Bacteria | A Guide On The Life Cycle of Bacteria 2022

Bacteria are those organisms, which can be found throughout the biosphere. They are the simplest and smallest (< 1 µm — 50 µm width/diameter) organisms.

They belong to the group prokaryotes (Gr. Pro, before; karyote, nucleus), which can be studied even by a light microscope.

What is Bacteria or Give Bacteria Definition?

According to the microbiology definition of bacteria, they are one-celled and are classified as microscopic organisms found throughout the whole environment.

In soil, in air, in water (even in boiling water), and even in your own body. For example, the bacteria Escherichia coli.

E Coli Morphology

Here is the external and internal structure of E. coli.

Bacteria Escherichia Coli Structure

History of Bacteria Discovery

Father Of Microbiology

Antonie Van Leeuwenhoek (Father Of Microbiology)

Anton von Leeuwenhoek discovered bacteria in 1675 without any major understanding of the organism and give them the name animalcules.

Pasteur And Koch

Pasteur And Koch

But French Microbiologists, Pasteur, and Koch had the honor to make a major contribution in the field from 1870 to 1976.

They were able to identify several bacteria morphologically and physiologically.

Koch was able to demonstrate that bacteria can cause diseases such as Anthrax tuberculosis. This understanding leads them to suggest “The Germ Theory of Disease”. The idea was soon accepted and studies on it as a subject were established.

Father Of Mycology

Thomas Jonathan Burrill (Father Of Mycology)

Originally the idea was restricted to animals only.

T. J. Burrill was the first to demonstrate that even plants could develop diseases caused by bacteria.

A lot of information about the bacterial kingdom has been added to human knowledge since then.

Classification Of Bacteria

It is a traditional approach that classification is based on morphological characters. But in bacterial cases, because of their simple morphology, they cannot be divided into orders, families, genera, and species on the basis of their structure alone.

Thus, their classification studies are carried out in the field of their biochemistry, physiology, and cultural characteristics.

We have a number of classifications of bacteria but the most accepted classification is believed to be the classification carried out by Bergey (1957). Some other classifications are also quoted for a better understanding of the subject.

Erwin Frink Smith

Erwin F. Smith, a pioneer worker in this field of bacterial classification accepted a system of classification proposed by Migula, which was based on the number and arrangement of flagella. A general outline of the classification is as follows.

  • Bacterium included all atrichous rod-like forms.
  • Pseudomonas included polar flagellated forms.
  • Bacillus included peritrichous rod-like forms.

Smith in 1905, revised this system with slight modifications in the name of the divisions, thus the name

  1. Aplanobacter gave for atrichous, rod-like bacteria
  2. Bacterium for polar flagellate bacteria
  3. Bacillus for peritrichous bacteria

The commonly used and more widely accepted divisions of bacteria are,

  • Bacterium or Bacillus included all rod-like and idly shaped forms.
  • Coccus includes all-spherical bacteria.
  • 3, Spirillum included spiral, comma-like, and variously curved bacteria.

Bergey’s Manual Of Determinative Bacteriology (1957)

A system of classification of Schizomycetes that is used by all bacteriologists and that has an international standing was presented by Berger in 1957 in “Bergey’s Manual of Determinative Bacteriology”.

It represents the collaborative efforts of over 100 best-qualified microbiologists of that time. It was established by the International Committee of Bacteriological nomenclature in 1947.

The system was based on the international nomenclature rules of viruses and bacteria. As outlined in Bergey’s manual all bacteria were divided into about 1500 species.

These all bacteria were differentiated from one another primarily on the basis of type of motility and morphological characters. They are divided into a total of 10 orders.

Order Pseudomonadales

Cells are rigid, spheroids or rod-shaped, straight, curved, or spiral some groups form trichomes; motile species have four flagella.


In trichomes rod-like cells are present. They are often sheathed and iron hydroxide is deposited in the sheath. The flagella are sub-polar when present.


Cells spheroidal or ovoid, connected on stalk or thread, on trichomes; exhibits budding and longitudinal fission; flagella polar when present.


Typically, eubacteriales have rod-like or unicellular spheroidal cells present on trichomes. They consist of a sheath or other accessory structures. In eubacteriales, the motile species have peritrichous flagella.

