BACTERIA

Bacteria are microscopic, unicellular, prokaryotic organisms. They do not have

membrane-bound cell organelles and lack a true nucleus, hence are grouped under the
domain “Prokaryota” together with Archae.

In a three-domain system, Bacteria is the largest domain. (Living beings are classified into
Archae, Bacteria, and Eukaryota domain in the three-domain system).

Bacteria, a singular bacterium, is derived from the Ancient Greek word “backērion”
meaning “cane”, as the first bacteria observed were bacilli. The study of ‘Bacteria’ is
called ‘Bacteriology’; a branch of ‘Microbiology’.

Salient Features of Bacteria

Bacteriology is the branch of botany under which we study the metabolism and
reproduction of bacteria. The general characters of bacteria are:

1. They are omnipresent i.e. present in soil, air and water.

2. They are unicellular, prokaryotic microorganism.
3. The cell bears a thick rigid cell wall outside the plasma membrane (because of this
character they are kept in plant kingdom).
4. They have great variation in the mode of nutrition i.e. may be autotrophic and
heterotrophic. In heterotrophism mode of nutrition they may be parasite saprophyte
or symbiotic in nature.
5. They lack true chlorophyll but few photosynthetic bacteria have a special type of
chlorophyll called bacteriochlorophyll.
6. Because of the prokaryotic nature they lack true nucleus (lacking nuclear membrane
and nucleolus), genetic material is in the form of composite structure known as
genophore / nucleoid / incipient nucleus.
7. The cell wall of bacteria is made up of mucopeptide unlike the cell wall of plants (where
it is made up of cellulose).
8. They lack mitochondria, golgi apparatus, plastid and endoplasmic reticulum.
9. They lack basic protein histone in their DNA.
10. Ribosomes are of 70s type.
11. At some places the plasma membrane invaginate in folds to form mesosomes.
12. All the enzymes required for respiration are found in the cell membrane.
13. Both DNA and RNA are present in the bacterial cell. DNA is in the form of single circular
chromosome (therefore the ce.il is haploid).
14. Vegetative reproduction is generally by binary fission, cyst, budding and gonidia.
15. Asexual reproduction is by conidia, motile spores and endospore.
16. True sexual reproduction is absent in bacteria but there are examples of genetic
recombination which may be of following types viz. conjugation, transduction and
transformation.
Structure of a Bacterial Cell

Bacteria are unicellular i.e., made up of a single cell. They are prokaryotes and their cells are
different from animal and plant cells. In general, the structure of bacteria can be studied as
external and internal structures;

Structure of a Bacterial Cell

External Structure of a Bacteria

It includes a cell wall and all the structures outside the cell wall.

1. Flagella (sing. Flagellum)

Flagella are long hair-like filamentous structures of about 4 – 5 μm long and 0.01 – 0.03 μm in
diameter. They confer motility to the bacteria. Flagella are divided into three parts; filament,
hook, and the basal body.

The filament is a threadlike part extending outside the cell wall. It is made up of flagellin
protein.

The hook is a short-curved structure that joins filament with the basal body. It produces
repulsion like the propeller during the revolving of flagella.

The basal body is a set of rings embedded in the cell wall and plasma membrane. It consists of 2
pairs of rings in Gram-Negative bacteria and 1 pair of rings in Gram-Positive bacteria. It
synthesizes polymers of the flagellum, produces energy for revolution, and regulates
movements of the flagellum.

Functions of Flagella

• Responsible for motility

• Aids in chemotaxis

• Aids in bacterial pathogenicity and survival

2. Pili/Fimbriae

They are the short, hollow, non-helical filamentous structure of about 0.5 μm in length and 0.01
μm in diameter. They are exclusively found in Gram-Negative bacteria.

They are composed of protein ‘pilin’ arranged non-helically. They are short, numerous, and
straight than flagella.

Sex pili are a special kind of pili that take part in bacterial conjugation. They are larger than
usual pili; 10-20 μm in length. They are few in number, just 1-4 in number. They are further
classified into two types; F-pili and I-pili.

