Characteristics of Monera

Characteristics of Monera

Characteristics of Monera, Monera (Monos – single)

Monera (Monos – single) includes prokaryotes and shows the following characters :

(1) They are typically unicellular organisms (but one group is mycelial).

(2) The genetic material is naked circular DNA, not enclose by nuclear envelope.

(3) Ribosomes and simple chromatophores are the only subcellular organelles in the cytoplasm. The ribosomes are 70 S. Mitochondria, plastids, golgi apparatus, lysosomes, endoplasmic reticulum, centrosome, etc., are lacking.

(4) Sap vacuoles do not occur. Instead, gas vacuole may be present.

(5) The predominant mode of nutrition is absorptive but some groups are photosynthetic (holophytic) and chemosynthetic.

(6) The organisms are non-motile or move by beating of simple flagella or by gliding.

(7) Flagella, if present, are composed of many intertwined chains of a protein flagellin. They are not enclosed by any membrane and grow at the tip.

(8) Moneran cells are microscopic (1 to few microns in length).

(9) Most organisms bear a rigid cell wall (Peptidoglycan).

(10) Reproduction is primarily asexual by binary fission or budding. Mitotic apparatus is not formed during cell division.

(11) The kingdom Monera includes true bacteria, mycoplasma, rickettsias, actinomycetes (ray fungi) etc. Microbiologists also include blue green algae (i.e., Cyanobacteria) under the group bacteria because of the presence of prokaryotic cell structure. Studies have established that the members of archaebacteria group are most primitive and have separated from eubacteria group very early in the process of evolution.


Study of bacteria is called bacteriology. Linnaeus placed them under genus vermes. Nageli classified bacteria under schizomycetes. Bacteria are unicellular, microscopic and cosmopoliton organisms. The branch of science, which deals with the study of microorganism and their process is called as microbiology. Antony Von Leeuwenhoek is known as father of microbiology and father of modern microbiology is Robert Koch.

These are the smallest cell wall having prokaryotic cell. The bacteria constitute a highly specialised group of one celled plants. They differ from animals in having a rigid cell wall and being capable to synthesize vitamins. Bacteria were first seen by a Dutch lens maker, Antony Von Leeuwenhoek (1683) who named them animalcules. Louis Pasteur (1822-95) made a detailed study of bacteria and proposed germ theory of disease. Ehrenberg (1829) was the first to use the term bacterium. Robert Koch (1881) found that some diseases like tuberculosis, cholera in man, and anthrax in cattle is caused by bacteria. Lister introduced antiseptic surgery. He used carbolic acid for sterilization of surgical instrument. Pasturization theory was proposed by Louis Pasteur.

(1) Size : They are 3 to 5 microns (1m = 1/1000 millimetre or about 1/25,000 inch) in length. A few species of bacteria are approximately 15m in diameter.

(2) Shape : The bacteria possess the following forms :

Cocci (Gk. Kokkos = Berry) : They are oval or spherical in shape. They are called micrococcus when occur singly as in Micrococcus, diplococcus when found in pairs as in Diplococcus pneumoniae, tetracoccus in fours, streptococcus when found in chains as in Streptococcus lactis, staphylococcus when occurring in grape like clusters as in Staphylococcus aureus and Sarcine, when found in cubical packets of 8 or 64 as in Sarcina.

Bacilli : They are rod–shaped bacteria with or without flagella. They may occur singly (bacillus), in pairs (diplobacillus) or in chain (streptobacillus).

Vibrios : These are small and ‘comma or kidney’ like. They have a flagellum at one end and are motile, vibrio bacteria has curve in its cell e.g., Vibrio cholerae.

Spirillum (Spira = Coil) : They are spiral or coiled like a cork-screw. The spirillar forms are usually rigid and bear two or more flagella at one or both the ends e.g., Spirillum, Spirochaetes etc.

Filament : The body of bacterium is filamentous like a fungal mycelia. The filaments are very small e.g., Beggiota, Thiothrix etc.

Stalked : The body of bacterium possesses a stalk e.g., Caulobacter.

Budded : The body of bacterium is swollen at places e.g., Rhodomicrobium.

(3) Flagellation : Depending upon the presence or absence of flagella, the bacteria are of following types :

Atrichous : When the flagellum is absent it is called atrichous. e.g., Pasturella, Lactobacillus

Monotrichous : Only one flagellum is found at one end. e.g., Vibrio cholerae.

Lophotrichous : When a group of flagella is present at one end e.g., Spirillum volutans.

Amphitrichous : When single or group of flagella is present at both the end e.g., Nitrosomonas.

Peritrichous : A number of flagella are present all over the body. e.g., E. coli, Clostridium tetani.

Staining of bacteria

(1) Simple staining : The coloration of bacteria by applying a single solution of stain to a fixed smear is termed simple staining. The cells usually stain uniformly.

(2) Gram staining : This technique was introduced by Hans Christian Gram in 1884. It is a specific technique which is used to classify bacteria into two groups Gram +ve and Gram –ve. The bacteria are stained with weakly alkaline solution of crystal violet. The stained slide of bacteria is then treated with 0.5 percent iodine solution. This is followed by washing with water or acetone or 95% ethyl alcohol. The bacteria which retain the purple stain are called as Gram +ve. Those which become decolourised are called as Gram –ve. In general the wall of Gram +ve bacteria have simpler nature as compared to Gram –ve bacteria. E.coli is a Gram –ve bacterium. Gram negative bacterium can be seen with other stain safranin.

