General aspects & Morphology of Bacteria:
Bacteria are single cellular microscopic organisms. The study of bacteria
is known as bacteriology and it is a branch of microbiology. The singular
world of bacteria is bacterium. Bacteria have been grouped into
prokaryotic, which means absence of nucleus.
Cell Wall :
Cell walls of bacteria are made up of glycoprotein murein.
The main function of cell wall is it helps in providing support, mechanical
strength and rigidity to cell.
It protects cell from bursting in a hypotonic medium.
Gram staining bacteria are a method of differentiating bacterial species
into two large groups, which are based on their chemical and physical
properties of their cell wall.
Gram positive bacteria: Those bacteria when they are stained in gram
stain results in purple colour.
Gram negative bacteria: Those bacteria when they are stained in gram
stain results in pink colour.
It is also known as cytoplasmic membrane (or) cell membrane.
It is composed of phospholipids, proteins and carbohydrates, forming a
It helps in transportation of substances including removal of wastes
from the body.
It helps in providing a mechanical barrier to the cell.
Plasma membrane acts as a semi permeable membrane, which allows
only selected material to move inside and outside of the cell.
Helps in cellular growth, metabolism and replication.
Cytoplasm is the store houses of all the chemicals and components that are
used to sustain the life of a bacterium.
A tiny granule made up of RNA and proteins.
They are the site of protein synthesis.
They are freely floating structures that helps in transferring the genetic
Plasmids are small circle of DNA.
Bacterial cells have many plasmids.
Plasmids are used to exchange DNA between the bacterial cells.
Flagella: It is a rigid rotating tail.
It helps the cell to move in clockwise and anticlockwise, forward and also helps the
cell to spin.
The rotation is powered by H+ gradient across the cell membrane.
Short protein appendages.
Smaller than flagella.
Fixes bacteria to surfaces.
It also helps in reproduction during conjugation.
Capsule is a kind of slime layer, which covers the outside of the cell wall.
They are composed of a thick polysaccharide.
It is used to stick cells together and works as a food reserve.
It protects the cell from dryness and from chemicals.
Respiration in bacteria:
Anaerobic bacteria: does not require oxygen for respiration.
Aerobic bacteria: require oxygen for respiration.
Locomotion of bacteria: They move around by using their locomotion
organs such as cilia and flagella.
Reproduction in Bacteria: Most organism have one or two methods of
reproduction. But bacteria have many.
a) Asexual reproduction: The bacteria cell undergoes to produce two
bacteria by Binary fission and also by endospores.
b) Sexual reproduction: It is also by two distinct methods like
transformation, conjugation and transduction. In transformation, one
bacteria releases naked DNA strand into the surroundings. The other
bacteria takes up and incorporate into their genome. Transduction happens
due to virus.
• Nutrition of bacteria:
• They exhibits different modes of nutrition level such as-
Autotrophic bacteria: These bacteria are able to synthesize their own
food. For e.g.: Phototropic bacteria and chemosynthetic bacteria.
Heterotrophic bacteria: These bacteria are unable to synthesize their own
food, hence they depends on other organic materials. For e.g.:
saprophytic bacteria-these bacteria feeds on dead and decaying matter.
Symbiotic bacteria: These bacteria have a mutual benefit from other
organisms. For e.g.: nitrogen fixing bacteria (or) rhizobium.
Parasitic bacteria: These bacteria are present in plants, animals and
human beings. These bacteria feeds on host cells and causes harm to the
• Cosmopolitan distribution: Bacteria are the only organisms
which show cosmopolitan distribution. They are present in
air, water, soil, snow etc. They are present at very cold
(minus degree) temperature regions and also very high
temperature regions. They can live in acidic environment
(helicobacter pyroli) and even basic environment (iron
• Resistance and tolerance: Some of the bacteria are highly
resistant to adverse environments. Even they get
tolerance to harsh chemical and other stuff which destroy
them. Hence irrational use of antibiotics. This has become
so severe that World Health Organization warns of
rampant prevalence of resistant bacteria which can be
incurable by currently available drugs.
Size of a Bacterial Cell:
• There are great variations in size of bacteria. They
measure from 0.75 µ to 1.5 µ but on an average each cell
of bacterium measures about 1.25 µ to 2 µ (in diameter).
Some times their size varies due to slide preparation.
• The smallest rod shaped eubacteria is Dialister Pneum-
osintes and measures between 0.15 µm to 3.0 µm size.