Order Actinomycetales

Cells branch and many species form mycelia and mold-like conidiophores and sporangiophores, polar flagellate, and sporangiospores in only one species.


Trichomes are often very long, peritrichous flagella.


Alga like trichome or coccoid cell, accumulate elemental S, gliding and oscillatory or rolling motion; no flagella.


Cells coccoid, rod-like or fusiform, communal slim, fruiting bodies, cells flexuous, gliding motility in contact with the solid surface; no flagella.


Elongated, spiral cells rotatory and flexing motions and translator motility; no flagella.


Extremely pleomorphic and easily distorted cells without cell walls; complex life cycle; non-motile.

Table of Comparative Study of Different Workers

A comparative study of different workers in this field is as follows:

Bacteria Classification Table

Phylogenetic Relationship

The phylogenetic relationship of the bacteria is not certain. This uncertainty primarily becomes the cause of a long delay in the proper classification of this group. According to one scheme, they were placed in the phylum Schizomycophyta in the Sub Kingdom Thallophyta, which includes all plants and does not form embryos during development. According to another modern taxonomic system these organisms have been placed in the Kingdom Protista along with algae, fungi, and protozoan. But the most modern understanding of the five Kingdom systems proposed by Robert Whittaker in 1969 placed them in the kingdom Monera along with blue green algae, on the basis of their Prokaryotic nature.

According to one school of thought the bacteria have been, descended from the blue-green algae, after becoming adopted to a saprophytic or parasitic life style. This view has its general understanding based on the similarity of the ceil structure of these two forms.

According to other investigators, the fact that many bacteria possess flagella indicates that these organisms descended from simple flagellated forms and perhaps these forms have also given, rise to the, green algae. There are others who believe that the heterotrophic bacteria of today have been evolved from autotrophic ones. Well, according to these workers, before the appearance of any chlorophyll containing plants the autotrophic bacteria may have appeared. It is also suggested that different groups of bacteria from different ancestor have descended independently. Many others still believe that bacteria have given rise to other forms as being a terminal group in evolution or if they have not descended from the blue-green algae, perhaps the blue green algae have descended from them.

Characteristics of Bacteria


They are omnipresent and found almost everywhere but they are in great abundance in tropical and temperate regions.

On one hand, they can be found in the snow at temperatures less than zero but on the other hand, they can also be found in hot springs with a temperature of 78°C.


The majority of them are unicellular but the filamentous and colonial forms are quite common. Usually, they are found embedded in the mucilaginous envelope. Even when the bacteria are held together in chains each individual cell carries on its own metabolic process quite independently.

They may be devoid of chlorophyll and mostly heterotrophic in nutrition. They produce asexually chiefly by fission while sexually they have different three modes like conjugation, transformation, and transduction. Before going towards the morphology of bacteria, here we have a few examples of bacteria that affect humans. The chart is given below.

Examples of Bacteria

General Morphological Characters Of Bacteria

Size And Shape of Bacteria

The bacterial cell varies greatly in their size. An average bacterial cell measures 3µ long and 1µ broad whereas an average spherical cell is 1µ in diameter. The average volume of a bacterial cell is 1 cubic micron. It has various forms but the abundant three basic forms of bacteria are: Coccus, Bacillus, and Spirillum.


They are rounded or spherical. They are the smallest forms of bacteria. The cell may either separate from each other or may remain joined ‘together after the division to form groups of two (Diplococcus) or four (Micrococcus tetra-genus) or it may be a chain of cells (Streptococcus). The average size of coccus bacteria ranges in diameter from 0.5µ to 1.25µ.


They are straight rod-like, which possess rod-like, kidneys or elongated cells. They vary greatly in their length and diameter ranging from 0.6µ to 1.2µ long and 0.5µ to 0.7µ wide to 3.8µ long and 1 to 1.2µ wide.


They are spiral or curved forms. In this case, the cells vary in size from 1.5µ to 4µ long and 0.2µ to 0.4µ wide in vibrio and up to 50µ long in spirillum.

Pleomorphism: Under favorable conditions of growth and development the shape of one species remains constant, whereas under unfavorable conditions some bacteria such as nitrogen-fixing bacteria change into at least three different forms. Besides the original structure of the parent type many bacteria show a mixture of several integrating forms in young cultures. This phenomenon is known as pleomorphism.