Gram positive bacteria : e.g., Pneumococcus, Streptococcus, Staphylococcus, Bacillus, Clostridium, Mycobacterium, Streptomyces.

Gram negative bacteria : e.g., Salmonella, Pseudomonas, Escherichia, Haemophilus, Helicobacter, Vibrio, Rhizobium.

Structure of bacteria

(1) Capsule : In a large number of bacteria, a slimy capsule is present outside the cell wall. It is composed of polysaccharides and the nitrogenous substances (amino acids) are also present in addition. This slime layer becomes thick, called capsule. The bacteria, which form a capsule, are called capsulated or virulent bacteria. The capsule is usually found in parasitic forms e.g., Bacillus anthracis, Diplococcus pneumoniae, Mycobacterium tuberculosis.

It provides protection against phagocytosis and antibiotics. Capsule also protects the cell against dessication and viral attack. The capsulated bacteria are usually non-flagellated (i.e., Atrichous).

Capsulated bacteria form smooth colonies and are known as S-type bacteria, which are highly virulent. Non-capsulated bacteria form rough colonies and are known as R–type bacteria.

(2) Cell wall : All bacterial cells are covered by a strong, rigid cell wall. Therefore, they are classified under plants. Inner to the capsule cell wall is present. It is made up of polysaccharides, proteins and lipids.

In the cell wall of bacteria there are two important sugar derivatives are found i.e., NAG and NAM (N-acetyl glucosamine and N-acetyl muramic acid) and besides L or D – alanine, D-glutamic acid and diaminopimelic acid are also found.

One of the unique components of cell wall of bacteria is peptidoglycan or mucopeptide or murien (made of mucopolysaccharide + polypeptide).

In peptidoglycan, NAG and NAM are joined by short peptide chains or cross bridges of amino acids.

Outer layer of cell wall of Gram –ve bacteria is made up of lipopolysaccharides and cell wall of Gram +ve bacteria of teichoic acid.

The cell wall of Gram positive bacteria is much thicker and contains less lipids as compared to that of Gram –ve bacteria. The enzyme lysozyme can dissolve the bacterial cell wall.

(3) Plasma membrane : Each bacterial cell has plasma membrane situated just internal to the cell wall. It is a thin, elastic and differentially or selectively permeable membrane. It is composed of large amounts of phospholipids, proteins and some amounts of polysaccharides but lacks sterols. It is characterised by possessing respiratory enzymes.

Mesosome : On the plasma membrane generally at mid point, there are present some circular coiled bodies called mesosomes. So mesosomes are simply infoldings of plasma membrane. Mesosomes contain respiratory enzymes like oxidases and dehydrogenases and hence they help in respiration. Hence mesosomes are also known as “mitochondria of bacterial cell” or chondrioides. Mesosomes are more prominent in Gram +ve bacteria.

1. It receive DNA during conjugation and DNA replication enzyme.

2. Mesosome participate in the formation of septa during cell division.

(4) Cytoplasm and Cytoplasmic inclusions : The cytoplasm is a complex aqueous fluid or semifluid ground substance (matrix) consisting of carbohydrates, soluble proteins, enzymes, co-enzymes, vitamins, lipids, mineral salts and nucleic acids. The organic matter is in the colloidal state.

The cytoplasm is granular due to presence of a large number of ribosomes. Ribosomes in bacteria are found in the form of polyribosome. Membranous organelles such as mitochondria, endoplasmic reticulum, golgi bodies, lysosomes and vacuoles are absent. In some photosynthetic bacteria the plasma membrane gives rise to large vesicular thylakoids which are rich in bacteriochlorophylls and proteins.

Volutin granules : They were first reported in Spirillum volutans bacteria. These are also known as metachromatic granules, which are composed of polyphosphate. Volutin serves as a reserve source of phosphate.

Poly-b-hydroxy butyric acid granules (PHB) : These are polymer of lipid like material and chloroform soluble which are often found in aerobic bacteria especially under high carbon low nitrogen culture conditions. Granules can serve as a reserve carbon and energy source.

Glycogen and Sulphur granules : Glycogen are also known as polysaccharide granules. It can be stained brown with iodine. By electron microscopy they appear as dark granules.

(5) Nucleoid : It is also known as genophore, naked nucleus, incipient nucleus. There is present nuclear material DNA which is double helical and circular. It is surrounded by some typical protein (polyamine) but not histone proteins. Histones (basic proteins) are altogether absent in bacteria. This incipient nucleus or primitive nucleus is named as nucleoid or genophore.

(6) Plasmid : In addition to the normal DNA chromosomes many bacteria (e.g., E.coli) have extra chromosomal genetic elements or DNA. These elements are called plasmids. Plasmids are small circular double stranded DNA molecules. The plasmid DNA replicates independently maintains independent identity and may carry some important genes. Plasmid terms was given by Lederberg (1952). Some plasmids are integrating into the bacterial DNA chromosome called episomes.

(7) Flagella : These are fine, thread-like, protoplasmic appendages which extend through the cell wall and the slime layer of the flagellated bacterial cells. These help in bacteria to swim about in the liquid medium.