Sulphur bacteria, Thiophysa volutans has diameter of
about 18 µm and is considered as largest amongst all
Shape and Arrangement of Bacterial Cell
Bacterial cells exhibit greater variation in their shape but usually
four conventional shapes of cell have been recognized.
1. Cocci: Simplest form of bacteria in which bacterium appears like a
(i) Micrococci: When bacterium appears singly.
(ii) Diplococci: When they appear in pairs of cells.
(iii) Streptococci: When they appear in chain form.
(iv) Tetrad: Arranged in square of four.
(v) Sarcinae: When arranged in cuboidal or in different geometrical or
(vi) Staphylococci: Arranged in irregular clusters like bunch of grapes.
2. Rod Shaped: They are also called bacilli and are commonest in microbial
world. They are of two kinds:
(i) Short rods: Very short rods, occurring mostly singly.
(ii) Long rods: Cylindrical shape, are known as Bacilli or rods, occurring singly,
in pairs or in chains.
3. Vibrios: They are curved rods or comma shaped, their curvature is always
less than a half turn.
4. Spirilla: They are curved or spiral shaped cells, their curvature exceeds that
of a half turn. They may be classified as either spirilla or spirochetes.
• Few bacteria actually are flat. For instance, Anthony E. Walsy has
discovered square bacteria living in salt ponds. These bacteria are shaped
like flat, square to rectangular box about 2 µm to 4 µm and only 0.27 µm
• However, some bacteria are variable in shape and have a single
characteristic form. These are called Pleomorphic (corynbacterium,
Examination of bacterial cells with electron microscope
reveals various component structures. Some of these are
outside the cell membrane; others are internal to cell
I. Ultrastructure of Bacteria:
Structures Outside the Cell Membrane:
Some prokaryotic organisms secrete slimy or gummy materials
(exopolymers) on their surface. A variety of these structures
consist of polysaccharides and a few consist proteins. The
more general term glycocalyx is also used.
The glycocalyx is defined as the polysaccharide containing
material lined outside the cell. Composition of layers varies in
different organisms but can contain glycoprotein and different
polysaccharides including polyalcohol and amino sugars.
These layers may be thick, or thin, and rigid or flexible, depending
on their chemical nature in specific organism.
The rigid layers are organized in a tight matrix, this form is
referred to Capsule.
Most bacterial capsules are composed of polysaccharides. Capsules
composed of single kind of sugar are termed homopolysaccharides;
are usually synthesized outside the cell from disaccharides by
exocellular enzyme. Other capsules are composed of several types
of sugar and are termed as heteropolysaccharides, for example,
capsule of Klebsiella pneumoniae.
A few capsules are polypeptides. For example B. anthracis, is
composed entirely of a polymer of glutamic acids.
a) Functions of Capsule:
Capsule is not essential for survival of organism under
favourable growth conditions. It, however, provides
protection in un-favourable environments. Presence of
capsules is of importance in disease causing ability of
The in-capsulated strains of the bacterium are protected
from phagocytosis and take part in virulence in addition
because outer polysaccharide layer bounds a significant
amount of water so it is resistant to desiccation.
Most motile prokaryotes move by use of flagella thread
like locomotors appendages, extending outwards from the
cell membrane and cell wall.
Bacterial flagella are slender, rigid structures, about 20
nm across and up-to 15 to 20 µ m long.
Flagella are so thin that they cannot be observed directly
with bright field microscope. But must be stained with
special techniques designed to increase their thickness.
Bacteria species often differ distinctively in their patterns
of flagella distribution and these patterns are useful in
identifying bacteria. Mono-trichous bacteria have one polar
flagellum. Amphi-trichous bacteria have either singe or
duster of flagellum at both pole.
In contrast, Lopho-trichous bacteria, have a cluster of
flagella at one ends. Bacteria surrounded by lateral flagella
c) Flagella Ultra-Structure:
Transmission electron microscope studies have shown
that flagellar apparatus is made up of three distinct
(1) The outermost region is filament which is extended
from the cell surface to tips.
(2) Basal bodies consist of small central rods inserted
(3) A short curved segment, the flagellar hook, links the
filament to basal bodies and acts as flexible coupling.
The filament is a hollow, rigid cylinder made up of protein subunits
flagellin which ranges in molecular weight from 30,000 Dalton to
6,000 dalton, depending on the bacterial species.
