Page 242 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 211

NaBH + 2 H O = NaBO + 4 H a readily available source of nitrogen and car-
4 2 2 2
bon. Some bacteria require the addition of other
C H O(COOH) + 2 NaHCO + [CoCl ] = nutritive substances such as serum or blood.
3 5 3 3 2
C H O(COONa) + 3 CO + 3 H + [CoCl ] Carbohydrate supplements such as dextrose
3 5 3 2 2 2
may also be desirable along with salts of calcium,
2H + O + [Catalyst] = 2H O + [Catalyst] manganese, magnesium, sodium or potassium.
2 2 2
Dyes and chemicals may be added to media to
Essentially, the chemicals react with water to (1) indicate specific metabolic activity, or (2)
produce hydrogen and carbon dioxide along with to aid or inhibit the development of certain types
sodium citrate and water as by products. of bacteria (these are then known as ‘selective
media’).

Media
Sterility
A satisfactory microbiological culture medium
must contain available sources of carbon, nitro- The media used for isolating bacterial pathogens
gen, inorganic salts and in certain cases, vitamins must be sterile, that is, free from all other organ-
or other growth promoting substances (see isms whose development might influence or
Table 4.2a). These can be supplied in the form prevent the normal growth of the bacterial inoc-
of meat infusions which are still widely used ulum. Autoclaving is the usual method for the
in culture media. Beef extract can replace meat sterilization of culture media. Excess steriliza-
infusion and the addition of peptone provides tion or prolonged heating should be avoided as
this will change the composition of the medium,
for example, agar in the medium may form a pre-
cipitate. Culture media which may be damaged
by autoclaving can be sterilized by discontinuous
or intermittent heating at lower temperatures or
by filtration (check the instructions which come
with specific media or contact the manufacturer
for more detailed guidance).

Storage
Media should always be stored in a cool moist
Figure 4.7 Classical microbiology requires a wide atmosphere to prevent evaporation and drying.
range of reagents and relies on the technical exper- Prepared agar plates can be sealed and stored in
tise and experience of the laboratory technician a refrigerator but prolonged storage (> 10 days)
for the successful culture and identification of dis- of sterile media is not recommended. Once a
ease causing agents. This photograph illustrates laboratory can gauge how many plates are going
the range of media and biochemical tests required to be required it is easier to plan ahead, how-
to identify a strain of Yersinia pseudotuberculosis ever, when starting out it is advisable to make
isolated at necropsy from a spleen abscess in a up batches of media on a regular basis as plates
parrot. See also case study example (collibacillosis are needed.
in poultry) and diagnostic flow charts Appendix 5,
A5.1 and A5.2.

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