Targeting the tcdA gene: is this appropriate for detection of A and/or B Clostridium difficile toxin-producing strains?

摘要

In a recent article (3), Noren et al. reported an extensive evaluation of illumigene Clostridium difficile compared with the cell culture cytotoxin B assay (CTBA) and/or toxigenic culture (TC) of cytotoxin-producing C. difficile isolates. The illumigene assay utilizes loop-mediated isothermal DNA amplification (LAMP) technology to detect the pathogenicity locus (PaLoc) of toxigenic Clostridium difficile. The authors reported a sensitivity of 98 (49/50 isolates) and a specificity of 98 (218/222 isolates) for the illumigene assay, using combined CTBA plus TC as the gold standard. While demonstrating good clinical performance, the authors raised the question of whether the toxin A fragment amplified by the illumigene assay is the optimal target for detecting toxin A-negative/toxin B-positive (A~+ B~-) strains. The Clostridium difficile PaLoc segment encodes both the toxin A gene (tcdA) and the toxin B gene (tcdB), has conserved border regions, and is found at the same site on the C. difficile genome for all toxigenic strains. Both the tcdA and the tcdB genes have similar structures consisting of three distinct fragments: a catalytic domain, a putative translocation domain, and a repetitive domain (4). The repetitive regions are prone to homologous recombination resulting in various deletions at the 3' ends of the toxin genes. For example, the well-characterized A~ -B+ strains (toxinotypes VI and VII) and the most frequently occurring strains (toxinotypes VIII and X) all have various deletions at the 3' end of the tcdA gene. However, the 5' portion of the tcdA gene remains intact for all of these strains.