Functions of Pili/Fimbriae

• Aids in adherence to host cells

• Sex pili helps in bacterial DNA transfer during bacterial conjugation

3. Capsule

It is a viscous outermost layer surrounding the cell wall. It is composed of either polysaccharides
or polypeptides of both (~2%) and water (~98%). They are present only in some species of
bacteria. The capsule is of 2 types; macro-capsule (capsule with a thickness of 0.2 μm or more)
and micro-capsule (capsule with thickness less than 0.2 μm).

Capsule of a Bacteria
Instead of viscous covering, some bacteria are surrounded by amorphous/paracrystalline
colloidal protein materials called the slime layer.

Functions of Capsule

• Aids in adherence

• Prevents from desiccation

• Confer resistance against phagocytosis

• The Slime layer protects from proteolytic enzymes

4. Sheath and Prosthecae

• A sheath is a hollow tube-like structure enclosing chain-forming bacteria, mostly aquatic

bacteria. It provides mechanical strength to the chain.

• Prosthecae is a semi-rigid extension of the cell wall and plasma membrane. It increases
nutrient absorption and also helps in adhesion.

5. Cell Wall

• The cell wall is a rigid structure made up of peptidoglycan that surrounds the plasma
membrane as an external coat. It is 10 -25 μm in thickness.

• Peptidoglycan is a cross-linked polymer of alternately repeating N-Acetylmuramic Acid

(NAM) and N-Acetylglucosamine (NAG) polysaccharide sub-units.

• Based on composition, bacterial cell-wall is classified into 2 types; Gram-positive, and

Gram-negative cell walls.

Gram-positive cell wall

The gram-positive cell wall is a thick cell wall containing a large amount of peptidoglycan, about
40 – 90% of the cell wall, arranged in several layers. This type of cell wall also contains acidic
sugars like teichoic acids, teichuronic acids, and neutral sugars like mannose, arabinose,
rhamnose, and glucosamine as matrix substances.

Teichoic acids are made of polyribitol phosphate or polyglycerol phosphate. They are major
surface antigens of gram-positive bacteria. They are of two types; wall teichoic acid and
lipoteichoic acid.

Teichuronic acid is a polymer of N-acetylmannuronic acid or D-glucuronic acid.

This type of cell wall takes up the crystal violet dye and confer the purple color of the gram-
positive bacteria in Gram staining.

Gram-Positive Bacteria Cell Wall Structure

Gram-negative cell wall

The gram-negative cell wall is a thin cell wall with significantly less amount of peptidoglycan. It
is comparatively more complex than the gram-positive cell wall. It contains lipoprotein,
lipopolysaccharide, and outer membrane in addition to peptidoglycan.

The lipoprotein layer is composed of Braun’s lipoprotein. It is embedded in the outer membrane
and stabilizes the outer membrane.

The outer membrane is a bilayered structure containing an inner layer resembling the plasma
membrane in composition, and an outer layer made up of lipopolysaccharide. It is rich in a
variety of proteins like ‘porin and outer membrane proteins.

Lipopolysaccharide is a complex molecule consisting of 3 components; Lipid-A, core

oligosaccharide, and O-polysaccharide. Lipid-A is composed of phosphorylated glucosamine
disaccharides, long-chain fatty acids, and hydrosymyristic acid. Core oligosaccharide is
composed of two sugars; keto-deoxy octanoic acid and a heptose sugar bounded together by
Lipid A. O-polysaccharide are composed of a wide variety of sugars that differ in between
bacterial strains. This confers different antigenic properties to these different bacterial strains.
They lose crystal violet during the Decolorization step and take up safranin during
counterstaining, hence providing characteristic pink color to Gram-Negative bacteria.

Gram Negative Bacteria Cell Wall Structure

Cell-wall of Acid-Fast Bacilli

It is unique with a large number of mycolic acids. They resist the Decolorization of acid alcohol
or sulfuric acid, hence called acid-fast.

Bacteria without a cell wall

Mycoplasma is a minute (50 -300 nm) bacteria without a cell wall. They do not have a fixed
shape. Besides these natural bacteria, there are several other cell walls deficient forms like
protoplasts, spheroplasts, and L-forms.
Gram-Positive Cell-Wall vs Gram-Negative Cell-Wall

Gram-Positive Cell-Wall Gram-Negative Cell-Wall

Thick (20 – 80 nm) Thin (10 – 15 nm)

Higher peptidoglycan content Lower peptidoglycan content

Lower lipid content (2 – 5%) Higher lipid content (15 – 20%)

The main components are peptidoglycan, teichoic The main components are peptidoglycan, lipoprotein,
acid, and teichuronic acid lipopolysaccharide, outer membrane

Very few amino acids without any aromatic amino Wide variety of amino acids with different aromatic amino
acids acids

Internal Structure of Bacteria

It includes the cell membrane and all the structures inside the cell membrane.

