Page 232 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 201

brief description of cell components
and their function (Figure 4.2)

Cell wall
The porous cell wall covers the whole organism.
Division of one bacterium into two (by binary
fission) is initiated by a constriction of the cell
wall in the centre. This splitting does not always
occur completely, and may result in character-
istic clumps of bacteria (Staphylococci), chains
(Streptococci) or pairs – diplococci. Figure 4.3 Scanning electron micrograph (SEM)
of a gram-negative bacterium (Yersinia sp.). SEM
allows the detailed examination of the external
Cytoplasmic membrane structures of bacteria. The elongated structures are

Between the cell wall and the cytoplasm is a fimbriae which form part of the ‘antigenic’ compo-
very thin cytoplasmic membrane. The cytoplas- nent of the bacteria (surface antigens).
mic membrane is permeable allowing water
and nutrients to pass into the cell and waste
products to leave the cell. The nature of the Capsule
membrane present in different groups of bacte- Many, but not all, bacteria have a gelatinous
ria determines the uptake of dyes and therefore layer of material on the outer surface of the
influences the result of Gram staining. cell called a ‘capsule’. The thickness of this
layer varies considerably but it is usually visible
when viewed under the light microscope. Well-
Cytoplasm
developed capsules are often slimy and form a
Bacterial cytoplasm is a gelatinous substance covering for the organism which may protect
that contains soluble metabolites and other it from the environment or from attack by the
material. Lying within the cytoplasm is the bac- defence mechanisms of the body.
terial chromosome.

Flagella
Genetic information
Bacterial flagella originate from basal granules
Bacteria do not have a true ‘nucleus’. The genetic inside the cell membrane and are usually about
information, deoxyribonucleic acid (DNA) is 0.02 µm thick and can be longer than the body
contained in the chromosome (nucleoid) which of the bacterium. There may be one single fla-
does not have a membrane cover. The nucleoid gellum or several. Flagella allow movement
divides into two during bacterial multiplication. (motility) which enables bacteria to come into
Some bacteria also have an additional short piece contact with nutrients. Motility can sometimes
of circular extra-chromosomal DNA (plasmid) be observed when preparations are examined
which can be exchanged with other bacteria. under the microscope or when cultures are
The transfer of plasmids can be important in the inoculated into specialized media.
development of antibiotic resistance.

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