The Kingdom Monera includes all prokaryotes.

Monerans are the most primitive forms of life, originating from more ancient living stock termed progenote.

The kingdom Monera includes eubacteria and archaebacteria.

Eubacteria includes Cyanobacteria, Actinomycetes, Mycoplasma, Rickettsiae, Chlamydiae and Spirochaetes etc.

Classification of Monera

(1)  In 4 kingdom system, a new kingdom was created to accommodate all prokaryotic organisms i.e, eubacteria and archaebacteria. Copeland called it kingdom Mychota. It was called 'Monera' by Daugherty and Allen.
(2)  Actually, archaebacteria differ from eubacteria in many respects and resemble eukaryotes in some ways.
(3) (i) Carl Woese separated the archaebacteria from eubacteria on the basis of some major differences such as the absence of peptidoglycan in the cell walls of the former and the occurrence of branched chain lipids (a monolayer instead of a phospholipid bilayer) in the membrane.

(ii)  Therefore, 6 kingdoms given by Carl Woese are
Kingdom-1 -Archaebacteria    Kingdom-2 -Eubacteria
Kingdom-3 -Protista        Kingdom-4 -Fungi
 Kingdom-5 -Plantae        Kingdom-6 -Animalia

Salient Features of Monera

1. These are unicellular, colonial, multicellular prokaryotic organisms without nuclear membrane, nucleolus, chromatin and histone proteins.

2. Cell wall is made up of peptidoglycan (exceptions are Archaebacteria and Mycoplasma).

3. Membrane bound organelles are absent.

4. Cyclosis is absent and ribosomes are of 70 S type .

5. Respiratory enzymes are found associated with plasma membrane.

6. Nucleoid or genophore or incipient nucleus or prochromosome is composed of naked DNA, RNA and nonhistone proteins.

7. Reproduction by asexual method.

8. Cell division is amitotic type and lacks spindle formation.

Let us discuss various monerans in detail :


Bacteria are cosmopolitan and occur in every habitat wherever living or dead organic matter is present.

Anton Von Leeuwenhoek discovered bacteria in rain water which had been allowed to stand for many days and tartar scrapped from teeth.

In 1695, he published his work "The Secrets of Nature".

A.V. Leeuwenhoek termed these microorganisms as dierkens which was later translated as animalcules by the Royal society.

Term microbe for animalcules was coined by Se' dillot, but the term microorganism was proposed by Pasteur.

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Ehrenberg first of all coined the word 'bacteria'

Louis Pasteur is considered father of modern microbiology. He introduced the term aerobic and anaerobic for the life in the presence or absence of oxygen respectively.

Robert Koch, a German doctor, demonstrated that the anthrax disease of sheep was caused by bacteria. Koch had followed four experimental steps (Koch's postulates) which help to establish a relationship between a microorganism and a disease.

Smallest bacterium : Dialister pneumosintes

Largest filamentous bacterium : Beggiatoa mirabilis

 Shapes of Bacteria

Bacteria occur in four basic forms or shapes.

These are spherical (Cocci), rod shaped (Bacilli), Vibrio and Spiral.

Though most bacterial species have cells that are of a fairly constant and characteristic shape, some species are pleomorphic (i.e., these can exhibit a variety of shapes), e.g., Rhizobium leguminosarum.

 (a) Coccus : Spherical or nearly spherical, aflagellate, sub-divided into six groups on the basis of cell arrangement:

Monococcus -Only single cell represents the bacterium, e.g., Micrococcus luteus, M. roseus.

Diplococcus -Cocci divide in one plane and remain attached in pairs, e.g., Meningococcus, Gonococcus, Diplococcus pneumoniae.

Streptococcus -Cocci remain attached to form chains of different lengths, e.g., Streptococcus lactis.

Tetracoccus -Cocci divide in two planes at right angles to one another and form groups of four, e.g., Tetracoccus, Neisseria.

Staphylococcus -Cocci divide in several planes resulting in formation of irregular bunches of cells, sometimes resembling a cluster of grapes, e.g., Staphylococcus aureus.

Sarcinae -Cocci divide in 3 planes at right angles to one another and resemble cubical packets of 8 or more cells forming three dimensional geometrical figures, e.g., Sarcina lutea.

(b)    Bacillus : Rod-like forms, either singly or may be arranged differently. They are generally flagellate. It is the most common of all the shapes. They are of following types :

(i)  Monobacillus -The bacteria occur singly, e.g., Bacillus anthracis, Lactobacillus.

(ii) Diplobacillus -Bacteria are arranged in pairs. e.g., Bacillus subtilis

(iii) Streptobacillus -Bacteria form a chain of rods, e.g., Streptobacillus.

(iv) Palisade-like -If the cells are lined side by side like match sticks and at angles to one another.

e.g., Corynebacterium diphtheriae.

 (c) Spiral bacteria : Coiled forms of bacteria exhibiting twists with one or more turns are called spirilla, e.g., Spirillum volutans.

 (d) Vibrio : Bacteria with less than one complete twist or turn are called vibrio. These resemble a comma (,) in appearance, e.g., Vibrio cholerae.

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

 (f)  Budding bacteria : The body is swollen at places, e.g., Rhodomicrobium

Bacterial Cell Structure

Bacterial cell structure is very simple although they are very complex in behaviour. They show the most extensive metabolic diversity. Electron microscope can only reveal the detailed structure of bacterial cell. It consists of following structures:

 1.    Glycocalyx: 
(i)  It is outermost part of cell envelope (Glycocalyx, cell wall, plasma membrane)

(ii)  Represented by either slime layer or capsule

(a) Slime layer is composed of dextran, dextrin and lavan sugars and protect the cell against desiccation and loss of nutrients.

(b) Capsule is made up of polysaccharides and D-glutamic acid. It provides gummy or sticky character and virulent property to the cell.

 2. Cell wall :

It is present outside the cell membrane and is a rigid structure.

Due to its rigidity, it protects the internal structures of the cell and provides shape to the cell.

However, its main function is to prevent the cell from expanding and bursting because most bacteria live in hypotonic environments, and are likely to take in much water and eventually burst.

