Page 246 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 215

3 Describe the colour and consistency of the
colony types present (that is, cream coloured,
white, flat, convex, dry, mucoid and so on).
4 Describe the growth on blood agar and com-
ment on the degree of haemolysis around
the colonies (for example, beta, alpha or no
haemolysis )
3
5. Record the growth on MacConkey agar; are
the colonies pink (that is, lactose fermenting)
or colourless (non-lactose fermenting)?

In most cases bacterial cultures are plated
directly onto blood agar. MacConkey agar may
also be used for the initial plating of enteric
pathogens. The majority of bacteria of veteri-
nary importance grow reasonably well on blood
agar but, only the enteric bacteria (gut flora) will
grow well on MacConkey agar, this is because
Figure 4.8 Initial ‘streaking’ of a culture plate this media contains bile salts that are inhibitory
and preparation of a subculture (or ‘purity’ plate). to the growth of many other bacteria. Many com-
Initial plating out is usually done on blood agar mensal enteric organisms ferment lactose and,
and MacConkey agar unless special media are due to the presence of an indicator dye, these
requested. (A) Initial zone (if this is from a faecal bacteria produce pink colonies after overnight
sample the initial growth will probably be mixed growth on MacConkey media.
and prolific therefore streaking out should be care- To interpret microbiological findings, it is
fully done to avoid contamination of the rest of the important to know the characteristics of the nor-
plate). Flame the loop between each set of streaks mal commensal bacteria found in different species
(O ->). Incubate the plate at 37°C and check the of animal and to have a good case history. The
plate at 24 h and 48 h for growth and describe what latter should be provided by the submitter on
is seen. (B) If the procedure has been done carefully the submission form. Culture results from fae-
there should be individual colonies to select from cal samples and samples from the skin are very
for the subculture. Select one or two of any colo- difficult to interpret without knowledge of the
nies which may be pathogenic (disease causing) case. Microbiological examination of samples
for Gram staining. (C) The subculture is prepared collected aseptically from a catheterized bladder,
by selecting an isolated distinctive colony, this can correctly submitted necropsy samples or asepti-
then be grown overnight in broth before plating out. cally collected fluid aspirates (for example, joint
There is no need to flame the loop after the first fluid, cerebrospinal fluid) is usually more straight-
stage. (D) Check that there is a pure growth of bac- forward because the degree of contamination is
teria before harvest at 24 h (or 48 h if slow growing) considerably reduced and the chances of growing a
for biochemical tests. good culture of the causative agent much greater.
Once a bacterial culture has grown, it is rou-
present (that is, 2–3 mm,1–2 mm, pinpoint tine practice to sub-sample and stain bacteria
colonies, round, rough edge, entire edge, and from individual colonies using Gram stain (or
so on)? other stains) to assist in the identification of

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