Different Forms of Bacteria

When treated with antibiotic, attacked by T4 bacteriophage or cold shook they sometime develop soft protoplasmic forms, which is known as large bodies or L-forms. If transferred back to Normal conditions their form also reverts back to normal shape.

Structure of Bacterial Cell (Bacteria Anatomy)

The internal structure of bacteria cells contains vacuoles, ribosomes, and granules of stored food. It also contains granules of glycogen, proteins, and fats but lacks an endoplasmic reticulum.

Mitochondria (defined as rod-shaped structures or organelles that are classified as power generators/powerhouse of the cell, which converts nutrients and oxygen into (ATP) adenosine triphosphate) are absent.

Enzymes found in the mitochondria are localized on or near the cell wall. Water is an important constituent about 90 % of cell is water. The movement of materials in and out of the cell is regulated by a cell membrane.

Structurally the cytoplasm of a bacterial cell is similar to the cytoplasm found in the living cells of more complex organisms. The membrane plasmalemma invaginates to become a complex structure known as mesosomes.

It is believed that mesosomes have an active role in the cell division by wall synthesis and in the secretions of extra-cellular substances. Mode of cell division is amitotic.

Structure of Bacterial Cell: Diagrammatic representation of bacteria anatomy
Bacteria Nucleus

A nuclear membrane is not present. The prokaryotic nuclear region appears as an electron translucent area and can be shown to contain very fine fibrils in the electron microscope. These fibrils are molecular strands of DNA. The DNA is a centrally restricted area of the cell. It has been demonstrated for some species of bacteria that the DNA is about 1200 microns long. i.e. more than 500 times long as the bacterial cell that contains it. The adjustment is made by supper coiling of the single circular chromosome.

Bacteria Cell Wall

A definite thin cell wall consisting of a single layer is present. The cell walls become gelatinous in some cases and appear like a sheath or capsule which holds cells together to form colonies. The cell wall accounts for 20% of the dry weight of the cell. It gives shape and firmness to the cell.

The chemical nature of the wall demonstrates that it is made up of highly complex, which consists of proteins, polysaccharides, and lipids. The complexity of the wall is generalized into two categories i.e., Gram positive bacteria and Gram negative bacteria. These are discussed further down.

Define Flagellation and its types

Bacteria bear thin elongated thread-like structures called flagella (singular-flagellum), which help in their locomotion. There are different forms of bacteria that bear flagella. Almost all the spirilla, most of the bacilli, and some of the cocci are flagellated.

Flagella are the motile organs, which give a speed of about 50mµ per second, although they are much smaller than the eukaryotic flagella. They are about 120 to 180°A in diameter. Bacteria flagella consist essentially of a protein of a pure protein called “flagellin”.

The flagella arise from small basal granules as they are fixed onto the protoplast. The blepharoplasts are situated on the outside of the cytoplasm. The flagella may consist of either a simple filament onto the protoplast and arise from small basal granules, the blephroplasts situated on the outside of the cytoplasm.

Types of Flagella Arrangement

Here bacteria are divided into different types according to the presence of flagella.

  • Atrichous Bacteria: Those bacteria in which flagella are absent are termed as Atrichous.
  • Define Monotrichous: Those bacteria which have only one polar flagellum are known as Monotrichous e.g. Vibrio.
  • Lophotrichous Bacteria: Another type is Lophotrichous. Now these are types of bacteria which have a group of flagella present at one end. For example. Spirillum.
  • Amphitrichous Bacteria: On the other hand, those bacterial forms which have a group of flagella at both ends are named Amphitrichous.
  • Peritrichous Bacteria: Those types of bacteria which have flagella uniformly distributed all over the body are known as Peritrichous. e.g. Salmonella.
Bacterial Flagella

What is the Structure of a Flagella

The bacterial flagella is a non-flexible structure consisting of a single filament composed of many subunits of the protein flagellin. The filament of the bacterial flagellum is attached to the cell by a hook and a basal body, which has a set of rings that attach to the cytoplasmic and a rod that passes through the rings to anchor the filament to the cell.

In Gram negative bacteria the basal body has two sets of rings, with each set containing two rings. The two rings which are attached to the cytoplasmic membrane are designated S and M and the two rings that attach to the outer membrane of the envelope are designed as L and P. But the Gram positive bacteria have only one set of rings and this set is attached to the cytoplasmic membrane. These rings are also designated as S’ and M. The hook structure attaches the filament of the bacterial flagellum to the rod of the basal body.