The cell walls of almost all the eubacteria (true bacteria) are made up of peptidoglycan, also called murein or mucopeptide.

It is found only in prokaryotes.

As the name suggests, the peptidoglycan consists of two components-a peptide portion which is composed of amino acids connected by peptide linkages, and a glycan or sugar portion.

The glycan portion, which forms the backbone of peptidoglycan, is composed of alternating units of amino sugars N-acetyl-glucosamine (NAG) and N-acetylmuramic acid (NAM) joined together by b-1, 4 linkages.

The peptidoglycan chains are laterally linked by short chains of four amino acids which are attached to N-acetylmuramic acid residues.

The four amino acids of this tetrapeptide are D-alanine, L-alanine, D-glutamic acid and L-Iysine (in Gram +ve bacteria) or diaminopimelic acid (in Gram -ve bacteria).

The tetrapeptide chains are also interlinked by a peptide bridge between the carboxyl group of an amino acid in one tetrapeptide chain and amino group of an amino acid in another tetrapeptide chain.

The cross linkages can occur between tetrapeptides in different chains, as well as between adjacent tetrapeptide chains. As a result, peptidoglycan forms a rigid, multilayered sheet.

Another component, teichoic acid, an acidic polymer consisting of a carbohydrate (e.g., glucose), phosphate and an alcohol is found in cell walls of Gram +ve bacteria.

Teichoic acid has several functions such as binding metals, acting as receptor sites for some viruses and maintaining cells at low pH to prevent degradation of cell walls by self-produced enzymes.

The walls of Gram positive bacteria contain very little amount of lipids.

The cell walls of Gram negative bacteria are much more complex.

The peptidoglycan layer is very thin making up only 10% or less of the cell wall.

However, the most interesting feature is the presence of an outer membrane that covers a thin underlying layer of peptidoglycan.

The outer membrane is a bilayered structure consisting chiefly of phospholipids, proteins and lipopolysaccharides (LPS).

The outer membrane serves as a barrier to prevent the escape of important enzymes from the space (periplasmic space) between the cytoplasmic membrane and the outer membrane.

It also prevents the entry of various chemicals that could damage the cell.

It acts as main surface antigen in cell wall.

However, permeability of outer membrane to nutrients is provided by proteins called porins which form channels in the membrane through which substances of hydrophilic nature and low molecular weight can diffuse.

Christian Gram (1884) developed a staining method for bacteria, using Gram stain (crystal violet).

On the basis of stainability with Gram Stain, bacteria are classified into two groups; Gram positive and Gram negative.

Flow chart representing Gram staining technique

 3. Surface Appendages : These include flagella and fimbriae (or pili).
(a) Flagella are long, fine, wavy, filamentous appendages that protrude through the cell wall, responsible for the motility of bacteria. These are much thinner than the flagella or cilia of eukaryotes.

Structure of Flagella: The entire flagellar apparatus is made up of three distinct regions: basal body, hook and filament.

Basal body: It is most complex portion of flagellum and has four rings (L, P, S and M), only two rings S and M are present in gram +ve bacteria. L and P rings in cell wall constitute distal set, while S and M rings are present in plasma membrane, forming proximal set.

Hook: Made up of different protein units.

Filament: Bacterial flagella are made up of identical spherical subunits of a protein called flagellin. Longitudinal chains of flagellin molecules run longitudinally around each other to form a wavy helical or rope-like structure. Therefore, a cross section of the flagellum reveals a number of flagellin molecules around a central space.

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Depending upon the presence or absence, number and position, following types of flagellar arrangements are observed among bacteria :

Atrichous: Flagella absent, e.g., Pasteurella, Lactobacillus.

Monotrichous: Only one flagellum attached at one pole of the organism, e.g.,Thiobacillus, Vibrio.

Amphitrichous: One flagellum at both ends, e.g., Nitrosomonas.

Cephalotrichous: Two or more flagella attached at one end, e.g., Pseudomonas fluorescence.

Lophotrichous: Two or more flagella attached at both ends, e.g., Spirillum volutans.

Peritrichous: Flagella distributed all over the surface of the cell, e.g., Escherichia coli, Clostridium tetani.

(b) Pili and fimbriae are hollow, non helical, filamentous appendages projecting from the walls of Gram-negative bacteria. These are thinner and shorter and more in number than the flagella. These are made up of specific proteins called pilin.
There are different types of pili which serve different functions. One type, known as type I pili, (somatic pili) play a major role in infection by facilitating the attachment of bacterial cell to the host cell. Another type, termed sex pili, serve as portals of genetic material from donor to recipient cell during conjugation.

Differences between Gram positive and Gram negative bacteria

4. Protoplast: Cell wall encloses the protoplast, the living matter. It includes (i) Cell membrane (ii) Cytoplasm, (iii) Nucleoid and may have plasmid and episome.
(i) Cell membrane: 

It lies inner to the cell wall, actually representing the outermost layer of the protoplast.

It is living and semipermeable, controlling the movements of various dissolved substances in and out of the cells.

Functionally, the cell membrane of bacteria resembles mitochondria of eukaryotic cells as respiratory ETS enzymes and succinate dehydrogenase (Kreb's Cycle) are associated with the membrane.

The cell membrane gets invaginated and folded to form a structure called mesosome (chondroid) in some bacteria, particularly the Gram positive bacteria.

These may be central or peripheral in position and they are supposed to play a role in replication of DNA during cell division, as these are often attached to the nuclear body.

Besides, these increase the surface area of absorption and help in septa formation during binary fission.

(ii)    Cytoplasm:

It is homogenous colloidal mass of carbohydrates, fats, proteins, lipids, nucleic acids, minerals and water.

It does not show streaming movements.

It lacks sap vacuoles and gas vacuoles (may be present in some bacteria which live in aquatic condition).

Typical membrane bound organelles of eukaryotic cells like endoplasmic reticulum, mitochondria, golgi complex and plastids are absent.

The cytoplasm appears granular due to the presence of ribosomes.

However, these are 70S type in bacteria as compared to 80S type in eukaryotes.

