Variant forms of the binary toxin CDT locus and tcdC gene in Clostridium difficile strains.

Variability in the genes for toxin A, toxin B and other pathogenicity locus regions is well known and is the basis for the distribution of Clostridium difficile strains into variant toxinotypes. Previous data have indicated that some C. difficile strains have a non-functional truncated form of the binary toxin (CDT) locus. This study analysed variability in the CDT locus and the presence of deleted tcdC genes in C. difficile strains. A total of 146 strains were screened, including known variant toxinotypes and non-variant A+B+ (toxinotype 0) and A-B- C. difficile strains. In all of the strains studied, only two forms of the CDT locus were found: a full-length 4.3 kb fragment encoding the functional binary toxin or a truncated 2.3 kb fragment. Whilst the full-length CDT locus was found almost exclusively in variant toxinotypes, the truncated form was detected in 79% of toxinotype 0 strains. Non-toxinogenic A-B- strains with a truncated version were not found and only rarely possessed the full-length CDT locus (A-B-CDT+ strains). Four different forms of the tcdC gene were found; three represented deleted versions and typically were found in toxinotypes III-VII, XI, XIV-XVI and XXIV.