Bacterial flagella are the most primitive of all motile organs. Each is composed of a single thin fibril as against the 9+2 fibrillar structure of eukaryotic cells. The flagellum is composed entirely of flagellin protein.

The diameter of each subunit is about 40-50Å. These subunits are arranged around a hollow axis. A flagellum is usually 4.5m long and 120-185Å in diameter. Flagellum is attached to cell membrane by a special terminal hook, which is attached to the basal body called (blepharoplast). A bacterial flagellum can be divided into three parts – (Basal granule, Hook and Filament).

(8) Pili or Fimbriae : Besides flagella, some tiny or small hair-like outgrowths are present on bacterial cell surface. These are called pili and are made up of pilin protein. They measure about 0.5–2mm in length and 3–5mm in diameter. These are of 8 types I, II, III, IV, V, VI, VII, and F types. I to F are called sex pili. These are present all most all gram –ve bacteria and few gram +ve bacteria. Fimbriae take part in attachment like holding the bacteria to solid surfaces.

The function of pili is not in motility but they help in the attachment of the bacterial cells. Some sex pili acts as conjugation canals through which DNA of one cell passes into the other cell.

Reproduction in bacteria

Vegetative reproduction

(1) By budding : According to Bisset and Hale, reproduction by budding takes place in Bigidi bacterium bifidus.

(2) By binary fission : This type of reproduction is most common in all kinds of bacteria. Under favourable conditions bacterial cell expands. Cytoplasm divides into two parts due to constriction and formation of a transverse septum in the centre of the cell. Later on, these two parts separate from each other and give rise to two cells.

The speed of binary fission is decreased due to low temperature. Therefore, food is preserved in the cold storage. The cause of food spoilage and bacterial infection is the rapid multiplication of bacteria.

Asexual reproduction

(1) By endospores : Endospores are formed in all species of the genera Bacillus and Clostridium. In each cell only one endospore is formed. Endospore is highly resistant to very high and very low temperature.

Endospore is found either in the centre or near the cell wall. Under unfavourable conditions cytoplasm shrinks and gets rounded and around it a hard protective three layer is formed. Each endospore may be either circular, ellipsoidal or semicircular. When favourable conditions come, outer layers rupture and active bacterial cell comes out. So this is a method of perennation (i.e., to tide over unfavourable condition) and some people say it “reproduction wihtout multiplication”.

(2) By conidia : Some filamentous bacteria e.g., Streptomyces reproduce by means of conidia. The conidia are spore like in structure and are formed in chains. Each conidium gives rise to a new bacterium.

(3) By zoospores : In rare cases bacterial cell forms some motile spores which give rise to new cells. This process has been rarely seen. e.g., Rhizobium.

Sexual reproduction (Genetic recombination)

Sometimes it was believed that sexual reproduction does not take place in bacteria. Lederberg and Tatum (1946) proved that sexual reproduction takes place in bacteria. On the basis of this discovery they were awarded Nobel Prize.

According to the present view, three types of sexual reproduction are found in bacteria :

(1) Transformation : In this process one kind of bacterium is transformed into another kind. It takes place by transferring DNA from capsulated to non-capsulated bacterium. For the first time Griffith (1928) reported transformation in mice. Later on, Avery, Mcleod and McCarty (1944) studied transformation in Diplococcus pneumoniae.

(2) Transduction : In this process DNA of a bacterial cell is transferred into another bacterial cell through bacteriophage – a kind of virus which is parasitic upon bacteria. Bacteriophage consists of DNA. It has been now accepted that DNA of a bacterial cell is transferred through bacteriophage to another bacterium. Transduction was first of all reported by N.D. Zinder and Lederberg (1952) in bacteria Salmonella typhimurium.

(3) Conjugation : In this process genetic material from one strain of bacterium which is known as male is transferred into another strain of bacterium which is known as female. On the experimental basis it is believed that genetic material of male enters into female bacterium in the form of a straight line. Lederberg and Tatum first of all reported conjugation in bacterial strain of E.coli called K12 (1946). In 1966, Wollman and Jacob described it in details.

In gram negative bacteria, there are two strains, F+ (with fertility factor and sex pili) and F– (without fertility factor and sex pili). These two can come together. Sex pilus of donor cell extrudes a protein that helps it in attaching to the recipient cell. Latter on, sex pilus is converted into conjugation tube between the two. The donor or F+ can transfer its fertility factor or plasmid to recipient cell or F– and convert it into donor as well. Sometimes the F+ plasmid attaches to nucleoid, becomes episome and converts the donor into HFr (high frequency of recombination 1 : 100). There is a transfer of few genes of nucleoid from HFr to F–. HFr quality can also be transferred occasionally (HFr × F– = F– plus a few genes and a few HFr). The transferred segment is called exogenote which ‘similar’ segment of the recipient bacterium is known as endogenote. The recipient bacterium is called merozygote or partial zygote. Some of the donor genes integrate into recipient genome.

In gram positive bacteria sex pili do not develop. Donor cells produce a protein adhesion over their surface for bringing recipient cells in contact with them. In Streptococcus faecalis, the recipient cells excrete a small peptide (sex hormone) for inducing clumping. Wall dissolves in the region of contact. Transfer of DNA segment occurs from donor to recipient cell.

