Page 244 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 213

Bacterial species Blood agar** MacConkey agar* Comments
Haemolytic Grey, smooth, shiny round Good growth, Characteristic ‘coliform’
Escherichia coli colonies, 2–3 mm, clear colonies pink smell, bright pink colonies on
zone around colonies indicating lactose MacConkey the same size as on
fermentation blood agar
Non-haemolytic Grey, round and shiny Good growth, As above
Escherichia coli colonies, 2–3 mm, no clear colonies pink
zone indicating lactose
fermentation
Salmonella spp. Grey, round and shiny Good growth Not lactose fermenting, pale
colonies, 2–3 mm, no clear colonies on MacConkey agar.
zone No smell
Yersinia spp. Grey, round and shiny Good growth Not lactose fermenting
colonies, 2–3 mm, no clear
zone
Pseudomonas Blue-green, flat round Growth but pale Not lactose fermenting, some
aeruginosa colonies, 2.5–4.0 mm, may colonies strains have metallic sheen on
have clear zone blood agar
Corynebacterium Grey-white, moist colonies, No growth Colonies may appear dry over
renale 0.5–1.0 mm, no clear zone time
Pasteurella Transluscent, round shiny No growth Colonies may appear pinkish on
multocida colonies, 1–2 mm, no clear blood agar, characteristic sweet
zone odour
Campylobacter Small opaque colonies, No growth May require 2–3 days for
fetus 0.5 mm diameter, no clear culture, requires reduced
zone oxygen conditions to grow
(Micoaerophilic)
Clostridium Grey, flat and often an No growth Requires anaerobic conditions to
perfringens irregular edge to colonies, grow. Double zone of haemolysis
2–3 mm diameter, some is possible
have a clear zone
Bacillus spp. Grey, dry granular No growth Many haemolytic but not Bacillus
appearance, 3–5 mm anthracis. Dry colonies as no
diameter, may have clear capsular material is produced on
zone lab. media

Notes: *Colony characteristics may vary but the descriptions given are for cultures read at 48 h. **The type of haemolysin
present will determine the appearance of the zone around the colonies. Note that alpha haemolytic staphylococci appear
different to alpha haemolytic streptococci.

pH requirements Brucella spp.) and others require an alkaline pH
(for example, some Salmonellae). Essentially,
The pH of a culture medium is extremely impor- the optimum pH for bacterial growth in the labo-
tant for the growth of microorganisms. The ratory is the same pH as that in the animal host
majority of bacteria prefer culture media which from which the sample was collected. Optimum
are neutral (pH = 7.0) while some may require pH for bacterial growth varies and therefore this
a medium which has an acid pH (for example, is often used in media as a means of ‘selecting’

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