Ribosomes lie scattered freely in the cytoplasm, but sometimes may form a small chain of 4-6 ribosomes attached to mRNA constituting polyribosome or polysome.

Various non living inclusions like glycogen particles, fat bodies, volutin granules (polymetaphosphate -source of energy) and lipid molecules acting as food reserve lie dispersed in the cytoplasm.

The cytoplasm is usually colourless, lacking pigments.

However, in photosynthetic bacteria, the cytoplasm contains pigments like bacteriochlorophyll and bacterioviridin.

The pigments either lie dispersed in the cytoplasm or present in membrane bound spherical vesicles called chromatophores.

These pigments are capable of entrapping solar energy for photosynthesis

(iii)  Nucleoid (Prochromosome, Genophore, Incipient nucleus)

Bacterial cell lacks a well organized nucleus.

It consists of a long double stranded DNA molecule repeatedly folded with the help of RNA to form a circular ring.

DNA has no free ends and not associated with histone proteins (polyamines present). Circular DNA ring, without histones is often termed bacterial chromosome.

Plasmid (Minichromosome) :

Term plasmid was given by Lederberg and Hays.

These are small, extrachromosomal, non-essential, circular, double stranded, free naked DNA molecules.

The genes present on them have no vital role in survival and growth of bacteria.

These perform autonomous replication.

If plasmids temporarily integrate with bacterial chromosome, then they are called episomes.

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Types of plasmids :

(a)  F-Plasmid: It forms sex pilus and is responsible for process of conjugation or fertility factor transfer.

(b)  R-Plasmid: These plasmids have resistance gene (Resistance Transfer Factor, RTF) for antibiotics like penicillin, tetracycline.

(c)  Col-Plasmid: Genes of this plasmid are responsible for production of colicins (bacteriocin) for killing other bacteria.

(d)  Ti Plasmid: From Agrobacterium tumefaciens, used in genetic engineering

(e)  Degradative plasmid of Pseudomonas putida (superbug) helps to decompose hydrocarbons of petroleum in oil spills

5.  Bacterial Life Processes

Discussion of bacterial life processes revolves around the study of the prominent metabolic activities like respiration and nutrition.

(A) Respiration: On the basis of mode of respiration, the bacteria are divided into two main groups: i.e., aerobes and anaerobes. Each group is further of two types i.e. strict or obligate and facultative.

(a) Obligate or strict aerobes : These bacteria can live only in presence of oxygen they possess the enzyme system for aerobic respiration only. In the absence of oxygen, they cannot respire and thus, die, e.g., Bacillus subtilis.

(b) Facultative anaerobes : They normally respire aerobically. However, they are capable of switching over to anaerobic mode to get energy for their survival, if sufficient oxygen to sustain aerobic respiration is not available in the environment, e.g., Pseudomonas.

(c) Obligate or strict anaerobes: These bacteria respire anaerobically only. The growth of such bacteria will certainly be slower as anaerobic respiration liberates much less amount of energy as compared to aerobic respiration. They lack enzymes necessary for carrying out aerobic respiration e.g., Clostridium botulinum.

(d) Facultative aerobes: They normally respire anaerobically, but are capable of respiring aerobically as well, if oxygen is available. Most of the photosynthetic bacteria are facultative aerobes e.g., photosynthetic bacteria Chlorobium.

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(i)  Aerotolerant anaerobes: Bacteria that continue to perform anaerobic respiration even in the presence of oxygen, e.g., Lactic acid bacteria.

(ii)  Anaerotolerant aerobes: Aerobic bacteria continue to perform aerobic respiration even in absence of free oxygen by using oxygen of oxidised salts, e.g., Denitrifying bacteria

(B) Nutrition :

Nutrition in bacteria is of two types i.e. autotrophic and heterotrophic.

Bacteria having autotrophic mode of nutrition may be photoautotrophs and chemoautotrophs, carrying out photosynthesis and chemosynthesis, respectively.

(I)    Autotrophic Nutrition
(i)    Photoautotrophic bacteria: 

These bacteria are capable of entrapping solar energy and utilizing it for the synthesis of complex food materials due to the presence of pigments like bacteriochlorophyll (bacteriopurpurin) and bacterioviridin.

Purple sulpher bacteria (e.g., Thiospirillum) and green sulpher bacteria (Chlorobium limicola) are the most familiar examples containing pigment bacteriochlorophyll, bacteriopurpurin and bacterioviridin respectively.

Concept Builder

Bacterial photosynthesis, however, differs from photosynthesis of higher plants in not liberating oxygen.

This type of photosynthesis, characteristic of bacteria, is termed as anoxygenic.

Normal photosynthesis, occurring in higher plants, is termed as oxygenic.

In bacterial photosynthesis water is not the source of electron that acts as reducing power to convert CO2 into glucose.

The bacteria obtain reducing power from various compounds such as hydrogen sulphide, thiosulphate or even some organic compounds.

No oxygen is evolved as it does not involve splitting of water.

Hydrogen released by various compounds mentioned above is picked up by NAD+ which gets reduced to NADH2 acting as reducing power.

NADH2 alongwith ATP, produced generally by entrapping solar energy are used to reduce CO2 to glucose.

Simple equation for anoxygenic photosynthesis may be written as follows :

Most of the photosynthetic bacteria are anaerobes (facultative aerobe

(ii) Chemoautotrophic Bacteria:

Bacteria belonging to this category obtain energy for the synthesis of food by oxidising certain inorganic substances like ammonia, nitrates, nitrites, ferrous ions etc.

Thus, they do not utilise light as energy source.

The chemical energy thus obtained, is trapped in ATP molecules.

This energy is then used in carbon assimilation with the help of hydrogen from some source other than water, e.g., hydrogen bacteria, nitrifying bacteria, sulphur bacteria, etc.

They play a great role in recycling nutrients like nitrogen, phosphorous, iron, sulphur.

 (a) Hydrogen bacteria. These bacteria oxidise hydrogen in the presence of oxygen, e.g., Hydrogenomonas.

