Booklet No. 288
III. Shape of Bacteria
IV. Size of Bacteria
V. Bacterial Cell
VI. Cell Wall
X. Spore Formation.
Bacteria are very small organisms which live in our atmosphere. There are two types of
bacteria aerobic and anaerobic. Those living in presence of oxygen are aerobic, and in absence
of oxygen are anaerobic bacteria. These organisms are very much useful as well as harmful for
man and animal. This depends on the species of the bacteria. There are certain strains of
bacteria which fixes atmospheric nitrogen into soil, and thus helps in increasing our agricultural
production. This booklet describes the habitat and life cycle of bacteria in detail.
Dr. K T. Chandy, Agricultural & Environmental Education
Bacteria are regarded as the simplest forms of plant life; but they grow without the aid of
chlorophyll. We know that of all the organisms, bacteria are most closely related to man's life.
There are about 1600 species of bacteria present on the earth. Except pits of volcanoes and
deep water s1:rata of rocks, bacteria are present almost everywhere. Bacteria include the
photosynthetic and chemo-synthetic auto-trophs, saprophytes, capable of decomposing all
imaginable organic materials, para sites, pathogens, symbionts of plants and animals and other
protista and predators. The average size of bacteria is about 1.25 microns in diameter. The
smallest bacteria is about 0.15 micron in diameter, which is almost last limit of visibility. It is very
surprising to know that a single drop of water may contain fifty thousand million bacteria.
Bacteria are unicellular (single celled) and simple in structure, possessing no definite
nucleus. Under suitable conditions, bacteria multiply very rapidly by splitting or 1:ransverse
Bacteria are present everywhere in nature where living conditions exist. They may
flourish at any temperature between 00 and 750 centigrades. They are found in large numbers
on the body surface of man, animals and plants. Normally they are found in the living tissues.
Majority of bacteria live on dead organic matter. Soil, sewage, water and air teem with forms of
bacterial life. Only a few are pathogenic and the rest are useful directly or indirectly in the
economy of life.
III. Shape of Bacteria
Bacteria exhibit three fundamental forms, spherical or cocci, straight rods or Bacilli, and
the curved reds or spirilla. The different shapes of bacteria have been explained in detail here.
The fully developed cocci are, perfectly spherical but when two or more are in
opposition, they may be slightly flattened along the tangential surface, giving an oval
appearance. When they are present singly they are called monococci or micrococci. When they
are presented in chains of more than two, they are called streptococci. If they are found in
packets of eight cells or of multiple of eight cells the are called sarcina. When the cells remain
attached in irregular masses, resembling clusters of grapes, they are known as staphylococci.
The bacilli are straight rods whose length varies from two to ten times their width. The
ends may be gently rounded like those of Lactobacillus bulgaricus, or square like Bacillus
anthracis. Bacilli may separate at once after cell division or they may remain united in chains.
An organisms producing such chains is sometimes termed a streptobacillus. Scientist Graham-
Smith divided rods into four groups.
1. Loop forming group: In this group the cell branches do not break apart upon formation of new
cell resulting in forming a loop of the chain. Anthrax bacillus is an example of this group.
2. Folding group: In this group the cells are less firmly attached forming a zig-zag pattern
3. Snapping group: In this the membrane connecting two cells suddenly ruptures at the side
under tension giving rise to an angle.
4. Slipping group: In this group the cells break apart readily forming more or less parallel lines.
The spirilla vary form small comma-shaped organisms with single curve, to longer
sinuous forms from 4 to 20 curves which suggest the appearance of an animated cork-screw.
Rods with a single curve are known as vibrio, those with a few curves is true spirilla and with
many curves as spirochete.
Besides the usual forms of bacteria, under certain conditions, bacteria may show abnormal or
unusual shapes. Environmental factors, such as temperature of incubation, age of the culture,
concentration of the substrate and composition of tile medium determine the production of these
Bacteria usually exhibit their characteristic morphology in young cultures and on media
possessing favourable conditions for growth. Those form depart widely from the standard
morphological picture, when one or more environmental factors are changed. It is called
involution forms. Some species of bacteria show this characteristic much more readily than
others. For example, Rhizobium leguminogarum and Mycobacterium tuberculosis generally
produce involutions forms.
