Which Toxin Causes C. difficile Disease?

Clostridium difficile produces two virulence factors, toxins A and B, that have been implicated in the development of colitis. Although studies using purified toxins have suggested that C. difficile–associated disease (CDAD) is caused primarily by toxin A, clinical isolates from some patients with CDAD have yielded strains that produced only toxin B. To further delineate the roles of these toxins, a multinational team of investigators developed and studied isogenic mutants of a virulent C. difficile strain.

Using molecular techniques, the researchers created two sets of C. difficile mutants that lacked either toxin A (A–) or toxin B (B–). In vitro analysis confirmed that the A– mutants produced only toxin B and the B– mutants produced only toxin A. In a hamster model, both A– mutants produced severe CDAD similar to that seen with wild-type C. difficile, with 16 of 17 (vs. 9 of 10) colonized animals dying. In contrast, only 4 of 19 animals colonized with the B– mutants died. Testing of C. difficile isolates obtained from the four animals that died showed that some of the B– mutants had reverted back to wild-type and produced toxin B.