(b) Nitrifying bacteria obtain energy by oxidizing ammonia into nitrate, oxidation of ammonia occurs in two steps. Each step is carried out by a specialised group of bacteria.

In the first step, ammonia is oxidised into nitrites by the species of the genus Nitrosomonas and Nitrococcus.

In the second step, the nitrite is converted into nitrate. This is brought about by species of the genus Nitrobacter and Nitrocystis which use this energy for chemosynthesis.

(c)  Sulpher bacteria. These bacteria obtain energy either by the oxidation of elemental sulphur or oxidation of H2S.

Oxidation of elemental sulpher. Sulpher bacteria (Thiobacillus thioxidans) oxidise elemental sulpher to sulphuric acid and utilise energy produced in this process. These bacteria can survive even in extreme acidic environment.

Oxidation of H2S to S. Some bacteria like Beggiatoa use the energy from oxidation of H2S and store the sulphur so produced in the form of granules.

(d)  Iron bacteria. These bacteria (e.g., Ferrobacillus, Leptothrix) inhabit water which contain iron compounds. These bacteria convert ferrous ions to ferric form. The ferric ion is deposited as insoluble ferric hydroxide. The energy so released, is utilised in the assimilation of CO2.
    (e)    Methane bacteria. Methanomonas, is one example which oxidises methane to carbon dioxide.
    (II)    Heterotrophic bacteria

These bacteria are most abundant in nature and are incapable of synthesizing their own food from simple raw materials.

They obtain nourishment either from dead and decaying organic matter or directly from a living host.

All heterotrophic bacteria are segregated into three main categories, i.e., saprophytic, symbiotic and parasitic forms.

(i)   Saprophytic bacteria :

They are free living bacteria, obtaining nourishment from organic remains such as dead animals, animal excreta, fallen leaves, decaying vegetables, fruits, bread and other products of animal and plant origin.

These bacteria secrete digestive enzymes into the substrate and the complex insoluble Substances are converted into simple soluble compounds like water, hydrogen sulphide, ammonia, CO2 etc.

Some of the simpler substances are absorbed and assimilated by the bacteria, whereas the others are added to the soil and atmosphere to complete the nature's material cycle.

Anaerobic breakdown of carbohydrates and proteins is termed fermentation and putrefaction respectively.

Aerobic breakdown of organic compounds is called decay.

(ii)    Symbiotic bacteria :

They are mainly Gram negative type.

A familiar example of symbiotic bacteria is Rhizobium leguminosarum, associated with roots of leguminous plants.

They are capable of fixing atmospheric nitrogen as ammonia, inside the nodule only and not in free state.

However, some bacteria like Azotobacter, Beijerinckia, Klebsiella are free living, aerobic and capable of nitrogen fixation in free state, enriching the soil.

Clostridium pasteurianum is anaerobic N2 fixing bacteria.

(iii)   Parasitic bacteria

These bacteria draw nourishment and obtain special organic compounds required for growth from living organisms, either plants or animals, called hosts.

The disease causing bacteria are termed pathogenic and the ones not causing any disease are termed as non-pathogenic.

6.. Reproduction

Bacteria reproduce mainly by asexual method and also show sexual recombination (True sexual reproduction is absent).

A.  Asexual Reproduction :

Bacteria produce several types of asexual spores like, sporangiospores, oidia, conidia and endospores. However, the most common mode of asexual reproduction is binary fission.

Under favourable conditions of nutrient availability, moisture and temperature, daughter cells may repeat binary fission many times and may forms a large population.

Fortunately, such a rapid rate is seldom achieved.

The process gradually slows down and ultimately stops because of:

(i)    Shortage of space.

(ii)    Lack of nutrient availability.

(iii)    Accumulation of waste products (making environmental conditions unfavourable for growth).

(iv)    Development of bacteriophages, destroying bacteria.

(a)  Binary Fission :

It is the most common method under favourable conditions of temperature, moisture and availability of nutrients.

Mature bacterial cell divides into two daughter cells.

In this process the cell division is amitotic type i.e., not involving the spindle formation

(b)   Endospores :

Cells of certain bacteria, e.g., Bacillus, Clostridium etc. form thick-walled, highly resistant bodies within the cell, called endospores.

One bacterial cell normally produces only a single endospore.

The endospores may be spherical or oval in shape and are terminal or central in position.

Anticoagulant nature of endospore is due to the presence of Ca-dipicolinic acid in cortex layer of wall.

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            Structure of Endospore :

The endospore consists of a central core made up of nuclear material and spore cytoplasm.

The cytoplasm includes DNA, RNA and proteins, lipid, Ca, Mn.

The central core is surrounded by a delicate membrane called core wall.

Around the wall is another layer, which is much thicker and is of relatively low density.

This layer, called cortex, mainly consists of peptidoglycan and calcium dipicolinic acid (CaDPA).

It is an anticoagulant which provides resistance to heat.

This complex may also be present in spore cytoplasm.

Endospore is highly resistant to desiccation, chemicals and radiations.

The cortex, in tum, is enclosed in spore coat which may be smooth, grooved, or raised into ridges. The whole structure may be wrapped in an exosporium.

The endospores are formed probably upon induction by the exhaustion of nutrients (unfavourable conditions).

During endospore formation, a part of the protoplast containing nuclear body undergoes dehydration, stores food material and gets separated from rest of the protoplast to form endospore.

It gets surrounded by different layers.

These can withstand temperature as high as 1000e or as low as -100°C, so these can remain unharmed during pasteurisation.

The remarkable resistance shown by endospores is due to :

(i) Thick and impermeable spore coat

(ii) Low water content

(iii) Low metabolic activity

(iv) Ca-DPA Complex

Under favourable conditions, endospore absorbs water, becomes metabolically active, ruptures thick spore coat and the bacterium surrounded by thin cell wall emerges out.

Endospores are actually the means of perennation and not reproduction as only one endospore is formed per cell and subsequently each endospore grows into single bacterium cell after the commencement of favourable conditions.

 B.  Sexual Recombination (Genetic Recombination) :

The bacteria exhibit a primitive form of sexual reproduction which differs from eukaryotic sexual reproduction because there is no gamete formation and fusion.