Flagella of Gram Negative Bacteria: The flagellum is anchored to the cell via a hook and basal body structure. Four rings attach the flagellum to the outer and cytoplasmic membranes of a Gram-negative cell. The four rings are designated L (lipopolysaccharide associated), P (peptidoglycan associated), S (periplasmic space associated); and M (cytoplasmic membrane-associated). This structure permits the flagellum to rotate. The energy for rotation comes from the protonmotive force

The structure of the bacterial flagellum allows it to spin like a propeller, with the basil body acting like a motor to rotate the filament and thereby propel the bacterial cell. Rotation of the flagellum requires energy, which is supplied by the proton gradient across the cytoplasmic membrane. Approximately 256 protons must cross the cytoplasmic membrane to power a single rotation of the flagellum. The flagellum can rotate at speed of up to 1,200 revolutions per minute, thus enabling the bacterial cell to move at speed of 100 µm/second (0.0002 mile/hour). Considering that a typical bacterial cell has a maximum length of 2 µm, a rapidly swimming bacterial cell can move 50 times its body length per second – or in relative terms, twice as fast as cheetah.

Bacterial Types on Basis of Cell Wall

As we studied above in the cell wall section, the chemical nature of the wall is made up of muco-complex, which consists of proteins, polysaccharides, and lipids. That’s why due to this complexity, the wall is divided into two types i.e., Gram positive bacteria and Gram negative bacteria as below.

Cell Wall of Gram Positive Bacteria

The Gram-positive cell wall has a peptidoglycan layer that is relatively thick (ca.40nm) and comprises approximately 90% of the cell wall. This thick peptidoglycan layer, which is considerably hydrated, accounts for the staining reaction observed in the Gram stain procedure. The primary stain (crystal violet) passes across the wall freely and is firmly attached to cell structure by the addition of the mordant (Gram’s iodine). The decolorizing agent (ethanol or acetone) dehydrates the wall, causing it to shrink and trap the primary stain iodine complex. Thus, Gram positive bacterial cells retain the primary stain and appear blue-purple following Gram-staining.

Cell Wall of Gram Positive Bacteria

The cell walls of most Gram positive bacteria also have teichoic acids, which are acidic anionic polysaccharides. Teichoic acids contain a carbohydrate such as glucose, phosphate and an alcohol (either glycerol or ribitol). The teichoic acid is bonded to the peptidoglycan, making them an integral part of the Gram-positive cell wall structure. Teichoic acids can bind protons, thereby. maintaining the cell wall at a relatively low pH. This low pH prevents autolysins from degrading the cell wall. Teichoic acids also bind cations such as Ca2+ and Mg2+ and act as receptor site for some viruses.

When phosphate concentration is low, Gram positive bacteria replace the phosphate-rich teichoic acids of the cell wall with teichuronic acids. This enables them to conserve phosphate that is essential for ATP, DNA, and other cellular components. Teichuronic acids are polysaccharides chains of uronic acids and N-acetylglucosamine, which fulfil the cell requirements for the acidic, anionic polysaccharides in the cell wall. Here we are discussing some of the gram positive bacteria examples you might like.

Gram Positive Bacteria Examples

Gram Negative Bacteria Cell Wall

The Gram-negative cell wall is far more complex than gram positive cell wall. The peptidoglycan layer of the Gram negative bacteria is very thin (2nm). An often comprises only 10% or less of the cell wall. The gram-negative staining reaction occurs because the wall is too thin to retain the crystal violet iodine complex when treated with the decoloring agent. Teichoic acid does not occur in Gram negative bacteria cell wall. Rather lipoprotein is bonded to the peptidoglycan, forming an integral part of the Gram negative bacteria wall, additionally there are layers of lipopolysaccharide, phospholipids, and protein outside the peptidoglycan layer. Although these layers are sometimes considered part of the cell wall, it is now more common to view the peptidoglycan layer as the wall component of a large, more complex structure, called the cell envelope, of the Gram negative bacterial cell.

Gram Negative Bacteria Cell Wall

Cell Wall Structure of Gram Negative Bacteria

The cell wall structure is divided into two descriptive portions: Outer Membrane and  Periplasm.