Young cells are in general, larger than old organisms of the sample species. Bacterial
variations resulting from changes in age are only temporary, the original forms reappear when
the organisms are transferred to fresh medium.
IV. Size of Bacteria
The unit used in measurement of microscopic objects is the micron. A micron, is the one
thousand part of a millimeter.
I Bacteria vary greatly in size according to the species. Some are so small that they approach
the limit of visibility when viewed with the light microscope. Others are so large that they are
almost visible with the normal sight. However, the sizes of the majority of bacteria occupy a
range intermediate between these two extremes. A coccus form is measured by the size of its
diameter, a rod or spiral form by its length and width. Cocci may vary from 0.15 to 2 micron in
diameter, although the usual cocci found in the pus are in the range of 0.8 to 1.2 micron. There
is even greater variation in the size of bacilli which may vary from small Haemophilus influenza
which may be 0.54 micron in length and 0.2 micron in width. Sulphur and iron bacteria measure
as large as 80 microns in length. Here, size of some of the important bacteria are given in table
Table-l: Sizes of some bacteria
Sl.No Particulars Bacteria size (microns)
1 Micrococcus winus 0.2-0.5 -
2 Streptococcus lactis 0.5-0.6 -
3 Bacillus coli 0.5-0.8 1.5-3.0
4 Bacillus subtilis 0.8-1.2 1.2-3.0
V. Bacterial Cell
The bacterial cell consists of cell wall and cytoplasm. Cytoplasm has a very complex and
easily changeable chemical composition. It is semi-fluid transparent living mass present in
colloidal state and it possesses primarily the characteristic of protein. The outer layer is more
dense. This membrane is physiologically active and plays an important role in the metabolism of
cells. In the cytoplasm of older bacterial cells vacuoles are present. In these vacuoles mineral
cells organic compounds remain in solution. In the cytoplasm many types of granules are also
present. Some of these granules are known as metachromatic granules or polar bodies.
Metachromatic granules may be distributed throughout the cytoplasm and they stain differently
from the cytoplasm. Besides these granules there may be carbon dioxide, fat globules,
glycogen, volutin, etc. in the cytoplasm. Cytoplasm is very sensitive-to the externa1 environment
and it coagulates at a temperature of 50-600 centigrades. It is also destroyed in a high acid or
alkaline medium and in poisonous surroundings.
In most of the bacterial cells it is difficult to isolate the nucleus from the cytoplasm. In the
composition of the cytoplasm of the bacterial cells nucleoprotein forms a considerable element
and specially the thymonucleic acid which is a major constituent of the nucleus found in the
bacterial cytoplasm. From this it may be concluded that the whole mass of the cytoplasm in a
bacterial cell may be considered as a nucleus.
VI. Cell Wall
The cell wall of bacteria consists of nitrogenous and non- nitrogenous substances. It is
semi permeable membrane covering through which food and waste products pass in the
general life process of the cell. Cell wall of younger bacteria are elastic and thinner. With
maturity they become thicker and less elastic. In certain bacteria the cell-wall may give rise to
capsules. The cell wall is not stained by ordinary bacterial stains unless they are previously
treated with the proper mordant. The chemical structure of the cell wall varies with the type of
microorganisms. It consists of complex carbohydrate true cellulose or hemicellulose in some
species, and of nitrogenous substances related to mucins in others.
Some bacteria possess a gelatinous coating along the cell. This is either an excretory
product which happens to be of such consistency that it is held about the cell or an outer layer
and a constituent part of the wall. It is called a capsule. Capsules are believed to be distinct
from the morphological and biochemical point of view. Its formation depends upon the
composition of the culture medium, but specially upon the variant phase of the organisms.