However, the essential feature of sexual reproduction, i.e., exchange of genetic material does take place and is called genetic recombination.

Three methods are known by which genetic recombination is achieved by bacteria.

In the order of their discovery, these are transformation, conjugation and transduction.

 (a)  Transformation :

Griffith (1928) worked on the effect of Diplococcus or Streptococcus pneumoniae bacteria on mice and discovered the process of transformation.

In transformation, the donor and recipient do not come in contact.

The donor cell releases a piece of DNA which is actively taken up by the recipient cell from the solution.

This ability to pick up DNA from the solution is called competence.

Two strains of D. pneumoniae are :

  • Capsulated or S-III (Virulent strain) and Non-capsulated or R-II (non-virulent strain).
  • Four steps were performed in experiment:


(But R-II is not virulent and S-III lost its virulent capacity upon heating).

Griffith concluded that 'something' passed from heat killed S-III to R-II bacteria, so that non virulent strain changed or transformed into virulent bacterial strain.

Avery, Macleod and McCarty (1948) repeated this experiment using various enzymes and proved that the transformation principle is DNA of heat killed S-III strain.

They proved that DNA is a genetic material.

(b)  Conjugation:

Lederberg and Tatum (1946) demonstrated in E. coli that during conjugation, one cell containing F-plasmid acts as donor (F+ or male) cell and the other lacking F-plasmid as recipient (F or female) cell.

The plasmid contains fertility factor or F gene which produces protrusions termed sex pili.

These help the donor F+ cell in attaching to the recipient cell.

The plasmid replicates and a replica is transferred to recipient cell, changing it into F+.

Often the plasmid integrates with bacterial chromosome, converting it into Hfr (High frequency of recombination) cell or super male and a part or whole of bacterial chromosome is transferred to recipient cell through-conjugation tube.

Such association of episome with the endogenote increases the efficiency of genetic transfer.

The number of genes transferred depends upon the time for which the two cells remain joined together.

When F-conjugates with super male, the frequency of recombination increases by 1000 times, that is why it is called as Hfr (super male).

(c)       Transduction :

During transduction, a small double stranded piece of DNA is transferred from donor to recipient by a bacteriophage.

This mode of genetic recombination in bacteria was first demonstrated by Zinder and Lederberg (1952) while working with Salmonella typhimurlum.

Some viruses have the ability to integrate their DNA with bacterial DNA, which is replicated at the same time as the host DNA and is passed from one bacterial generation to the next.

Such bacteria carrying phage (viral) DNA with their own DNA are called lysogenic bacteria.

Occasionally, the phage DNA becomes active and codes for the production of new virus particles.

A number of phage particles are synthesised followed by the destruction of the host cell and release of phage particles.

Upon release, the phage particles attack sensitive bacterial cells, multiply and release more phage particles.

However, sometimes faulty deatchment of phage DNA from bacterial DNA results in the incorporation of a small amount of bacterial DNA into the phage DNA.

Subsequent infection of another bacterium with this aberrant phage called transducing phage, introduces the piece of foreign bacterial DNA into the recipient's chromosomes, producing a genetic change.

Types of transduction:

The ability of the bacteriophage to carry the genetic material from any region of bacterial DNA is called generalised transduction, e.g., T4-phage.

On the other hand, there are bacteriophages such as lambda phage (l) of E coli which can carry only a specific region of the bacterial DNA to a recipient.

This is called specialised transduction (or restricted transduction).

Sometimes, the DNA brought by the phage does not integrate with the genome of the recipient bacterium and is lost after one or two generations.

Such a transduction is called abortive transduction

7. Economic Importance of Bacteria

Bacteria play significant role in day to day activities of human beings.

A.    Beneficial activities

(a)    Role in agriculture

(i)    Decay and decomposition of organic matter : 

They bring about decay and decomposition of dead remains of plants and animals.

These are the most important for mineral cycling to occur.

(ii) Sewage disposal :

The bacteria decompose the organic matter present in the sewage, converting into simpler inorganic substances.

The inorganic substances thus formed, being soluble, pass out through filter alongwith water which is highly useful for irrigation purposes, e.g., Clostridium, E. coli.

 (iii)  Nitrogen cycle :

The proteins present in dead remains of living organisms are converted into amino acids by different types of saprophytic bacteria.

Amino acids are converted into ammonia by ammonifying bacteria (Bacillus vulgaris, B. ramosus).

Nitrifying bacteria convert ammonium salts first into nitrites (Nitrosomonas, Nitrococcus) which are subsequently converted into nitrates (Nitrobacter).

In presence of denitrifying bacteria (Pseudomonas denitrificans), nitrates and nitrites of soil are converted to gaseous nitrogen.

 (iv)  Nitrogen fixation :

It is the biological process in which atmospheric nitrogen is converted into nitrogenous compounds by nitrogen fixing bacteria.

These bacteria are of 2 types :

(a) Free living 

(b) Symbiotic

(a) Free living bacteria are Azotobacter and Beijerinckia (aerobic) & Clostridium (anaerobic).

(b) Common symbiotic bacteria are Rhizobium leguminosarum and Xanthomonas. These convert nitrogen into ammonia which is directly converted into amino acids by plants.

(v)    Manure preparation :

Saprotrophic bacteria help in preparation of farmyard manure by converting farm refuse, dung and other organic wastes into humus.

(b)    Role in Industry :

Man has utilized the metabolic activities of bacteria in preparation of a number of industrial products as listed below :

(i)   Butter milk and sour cream

(ii)  Yoghurt

(iii)  Cheese

(iv)  Vinegar

(v)   Retting of fibres : Retting is a controlled microbial decomposition for separation of fibres. The tissues are immersed in water tanks where anaerobic butyric acid bacteria dissolve the pectin of middle lamella of cells, thus, separating the fibres. Clostridium perfringens and Pseudomonas fluorescence are useful in this process.

(vi)  Curing of leaves: To improve the flavour and taste in tea, using Micrococcus candidans and in tobacco leaves by Bacillus megatherium.

