Page 240 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 209

(a) (b)

(c) (d)

Figure 4.5 Agar plates showing culture media and growth characteristics of colonies of different microor-
ganisms. (A) Agar plate with a culture of a fungus (Paecilomyces sp.) obtained from the skin of a reptile.
Fungal identification depends on the colour and type of growth on specialized agar media (for example,
sabouraud dextrose agar) and on the morphology of the fruiting bodies or Hyphal structure (see also
Figures 4.14a and b). (B) Specialized media (CIN) with a culture of Yersinia pseudotuberculosis. This agar
is selective for some enteric bacteria, Yersinia spp. colonies appear small and pigmented (dark grey). CIN
= Cefsulodin-Irgasan-Novobicin agar (Difco). (C) This is an example of a positive (left) and negative (right)
catalase test. Catalase is an enzyme which breaks down hydrogen peroxide (H O ) into H O and O . When
2 2 2 2
a drop of 3% H O is added to a colony of catalase positive bacterium, the reaction produces bubbles.
2 2
Photo: Paul Gadja. (D) This is an example of the appearance of Staphylococcus spp. grown on blood agar.
On the left side of the plate is S. aureus showing characteristic golden colonies surrounded by clear zones
of beta haemolysis. On the right side of the plate is S. xylosus, which is coagulase negative and does not
produce haemolysis. Photo: Karen Liljebjelke. See also Plate 8a–d.

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