Outer Membrane: The cell envelope of gram negative bacterial cell extends, outwards from the cytoplasmic membrane – the outer membrane. The outer membrane is a lipid bilayer containing phospholipids, proteins, lipoprotein, and lipopolysaccharides; unlike the cytoplasmic membrane, it is relatively permeable to most small molecules. Electron microscope revealed that cytoplasmic membrane in some Gram positive bacteria may be joined or fused at many points around the cell. Whereas the inner surface of Gram negative bacteria outer membrane is bridged to the peptidoglycan layer through lipoprotein. The outer membrane contains lipopolysaccharides (LPS), which are not found in cytoplasmic membrane. LPS is a complex molecule composed of distinct regions and is able to caused toxicity to the host cells.

Functionally, the outer membrane of the Gram negative bacterial cell is a course molecular sieve. The permeability of the outer membrane to the nutrients is due to the outer membrane proteins (Omp), collectively called porins. The porins usually occurs in aggregates of three, form cross-membrane channels through which some molecules can diffuse. Hydrophilic and hydrophobic molecules can diffuse through the outer membrane but cytoplasmic membrane excludes almost all hydrophilic substances except water.

Periplasm: The region between the cytoplasm and outer membrane is known as periplasm (also called as periplasmic space or periplasmic gel). This is an important region in Gram negative bacteria where diverse reactions occur, including oxidation reduction reaction, osmotic regulation, solute transport, protein secretion and hydrolytic activities.

Moreover, many proteins are found in this region, which includes binding proteins, chemoreceptors, and  enzyme (protein molecules inside the cells, they perform their function as catalysts as they speed up the chemical reactions) including those involved in the oxidation reduction process. The binding proteins facilitate the transport of substances to carriers into the cell by delivering substances to carrier that are bound to the cytoplasm.

The chemoredeptors bind with substances and direct the cell movement towards or away from these substances. Hydrolytic enzymes (Hydrolytic (Hydro- water, lytic- break down) enzymes are generally catabolic enzymes that break the chemical bond between atoms of large molecule in the presence of water) are also present in periplasm which breakdown large molecules into smaller ones on their way to be used to produce ATP and cellular constituents. Few of the gram negative bacteria example species are as follows:

Gram Negative Bacteria Example Species

Bacterial Capsules, Slime Layer, and S Layers

What is Glycocalyx: Various external layers may surround the bacterial cell wall, playing various roles, including protection of the cell. Collectively these structures are called the glycocalyx. In this general term includes the complex polysaccharides or protein and include capsules, slime layer and S layers.

Some bacteria form a protective layer called a capsule. The capsule surrounds the cell wall. Chemical composition of the capsule varies among the species of the bacteria. It often is composed of polysaccharides attached externally to the cell wall. In contrast some bacteria have cell wall composed of glutamic acid. The capsule protects the bacteria against the phagocytosis (phagocytosis definition – the process by which certain cells i.e. Phagocytes engulfs a large particle to form an internal part known as a phagosome) by various protozoa and human white blood cells. Thus, having capsule is the major factor in determining the pathogenicity of a bacterium.

Although capsules and slime layer are similar in composition a distinction can be made between them. Although both capsule and most slimy layers are composed of polysaccharides slimy layers are not as tightly bound to the cell as capsule. These external layers may protect the bacterium against the dehydration and loss of nutrition. In some cases, they act as trap for nutrients. In some cases, they act as traps for restricting the flow of substrates away from the cell.

In addition to these layers some bacteria have a crystalline protein layer surrounding the bacteria cell. It is known as S layer. This layer occurs outside the wall of gram positive bacteria and is external to the outer membrane of Gram negative bacteria. It is also the only layer observed surrounding the cytoplasmic membrane. The function of the S layer is not yet known.

The Life Cycle Of Bacteria | Growth In Bacteria

Cell Growth

If conditions are favorable, cell division is normally followed by a period of elongation and growth. Under favorable conditions of moisture, nutrition, pH, and temperature some bacteria may double in about 20 minutes.

Different types of bacterial species show variety of growth shape curves depending upon the generation time and the maximum population attainable under the growth conditions that prevail. During the period of one to several, hours there may be lag phase in which there is little or no increase in cell number. During the first part of lag phase the cells adoption to the new environment.

