Page 231 - The Veterinary Laboratory and Field Manual 3rd Edition

200 Susan C. Cork and Roy Halliwell

normal flora that might be isolated from samples anatomy of the bacterial cell
such as faeces and skin. (see also Appendix 3).
Although these are not usually considered The principal structures of a bacterial cell are
significant, some of these (usually harmless) shown in Figure 4.2. The cell is bound periph-
bacteria can invade tissues and cause disease if erally by a very thin, elastic semi-permeable
the animal is debilitated or injured, that is, they cytoplasmic membrane. Outside and covering
can be opportunistic pathogens. It is important the membrane is the rigid supporting cell wall,
to make the distinction between primary patho- which is porous and relatively permeable. The
gens and opportunistic pathogens because in cytoplasm consists of a watery fluid packed with
the case of the latter, treatment would need to large numbers of small granules including ribo-
focus on determining the underlying cause(s) of somes and other structures. Other intra-cellular
the disease as well as dealing with the bacterial and extra-cellular structures may be present in
infection. some types of bacteria and this is dependent on
Clinical bacteriology involves the isolation the presence of favourable growth conditions.
and identification of bacteria that cause dis- For example, a protective gelatinous cover-
ease. Pathogenic bacteria cause disease either ing layer called a capsule may be produced (as
by the direct effect of the bacteria and the for some Clostridium spp.). Some bacteria (for
associated host response (for example, inflam- example, Salmonellae) have flagella and fimbria
mation, pus formation and so on), or due to the (pili) which facilitate adhesion and motility.
toxins they produce (for example, Clostridial Differences in the latter are used to subtype
diseases and some enteric pathogens). Disease strains of Salmonella enterica.
causing organisms are called pathogens. Only
a small proportion of bacteria present in
and on the body and in the environment are
pathogenic.

Figure 4.2 The principal
structures of a bacterial cell.
(a) Cell wall; (b) capsule of slime
layer; (c) cytoplasmic (cell)
membrane; (d) cytoplasm;
(e) chromosome (nucleoid);
(f) flagellum; (g) pili or fimbriae.
See also Figure 4.3.

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