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

CULTURAL AND BIOCHEMICAL CHARACTERISTICS

• It is an anaerobe, but can also grow under micro aerophilic conditions.
• Oxygen is not actively toxic to the bacillus and cultures do not die on exposure to air.
• Though, this bacillus is grown at 37⁰C, pH 5.5-8.0, the temperature of 45⁰C is optimal for many strains.
• The generation time at this temperature is ten minutes only.
• This property can be utilized for obtaining pure cultures of C.welchii.
• Robertson's cooked meat broth inoculated with mixtures of C.welchii and other bacteria and incubated at 45⁰C for 4-6hrs serves as enrichment.
• Subcultures from this onto blood agar plates yield pure or predominant growth of C.welchii.

• Good growth occurs in Robertson’s cooked meat medium.
• The meat is turned pink but it is not digested.
• In litmus milk, fermentation of lactose leads to formation of acid, which is indicated by the change in the color of litmus from blue to red.
• The acid coagulates the casein (acid clot) and the clotted milk is disturbed due to the vigorous gas production.
• The paraffin plug is pushed up and shreds of clot are seen sticking to the sides of the tube. This is known as stormy fermentation. (Click here for visual)
• After overnight incubation on rabbit or sheep blood agar, colonies of most strains show a target haemolysis resulting from a narrow zone of complete haemolysis due to theta toxin and a much wider zone of incomplete haemolysis due to alpha toxin. This double zone haemolysis pattern is characteristic for C.welchii.

• All the five types of C.welchii (A –E) produce alpha toxin.
• This is a lecithinase C, which, in the presence of calcium and magnesium ions splits lecithin into phosphotidyl choline and diglyceride.
• This specific lecithinase effect can be demonstrated by Nagler’s reaction.
• Type A antitoxin is spread over half of an egg yolk agar plate and allowed to dry.
• The suspect C.perfringens is streaked across both sides of the plate. Click here for visual

• All the types of C.perfringens produce the alpha toxin, that is a lecithinase.
• On the half of the plate without the antitoxin, the lecithin in the medium is attacked causing opalescence around the streak.
• The lecithinase reaction is neutralized on the other half of the plate with the antitoxin but the growth of C.perfringens is unaffected.
• C.welchii ferments several sugars (glucose, maltose, lactose and sucrose) and produces acid and gas. Indole –ve, MR +ve, VP-ve, H₂S +ve.