(vii)  Single cell proteins (SCP) : Like -Methylophilus methylotropus and Rhodopseudomonas capsulata.

(c)  Role in Medicine :

Bacteria have been used extensively in preparation of antibiotics, vaccines, serums and vitamins.

(i)     Antibiotics :

The term antibiotic was given by Waksman, who discovered streptomycin.

These are the organic substances produced by microorganisms which inhibit the growth of other organisms (mostly pathogens) but do not affect the growth of organisms secreting these.

The first commercial antibiotic penicillin was discovered by Flemming (1959) from a fungus called Penicillium.

Some antibiotics of eubacterial origin:
(a)    Bacitracin ........................... Bacillus licheniformis
(b)    Polymixin ........................... Bacillus polymyxa
(c)    Gramicidin ........................... B. brevis
(d)    Subtilin ........................... B. subtilis

(ii)    Vaccine production

Vaccines and serums against typhoid, cholera, TB, pertussis, tetanus and diphtheria are made with the help of bacteria or their toxins e.g., DPT (against diphtheria, pertussis and tetanus), TI(against tetanus), BCG (Bacille of Calmette-Guerin against TB), DT (against diphtheria and tetanus).

(iii)   Vitamins :

Escherichia coli present in human intestine produces large quantities of B-complex vitamins and vitamin K. Bacteria are utilized in industrial production of a number of vitamins like riboflavin from Clostridium butylicum, Cobalamine (B12) from Bacillus megatherium and Pseudomonas denitrificans.

(iv)   Pollution control :

Pseudomonas putida degrades petroleum wastes.

Flavobacterium can decompose 2, 4-D. DDT can be decomposed by Acetobacter aerogens.

Ganga's water contains Bdellovibrio bacteriovorus that maintains purity of its water.

(v)    Poly--hydroxybutyrate is used to produce biodegradable plastic.

B.    Harmful activities

(i) Spoilage of food : Saprophytic bacteria cause decay of vegetables, fruit, meat, bread and other foods, making these unfit for human consumption. Some bacteria even produce strong toxins in the infected food stuffs which cause food poisoning when consumed.

(ii)  Deterioration of Domestic Articles : Some saprophytic bacteria like Cellulomonas, Spirochaete cytophaga cause deterioration of domestic articles of daily use such as leather, woolen, canvas articles etc.

(iii) Denitrification of Soils : Denitrifying bacteria like Thiobacillus denitrificans, Micrococcus denitrificans and Pseudomonas convert nitrates and nitrites present in the soil into gaseous nitrogen, thus depleting the soil nitrogen, thereby decreasing soil fertility.

(iv) Desulphurification : Desulphovibrio desulphuricans.

(v)  Diseases : Common disease of humans, animals and plants are listed below respectively –

Table: 2

Table : 3


Cyanobacteria are Gram negative photosynthetic prokaryotes, being the most primitive organisms to have oxygenic photosynthesis.

They added oxygen to the atmosphere, which is indispensible for the existence of aerobic forms of living organisms.

They are also known as BGA (Blue green algae) and are classified variously under cyanophyceae or myxophyceae.


They are mainly fresh water forms, though few are marine.

Red sea is named so because of abundant occurrence of a cyanobacterium Trichodesmium erythraeum, which imparts red colouration to water.

They occur in symbiotic association with almost every group of eukaryotes i.e. green algae, fungi, bryophytes like mosses and Anthoceros, ferns, gymnosperms, angiosperms, sponge, shrimps, mammals etc.

Anabaena azollae is associated with Azolla, an aquatic fern.

Anabaena cycadeae is associated with coralloid roots of Cycas.

In many lichens (symbiotic association of algae and fungi), the algal partner may be a cyanobacterium.

When they live endozoically in protozoans they are called cyanelle.

Structural Organization

These may be unicelled or multicelled. The latter may be filamentous or colonial.

Filamentous form consists of one or more cellular strands, called trichomes, surrounded by mucilagenous sheath.

Cyanobacteria are characterised by the absence of flagellum throughout life cycle.

Cell Structure

  • The cell structure in cyanobacteria is typically prokaryotic.
  • The cell lacks a well defined nucleus and the chromatin material is centrally located, resembling the bacterial chromosome.
  • The cell wall is 4 layered and is invariably covered by mucilagenous sheath, composed largely of mucopeptides.
  • Protoplasm in cyanobacterial cell can be distinctly divided into two parts the centroplasm and chromoplasm.
  • The central colourless centroplasm contains the chromatin material.
  • The peripheral protoplasm is coloured or pigmented because of the presence of thylakoids, called as chromoplasm.
  • The protoplast lacks membrane-bound organelles like endoplasmic reticulum, golgi bodies, mitochondria, lysosomes, plastids and contains 70S ribosomes.
  • Similar to the mesosome of bacteria, a group of coiled membrane called lamellasome is found which connects nucleoid to the cell membrane.
  • It helps in respiration and replication of DNA.
  • The cell membrane lack sterols. The sap vacuoles are absent.
  • Instead, the cell may contain gas filled vacuoles which help to regulate the buoyancy of the organism in water.
  • The characteristic feature of cyanobacterium cell is the presence of a system of photosynthetic lamellae called thylakoids.
  • The characteristic photosynthetic pigments present in the thylakoids are chlorophyll a and phycobilins i.e., phycocyanin (blue coloured), phycoerythrin (red coloured) and allophycocyanin (light blue coloured).

The cyanobacterial cell contains reserve food material in the following forms
(i)    Cyanophycean granules (Protein)


(iii)    Cyano-or myxophycean starch or -granule (Similar to glycogen but negative to iodine test)

(iv)    Volutin body (Reserve phosphate)

(v)    Polyhedral body (Rubisco rich)

Concept Builder

Gaidukov's phenomenon or complementary chromatic adaptation -Cyanobacteria or blue green algae can adaptively change their body colour according to different wavelengths of available light, e.g., Trichodesmium erythraeum. It is also known as "red sea" causing alga.


They are the most self-dependent organisms, because most of these are capable of converting atmospheric nitrogen into ammonium compounds besides utilizing atmospheric CO2 for synthesis of organic food during photosynthesis.

