INTRODUCTION
Bacterium
functionally specific components of metabolism that belongs to the kingdom
Prokaryotae (Monera).
The internal cytoplasm is invariably surrounded by one-or two-layered rigid cell wall composed of
phospholipids. Some bacteria also produce a specific mucoid extracellular capsule for additional protection,
particularly from phagocytosis by white-blood cells (WBCs). Bacteria can synthesize nucleic
acids (DNA, RNA), other important proteins and can reproduce independently, but may essentially need
a host to supply food and also a supportive environment. In reality, millions of nonpathogenic bacteria
live on the skin and mucous membranes of the human gastrointestinal tract (GIT) ; these are termed as
normal flora. Importantly, bacteria that cause disease are usually known as pathogens.
CHARACTERISTIC FEATURES
A few vital and cardinal characteristic features of ‘bacteria’ are as enumerated under
Shape
There are
three principal forms of bacteria, namely :
(
a) Spherical or Ovoid — bacteria occur as single cells (micrococci), or in pairs (diplococci),
clusters
(staphylococci), chains (streptococci) or cubical groups (sarcinae) ;
(
b) Rod-shaped — bacteria are termed as bacilli, more oval ones are known as coccobacilli, and
those forming a chain are called as
streptobacilli ; and(c) Spiral — bacteria are rigid (spirilla), flexible (spirochaetes) or curved (vibrios).
Size
An average rod-shaped bacterium measures approximately 1
μm in diameter and 4 μm in length.
They usually vary in size considerably from < 0.5 to 1.0
μm in diameter to 10–20 μm in length in someof the longer spiral forms.
Reproduction
It has been observed that
simple cell division is the usual method of reproduction, whereas certain
bacteria give rise to
buds or branches that eventually break off. The growth rate is substantially
affected on account of changes in temperature, nutrition, and other factors.
Importantly,
bacilli can produce reproductive cells invariably termed as spores, whose relatively
thick coatings are highly resistant to adverse environmental conditions. In the event of a better congenial
environment the
spores commence to grow. Besides, spores are difficult to kill as they are highlyresistant to heat as well as disinfectant action.
Formation of Colony
A group of bacteria growing in one particular place is known as a
colony. A colony is invariably
comprised of the
‘descendants of a single cell’. It has been found that colonies differ in shape, size,
colour, texture, type of margin, and several other characteristic features. Interestingly, each species of
bacteria has a characteristic type of colony formation.
Mutation
Evidently, a majority of bacteria, like all living organisms, do possess the ability to adapt their
shape or functions when encountered with distinct changes in their environment, but there are certain
degree of limits to this ability. However, they may also mutate to adapt to some potentially lethal substances,
for instance : antibiotics.
Motility
It has been duly observed that none of the ovoid or spherical cocci are capable of moving, but
certain bacilli and spiral forms do exhibit absolute independent movement. It is, however, pertinent to
mention here that the power of locomotion exclusively depends on the possession one or more flagella,
slender whiplike appendages which more or less work like propellars.
Food and Oxygen Requirements
Bacteria are of different types based upon their
food and oxygen requirements as given below :
(
a) Heterotrophic : require organic material as food,
(
b) Parasites : feed on living organisms,
(
c) Saprophytes : feed on non-living organic material,
(
d) Autotrophic : i.e., self-nourishing–obtain their energy from inorganic substances, including
most of the soil bacteria,
(
e) Aerobes : essentially require oxygen for their very existence and growth, and
(
f) Anaerobes : do not require oxygen for their existence and growth. e.g., most bacteria foundin the GIT
Temperature Requirements
Although some bacteria live at very low temperature or very high temperature ; however, the
optimum temperature for a majority of pathogens is 37 °C (98.6 °F).
ACTIVITIES
Following are some of the predominant
activities of bacteria, namely :
(
a) Enzyme Production. Bacteria invariably give rise to the production of enzymes that act on
complex food molecules, breaking them down into much simpler components ; they are the
principal agents responsible for causing decay* and putrefaction.**
(
b) Toxin Production. Special molecules called adhesins bind bacteria to the host cells. Once
the attachment gets materialized, the
bacteria may produce poisonous substances usually
known as
toxins.
Toxins are commonly of
two kinds, such as :
(
i) Exotoxins — enzymes that virtually disrupt the cell’s function or kill it, and
(
ii) Endotoxins — stimulate production of cytokines*** which may produce widespread
vasodilation and shock.
(
c) Miscellaneous. A host of bacteria produce several chemical and physical characteristic products,
such as :
Pigments
— colouring matter,
Light
— exhibiting luminescent at night,
Chemical substances
— e.g., acids, alcohols, aldehydes, ammonia, gases, carbohydrates,
and indole, and
Hemolysins, leukocidins, coagulases, and fibrolysins
— produced by pathogenic bacteria ;
The
soil bacteria play a vital and important role in different phases of the nitrogen cycle viz.,nitrification, nitrogen fixation, and denitrification