Respiration in bacteria

With respect to oxygen requirement and mode of cellular respiration, bacteria distinctly belong to two broad categories :

(1) Aerobic respiration

Obligate aerobes : These bacteria grow exclusively in presence of molecular oxygen and fail to survive in its absence, e.g., Bacillus subtilis, Azotobactor, Arthrobactor, Mycobacterium etc.

Facultative anaerobes : The aerobic bacteria which can also survive in absence of oxygen, e.g., Aerobacter, Klebsiella, Pseudomonas, etc.

(2) Anaerobic respiration

Obligate anaerobes : These bacteria grow and multiply in the absence of free oxygen. They fail to survive under aerobic conditions, e.g., Clostridium botulinum.

Facultative aerobes : The anaerobic bacteria which can also survive in presence of oxygen, e.g., Chlorobium limicola.

Mode of nutrition in bacteria

On the basis of mode of nutrition, bacteria are grouped into two broad categories. First is autotrophic and second is heterotrophic bacteria.

Autotrophic bacteria : These bacteria are able to synthesize their own food from inorganic substances, as green plants do. Their carbon is derived from carbon dioxide. The hydrogen needed to reduce carbon to organic form comes from sources such as atmospheric H2, H2S or NH3. These are divided into two categories.

(1) Photoautotrophic bacteria : These bacteria are mostly anaerobic bacteria. They use sunlight as source of energy to synthesize food.

They possess a pigment called bacteriochlorophyll which is different from the chlorophyll pigment found in higher plants. This is known as anoxygenic photosynthesis. e.g., Green sulphur (Thiothrix) and purple sulphur (Chromatiun) bacteria. They can perform photosynthesis in far-red light. Rhodospirillum bacteria fixes CO2 into carbohydrate (Photoautotrophic).

Green sulphur bacteria : They are autotrophic. The hydrogen donor is H2S and the pigment involved in the process is chlorobium chlorophyll (Bacterioviridin) e.g., Chlorobium.


Purple sulphur bacteria : They are also autotrophic. The hydrogen donor is thiosulphate and the pigment involved in photosynthesis is bacteriochlorophyll e.g., Chromatium.


Purple non-sulphur bacteria : They are heterotrophic utilizing succinate or malate or alcohol. e.g., Rhodospirillum, Rhodopseudomonas.


(2) Chemoautotrophic bacteria : Some bacteria manufacture organic matter form inorganic raw materials (such as carbon dioxide) and utilize energy liberated by oxidation of inorganic substances present in the external medium such as ammonia, ferrous ion, nitrates, nitrites, molecular hydrogen, etc. The energy liberated from exergonic chemical reactions is trapped in the ATP molecules which is used in carbon assimilation to synthesize organic matter.

Sulphur bacteria : These bacteria derive energy by oxidizing hydrogen sulphide or molecular sulphur. Beggiatoa, a colourless sulphur bacterium oxidises hydrogen sulphide to water and sulphur. The energy released is used up and the sulphur granules are deposited inside or outside the body of bacterial cell.


Iron bacteria : Some chemoautotrophic bacteria such as Gallionella, Sphaerotilus, Ferrobacillus, etc, inhabit the environments where irons to ferric form. The Ferric ions are deposited in the form of soluble ferric hydroxide and the energy released during the conversion is used in the production of carbohydrates.


Hydrogen bacteria : These bacteria utilize free molecular hydrogen and oxidize to hydrogen into water with the help of either oxygen or oxidize salts e.g., Hydrogenomonas. $2{{H}_{2}}+{{O}_{2}}to 2{{H}_{2}}O+text{Energy}$ (56 kcal).

Amonifying bacteria : They oxidise protein and amino acid into NH3 (ammonia). e.g., Proteus vulgaris, Bacillus mycoids.

Nitrifying bacteria : They oxidise ammonia to nitrites and then into nitrates.

$N{{H}_{3}}+{{O}_{2}}xrightarrow{text{Nitrosomonas}}N{{O}_{2}}+{{H}_{2}}O+text{Energy}$ and $2N{{O}_{2}}+{{O}_{2}}xrightarrow{text{Nitrobacter}}2N{{O}_{3}}+text{Energy}$.

Denitrifying bacteria : They change nitrogen compound into molecular nitrogen. So that they reduce fertility of soil e.g., Micrococcus denitrificans, Pseudomonas denitrificans.

Methane bacteria : The bacterium Methanomonas utilizes methane as source of carbon and energy.


Methane producing bacteria : These are spherical or rod shaped bacteria which produce methane from hydrogen gas and carbon dioxide e.g., Methanobacterium. $C{{O}_{2}}+4{{H}_{2}}xrightarrow{{}}C{{H}_{4}}+2{{H}_{2}}O$

Methane (swamp gas) is produced under anaerobic conditions and can be used as a “biogas”, otherwise it is a pollutant that contributes to the green house effect and global warming.

Carbon bacteria : These bacteria oxidize carbon monoxide into carbon dioxide and use the liberated energy, e.g., Bacillus oligocarbophilus. $2C{{O}_{2}}+{{O}_{2}}xrightarrow{{}}2C{{O}_{2}}+text{Energy}$.