1. Cell membrane/Plasma membrane

• It is the innermost phospholipid bilayer, just beneath the cell wall, enclosing cytoplasm.
It is a thin (~ 5 -10 nm) semipermeable layer.

• Unlike eukaryotic plasma membrane, they lack sterols (except in Mycoplasma), and
comparatively have a higher proportion of proteins. In place of sterols, they have sterol-
like compounds, called ‘hapanoids’. They contain a wide variety of fatty acids like usual
saturated and unsaturated types and additionally methyl, hydroxyl, or cyclic groups too.

• The plasma membrane is equipped with several porin proteins for the passive transport
of nutrients and ions.

Functions of Cell membrane/Plasma membrane

• Selective permeability regulates the inflow and outflow of nutrients, ions, and
metabolites

• Electron transport and oxidative phosphorylation

2. Cytoplasm
 • It is a colorless, colloidal, viscous fluid with suspended organic and inorganic solutes
enclosed within the plasma membrane.

• Unlike eukaryotic cytoplasm, they lack membrane-bound organelles. They have

ribosomes, mesosomes, inclusion bodies, nucleic acids floating in them.

2.1 Ribosomes

Bacterial ribosomes are of 70S type and quite smaller than eukaryotic 80S types. They are made
up of 2 subunits, the 50S, and 30S. Their main role is to synthesize bacterial proteins and
enzymes. They are target sites for different antibiotics like erythromycin, macrolides,
aminoglycosides, etc.

2.2 Mesosomes

They are vesicular or branched structures formed by invaginated of the plasma membrane. They
represent the eukaryotic mitochondria in function and are the site of action of the bacterial
respiration enzymes.

2.3 Inclusion bodies

They are believed to be storage food. They are of two types; (i) organic inclusion bodies,
containing glycogen or polyhydroxybutyrate granules, and (ii) inorganic inclusion bodies,
containing polyphosphate or sulfur granules.

3. Bacterial Nucleus

They are called nucleoids. Unlike eukaryotic nuclei, they are not enclosed in the nuclear
membrane and lack nucleolus and nucleoplasm. It is represented by a dsDNA molecule either in
a closed circular form or in coiled form.

Bacterial DNAs are found either in nucleoid as chromosomal DNA or outside nucleoid as a
plasmid.

Endospore of a bacteria

Some bacteria under stress form a dormant stage called an endospore. They are produced
during unfavorable environmental conditions. They grow to vegetative form when the
conditions become favorable.

They have four distinct structural components; (i) core, containing nucleoid and condensed
cytoplasm, (ii) spore wall, the innermost wall of peptidoglycan, (iii) cortex, the thickest wall
with unusual peptidoglycan, and (iv) protein coat, an outer impermeable layer made of keratin
like protein.

Shapes and Arrangement of Bacteria

Basically, bacteria are of four distinct shapes, cocci, bacilli, spiral, and comma-shaped.

a. Cocci shape bacteria

They are spherical bacteria. Based on the arrangement of cells they are further sub-grouped as;

1. Monococci; singular cocci. Eg. Micrococcus luteus,

2. Diplococci; two spherical bacteria are arranged in pairs. Eg. Neisseria spp., Moraxella
catarrhalis, Streptococcus pneumoniae, etc.

3. Streptococci; spherical bacteria are arranged in a long chain. Eg. Streptococcus

pyogenes, S. agalactiae, etc.

4. Staphylococci; spherical bacteria arranged in irregular clusters like a bunch of grapes.

Eg. Staphylococcus aureus, S. saprophyticus, etc.

5. Tetrad; arrangement in a group of 4 cocci. Eg. Aerococcus urinae, Pediococcus spp., etc.

6. Sarcinae; arrangement of cocci in a group of 8. Eg. Sarcina spp., Clostridium maximum,

etc.
b. Bacilli shape bacteria
They are rod-shaped bacteria. Based on the arrangement of cells they are also sub-grouped as;

1. Bacillus /Mono–bacillus; single unattached rod-shaped bacteria. Eg. Salmonella

enterica serovars, Bacillus cereus, etc.

