The role of toxin A and toxin B in Clostridium difficile infection

Clostridium difficile toxins revisited Clostridium difficile , the most common cause of infectious diarrhoea in hospitals in Europe and North America, produces two toxins. Their relative importance has been widely debated, and although animal studies had indicated that purified toxin A alone can induce most of the pathology observed in C. difficile infections, a recent Nature paper ( http://go.nature.com/oh6un5 ) suggested that the other toxin, toxin B, was the main cause of disease symptoms. Now a new study, involving C. difficile strains producing either toxin A or toxin B alone and — for the first time — a double-mutant strain producing neither, demonstrates that both toxins are important for disease, and need to be considered for diagnosis and treatment. Clostridium difficile , an important nosocomial pathogen, produces two toxins. Studies with purified toxins have indicated that only toxin A is important for pathogenesis, but recently it has been suggested that toxin B causes the majority of the disease symptoms in the context of a bacterial infection. These authors demonstrate that both toxins are important for disease and will need to be considered for diagnosis and treatment. Clostridium difficile infection is the leading cause of healthcare-associated diarrhoea in Europe and North America 1 , 2 . During infection, C. difficile produces two key virulence determinants, toxin A and toxin B. Experiments with purified toxins have indicated that toxin A alone is able to evoke the symptoms of C. difficile infection, but toxin B is unable to do so unless it is mixed with toxin A or there is prior damage to the gut mucosa 3 . However, a recent study indicated that toxin B is essential for C. difficile virulence and that a strain producing toxin A alone was avirulent 4 . This creates a paradox over the individual importance of toxin A and toxin B. Here we show that isogenic mutants of C. difficile producing either toxin A or toxin B alone can cause fulminant disease in the hamster model of infection. By using a gene knockout system 5 , 6 to inactivate the toxin genes permanently, we found that C. difficile producing either one or both toxins showed cytotoxic activity in vitro that translated directly into virulence in vivo . Furthermore, by constructing the first ever double-mutant strain of C. difficile, in which both toxin genes were inactivated, we were able to completely attenuate virulence. Our findings re-establish the importance of both toxin A and t