VMC 311: Systematic Veterinary Bacteriology and Mycology (2+1)

MORPHOLOGY AND CULTURAL CHARACTERISTICS

Morphology

• Gram –ve, small, rod shaped organism. They are non-motile, non-sporing and non-acid fast.
• They are non capsulated (except A.pleuropneumoniae) but extracellular slime is present in three major species (A.lignieresii, A.equuli and A.suis)
• In media containing fermentable carbohydrates, the occurrence of rather long, almost filamentous forms is seen.
• Small granules are found scattered along the bacilli, often lying at the pole of a bacillary or filamentous form, giving a characteristic ‘Morse code’ form.
• In lesion in the animal body small grayish white granules are present.
• If these granules are crushed on a slide and stained, club colonies are seen consisting of club-like processes of calcium phosphate, with Gram-negative rods of A. lignieresii in the center.
• Both bacilli and club forms are Gram negative.
• They can be distinguished with ZN stain in which the club appears red and the bacilli blue.

Cultural characteristics

• They are aerobic, or micro aerophilic, and facultative anaerobe.
• The optimum temperature is 37⁰C on blood agar, A.lignieresii develops small, glistening, non-haemolytic colonies within 24 hrs.
• They are usually slightly sticky (cohesive properties) on primary isolation, but lose this character on subculture.
• The organism grows well on MacConkey agar and it is a late lactose fermenter. The colonies are first pale but become pinkish.
• A. equuli strains are haemolytic. The colonies are sticky with this feature remaining on subculture. It is lactose fermenter on MacConkey agar.
• A.suis are also haemolytic. Colonies are stickier. It grows well on MacConkey agar.
• In CAMP test A.pleuropneumoniae enhances beta haemolysis of Staphylococcus aureus. (i.e. Positive).