Chemical composition of the capsular material shows a complex carbohydrates known as
Bacterial motion is due to the presence of organs of locomotion known as flagella.
flagella are very thin, whiplash-like appendage on the cell. A few of the sulphur bacteria are
exceptions in that they do not possess flagella but exhibit a slow creeping motion caused
probably by a contraction of their protoplasm. Independent bacterial movement is a true
movement of translation and must be distinguished exhibited by very small particles suspended
in a liquid. This latter type of motion is known as brownian movement and is caused by the
bombardment of the bacteria by the molecules of the suspending fluid Flagella are very delicate
structure and are present only in the young cells. Flagella may show considerable variation in
length, depending upon age and changes in the environment. Number and arrangement of
flagella vary with different species. Organisms may be classified on the basis of the numbers
and arrangement of flagella as mentioned here.
a. Atrichous -absence of flagella.
b. Monotrichous -two flagellum on one end.
c. Liphotrichous -two or more flagella at one or both ends of the cell.
d Amphitrichous -one flagella at each end.
e. Peritrichous -flagella surrounding the cell.
Simple Binary fission is the usual mode of reproduction among bacteria. This, of course
is asexual in character. This means that new bacterial cells are formed by division of pre-
existing cells into two cells. Division is proceeded by an in- crease in the size of the cell. Just
before division a constriction appears in the middle and ultimately each cell divides into two
similar daughter cells. The daughter cells may separate almost immediately after division or
may remain together for a varying period.
Among the cylindrical and spiral forms, fission occurs at right angles to the long axis, but
in coccal form it occurs in any diameter. The process can be readily seen under the microscope.
The average life of a bacterium is 20 -30 minutes. Hence bacteria grow and multiply with
enormous speed. A single cell in 24 hours give birth to more than fifteen million descendants.
The rate of multiplication, however, is profoundly influenced by environmental conditions.
The process of the development of bacterial growth follows certain phases. The different
phases of their growth are explained in detail here.
1. Log phase
This is the phase immediately following the inoculation of fresh sterile media when the
culture is started. During this
phase bacteria do not multiply but remain constant in numbers or even decrease in numbers.
The cells adjust themselves in the new medium and do not start to multiply. It is an important
phase in the development of a culture.
2. Rapid growth phase
After the period of adjustment during the log phase the organisms begin to divide at an
increasing rate until the maximum rate of multiplication is attained. The cells are dividing with
the shortest generation time when best conditions of food concentration and temperature is
3. Decreasing numbers phase
After the phase of rapid growth, retarding influences soon come into play, accompanied
by a reduction in the rate of cell division, and increase in the death rate. The reason for the
decrease in the growth race of bacteria is the lack of food and the accumulation of products of
X. Spore Formation
Among the rod shaped bacteria about 100 species of aerobes and some 50 species of
anaerobes are known to have the capacity to develop a sort of encysted or resting stage by a
process known as sporulation or spore formation. The body thus formed is known as
endospore. The cocci or the spiro-chetes do not produce endospore. The ability to produce
spore is a distinguishing feature of the organisms of the family Bacillaceae. The aerobic genus
is called Bacillus and the anaerobic genus, Clostridium.
Spores are generally formed when there is an unfavourable condition of growth, such as,
decrease of the food material, unfavourable reaction of the nutrient medium, accumulation of
the produces of metabolism etc. This process should not be considered as a method of
reproduction, but as the ability of the cells to overcome unfavourable conditions of growth.
During the production of spores the cell contents of the bacteria become thicker and the
moisture content is reduced. For the production of spores at least a time of 24 hours is needed.
If unfavourable conditions of growth, like increase of temperature takes place before this period
the cells do not find time to form spores but they die. During the process of spore formation the
spores retreat form the cell-wall and form their own walls.
Spores are found by the condensation of the protoplasm and the formation of a tough
envelope around it. They are very resistant to heat and chemical antiseptics. They are difficult to
stain and special methods have to be employed for it, but they are easier visible by the indirect
staining method. Some bacteria produce their spores early in their life and some late. When
favourable conditions are restored. Spores germinate into vegetative cells which begin to grow
and reproduce in the usual manner.
Spores are very refractile, oval or round in shape and bear a definite positional relation
with the cell body. When spores are located at the centre of the cell, they are spindle shaped,
and when they are located at the end of the cell then they are known as drum-sticks. Some
spores remain viable in the resting stage even up to ten years. When favourable conditions for
germination are available they germinate in 4 -6 hours.