2. Diplobacilli; bacilli arranged in a pair. Eg. Moraxella bovis, Bacillus licheniformis, etc.

3. Streptobacilli; bacilli arranged in a chain. Eg. Streptobacillus moniliform, etc.

4. Palisade; bacilli arranged in fence-like form. Eg. Corynebacterium diptheriae, etc.

5. Coccobacilli; bacilli with rounded ends or oval-shaped. Eg. Chlamydia spp., H. influenzae,
etc.

c. Spiral

They are long helical-shaped or twisted bacteria. Eg. Spirilla spp. , Spirochetes spp. , etc.

d. Comma shaped

They are comma (,) like in structure. Eg. Vibrio spp.

Besides these four basic shapes, several bacteria are found in other shapes like;

1. Filamentous (E.g. Actinobacteria, Candidatus savagella, etc. )

2. Star shaped (E.g. Stella vacuolata, Stella humosa, etc)

3. Appendaged / Budding (E.g. Hypomicrobium, Rhodomicrobium, etc.)

4. Pleomorphic (E.g. Mycoplasma spp.)

5. Chinese letter like (E.g. Corynebacterium spp.)

6. Lobed (E.g. Sulfolobus spp.)

7. Stalked (E.g. Caulobacter crescentus )

8. Sheathed (E.g. Leptothrix, Clonothrix)

Size of Bacteria
• Bacteria are microscopic with a wide range of sizes from 0.2 μm to 100 μm.

• Cocci are generally of 0.2 to 1.0 μm.

 • Bacilli are generally of 1.0 μm 5 μm in length and 0.5 to 1.0 μm in diameter.

• Spirochetes are generally 20 μm in length and 0.1 to 1.0 μm in diameter.

• The smallest bacilli are Pelagibacter ubique ( 370 – 890 nm in length and 120 – 200 nm
in diameter).

Size of Bacteria

• The smallest cocci are Mycoplasma genitalium with a diameter of 200 – 300 nm.

• The largest bacteria is Thiomargarita namibiensis with a diameter of 0.75 mm.

Classification of Bacteria
There are different schemes for the classification of bacteria. Some of the most common
schemes of classifications are:

a. Classification of Bacteria based on Gram Staining

It is the most common mode of classification used widely in medical and research purposes.
Bacteria are grouped into two groups as;

1. Gram-Positive Bacteria

Bacteria having a thick peptidoglycan layer and retaining the purple color of crystal violet during
Gram staining are Gram-positive bacteria. E.g. Staphylococcus, Streptococcus, Enterococcus,
Corynebacterium, Streptomyces, Bacillus, Haemophilus, Clostridium, Listeria, etc.

2. Gram-Negative Bacteria
Bacteria having a thin peptidoglycan layer and losing crystal violet but retaining pink / red color
of counterstain safranine during Gram staining are Gram-negative bacteria. E.g. Escherichia,
Salmonella, Shigella, Neisseria, Klebsiella, Proteus, Pseudomonas, Enterobacter, Citrobacter,
etc.

b. Classification of Bacteria based on Oxygen Requirements

Bacteria are classified into 3 types as;

1. Aerobic bacteria

They respire aerobically and can’t survive in anoxic environments. E.g. Pseudomonas
aeruginosa, Nocardia spp., Mycobacterium tuberculosis, etc.

2. Facultative aerobes

They survive in very low oxygen levels and can survive in both oxygenic and anoxic
environments. They are Microaerophiles. E.g., E. coli, Klebsiella pneumoniae, Lactobacillus spp.,
Staphylococcus spp., etc.

3. Anaerobic bacteria

They respire anaerobically and can’t survive in an oxygen-rich environment. E.g., Clostridium
perfinges, Campylobacter, Listeria, Bifidobacterium, Bacteroides, etc.

c. Classification of Bacteria based on Optimum Temperature

Bacteria are classified broadly into 3 types as;

1. Psychrophiles

They have optimum growth temperature at 150C or below. E.g. Chryseobacterium,

Psychrobaceter, Polaromonas, Sphingomonas, Alteromonas, Hyphomonas, Listeria
monocytogenes, etc.