Heterotrophic bacteria : Most of the bacteria can not synthesize their own organic food. They are dependent on external organic materials and require atleast one organic compound as a source of carbon of their growth and energy. Such bacteria are called heterotrophic bacteria. Heterotrophic bacteria are of three types. Parasites, Saprotrophs and Symbionts.

(1) Parasites : They obtain their organic food or special organic compounds required for their growth from living cells of plants and animals. Some parasitic bacteria are relatively harmless and nonpathogenic, i.e., do not produce disease in hosts. However, majority of parasitic bacteria are pathogenic and cause serious diseases in plants and animals either by exploiting them or by secreting poisonous substances called toxins.

(2) Saprotrophic bacteria : These bacteria obtain their nutritional requirements from dead organic matter (such as animal excreta, corpses, fallen leaves, bread, fruits, vegetables, jams, jellies, etc.). These bacteria breakdown the complex organic matter into simple soluble forms by secreting exogenous digestive enzymes. Then they absorb the simple nutrient molecules and assimilate them. Aerobic break down of organic matter is called decomposition or decay.

(3) Symbiotic bacteria : Symbiosis is the phenomenon in which the two organisms live in close association in such a way that both the partners get mutual benefit from this association. For example, a very well known nitrogen fixing bacteria – Rhizobium forms a symbiotic association with roots of leguminous plants (soyabean, clover, alfalfa, etc.) and producing root nodules.

Another example of symbiosis is the presence of enteric bacterium Escherichia coli (E. Coli) in human intestine. The bacteria shares our food but at the same time checked the growth of harmful putrefying bacteria and releases vitamin K and B12, which help to produce blood components. 


These are free inhabitants of mud and water, and are chemoheterotrophic unicellular bacteria. These are spiral or helicoid in shape, covered by flexible cell wall. In spirochaetes flagella are absent but the cells are able to swim over solid surface by the fibrillae. Many diseases are caused by them as Treponema pallidum causes syphilis, Leptospira causes infectious jaundice and Borrelia causes relapsing fever. Besides some spirochaetes are found in teeth.


They are present in rumen (first part of stomach) of cattles. This is simplest and most primitive group of bacteria. The cell wall of these bacteria is made of polysaccharides and proteins (peptidoglycans and muramic acid are absent in cell wall). Further branched chain lipids are present in plasma membrane of archaebacteria, due to which these can face extremes of conditions of temperature and pH. Archaebacteria are considered to be ‘oldest of living fossils’. Three main groups of archaebacteria are following.

(1) Methanogens : These are strict anaerobic bacteria and mainly occur in muddy areas and also in stomach of cattle, where cellulose is fermented by microbes. These are responsible for methane gas formation in bio-gas plants, because they have capacity to produce CH4 from CO2 or formic acid (HCOOH). e.g., Methanobacterium, Methanobacillus, Methanosarcina and Methanococcus.

(2) Salt lovers archaebacteria or Halophiles : These are also anaerobic bacteria, which occur in extreme saline or salty conditions (upto 35% of salt or NaCl in culture medium). A purple pigmented membrane containing bacteriorhodopsin is developed in sun-light in these bacteria, which utilizes light energy for metabolic activities (different from photosynthesis). e.g., Holobacterium and Halococcus.

(3) Thermoacidophiles : These are the bacteria which are found in hot sulphur springs (upto 80oC). As against first two groups of archaebacteria, these are aerobic bacteria. These have the capacity to oxidize sulphur to ${{H}_{2}}S{{O}_{4}}$ at high temperature and high acidity (i.e., pH 2.0), hence given the name Thermoacidophiles, i.e., temperature and acid loving.

$2S+2{{H}_{2}}O+3{{O}_{2}}to 2{{H}_{2}}S{{O}_{4}}$+ energy. e.g., Sulfobolus, Thermoplasma, Thermoproteus.


It is a group of unicellular branched filamentous bacteria which resemble fungal mycelia. They grow in the form of radiating colonies in cultures and therefore, commonly called ray fungi. They are Gram +ve chemo-organotrophic, saprotrophic bacteria. Most species are facultative anaerobic. These are generally present as decomposers in soil. The filaments are aseptate (non-septate) branched and very thin (about 0.2 to 1.2 mm in width). The wall contains mycolic acid. They reproduce asexually by means of conidia, which are produced at tips of filaments. The endospores are not formed. Most of these secrete chemical substances having antimicrobial activities called antibiotics. Some of the most common and effective antibiotics are obtained from the different species of the genus streptomyces.

Some species are pathogenic and cause diseases in plant, animal and human beings, e.g., Mycobacterium. Some common diseases in plants are yellow ear rot of wheat (Tundu disease) caused by Corynebacterium tritici and scab of potato by Streptomyces scabies.

Rickettsias (H.T. Ricketts 1909)

They are gram negative obligate pleomorphic but walled obligate intracellular parasites which are transmissible from arthropods. They are intermediate between true bacteria and viruses. Rickettsias require exogenous factors for growth. Cell wall is like typical bacterial wall. ATP synthesis is absent but ADP is exchanged with host cell ATP. They have genome and size (0.3-0.5) smaller than true bacteria but have a longer generation time. Internally the cells of rickettsias contain DNA as well as RNA in a ratio of 1 : 3 .5. The cell walls contain muramic acid and are sensitive to lysozyme. Flagella, pili and capsule are absent. Reproduction occurs by binary fission. The natural habitat of rickettsiae is in the cells of arthropod gut. They cause typhus group of fevers. Spread by droplet method, lice, ticks, fleas, etc.

