Pseudomembranous colitis caused by toxin A-negative/toxin B-positive variant strain of Clostridium difficile.

We report the first case of pseudomembranous colitis (PMC) due to a toxin A-negative, toxin B-positive strain (toxin A variant strain) of Clostridium difficile in Japan. The toxin A variant strain of C. difficile is capable of causing PMC and is undetectable in clinical laboratories that use only toxin A immunoassays for C. difficile testing. If C. difficile-associated diarrhea is clinically suspected and toxin A is not detected, then the possibility of a toxin A variant strain should be considered, and further diagnostic testing, such as polymerase chain reaction (PCR) analysis for the detection of C. difficile toxin genes should be performed.