2. Mesophiles

They have optimum growth temperature at 15 – 450C. Pathogenic bacteria fall in this category.
E.g. E. coli, Staphylococcus aureus, Salmonella Typhi, Streptococcus pyogenes, Klebsiella spp.,
Pseudomonas spp., etc.

3. Thermophiles

They have optimum growth temperature at above 450C. E.g. Bacillus thermophilus,
Methanothrix, Archaeglobus, Thermophilus aquaticus, Geogemma barosii (at 1220C), Pyrolobus
fumarii (at 1130C), Pyrococcus spp., etc.

d. Classification of Bacteria based on Arrangement of Flagella

Bacteria are classified into 5 types as;

1. Atrichous

They are bacteria without flagella. E.g. Lactobacillus spp., Bacillus anthracis, Staphylococcus
spp., Streptococcus spp., etc.

2. Monotrichous

They are bacteria with only one flagellum at one pole. E.g. Campylobacter spp., Vibrio cholerae,
etc.

3. Lophotrichus

They are bacteria with multiple flagella at one end. E.g. Spirillum, Helicobacter pylori,
Pseudomonas fluorescence, etc.
4. Peritrichous

They are bacteria with multiple flagella projecting in all directions. E.g. E. coli, Klebsiella,
Proteus, Salmonella Typhi, etc.

5. Amphitrichous

They are bacteria with one flagellum at each pole. E.g. Alcaligenes faecalis, Nitrosomonas, etc.

e. Classification of Bacteria based on mode of nutrition

1. Autotrophic bacteria

They are bacteria capable of assimilating inorganic matters into organic matters i.e. capable of
preparing their food like plants. They are of 2 types;

Photoautotrophs; They use energy from sunlight for assimilation. It includes cyanobacteria
(Nostoc, Prochlorococcus, etc.), purple sulfur bacteria (Nitrosococcus, Thiococcus,
Halochromatium, etc.), purple non-sulfur bacteria (Rhodopseudomonas spp.), green sulfur
bacteria (Chlorobium, Chromatium, etc.)
 Chemoautotrophs; They use chemical energy for assimilation. It includes sulfur bacteria
(Beggiatoa, Thiobacillus, Thiothrix, Sulfolobus, etc.), nitrogen bacteria (Nitrosomonas,
Nitrobacter, etc.), hydrogen oxidizing bacteria (H. pylori, Hydrogenbacter, Hydrogenvibrio
marinus, etc.), methanotrophs (Methylomonas, Methylococcus, etc), iron bacteria (Thiobacillus
ferroxidans, Ferrobacillus, Geobacter metallireducens, etc.)

2. Heterotrophic bacteria

They are bacteria that derive energy by consuming organic compounds, but they do not convert
organic compounds to inorganics. They are parasitic or symbiotic types. E.g., E. coli, Rhizobium
spp., Staphylococcus spp., Mycobacterium spp., Klebsiella pneumoniae, etc.

3. Saprophytic bacteria

They are bacteria that decompose organic compounds into inorganic and derive energy. They
are decomposers and feed on dead plants and animals. E.g. Cellulomonas, Clostridium
thermosaccharolyticum, Pseudomonas denitrificans, Acetobacter, etc.

Feeding in Bacteria
Bacteria feed on several organic or inorganic compounds. The food enters the bacterial body
either by phagocytosis (active transport) or by osmosis and diffusion or through protein
channels (passive transport). They obtain energy by either photo- or chemosynthesis
decomposing organic compounds or breaking down inorganic compounds. Based on feeding
habits, they are grouped as autotrophs, heterotrophs, and saprophytes.

Reproduction in Bacteria
Bacteria have a very short generation time i.e., they reproduce very quickly. Their reproduction
is an asexual type and can be classified into the following types;

1. Binary fission

It is the most common type. Under favorable conditions, each bacterium divides into two
identical bacteria. The bacterial cells first acquire nutrition grow at their maximum size and
replicate their DNA. The new replicated DNA called an incipient nucleus, migrates towards
opposite poles. A transverse septum begins to develop and separate the two daughter cells.
2. Conidia formation

It is mostly seen in filamentous bacteria like those in actinomycetes, e.g. Streptomyces,

Micromonospora, Rhodomicrobium, etc.