Growing bacteria continue to do so at regular intervals until the maximum growth that can be supported by their environment is appreciated. This period of rapid cell division is known as the Logarithmic Growth Phase. The stationary phase occurs when rapid growth is halted by the depletion of nutrition’s, accumulation of the waste products, or other factors. The cells will eventually die, if they are not transformed to new environment which is capable of supporting the continuing support.

The Life Cycle Of Bacteria | Reproduction in Bacteria

In bacteria both asexual and sexual reproduction is reported.

Asexual Reproduction

Common method is Binary Fission. Under ideal conditions of temperature, moisture, pH and food bacteria shows a rapid cell division after every 20 minutes. If all the conditions remain optimum a man with simple knowledge can calculate the huge mass of bacteria accumulated within few hours. But practical knowledge proves this calculation to be wrong. In some cases, there may be an exhaustion of the food supply or an accumulation of waste products, which may be alcohol or some acid. Their accumulation retards the growth of the bacteria.

Binary Fission

The division begins with the doubling of the DNA, which is followed by the division of all constituent distributed equally among the two daughter cells. Method is very fast and son their number increases proportionate to the prevailing conditions.

Sexual Division – Types of Sexual Reproduction in Bacteria

Sexual reproduction is not a common phenomenon in bacteria. In general, understanding the sexual reproduction involves exchange of genetic material among the two individuals. In bacteria at least three different modes of exchange of genetic material has been demonstrated. List includes bacterial conjugation transduction and transformation.

Explain Conjugation in Bacteria: (A) A mating pair; (B) DNA transfer; (C) Separation


Conjugation is the mechanism in which genetic material is exchanged between the two bacteria through a cytoplasmic bridge. Whole DNA is not involved but a plasmid known as conjugative plasmid (F+ fertility+) forms a mating pair with bacteria that does not contain a conjugative plasmid(F-) by means of a F-pilus on the surface of the bacteria. The pilus contracts pulling the two bacteria together and the DNA is transferred.

This phenomenon is best studied in the bacteria E. coli, the conjugative plasmids have been demonstrated to have the ability to transfer themselves between bacteria and in some cases, also to transfer pieces of chromosomal DNA. It has been found that F plasmids and allied plasmids carry a group of genes called the Tra (Transfer) Genes, which encode all the proteins required to form a mating pair with other bacterium not containing it. DNA is then transferred from plasmid containing bacteria called the donor or F+ bacteria to the recipient F bacteria. The plasmid is large enough to contain 95 kb (kilo bite) in size.

Cell containing F plasmids (F+) also produce a structure known as F-pili on their surface, which are encoded by the tra genes. These are proteinaceous, filamentous structure that attach to the surface of bacteria due to a phenomenon called surface exclusion. The F-pilus retracts on contact and thus pulls the two bacteria together.

Transduction in Bacteria

A mechanism in which a piece of DNA is picked up from one bacterium called donor by phage and is transferred to another-bacterium called recipient is known as transduction. This phenomenon has been demonstrated in a wide range of bacteria and is thought to play vital role in the transfer of genetic material between bacteria in the nature.

Steps of Transduction in Bacteria: The bacterial filter prevents mixing of strains A and B, but allows free passage of the bacteriophage


The phenomenon was discovered by Fred Griffith (1928). In Griffith’s bacterial transformation experiments, the free DNA molecule transferred into recipient cells. In this experiment he found that DNA released from dead bacterial cell is genetically effective in very small amount. In the laboratory, pure DNA extracts are used in transformation experiments. Transformation typically involves only one trait, although several may be acquired independently in this process.

Eyeing In Griffith’s Bacterial Transformation Experiments

Economic Importance Of Bacteria

Many investigators after lot of hard work have been able to established the multifold uses of bacteria in the field of Biotechnology, which has established the bacterial use as a tool of economic importance.

Its multi-dimensional uses can be divided into two major subdivisions on the basis of its effect on human life.

We will be discussing all those uses of bacteria that helps us. This can be further divided into the fields of Agriculture and Industry.

Economic Importance of Bacteria In Agriculture

Many species of saprophytic and symbiotic bacteria add to the fertility of the soil and develops Rhizophere providing Nitrogen and Sulphur cycle more effective. These needs to be discussed in some detail.

