BACTERIA
The term bacteria was proposed by Ehrenberg in 1829.
They have widespread distribution be it air, water or soil.
They can even survive in extreme range of temperatures like
upto 780C and —1900C.
Important characteristics of bacteria are
(i) Bacteria are found in all kinds of habitats.
(ii) They are prokaryotic microorganisms.
(iii) They are unicellular.
(iv) Cell wall contains peptidoglycan.
(v) An organised nucleus is absent.
(vi) Extrachromosomal self-replicating DNA segments
called plasmids occur in most of the bacteria.
(vii) Mitochondria, plastids, Golgi apparatus,
endoplasmic reticulum and other membrane bound
cell organelles are absent.

Size
The size of bacterial cell ranges from 1-10 in length and
from 0.7-1.5 gun in width.
Shape
The bacteria possess the following forms
(i) Coccus (Pl. coccus) bacteria are oval or spherical
without flagella. The spheres occur as single celh
(Monococcus), a pair of cells (Diplococcus), in grouvx
of four cells (Tetracoccus), as chain of cells
(Streptococcus) or in sheets (Staphylococcus).
A few cocci may also occur in cube-like
arrangements of 8 or more cells (Sarcina).
(ii) Bacillus (Pl. bacillus) bacteria are rod-shaped cells
which may occur singly (Monobacillus), in pain
(Diplobacillus), in chains (Streptobacillus) or as a layer
(stack) with many cells called Palisade bacillus.
(iii) Spirillum (Pl. spirillum) bacteria are cells, which are
twisted, like a screw. They occur as free single cells,
e.g., Spirillum, Spirochaete, etc.
(iv) Vibrio are cells which are curved, C-shaped
or
comma-shaped, e.g., Vibrio cholerae.
also
Apart from these some other shapes of bacteria are
found such as mycelial, stalked, budding bacteria.

Structure
A bacterial cell is covered by mucilage. It is differentiated
into cell wall, plasma membrane, cytoplasm, nucleoid,
plasmids, inclusion bodies, flagella, pilli and fimbriae.
Membrane bound organelles are absent.
Details about the structure of bacteria (prokaryotes) will
be studied later in chapter 8th of this book.
Knowledge Plus
• Bacteria were discovered by Anton van Leeuwenhoek
(1632-1723). He observed bactena tn 1675.
• Louis Pasteur laid the foundation ot Bacteriology by
develop•ng culture techn.que€m
Nutrition
Bacteria show both autotrophic and heterotrophic mode
of nutrition, i.e., they are mixotrophic.
On the basis of mode of nutrition, bacteria are of two
types
i. Autotrophic Bacteria
These are of following two types
(a) Photosynthetic These bacteria have green
sunlight trapping pigment called
bacteriochlorophyll.
These are found at the bottom of ponds and lakes.
Bacterial photosynthesis does not release oxygen.
(b) Chemosynthetic These bacteria are able to
synthesise organic food from inorganic raw
materials with the help of energy derived from
exergonic chemical reactions. Examples include
nitrifying bacteria (Nitrosomonas), iron bacteria
(Ferrobacillus ferroxidants), sulphur oxidising
bacteria (Beggiatoa).
ii. Heterotrophic Bacteria
These bacteria are not capable of synthesising their own food.
Instead, they obtain food from different sources.
These may be of following types
(a) Saprophytes These obtain food by decomposing dead
bodies, excreta of animals, dead plants and their parts.
These are also called decomposers, detrivores or
transformers.
(b) Parasites These are disease causing bacteria called as
v.’hich
pathogens, e.g., Salmonella typhimurium,
causes typhoid in humans.
(c) Symbionts These bacteria live in mutually beneficial
associations with other organisms, e.g., Rhizobium and
Bacillus, species form nodules in roots of legurninous
plants. They obtain nutrition fronm the plant and in
turn, help the plant in nitrogen-fixation.
“‘c Plant in nitrogen-fixation.
‘rorti the plant and in

Reproduction
Bacteria reproduce by asexual and sexual
i. Asexual Reproduction
Asexual reproduction occurs by binary fission and endospore
(a) Binary Fission It is a simple cell division in which
bacterial cell divides into two parts. A constriction
appears at the equator of the cell, deepens further and
grows from margin to centre and finally two new cells
are produced.
(b) Endospore Formation Endospores are perennial
structures which help in survival even during harsh
environmental conditions, e.g., in Clostridium and
Bacillus, the endospore has many wall layers
containing heat resistant chemicals such as sialic acid
and dipicolinic acid.
ii. Sexual Reproduction
Sexual reproduction in bacteria occurs by a parasexual
process actually called genetic recombination.
The three methods involved are as follows
(a) Conjugation The male cell (donor cell) has fertility
factor or F-factor, on its plasmid which connects itself
to cell wall of female cell (recipient cell). The
connection is established by the formation of a
bridge-like structure called sex pilus.
(b) Transformation This process wrs discovered by
Griffith in 1928. It is a process where segments of
DNA are transferred from one bacterial cell to another
via the liquid medium.
(c) Transduction During this process, the segments of
DNA are transferred from one bacterium to another
by the viruses (bacteriophages).

Uses of Bacteria
Bacteria are useful in the following ways
(i) Bacteria are natural scavengers. They obtain their
nutrition by decomposing dead bodies, dead plants
and animal excreta. Such saprophytes improve the
fertility of soil by formation of humus, manure, etc.
(ii) Several species of bacteria (anaerobes) such as
Escherichia coli, Streptococci, Clostridium, Proteus,
Pseudomonas, etc., are useful for proper disposal of
sewage. They have the ability to breakdown organic
matter into simpler, less harmful substances.
(iii) Bacteria also play important role in different steps of
nitrogen cycle. Examples of some important bacteria
in nitrogen cycle are Clostridium, Azotobacter (soil
bacteria), Rhizobium legu
minosarum, Bacillus radicicola
(in nodules), Nitrosomonas, Nitrosobacter, Pseudomonas,
etc.
(iv) These are used in fermentation process for
Penicillium vinegar manufacturing Acetobacter,
yogurt making Lactobacillus, etc.
etc.
(iv) These are used in fermentation process for
Penicillium vinegar manufacturing Acetobacter,
yogurt making Lactobacillus, etc.
(v) Some bacteria help in retting of jute and coconut
plant fibres. The separated fibres are used in making
ropes or gunny bags.
(vi) Bacteria also find important uses in leather tanning
industry.
(vii) The genera Penicillium and Streptomyces have many
used to produce different antibiotics.
Some important antibiotics using various bacteria are
neomycin, chloromycetin, streptomycin, gramicidin,
bacitracin.

Harmful Effects of Bacteria
Bacteria are harmful in the following ways
(i) Bacteria are responsible for various plant diseases like
citrus canker in lemon leaves and fruits, soft rot in
carrot plants, blight disease in rice plants, crown gall
disease in apple trees and rose plants.
(ii) In animals like horse, cattle and sheep, anthrax
disease is caused by Anthracis.
(iii) In humans, bacteria cause diseases like cholera
( Vibrio cholerae), gastric ulcer (Heliobacter pylon),
tuberculosis (Mycobacterium tuberculosis), sexually
transmitted diseases like gonorrhoea (Neisseria
gonorrhoeae), syphilis (Treponema pallidum), etc.
(iv) Food poisoning occurs due to the production of
toxins by some bacteria like Clostridium botulinum.
They cause botulism, which can kill humans by,
respiratory paralysis.