Importance of bacteria

Bacteria are our ‘friends and foes’ as they have both useful and harmful activities.

Useful activities

(1) In agriculture or In soil fertility : Some bacteria increase soil fertility. Nitrogen is essential for all plants. Nitrogen occupies 80% of the atmosphere. The plants take nitrogen in the form of nitrates. In soil, nitrates are formed by three processes :

By nitrogen fixing bacteria : Bacteria are found in soil either free e.g., Azotobacter and Clostridium or in root nodules of leguminous plants e.g., Rhizobium leguminosarum. These bacteria are capable of converting atmospheric free nitrogen into nitrogenous compounds.

Nitrifying bacteria : These bacteria convert nitrogen of ammonia into nitrite (NO2) e.g., nitrosomonas and convert nitrite compounds into nitrates e.g., nitrobacter.

Decay of dead plants and animals : Some bacteria attack on dead bodies of plants and animals and convert their complex compounds into simpler substances e.g., carbon dioxide (CO2), water (H2O), nitrate (NO3), sulphate (SO4) etc.

(2) In dairy : Bacterium lactici acidi and B. acidi lactici are found in milk. These bacteria ferment lactose sugar found in milk to form lactic acid by which milk becomes sour.

Lactic acid bacteria bring together droplets of casein a protein found in milk and help in the formation of curd. On freezing of casein of milk protein it is fermented by bacteria with the result that foamy and soft substance, different in taste is formed.

(3) In industries : From industrial point of view bacteria are most important. Some of the uses of bacteria in industries are as follows

Vinegar industry : Vinegar is manufactured from sugar solution in the presence of Acetobacter aceti.

Alcohol and acetone : Clostridium acetobutylicum takes part in the manufacture of butyl alcohol and acetone.

Fibre retting : By this process fibres of jute, hemp and flax are prepared. In the preparation of flax, hemp and jute the retting of stems of Linum usitatissimum (Flax = Hindi Sunn), Cannabis sativa (Hemp = Hindi Patson) and Corchorus capsularis (Jute) respectively is done.

Tobacco industry : Bacillus megatherium is used for its fermentative capacity for developing flavour and taste in tobacco leaves.

Tea industry : By fermentative action of Mycococcus condisans curing of tea leaves is done. By this process special taste is developed in the tea leaves.

Tanning of leather : Some bacteria decompose fats which are found in skin of animal with the result that skin and hairs are separated from each other and this leather becomes ready for use.

Disposal of sewage : Some bacteria convert organic faecal substances e.g., cow dung, decaying leaves of plants, etc. into manure and humus.

Human symbionts : Escherichia coli inhabitats the large intestine of man and other animals and it synthesizes vitamins.

(4) In medicines : Some of the antibiotics are manufactured by bacterial actions e.g., Bacillus brevis – antibiotic thyrothricin and B. subtilis – antibiotic subtelin. Vitamin B2 is manufactured by fermentative action of Clostridium acetobutylicum.

Antibiotics : These are the chemical substances produces by living microorganisms capable of inhibiting or destroying other microbes. These are the products of secondary and minor metabolic pathways, mostly secreted extracellularly by the microorganisms. These are used in controlling various infectious diseases.

Harmful activities

(1) Food poisoning : Some saprotrophic bacteria cause decay of our food, i.e., they alter their normal form and induce unpleasant aroma, taste and appearance. Some bacteria produce powerful toxins in food to cause “food poisoning”. Consumption of such food may cause serious illness or even death.

Botulism : It is caused by Clostridium botulinum. The main symptoms are vomitting followed by paralysis and death.

(2) Spoilage of food : Some examples of bacterial food spoilage are :

Greening on meat surface is caused by Lactobacillus and Leuconostoc. Souring of milk is caused by Lactobacillus and Streptococcus. Explosion of curd (gas production) is caused by Clostridium and Coliform bacteria. Ropiness (i.e., slimy milk) is caused by Klebsiella and Enterobacter sp.

(3) Pollution of water : There are reports of epidemics of cholera, typhoid, jaundice and other infectious diseases, which were caused by polluted water. Many pathogenic bacteria such as, Vibrio cholerae, Salmonella typhi, Leptospira cetero-haemorrhagiae, etc. pollute water and make it unfit for drinking. These are eliminated by chlorination.

(4) Deterioration of textiles : Some bacteria (e.g., Cytophaga, Vibrio and Cellulomonas) damage cellulose of textiles.

(5) Abortion : Bacteria like Salmonella induce abortion in goats, horses, sheep etc.

(6) Biological warfare : Some bacteria which cause diseases like anthrax, black-leg, tuberculosis etc, are employed as secret war agents.

(7) Denitrification : Denitrification bacteria like Bacillus licheniformis, Pseudomonas aeruginosa convert nitrates and nitrites into free nitrogen, thus responsible for the process of denitrification. Thus soil is depleted of essential nutrient like usable form of nitrogen.

(8) Putrefaction : It is the spoilage of protein in the absence of O2 by the putrefying bacteria e.g., Proteus, Mycoides.

