| |
Bacteria
- Bacteria are generally simple structures. The bacterial cell
lacks a membrane-bound nucleus. Because of this, bacteria are
described as prokaryotes, "pro-" meaning "before" and "-karyon"
from the Greek word for a "nucleus". Despite their simplicity, bacteria
have an enormous range of metabolic capacities, and can be found in some
of the most extreme environments on earth. Only a small minority of
bacteria causes disease.
- There are three basic shapes that bacterial cells adopt. They are
either round, rod shaped or spiral. Round bacteria are referred to as
cocci (singular: coccus), and rod
shaped bacteria are known as bacilli (singular: bacillus). The
term 'bacillus' meaning a rod-shaped bacterium should NOT be confused
with the genus of bacteria known as 'Bacillus'. The shape of
bacterial cells is of fundamental importance in the classification and
identification of bacteria. Although bacteria are of three basic shapes,
they display an astonishing variety of forms when viewed
microscopically.
|
|
Sporing cells of
Clostridium tetani
Note spores do not stain and in this case cause the bacilli to swell
|
Irregular cells of
Corynebacterium diphtheriae
|
Diplococcal cells of
Streptococcus pnuemoniae
|
Sporing cells of
Bacillus anthracis
Note spores do not cause the bacilli to swell in this species.
|
Various shaped cells of
Haemophilus influenzae
|
Streptococci
|
Spiral cells of
Treponema pallidum
This bacterium causes syphilis and is so slender that it cannot be seen
using conventional light microscopy.
It is most easily visualised using dark-ground microscopy.
|
Curved rods of
Vibrio cholerae
|
- Bacteria may be conveniently divided into two
further groups, depending upon their ability to retain a crystal
violet-iodine dye complex when cells are treated with acetone or
alcohol. This reaction is referred to as the Gram reaction,
named after Christian Gram, who developed the staining protocol in
1884. It may seem a very arbitrary basis on which to build one's
classification system. This reaction, however, reveals fundamental
differences in the structure of bacteria. Electron microscopy shows
that Gram-negative and Gram-positive bacteria have fundamentally
different structures, related to the composition of the cell wall,
amongst other things.
- Flagella are responsible for the motility of pathogenic bacteria
and can play a role in the production of disease. Gram-negative
pathogenic bacteria may be covered in fine hairs called fimbriae
(singular: fimbria) these help to stick to body surfaces. Pili
can attach two bacterial cells together: sex pili are necessary for the
transfer of certain plasmids between bacteria.
- Gram-negative bacteria have additional structures. They exchange
genetic material in a process of conjugation that involves cells being
joined by sex pili: tube-like structures through which DNA is
passed. The surface of Gram-negative cells is also covered in fine,
hair-like structures called fimbriae (some microbiologists also
call these pili, confusing them with sex pili). These are important in
adhesion and can play a central role in virulence. If a microbe is to
cause an infection, first it must attach to its surface.
- Some bacteria are enclosed within a capsule. This protects
the bacterium, even within phagocytes, helping to prevent the
cell from being killed. Encapsulated bacteria grow as 'smooth' colonies,
whereas colonies of bacteria that have lost their capsules appear rough.
Rough colonies do not generally cause disease. Encapsulated bacteria do
not succumb to intracellular killing as easily as bacteria that lack
capsules. Strains of Streptococcus pnuemoniae that lack capsules
do not cause disease. All the bacteria that cause meningitis are
encapsulated. Suspending bacteria in India ink is an easy way of
demonstrating capsules. Ink particles cannot penetrate the capsular
material and encapsulated cells appear to have a halo around them. This
is the Quellung reaction.
- In the 'Quellung' reaction, bacterial cells are resuspended in
antiserum that carries antibodies raised against the capsule. This
causes the capsule to swell, and this can be easily visualised by
suspension in India Ink. The ink particles cannot penetrate the capsule,
which this appears as a halo around the bacterial cells.
- Some bacteria produce slime to help them to stick to surfaces. Slime is
produced by several types of pathogenic microbes, and is usually made up from
polysaccharides. The slime produced by Streptococcus mutans enables it
to stick to the surface of teeth, where it helps to form plaque, leading
eventually to dental caries. 'Coagulase-negative' staphylococci live on the
skin, and some strains produce a slime that enables them to stick to plastics.
These bacteria cause infections associated with implanted plastic medical
devices.
- A few species of bacteria have the ability to produce highly resistant
structures known as endospores (or simply spores). These resist a range of
hazardous environments, and protect against heat, radiation, and desiccation.
Endospores form within (hence endo-) special vegetative cells known as
sporangia (singular sporangium). Diseases caused by sporing
bacteria include botulism (Clostridium botulinum), gas gangrene
(Clostridium
perfringens), tetanus (Clostridium tetani) and acute food poisoning
(Clostridium perfringens, again) All these bacteria are 'anaerobic'.
- The aerobic sporing bacteria can also cause disease. Anthrax is
caused by Bacillus anthracis. Bacillus cereus causes two
types of food poisoning.
|
|