A) Rhizosphere

Bacteria of various kinds are associated with the leaves, stems, flowers, seeds and roots of the plants. They influence the plants in many ways. The plants release a wide range of potential microbial substrates, inhibitors and stimulants. The soil bacterial population respond to the release of organic material in the immediate vicinity of the bacterial community, this region is called the rhizosphere and is very important to the soil with low fertility. In the rhizosphere, nitrogen fixation performed by Azotobacter and Azospirillum increase ammonium ion availability for the plants. Rhizobium, a prominent member of the Rhizosphere community can fix nitrogen.

B) Nitrogen Cycle in Agriculture – Ammonifying and Nitrifying Bacteria

Ammonifying Bacteria: Bacillus subtilis, B. mycoides, B. ramosus etc., act upon the dead animals and decompose their complex organic compounds like proteins into ammonium compounds. They are also known as putrefying bacteria.

Nitrifying Bacteria: Nitrosomonas oxidize the ammonium compound into nitrites in presence of free oxygen and Nitrobacter oxidize nitrites into nitrates in the presence of free oxygen. Thus, ammonifying and nitrifying bacteria increase the number of nitrogenous compounds in the soil. Dead plants, animals etc., are converted into humus, which itself acts as fertilizer for the plants.

Nitrogen Fixing Bacteria: Azotobacter, Clostridium and Rhizobium fix free nitrogen of the soil and make it available to the nitrogenous compounds in the soil. The third one is a symbiotic type. They take the free atmospheric nitrogen as they live in the root nodules of leguminous plants and fix it within its tissues. These bacteria help plants grow in the soil having no other source of nitrogen. The leguminous plants make the soil rich in nitrogen supply thus are used as green manure.

Mechanism of Nitrogen Fixation: Mechanism of nitrogen fixation takes place with help of special type of bacteria, which fix free atmospheric nitrogen gas into ammonia by means of symbiosis with leguminous plants.

Nitrogen Cycle in Agriculture

Rhizobium: One of the bacteria taking part in the process includes Rhizobium leguminosarum, which is a soil born bacterium. They produce IAA (Indol Acetic Acid), which affects the root hair and made them curl. This rod like bacteria penetrates through the tip of the root hair forming a continuous infection thread that enters the cortical region in twenty-four hours. During its passage through the root hair, the infectious thread gets surrounded by the cellulose wall. The host as a reaction to the infection secretes this wall. The infectious thread ramifies in the cortical region and the bacterial rods are released in the cytoplasm of the cells, which are stimulated. These cells are enlarged and multiply to form the characteristic nodules all over the root system. On the outer side the root nodule has a cortical layer which is followed by an actively proliferating region, then the vascular system enclosing in the centre a zone of bacteria.

These bacteria absorb the atmospheric nitrogen and make it available to the host plant in the form of ammonia, which is being convened into nitrates. In turn they get shelter and nutrients in the form of carbohydrates from the plant. On the death and decay of the plant the bacteria are set free to attack the new plants and thus add to the fertility of the soil.

The fixation of free nitrogen from the atmosphere through ammonia into free nitrates and again, their conversion into ammonia and free nitrogen takes place by means of nitrifying and denitrifying bacteria, along with other organisms. This process known as nitrogen cycle.

C) Sulphur Cycle – Describe Sulphur Cycle and Role of Bacteria in it

Bacteria contribute greatly to the Sulphur cycle. Photosynthetic bacteria transform Sulphur by using sulfide as an electron source. In the absence of light, sulfide can be used by Thiobacillus and other lithoautotropic. In contrast, sulfate can undergo sulfate reduction when organic reductants are present; Desulfovibrio can derive energy by using sulfate as an oxidant under these conditions. Dissimilatory reduction occurs when sulfate is used as an external electron acceptor. (anaerobic respiration) to form sulfide. In comparison, the reduction of sulfate for amino acid and protein biosynthesis is described as assimilatory reduction. When pH and oxidation-reduction conditions are favorable, several key transformations in the sulfur cycle also occur as a result of chemical reaction.

