and help in the enlargement of trichomes. Reproduction occurs by means of fragmentation, hormogonia, akinetes and sometimes by heterocysts. Larger colonies of Nostoc are edible in China and nearby areas. Because of its ability to fix nitrogen, Nostoc enriches its habitat with nitrogen. It is also used in the reclamation of barren land and usar soil.
Anabaena is a fresh water floating alga in temporary and permanent pools, in the rice field. Some species are terrestrial and still others are found to grow endophytically in the leaves of Azolla, the water fern, and in the coralloid root of Cycas. The plant body is a trichome, consisting of many spherical or barrel-shaped and rarely subcylindrical cells arranged end to end. The trichome is uniseriate and unbranched, generally devoid of mucilaginous sheath. In Anabaena, the trichome shows continuing motility. Heterocysts and akinetes are present in trichomes. Reproduction takes place mainly by akinetes or endospores. Though Anabaena is useful as a nitrogen fixer, it often releases toxins which is harmful to animals.
Mycoplasma or mollicutes are the simplest and the smallest known prokaryotes characterized by the absence of a cell wall. These organisms were discovered in 1843 by Pasteur in the pleural field of cattle suffering from pleuropneumonia and were, therefore, called pleuropneumonia-like organisms (PPLO). Nocard and Rouse (1898) were able to grow these organisms in a culture medium. They also demonstrated that if healthy cattle were injected with these microorganisms, the symptoms of the disease appear. Mycoplasma are now known to infect a large number of animals (e.g., dogs, sheep, mice, and even man) and plants (e.g., potato, corn, brinjal, etc.)
In nature, they occur in soil, sewage water, and in plant and animal bodies. Cell size ranges from 0.1-0.15 µm. A cell wall is absent. The plasma membrane forms the outer boundary of the cell. Due to the absence of a cell wall, the organisms can change their shape and are pleomorphic. They occur in various forms such as coccoid, granular, filamentous, etc. Like other prokaryotes, mycoplasmas possess one envelope system. They lack a well-organized nucleus, ER, mitochondria, plastids, Golgi bodies, centrioles, and flagella. Genetic material is present in the form of a nucleoid. It consists of a single, circular, double-stranded DNA molecule without a nuclear membrane. Ribosomes are 70S and lie scattered in the cytoplasm. The cytoplasm also contains enzymes, RNA, proteins, and various kinds of granules. The DNA duplex is not compacted as in other prokaryotes but instead lies coiled throughout the cytoplasm. DNA possesses a replicating disc at one end to assist in the replication and separation of the genetic material. The amount of DNA and RNA in the mycoplasma cells is less than half what normally occurs in other prokaryotes. It is perhaps the lowest limit essential for a cellular organism.
Mycoplasma are gram-negative and usually nonmotile. However, some forms show gliding movements. They are heterotrophic in their mode of nutrition. Some of them are saprophytes but most of them are parasitic on plants and animals. Mycoplasma may reproduce by binary fission and budding. In many cases, nucleoid undergoes rapid multiplication without cytokinesis forming branching filaments. Later on, constrictions appear in between the nucleoids forming chains of cells that separate. Mycoplasma are always harmful being the causative agents of various plant and animal diseases. Commonly known plant diseases are aster yellows, potato witches broom, mulberry dwarf, maize stunt, rice yellow dwarf, bunchy top of papaya, little leaf of brinjal, etc. Common animal diseases are rheumatism, arthritis, several respiratory disorders, and primary atypical pneumonia (PAP) are caused by mycoplasma. M. hominis and M. fermentans, cause infertility in human males. Mycoplasmas are not affected by penicillin but are inhibited by tetracyclines.
The name is Rickettsiae was introduced by Da Rocha Lima (1916) in honour of Howard Taylor Ricketts. These are gram-negative, obligate, pleomorphic but intracellular parasites that are resident in the intestines of certain arthropods like lice, fleas, ticks, and mites. They are intermediate between true bacteria and viruses. Like virus Rickettsiae can not be grown in an inanimate culture media. But they differ from viruses in their inability to pass through the bacterial filter, large size, and microscopic visibility, and their possession of cellular metabolic systems like bacteria. The Rickettsiae exhibit a wide range in size and shape, some are spherical diplococcal or rod-shaped bodies 0.3 µm – 0.5 µm in diameter.
