Clostridium perfringens

Also called Cl. welchii. It causes gas gangrene and food poisoning.

Morphology

It is a Gram-positive bacillus that is capsulated and non-motile. It may be arranged singly, in chains, or in bundles. It forms bulging, oval, subterminal spores.

Remember: spores are usually not seen in clinical specimens. This is because sporulation typically occurs under unfavorable conditions (for example, outside the body), whereas in tissues the organism is usually in the vegetative form.

Classification

• Six antigenic types (A to F) are identified based on the type of toxin produced.
• About 80 serotypes are identified based on surface polysaccharide.

Human pathology

Gas gangrene and food poisoning are the major diseases caused by Cl. perfringens. It can also cause necrotizing enteritis, brain abscess, meningitis, panophthalmitis, septicaemia, cellulitis, and gangrenous appendicitis.

Gas gangrene: Also called myonecrosis or necrotizing fasciitis. It commonly follows war injuries, traffic accidents, etc., where a wound becomes contaminated with the organism. The bacteria then grow under anaerobic conditions in the wound and produce toxins that cause further tissue damage. As infection and necrosis spread, foul-smelling gas is produced.

Alpha toxin is the major toxin responsible for the infective process. It breaks down tissues (necrotizing) by damaging cell membranes. The patient presents with a foul-smelling, necrotic wound with edema and local pain. Crepitations occur due to gas formation, and in severe cases hypotension and shock can occur due to toxemia. It is fatal if not treated at the right time.

Food poisoning: Food becomes contaminated with heat-resistant spores of Cl. perfringens. When such food, especially meat dishes, is reheated, the bacteria grow rapidly and produce enterotoxin. The enterotoxin of Cl. perfringens is a superantigen. It acts on the small intestine to cause abdominal cramps and watery diarrhoea after an 8-16 hour incubation period.

Laboratory diagnosis of Cl. perfringens infections

Gas gangrene: Specimens are taken from the wound, including exudates and necrosed tissue.

• Gram stain shows many Gram-positive bacilli without spores, with scanty inflammatory infiltrate. This occurs because toxins from Cl. perfringens break down neutrophils.
• Anaerobic culture is done on blood agar and RCM.
  • On blood agar, it shows characteristic target hemolysis: a zone of complete hemolysis due to theta toxin, followed by a wider zone of incomplete hemolysis due to alpha toxin.
  • In RCM broth, it turns the meat particles pink (not black) because it has saccharolytic activity but no proteolytic activity.

Biochemical identification is done with the litmus milk test and Nagler reaction.

• The litmus milk test shows a change in color of litmus from blue to red due to stormy fermentation, with disruption of the casein clot due to gas production.
• In the Nagler reaction, when Cl. perfringens is inoculated on a medium containing egg yolk or serum, a zone of opacity is produced around the colonies due to lecithinase activity of alpha toxin.

Food poisoning: This is mainly a clinical diagnosis. Fecal matter and contaminated food can be obtained for anaerobic culture, followed by biochemical identification as in gas gangrene samples.