Economic Importance of Bacteria In Industry

Human understanding of bacterial metabolism made him employ different bacteria for the manufacture of different industrial products.

a) Butter Making Industry: Saprophytic bacteria such as Lacto bacilli popularly known as starters make the milk sour and produce various flavors. They are employed in butter industry for ripening milk and producing flavor in butter.

b) Cheese Making Industry: Bacteria are also employed in cheese industry. The casein of milk is ripened by certain bacteria after its coagulation process. Bacteria helps in making the casein spongy, soft and give it characteristic taste and flavor.

c) Vinegar Making Industry: Vinegar industry depends on the working of a bacteria Bacillus aceti, which convert the sugar solution into vinegar.

d) Alcohol and Acetone Manufacturer: Butyl alcohol and acetone are manufactured by the action of bacteria on molasses.

e) Tobacco Curing: Bacteria are utilized in both processes of curing and ripening of raw crud dry leaves of tobacco. These leaves are passed through curing and ripening process before they are ready for use. Due to bacterial activity the tobacco obtains a peculiar taste and smell.

f) What is Tanning: In leather tanning, the hides and skins after drying salting and cleaning are steeped in fluids containing specific bacteria. The process of fermentation goes on for some time aid then they are transferred to tan-pits and further allowed to be fermented. This whole process is known as tanning.

g) Tea Curing: Fresh crud leaves of tea are subjected to fermentation with the help of bacteria. The process is known as curing, which is employed to impart a peculiar taste and flavor to the tea, which is due to the bacterial activity.

h) Fibre Retting: The procedure of separation of fibres from the plant tissues is called as Retting. Bacteria are employed in this industry, which causes decay of the softer tissues and render fibre easily separable mechanically. Coconut, jute, flux, hemp and others fibrous plant fibres are obtained by immersion process. This involves the dipping of specific plant organs in stagnant pond water where bacteria develop and cause retting.

i) The Sewage Work: In order to overcome solid and semisolid sewage waste, it is allowed to putrefy. Putrefying bacteria are allowed to act upon sewage under anaerobic conditions. It gets decay and liquefied. It is filtered and the liquid is either drained to river or used as manure in the fields.

j) Ensilage: The process of preserving green fodder in pits is known as Ensilage. Certain bacteria help in the process of preservation of fodder.

k) Medicines: Those chemical substances which are produced in the host tissues in response to the attack of parasitic bacteria is called Antitoxins. Different vaccines and serums now prepared from these used in the treatments of specific ailments. The antibiotics such as streptomycin, aureomycin, etc., are obtained from certain actinomycetous bacteria.

Antibiotics Against Gram Positive and Gram Negative: In medicine field many questions arises about use of bacteria and one of the question is; “what antibiotics are used to treat gram negative bacteria”. The answer is that there are different antibiotics made for both gram negative and gram positive bacteria. Some of examples are as follows:

  • Gram Negative Bacteria Medicine
    Trimethoprim-sulfamethoxazole, Imipenem, Cephalosporins (ceftriaxone-cefotaxime, ceftazidime), Broad-spectrum penicillins with or without β-lactamase inhibitors (amoxicillin-clavulanic acid, piperacillin-tazobactam).
  • Antibiotics for Gram Positive Bacteria
    Erythromycin, Vancomycin, Lincomycin, Cloxacillin, Penicillins, Clindamycin, Fusidic Acid.

Harmful Bacteria

Besides its role as beneficial agents they are also harmful role to perform. Many bacteria are found performing harmful role for human beings thus they need attention.


Bacteria cause a lot of losses to animals including man and plants causing various diseases. Prominent human disease includes typhoid, pneumonia, dysentery, tuberculosis, tetanus etc., while plant disease includes ring disease of potato, yellowing rot of wheat, citrus canker etc.

Antibiotics Resistance

Sometimes the antibiotics doesn’t work because of the antibiotic resistance of bacteria that they develop overtime. That’s why bacteria become stronger and danger to human health and a cause of great number of diseases in humans and animals.

Food Spoilage

Human food is also a consumable item for bacteria. Thus, the food is spoiled by causing decay at can be prevented by fixing free water with the addition of salt, sugar and oil.

Loss of Soil Fertility

Anaerobic bacteria such as Bacillus denitrificans reduce the nitrates of poorly aerated soil to nitrates and then to ammonia compounds. Free nitrogen thus produced is liberated from the soil, rendering the soil nitrogen free and thus less fertile.

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