Systematic Veterinary Bacteriology and Mycology

CULTURAL AND BIOCHEMICAL CHARACTERISTICS

Cultural characteristics

• The bac­terium is comparatively easy to culture and grows well on sheep or ox blood agar.
• An atmosphere of 5-10 per cent CO2 enhances the growth of the organism, espe­cially on primary isolation.
• The inoculated plates are incubated at 37°C (Optimum temp. 37^oC.) for up to 5 days, although colonies may be seen after 24-48 hours incubation.

Colonial morphology

• Small (about 1 mm) greyish-yellow, distinctly haemolytic colonies can be seen after 24-48 hours incubation.
• They are firmly adherent to the medium and are embedded in the agar.
• After 3-4 days, isolated colonies can be 3 mm in diameter and are rough, wrin­kled with a golden-yellow colour. Older colonies can become mucoid.
• No growth occurs on Sabouraud dextrose agar.

Microscopic appearance

• Gram-stained smears from colonies do not show the characteristic 'tram-track' appearance seen on direct microscopy.
• Usually the smears reveal uniformly staining, Gram-positive, branching filaments but sometimes coccal forms predominate.

Biochemical characters

• D. congolensis is catalase-positive, urease-positive, gelatin-positive and produces acid from glu­cose, fructose and maltose.
• It is indole-negative, does not reduce nitrate and non-fermentative although acid is produced from certain carbohydrates.