3. Budding

The bacterial cells develop small swelling, called protuberance or bud, at one side. Bacterial
DNA replicates and one copy enters into the bud. The bud eventually separated and develop
into a daughter cell. E.g. Planctomyces spp, Rhodomicrobium vannielia, Hyphomicrobium spp.,
etc.

4. Endospore formation

It is seen in some Gram-positive bacteria during unfavorable conditions and environmental

stresses. The cytoplasm becomes concentrated around bacterial DNA and a thick, hard, and
resistant wall develops around it. E.g. Bacillus spp., Clostridium spp., Sporosarcina spp., etc.
 Endospore formation in Bacteria

5. Transformation

It is considered a sexual method. In this method, the DNA of one bacterium directly enters into
a cell of another bacterium of the same species and forms recombinant DNA. The DNA enters
through extracellular environments.

6. Conjugation

It is another sexual method where DNA transformation is by direct contact between donor and
recipient bacterium via conjugation tube. Sex pili are responsible for conjugation. Donor cell
develops sex pilus and attaches to the recipient cell. A conjugation tube or bridge is formed at
the connected point. DNA fragments transform from one bacterium (donor) to another
(recipient) through this tube.

7. Transduction

In this method, DNA fragments are transformed from donor bacterium to recipient bacterium
by bacteriophages.
Importance, Uses and Applications of Bacteria
1. They are responsible for recycling several nutrients like nitrogen, carbon, sulfur,
phosphorus, oxygen, etc. They play the most important role in assimilation and
dissimilation of the organic compounds during any biogeochemical cycle.
2. They play a very important role in regulating atmospheric oxygen levels.
Photosynthetic bacteria (Cyanobacteria, Green Sulfur bacteria) play a very
important role in the production of oxygen during photosynthesis.
3. They are responsible for biodegradation, composting, decomposition, and
bioremediation. They play a very important role in the management of organic
wastes and dead organisms and their parts.
4. Several bacteria are used industrially for the production of several enzymes. These
enzymes are used in industrial processes, medical purposes, food processing, etc.
Amylase, lipase, cellulases, proteases, hemicellulases, zymase, penicillinases,
polymerases, etc. are produced by bacteria.
5. Bacteria are genetically modified and used in biotechnological applications to
produce hormones like insulin and enzymes.
6. They are used in an anaerobic fermentation process to produce biogas (methane)
which is used as fuel.
7. Different genera of Actinomycetes and other bacteria are the source of antibiotics
used for pharmaceutical purposes.
8. Several bacterial species like Bifidobacterium, E.coli, Lactobacillus, etc. are used as
probiotics.
9. Bacteria are used in producing fermented food products like fermented dairy
products, sausages, fermented fruit juices, etc.
10. They are used in the bioremediation of oil spillage, xenobiotic, radiation wastes,
heavy metal wastes, bio-hazardous wastes, toxic wastes, and other organic and
inorganic wastes.
11. Bacteria are used in genetic engineering and molecular research. Their genes are
being used in producing different Genetically Modified Organisms (GMOs).
12. The bacterial fuel cell is new technology to convert chemical energy into electric
energy. They can be used as an alternative source of energy.
13. In agriculture, they are used as bio-pesticides, bio-fertilizers, and bio insecticides.
14. Bacteria are the pioneer of life forms in barren lands like deserts, rocks, etc. Every
living organism living today are evolved from some eukaryotes which were
developed from bacteria some 2.0 billion years ago.
15. Bacteria are present as normal flora in our body. They help fight against invading
pathogens, boost immune response, and help in the digestion process.
Disadvantages and Limitations of Bacteria
1. Different pathogenic bacteria are responsible for a wide variety of human diseases
from simple to life-threatening. Bacterial diseases are responsible for thousands
of deaths each year.
2. Bacterial spoilage of foods feeds and pharmaceutical products is another
disadvantage. The food and pharma industries have to bear huge losses due to
bacterial spoilage.
3. Several bacteria like denitrifying bacteria, sulfur-oxidizing bacteria, etc. are
responsible for decreasing the fertility of the soil, ultimately reducing crop yields.
4. Bacteria can cause disease to crop plants and domestic animals. This will reduce
agricultural production.
5. Bacteria cause deterioration and degradation of useful organic products like
furniture, textiles, etc.