Hook and basal bodies are quite different from filaments. Basal body
is more complex part of flagellum. In E. coli and most Gram-negative
bacteria, basal bodies bear 2 pairs of ring, outer pair (L and P ring)
is situated at the level of outer membrane and inner pair (S and M
ring) is located near the level of cell membrane.
The outer L and P ring associates with lipopolysaccharides and
peptidoglycan layer respectively. Inner M ring contacts the plasma
membrane while S ring lies just above attached to inner surface of
peptidoglycan,. Flagella of Gram-positive bacteria have only lower S
and M ring.
d) Fimbriae or Pili:
Some bacteria mostly (Gram negative bacilli) contain, non-flagellated,
extremely fine appendages called fimbriae or pile. The filament of
pilus is straight and diameter is 7 nm. It is made up of pilin protein.
Molecular weight is 17,000.
Pili are non-motile but adhesive structure. They enable the bacteria
to stick firmly to other bacteria, to a surface or to some eukaryotic
such as mold plants, plants and animal cells including RBC and
epithelial cells of elementary, respiratory and urinary tracts.
Pilli help in conjugation (e.q. F. pili or sex pili) of male bacteria, in
the attachment of pathogenic bacteria to their host cell. Pile are
known to be coded by the genes of plasmid that determine cell
capacity to carry out conjugative genetic exchange with other cells.
• Some Gram-positive bacteria have tubular unicellular and rigid
appendage of singe protein moity called spinae. They are
known to help the bacterial cell to tolerate some environmental
conditions such as salinity, pH and temperature etc.
• They are in-foldings of plasma membrane found in some
• It contains enzyme for performing respiration and
pigments for photosynthesis.
Structure of Cell Membrane
Cell membrane is a thin structure, it surrounds the cell; about
8 nm thick. This structure is critical barrier separating the
inside of cell from environment.
Biological membrane, is composed of Phospholipids (20 - 30
%) and proteins (60 - 70 %). The phospholipids form a bilayer
in which the proteins are strongly held (integral proteins) and
these proteins can be removed only by destruction of the
membranes, as with treatment by detergents.
Other proteins are loosely attached (Peripheral proteins) can
be removed by mild treatment such as osmotic shock. The lipid
matrix of membrane has fluidity, allowing the components to
move around laterally. Fluidity is essential for various
Components of Bacterial Cell Wall:
On the basis of cell wall composition, bacteria are classified into
two major group i.e., Gram Positive and gram negative. The two
different cell wall types can be identified by differential stain
known as the Gram stain. After applying this technique; the gram
positive bacteria will stain purple, while the gram negative
bacteria will stain pink.
Gram positive cell wall composition:
It is an ingredient that both bacterial cell walls have in common.
Peptidoglycan is porous cross linked polymer which is responsible for strength of
Peptidoglycan is composed of three components.
Tetra-peptide side chain (chain of 4 amino acids) linked to NAM
Peptide cross linkage
Glycan backbone is the repeated unit of N-acetyl muramic acid (NAM) and N-
acetyl glycosamine (NAG) linked by β-glycosidic bond.
The glycan backbone are cross linked by tetra-peptide linkage. The tetra-peptide
are only found in NAM.
More than 100 peptidoglycan are known with the diversity focused on the
chemistry of peptide cross linkage and inter-bridge.
Although the peptidoglycan chemistry vary from organism to organism the glycan
backbone i.e., NAG-NAM is same in all species of bacteria.
Teichoic acid is water soluble polymer of glycerol, which is
embedded within the peptidoglycan layers, present in gram
It constitutes about 50% of dry weight of cell wall.
It is the major surface antigen of gram positive bacteria.
It contributes to the overall rigidity of the cell wall, which is
important for the maintenance of the cell shape, particularly in
Teichoic acids can either be covalently linked to peptidoglycan
(wall teichoic acids or WTA) or connected to the cell membrane
via a lipid anchor, in which case it is referred to as lipo-teichoic
LPS is attached to outer membrane by hydrophobic bond and is
synthesized in cytoplasmic membrane and transported to outer-
It contributes net -ve charge for the cell, helping to stabilize the
outer membrane, and providing protection from certain chemical
substances by physically blocking access to other parts of the cell
In addition, LPS plays a role in the host response to pathogenic gram
LPS is composed of lipid-A and polysaccharide.
Lipid-A: it is phosphorylated glucosamine disaccharide.
Polysaccharide: it consists of core-polysaccharide and O-