Biological nitrogen fixation is an anaerobic process as nitrogenase enzyme required for the process acts efficiently in the absence of oxygen.

Nitrogen fixation under anaerobic conditions occurs mainly in large, specialized cells called heterocysts as in Nostoc.

Heterocyst has terminal pores which at maturity develops a polar granule.

Thickened cell wall of these cells is impermeable to oxygen so this creates anaerobic environment in the cell even under aerobic conditions.

Besides this, heterocysts also lack PS II activities and CO2 fixation is done only by vegetative cells.

Oxygen is not evolved due to absence of PSII.

However, PSI remains active in heterocyst which generates ATP required to fix nitrogen.

Besides N2 fixation heterocyst promotes fragmentation.

This property of nitrogen fixation, most of the BGA enrich the soil by releasing nitrogenous compounds in the surroundings.


Cyanobacteria reproduce asexually. Typical sexual reproduction is absent.

Asexual reproduction occurs by following methods :

(i) Binary fission: It occurs in unicellular forms. The daughter cells formed by amitotic division separate immediately after the division.
(ii)  Fragmentation: It occurs in filamentous forms. The filament breaks up into short pieces or fragments which grow to form new filaments.
(iii)  Heterocysts : Under special conditions, the heterocysts germinate to form new filaments.
(iv)  Hormogonia: Due to the formation of biconcave, mucilage filled dead cells called necridia, in between living cells of trichome, the filament breaks into hormogonia.
(v)  Akinetes: Vegetative cells are transformed into thick walled akinetes due to the deposition of food material followed by the thickening of wall. On the arrival of favourable conditions, they germinate to form new filaments.

Importance of Cyanobacteria

(i)    They are the most ancient organisms having oxygenic photosynthesis and thus, played a significant role in the evolution of aerobic forms of life.
(ii)    They convert atmospheric nitrogen into ammonium compounds and excess of these compounds is excreted out, enriching the soil. The death and decay of these also increase the soil fertility, particularly the nitrogen content of the soil. Tolypothrix and Aulosira fix N2 non-symbiotically in rice fields .
Cyanobacteria like Nostoc and Anabaena have been used for reclaiming usar soils. As they can live in damp or aquatic habitat, they enrich the root environment in any wetland condition as in rice fields.
(iii)    Cyanobacteria are associated in symbiotic relationship with almost every group of plants. They benefit the partner by providing nitrogenous compounds because of their capability of nitrogen fixation.
(iv)    Some cyanobacteria serve as food to several aquatic animals. Spirulina is edible, non-toxic, fast growing cyanobacterium. It is cultivated in tanks as source of protein rich animal food (SCP).
(v)    Extract of Lyngbya is used for the manufacture of antibiotic.
(vi)    Some cyanobacteria like Microcystis aeruginosa, Anabaena flos-aquae, Aphanizomenon flos-aquae are known to cause algal blooms in water bodies. These also secrete toxins into the surroundings, which are harmful to aquatic animals and even to human beings. Water from such sources is harmful and may even prove fatal for organisms drinking it. They also deplete the oxygen from the water reservoir and thereby, cause large scale death of the fishes and other aquatic animals.


E. Nocard and E.R. Roux (1898)-two French Scientists, discovered these organisms from pleural fluid of cattles suffering from pleuropneumonia.

These are pleomorphic and were called PPLO (Pleuropneumonia Like Organisms) or Jokers of plant kingdom.

This organism was later on given the name Asterococcus mycoides by Borrel et al. (1910).

Nowak (1929) placed Asterococcus mycoides under the genus Mycoplasma.

All such organisms are now called Mycoplasma, or MLO's (Mollicutes like organisms).

These are sometimes placed in a separate class called Mollicuta.

Mycoplasma infects animals (e.g., dog, sheep, mice and man) and plants (e.g., potato, corn, brinjal etc.).

  • They are generally found in soil, sewage water, plants and animals.


These are unicellular, simplest free living prokaryotes.

They do not have cell wall so they are highly pleomorphic and can assume various shapes like spherical, granular, filamentous, coccoid etc.

Cell membrane is the outermost limiting layer.

It is trilamellar unit membrane structure.

In culture, colonies of mycoplasma show a characteristic fried egg appearance with an opaque central area and translucent peripheral zone.

The cells are generally non motile, but a few are gliding type.

The protoplasmic matrix contains ribosomes (70S type), fatty acid substances and proteins.

Organized nucleus, endoplasmic reticulum, plastids, mitochondria, golgi bodies, Iysosomes, centrioles, flagella, etc. are absent.

Mycoplasma has both RNA and DNA.

RNA is single stranded, present in both ribosomes and cytoplasm and DNA is double stranded, long coiled thread extending almost throughout the cell.

Enzymes are present freely in the cytoplasm as well as associated with the plasma membrane.

Replicating disc assist in replication and separation of the genetic material.

Nature of Mycoplasma:

Mycoplasma can pass through bacteriological filters and lack cell wall.

This shows that they are not bacteria.

Since they can multiply in abiotic medium having sterols, so they are not considered as virus.

Due to many similarities with bacteria they are said to be "Bacteria with their coats off".

Mode of nutrition is heterotrophic.

Some are saprophytic, but mostly they are parasitic.

They are parasitic, because they are unable to synthesize required growth factors, e.g., M. gallisepticum (0.3 to 0.5 µm, smallest prokaryote).

They can survive without oxygen.

 Sensitivity to Antibiotics:

Mycoplasma are Gram negative, insensitive to penicillin but sensitive to streptomycin, erythromycin, chloramphenicol (metabolic inhibitors) etc.,

They are insensitive to penicillin because they are wall less and penicillin interferes in the synthesis of peptidoglycan, a component of cell wall of bacteria.

Reproduction - Much is not known about reproduction of Mycoplasma but they mainly reproduce by means of elementary bodies.

 Concept Builder

Mycoplasmas cause various diseases in plants, animals and human beings.

Some are given below :

1. Plant diseases: The Mycoplasma diseases are generally transmitted through insects such as leaf hopper, mites and flies.

