Page 236 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 205

smears of specimens submitted for culture can used in microbiology are selected to differenti-
also be prepared for microscopic examination. ate and highlight specific cell features (see Table
Prepared smears will usually need to be fixed 4.1). Motility is also used as a means of identi-
in methanol or heat fixed and then stained to fying bacteria and can be studied by examining
allow good visualization of any microorganisms an unstained wet preparation of the organism
1
present. The staining techniques commonly suspended in a fluid.

Table 4.1 The staining characteristics of some bacteria of veterinary importance.
The list is not comprehensive but provides an overview of the sort of differences
to look out for when stained smears or fixed bacterial colonies are examined.

Stain Specimen and some of the Appearance in stained sections
organisms that might be seen
Gram stain Pus or exudate
Staphylococcus spp. Gram +ve cocci*
Gram +ve (purple – dark blue) (usually in clumps)

Gram –ve (pink) Streptococcus spp. Gram +ve cocci* (usually in chains)
Corynebacterium spp.*** Gram +ve rods**
The Gram stain is used widely Pasteurella spp. Gram –ve rods**
for routine staining of bacteria
in smears. Gram +ve bacteria Scabs
retain crystal violet in their Dermatophilus spp. Gram +ve filamentous and
cell wall so appear blue, Gram branching with coccal zoospores
–ve bacteria to not retain the Aspirated material from a lump
crystal violet stain and are
counterstained red As above for exudates
Actinobacillus spp. Gram –ve rods
Actinomyces spp. Gram +ve filamentous and
branching
Feacal sample (enteritis)
Escherichia coli Gram –ve rods
Salmonella spp. Gram –ve rods
Enterotoxaemia, deep infections,
cellulitis
Clostridium spp. Gram +ve fat rods, with or without
spores
Carbol fuchsin Faecal samples (enteritis)
Pink-purple colour Campylobacter spp. Curved rods, ‘seagull’ forms

Especially used for Foot rot exudates
recognizing Campylobacter Bacteriodes spp. Rods with knobs at one or both
spp. ends
Long slender filaments, irregular
staining

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