The cell wall is like a typical bacterial wall. ATP synthesis is absent, but ADP is exchanged with host cell ATP. Flagella, pili and capsule are absent. Within the cytoplasm, the nucleoid is a circular DNA associated with RNA, enzymes, and ribosome granules. Reproduction takes place by binary fission only so long as they are present within the living cells of host organisms. They often become pathogenic in mammals and humans where they cause typhus group of fever spread by droplet method, lice, ticks, fleas, etc. Endemic typhus is caused by Rickettsia typhi, epidemic typhus is caused by R. prowareckii. Rocky Mountain spotted fever is caused by R. rickettsiae. Unfortunately, Sir Howard Ricketts died in 1910 due to typhus fever while studying the same organisms that cause this disease.
The Spirochaetes are long, thin, spirally twisted micro-organisms. The term Spirochaeta was first introduced by Van Ehrenbergh (1838). Their length varies from 6 pm to 500 pm and their breadth from 0.2 µm to 0.75 µm. The cells are without cell walls, plasma membrane is spirally twisted. The nucleoid is double-stranded circular DNA. The cytoplasm contains RNA, ribosomes, and various enzymes. Reproduction takes place by binary fission. Spirochaetes are parasitic, commensal or free-living. Many diseases are caused by them as Treponema pallidum causes syphilis. Leptospira causes infectious jaundice and Borrelia causes relapsing fever.
Chlamydiae are gram-negative intracellular parasites of about 0.25 µm size, often grouped along rickettsiae but differ from them in a reproductive cycle that involves the formation of initial or reticulate bodies (RB) and elementary bodies inside the host phagosome. Their cell wall is like a typical bacterial wall. ATP generation is absent. ATP is obtained from outside. Chlamydiae are thus energy parasites. Chlamydia trachomatis causes conjunctivitis, sexually transmitted non-gonococcal urethritis, and epididymitis. C. pneumoniae causes pneumonia and- bronchopneumonia, C. psittaci is a bird parasite but contaminated feces cause psittacosis in humans.
Actinomycetes are mycelial (aseptate branched filaments) bacteria that form radiating colonies in culture. Because of this, they were formerly called ray fungi. The mycelial form is reduced in Mycobacterium and Corynebacterium. They occur abundantly in soil, water, manure, mud, milk, and other food products. Most of them are saprophytes, some are parasites. Mycelia have a diameter of 1 µm or less. The cell wall is rich in lipids including waxes and mycolic acid (fatty acid). Like true bacteria, actinomycetes are prokaryotic. They lack a well-organised nucleus. Instead, chromatin granules lie scattered in the cytoplasm. Most of them are Gram-positive and nonmotile. Actinomycetes reproduce vegetatively by fragmentation. The mycelium, under favourable conditions of food and temperature, breaks up into small fragments each of which grows into a new mycelium. Some species also reproduce by means of conidia, oidia, or arthrospores and by the formation of sporangiospores. Conidia and oidia develop singly or in chains on conidiophores and sporophores respectively.
Most of the actinomycetes are saprophytic and constitute an important component of decomposers, e.g. Actinomyces, Streptomyces, etc. A few are pathogenic in plants, animals, and humans, e.g., Actinomyces (Actinomycosis), Nocardia (Nocardiosis), Corynebacteria, Mycobacteria, etc. In pathogenic actinomycetes (e.g., Mycobacterium) a derivative of mycolic acid called mycoside (= cord factor) is involved in causing disease. A number of antibiotics are produced by actinomycetes, especially the genus Streptomyces (Streptomycin, Chloramphenicol, tetracyclines, terramycin, erythromycin, novobiocin, nystatin, etc.) Frankia produces root nodules in some nonlegumes (e.g., Alnus) and takes part in nitrogen fixation.