Clostridia

GENUS CLOSTRIDIA

This genus contains many species of Gram-positive anaerobic spore-forming rods; a few are aerotolerant. Widely distributed in soil and in the gut of man and animals. The spores are resistant to environmental conditions. The major diseases associated with species of the genus are gangrene, tetanus, botulism, food poisoning and pseudomembranous colitis. In each of these the production of potent protein exotoxins is an important cause of pathology and in several species the genes encoding toxins are carried by plasmids or bacteriophages.

 

Clostridium perfringens

 

Characteristics:

  • Anaerobic
  • Gram-positive
  • rods
  • spore forming, but spores rarely seen in infected material.
  • More tolerant of oxygen than other clostridia.

 

Transmission:

Spores and vegetative organisms widespread in soil and normal flora of man and animals. Infection acquired by contact; may be endogenous (e.g. contamination of a wound with soil, ingestion of contaminated food).

 

Diseases:

  • Gas gangrene resulting from infection of dirty ischemic wounds.
  • Food poisoning following ingestion of food contaminated with enterotoxin-producing strains.

 

Laboratory Diagnosis:

  • Hemolytic colonies on blood agar incubated anaerobically.
  • Identification confirmed by demonstration of alpha-toxin (lecithinase) production in the Nagler’s test.
  • Heat resistant spores may be responsible for food poisoning.
  • Five types of Cl. perfringens (A-E) identified on the basis of toxins produced.

 

Clostridium tetani

 

Characteristics:

  • Gram-positive
  • spore-forming
  • rod
  • terminal round spore (drumstick)
  • Strict anaerobe.

 

Transmission:

  • Organisms widespread in soil.
  • Acquired by man by implantation of contaminated soil into wound.
  • Wound may be major (e.g. in war, in road traffic accident) or minor (e.g. rose thorn puncture while gardening).
  • No person-to-person spread.

 

Diseases:

  • Tetanus (lockjaw) results from neurotoxin (tetanospasmin) produced by organism in wound.
  • Severe disease characterized by tonic muscle spasms and hyperflexia, trismus, opisthotonos and convulsions.

 

Laboratory Identification:

  • Grows on blood agar in anaerobic conditions as a fine spreading colony; ‘ground glass’ appearance.
  • Has very little biochemical activity useful for identification purposes.
  • Demonstration of toxin in a specimen is possible in a two-mouse model in which one animal is protected with antitoxin; the other unprotected. Injection of suspect material is via the root of the tail.

 

Clostridium botulinum

 

Characteristics:

  • Anaerobic
  • Gram-positive
  • Rods
  • Not easily cultivated in competition with other organisms
  • Produces most potent toxins known to man.
  • Eight immunologically distinct toxins (A, B C, D, E and F) produced by different strains of Cl. botulinum.
  • Three are most commonly associated with human disease: serotypes A and B associated with meat, E with fish.

 

Transmission:

  • Soil is the normal habitat.
  • Intoxication most often by ingestion of toxin in foods that have not been adequately sterilized (e.g. home-preserved foods) and improperly processed cans of food.
  • Toxin is associated with germination of spores.
  • There is no person-to-person spread.

 

Diseases:

  • Major pathogen of birds and mammals, but very rare in humans.
  • Botulism acquired by ingesting preformed toxin.
  • Disease entirely due to effects of   toxin.
  • Infant botulism results from ingestion of organisms and production of toxin in infant’s gut.
  • Associated with feeding honey contaminated with spores of Cl. botulinum.
  • Extremely rare: Wound botulism: toxin produced by organisms’ infecting a wound.

 

Laboratory Identification:

  • Requires strictly anaerobic conditions for isolation.
  • Grows on blood agar, but very rarely isolated from human cases of disease.
  • Detection of the toxin in the food or serum from the patient is the way of confirming the diagnosis.

 

Clostridium difficile

 

Characteristics:

  • Slender
  • Gram-positive
  • Anaerobic
  • Rod
  • Spore-former
  • Motile.

 

Transmission:

  • Component of normal gut flora; flourishes under selective pressure of antibiotics.
  • May also be spread from person to person by the fecal-oral route. 

 

Diseases:

  • Pseudomembranous colitis (antibiotic-associated diarrhea) is toxin mediated. It can be rapidly fatal especially in the compromised host.

 

Laboratory Identification:

  • Difficult to isolate in ordinary culture because of overgrowth by other organisms; selective medium containing cefoxitin, cycloserine and fructose is effective.
  • The mere presence of this organism is not indicative of infection, but a marker to note.
  • Diagnosis by detection of toxin in feces is practicable.