(9) Retting of fibres : It is the hydrolysis of pectic substances that bind the cells together. e.g., Clostridium sp., Pseudomonas fluorescence.


Mycoplasmas were discovered by E. Nocard and E. R Roux (1898). They were first isolated from bovine sheep suffering from pleuropneumonia. They are often designated as pleuropneumonia–like organisms (PPLO). These organisms were later put under the generic name mycoplasma by Nowak (1929). In 1966 international commitee of Nomanclature of bacteria, placed mycoplasmas under the class mollicutes, which consists of two genera Mycoplasma and Acholeplasma. These are the simplest and smallest unicellular non-motile known aerobic prokaryotes without cell wall. So that they can change their shape therefore called Jockers of microbiological park.

Structure : They are one of simplest prokaryotic and gram negative organisms. Their size varies from 0.1 – 0.15 mm. They lack the cell wall. Due to the absence of the cell wall, these organisms are highly elastic and readily change their shape; hence the mycoplasmas are irregular and quite variable in shape. That is called pleomorphism. They may be coccoid, granular, pear–shaped, cluster–like or filamentous. Mycoplasma cells are covered with three layered plasma membrane.

Unit membrane is made up of lipoprotein. They lack the well organised nucleus, endoplasmic reticulum, mitochondria, plastids, golgi bodies, centrioles, flagella, etc. The genetic material is a single, linear, double–stranded molecule of DNA, without a nuclear envelope. Unlike other prokaryotes, it is coiled throughout the cytoplasm. The cytoplasm contains the ribosomes which are 70S. It also contains RNA, proteins, lipids and many kinds of enzymes used in biosynthetic reactions. Lipids include cholestrol and cholestrol esters which are characteristic of animal cells and are not found in true bacteria and cyanobacteria. The amount of DNA and RNA in the cells is usually less than half of that which occurs in other prokaryotes. There is 4% DNA and 8% RNA. Mycoplasmas are gram-negative.

Physiology and reproduction : Mycoplasma are usually non–motile. They are sensitive to tetracycline and resistant to penicillin. These are destroyed usually by treatment of heat at 50o C for 6 hours mycoplasma are osmotically inactive. Mycoplasmas are heterotrophic in their mode of nutrition. Some of them are saprotrophs, but most of them are parasitic on plants and animals including man. They reproduce by budding or binary fission. Fragmentation specially in filamentous forms. Besides this, Mycoplasma reproduces by elementary cell bodies also. It is also called bleb particle. It is a kind of vegetative reproduction.

Importance of Mycoplasma

(1) Diseases in human beings : Mycoplamsa hominis causes pleuropneumonia, inflammation of genitals and endocarditis, etc. Mycoplasma pneumoniae causes primary a typical pneumonia (PAP), haemorrhagic laryngitis, etc. Mycoplasma fermentatus and M. hominis cause infertility in man, otitis media (inflamation of middle ear).

(2) Diseases in animals : Mycoplasma mycoides causes pneumonia in cattle. Mycoplasma bovigenitalum, causes inflamination of genitals in animals. Mycoplamsa agalactia causes agalactia of sheep and goat.

(3) Diseases in plants : Common mycoplasmal diseases of plants are: Bunchy top of papaya, witches’ broom of legumes, yellow dwarf of tobacco, stripe disease of sugarcane, little leaf of brinjal, clover phylloidy, big bud of tomato etc.


The new name of cyanobacteria has been given to myxophyceae or cyanophyceae. Cyanobacteria form a group of ancient Gram negative, photosynthetic prokaryotes. Many botanists prefer to call them blue-green algae. They have survived successfully for about 3 billion years. They may cause water blooms.

Cyanobacteria are predominantly fresh water forms, a few are marine. They impart unpleasant taste and smell to the water. One species of cyanobacteria containing red pigment (Trichodesmium erythraeum) flourishes in red sea and is responsible for the red colour of its water.

A few species grow in hot water springs having a temperature range of 70°–75°C (e.g., Phormidium, Hastigocladus) and other grow at very low temperature in the polar regions (e.g., Nostoc, Schizothrix, Microcoleus etc.).Some grow in the soil and help in fixation of nitrogen and utilize it in metabolism. Nostoc colony is found into the thallus of Anthoceros. Colonies of Nostoc and Anabaena grow in paddy fields. Anabaena cycadeae is found in coralloid roots of cycads.

Characteristics of Cyanobacteria

(1) They have prokaryotic type of cells.

(2) Cells do not have any organised nucleus. The nucleolus is absent and the nucleoid is not to be bounded by a nuclear membrane. The type of nucleus called incipient nucleus.

(3) The photosynthetic pigments present in the cell are – chlorophyll a, b carotene, myxoxanthophyll, myxoxanthin, C-phycocyanin and C-phycoerythrin. The C-phycocyanin is blue and C-phycoerythrin is red in colour. If C-phycocyanin is more as compared to C-phycoerythrin, it gives characteristic blue- green colour to the algae.

(4) The photosynthetic pigment are present in lamellae, called thylakoids.

(5) The presence of chlorophyll–a, cyanobacteria synthesis their own food from carbon dioxide and water in the presence of sunlight. Certain cyanobacteria fix atmospheric nitrogen in the presence of oxygen.