(i) Witches' broom        (ii) Aster yellow
(iii) Brinjal little leaf        (iv) Bunchy top of Papaya
(v) Sesame phyllody        (vi) Sandal spike

2. Diseases in animals
(i)    Pleuropneumonia in cattle
(ii)    Inflammation of genitals

3. Diseases in human beings
(i)    Infertility in man
(ii)    Primary atypical pneumonia


They are believed to have evolved immediately after the origin of life on earth, as even now these are living under extremely adverse conditions such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).

Very few other organisms can survive under such environmental conditions. So these are termed as "living fossils".

These possess introns in DNA, their ribosomal proteins are highly acidic, these prokaryotes possess histone proteins different from that of eukaryotes.

These being the most primitive and ancient most bacteria.

Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions.

The cell wall in archaebacteria contains proteins and non-cellulosic polysaccharides.

It lacks peptidoglycan, the characteristic cell wall material in bacteria and cyanobacteria.

It consists of glycoprotein, pseudomurein and non cellulosic polysaccharide.

Pseudomurein is like bacterial peptidoglycan, but contain N-acetyltalosaminuronic acid instead of NAM and lacks D-Amino acid.

The cell membrane contains branched chain lipids (phytanyl side chains) which decreases membrane fluidity.

This chemical composition of the cell membrane enables these organisms to withstand extremes of temperature and pH.

Archaebacteria are divided into three groups· methanogens, halophiles and thermoacidophiles

(a) Methanogens:

They are obligate anaerobes occurring in marshy habitats.

They are capable of converting CO2, methanol and formic acid (HCOOH) into methane and hence the name methanogens.

This property is exploited commercially in the production of fuel gas and methane in gobar gas plants (biogas fermenters).

Some of the methanogens live in rumen of herbivorous animals like buffalo, cow etc. (ruminants).

These microorganisms assist in fermentation of cellulose in such animals, e.g., Methanococcus, Methanobacterium, Methanosarcina, Methanospirillum.

(b) Halophiles:

They are aerobic chemoheterotrophic coccoid forms and are Gram negative. They occur in high salt concentration medium like sea, salt lake, brines, marshes, salted field etc.

In high light intensity a reddish pigment bacteriorhodopsin develops in their membrane to trap sun light to produce ATP, but they cannot use this ATP in food synthesis.

Sap vacuoles are absent in halophiles, hence, they cannot get plasmolysed in high salt concentration.

They maintain a high osmotic concentration of KCl in their cells.

These bacteria get lysed if NaCl level falls below 10%, e.g., Halococcus, Halobacterium.

These can grow well in a medium containing 25-30% of NaCl.

(c) Thermoacidophiles:

They are capable of tolerating high temperature as well as high acidity and hence, the name thermoacidophiles.

They often live in hot-water springs where the temperature is as high as 80°C and the pH as low as 2.

They oxidise sulphur to sulphuric acid under aerobic conditions and the energy obtained in this reaction is utilized for the synthesis of organic food.

The medium becomes highly acidic due to the production of sulphuric acid. Under anaerobic conditions sulphur is reduced to H2S, e.g., Thermoplasma, Thermoproteus, Thermococcus. Hence, these are chemosynthetic in nature.

 Concept Builder

Thermoacidophiles are capable of withstanding extremely low pH and high temperature due to:

(a) The cell membrane containing branched chain lipids

(b) The presence of resistant enzymes which can operate under acidic conditions.
It seems probable that archaebacteria, living in extreme environmental conditions got separated from the main line of bacterial evolution quite early. They survived through long period of geological changes successfully, existing at present and are perhaps the oldest "living fossil".


(1)    These are branched, filamentous bacteria and are considered as intermediate form between bacteria and fungi, e.g., Nocardia, Mycobacterium, Corynebacterium, Frankia, Streptomyces.

(2)    These filaments form radiating colonies in cultures so are also called mycelial bacteria.

(3)    Cell wall contains peptidoglycan and is Gram positive in nature. It contains mycolic acid.

(4)    They are facultative anaerobic, saprophytic, non-motile and are important decomposers of dead organic matter. 

(5)    They reproduce commonly by conidia, sporangiospore, oidia and fragmentation. 

(6)    They are acid fast in nature.

Concept Builder

Economic Importance of Actinomycetes : They are the source of more than 100 antibiotics. Most common antibiotics used in medicine are produced by filamentous bacteria and are listed below :

Frankia can fix N2 symbiotically by root nodule formation in non legumes like -Alnus, Myrica and Casuarina.

Earthy or musty smell of freshly ploughed soil or soil after first shower of rain, is due to oily secretion geosmin by some members like Nocardia, Micromonospora and Streptomyces

Concept Builder

1. In 1798, Edward Jenner used cowpox virus for immunising people against small pox.

2. Pasteurization simply kills bacteria but does not sterilize milk. It frees the latter of all vegetative cells of pathogens, but not the endospores.

3. Mycobacterium and Xanthomonas form nodules in leaves of Ardisia, Pavetta.

4. Bacterial cell membrane contains pentacyclic sterol like molecule termed as hopanoids as membrane stabiliser.

5. Kleinberger discovered tuberculosis and anthrax bacteria & developed "L-form" bacteria.

6. H. T. Ricketts described some new types of organisms (now considered a type of bacteria) in the blood of victims of Rocky Mountain spotted fever and similar organisms were called Rickettsiae, in honour of Ricketts. They occur as obligate intracellular parasites. The Q fever is caused by Coxiella burnetti. Rocky mountain fever is caused by Rickettsia rickettsii.

7. Chlamydiae: They are obligate intracellular parasite of vertebrates. They reproduce by elementary bodies. They do not have their own ATP generating system and obtain this energy from host. So that they are called energy parasite.

  1. Diseases
  • Trachoma (eye disease)                     – C. trachomatis
  • Conjuctivitis                                       – C. trachomatis
  • Psittacosis (influenza like)                – C. psittaci

8. Joseph Lister developed the technique of aseptic cultures.

9. Nostoc develops symbiotic associations with Gunnera stem and Trifolium roots.