(6) In cyanobacteria food is stored as cyanophycean starch or a–granules.

(7) Some members possesses simple unbranched filament with heterocyst like Nostoc, Anabaena, Aulosira, Cylindrospermum etc.

(8) Some members possesses simple unbranched filamentous forms without heterocysts and akinetes, e.g., Arthospira, Oscillatoria, Spirulina, Phormidium, Lyngbya, Symploca, Microcoleus, Schizothrix etc.

(9) Cyanobacteria reproduce asexually by fission and fragmentation. Unicellular forms multiply by binary fission. Sexual reproduction is totally absent.

(10) Flagella are completely absent but the movement occurs in some genera by special gliding motion. Such movements are connected with the secretion of mucilage. The genus Oscillatoria exhibits pendulum like oscillating movement of its anterior region.

(11) Cell wall is composed of a gelatinous sheath which is made up of three layers of microfibrils in which cellulose is not present.

(12) Cells contain organelles like cyanophycean granules, gas vacuoles, polyhedral bodies, ribosomes, polyglucoside bodies, polyphosphate bodies etc.

(13) Cyanobacteria are economically important because of having immense capability of fixing atmospheric nitrogen in the soil. An enzyme nitrogenase present in the heterocyst is responsible for the fixation of free nitrogen. Application of heterocysts blue–green algae as biofertilizer enhances the production in paddy field crops. e.g., Anabaena, Nostoc, Cylindrospermum, Scytonema etc.

Economic importance of Cyanobacteria

Cyanobacteria have both beneficial and harmful effects in human affairs.

Useful activities

(1) Growth of cyanobacteria in hard water is most probably responsible for the deposit of limestones.

(2) Since they grow, photosynthesis, multiply and ultimately die, thus adding organic matter to the soil and increasing its fertility.

(3) Whereas some cyanobacteria act to breakdown rock, the species that live in hot springs actually build rocks. This they accomplish by depositing salts of calcium and silica within the gelatinous sheath of the algal cell wall.

(4) Balls of Nostoc commune are collected, boiled and consumed as food by the Chinese and South Americans. The prepared food is called ‘Yoyucho’.

(5) Some cyanobacteria, such as Anabaena, Lyngbya etc. help in conservation of soil, thus checking soil erosion.

(6) Few species of Anabaena and Aulosira are inoculated in ponds to check the development of mosquito larvae.

(7) Certain cyanobacteria like Nostoc commune, Scytonema ocellantum, Aulosira fertissima are used for reclamation of usar (sterile alkaline) soil.

Harmful activities

(1) Their most harmful effect is undoubtedly the formation of blooms in bodies of water.

(2) They choke the intake of water supply systems and give the water a disagreeable odour giving a fishy taste to drinking water.

(3) Many cyanobacteria produce toxins. They are directly or indirectly harmful for human. e.g., Nostoc, Anabaena, Microcystes etc.


Habitat : Nostoc is found in aquatic and terrestrial habitat. The alga forms a jelly like mass in which numerous filaments are embedded. When young, they are more or less spherical, solid and microscopic in size. With advance in age, the colony grows and becomes macroscopic. In species like N. amplissimum it attains the diameter of 30cm or almost equals to the size of hen’s egg in N. punctiforme. A number of species of Nostoc on soil. They often swell up and glisten after rains and therefore called fallen stars.

Morphology : The plant is filamentous and trichome are unbranched and appear moniliform..

All the cells of the trichome are similar in structure but at intervals are found slightly larger rounded, light yellowish, thick walled cells called as heterocysts. Trichome mostly breaks near heterocyst and forms harmogonia and thus they help in its multiplication.

The heterocysts are intercalary and possess a very thick outer wall. Each heterocyst is connected with vegetative cells on two sides through prominent pores in the wall which later are occupied by a refractive cyanophycean granule called polar nodule.

Each cell trichome in Nostoc has primitive nucleus and chromoplasm and very much resembles in all details to a cyanophycean cell. Vacuoles and definite chromatophores are absent. The cellwall is differentiated into two layers. Outside the cellwall there is a mucilaginous sheath. Due to confluence of various mucilaginous sheaths of filaments, a mucilaginous colony is formed. The cell is prokaryotic.

Reproduction in Nostoc

There is no sexual reproduction in Nostoc but it reproduces asexually by following methods :
(1) Hormogonia : The filaments break at number of places into smaller pieces called as hormogonia by death and decay of an ordinary cell. They slip out of the mucilage sheath and grow into new plant. Frequently trichomes break near heterocysts.

(2) Resting spores or akinetes : Under certain conditions some of the vegetative cells enlarge and accumulate food material and develop thick walls. These are called akinetes and may be arranged on either side of the heterocysts or in between two heterocysts. In mature akinete the outer wall may be 2–3 layered and its protoplasm becomes highly granular. The akinetes germinate after a period of rest and the contents are liberated out through a pore. The protoplast by further division forms the filament.

(3) Heterocysts : In exceptional case like N. commune the heterocyst may be come functional and on germination produces a new colony.

Economic importance

(1) Many species of Nostoc fix atmospheric nitrogen and thus increases soil fertility.

(2) Reclamation of alkaline usar soils can be done by employing some species of Nostoc.

(3) N. commune is consumed as vegetable in China and Japan.

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