The role of toxin A and toxin B in Clostridium difficile-associated disease: Past and present perspectives

Abstract

Recently, we constructed and characterized isogenic tcdA and tcdB mutants of a virulent Clostridium difficile strain and used a hamster model of disease to demonstrate that toxin B, not toxin A, is essential for virulence of this emerging pathogen. Earlier studies had shown that purified toxin A alone was able to induce C. difficile disease pathology and that purified toxin B was not effective unless it was co-administered with toxin A, suggesting that the toxins act synergistically. In this addendum we discuss this paradigm-shifting conclusion in the context of current strain epidemiology, particularly with respect to naturally occurring toxin A-negative, toxin B-positive isolates and the NAP1/027 epidemic isolates. The role of toxin receptors and how variant toxins might exert their effects is also discussed in relation to the published data. We conclude that it is critical to use the natural infection process to dissect the role of toxins in disease, and that future studies are contingent on such work. The impact and importance of animal models of C. difficile virulence are therefore considered within this frame of reference.

Figure 1

Genetic organization of PaLoc and toxin structure. (A) Genetic organization of PaLoc from C. difficile. XbaI sites (X) are shown, with the PaLoc coordinate indicated, and red ovals show the gene-specific promoters and arrows the direction of transcription. (B) Structural organization of toxin B (toxin A is similar). Coordinates indicate the amino acid number. Relevant domains are shown. Figure modified from ref. .

Figure 2

Mechanism of action of toxin A and toxin B. These toxins glucosylate Rho GTPases, which control many cell signalling pathways, resulting in numerous functional consequences for intoxicated cells. Figure modified from ref. .

Figure 3

Comparative analysis of toxin production by wild-type and toxin mutant C. difficile strains. The wild type is JIR8094 (red bars) and the avirulent strain is CD37, which does not produce toxins. The toxin A mutants are designated as tcdA©1 and tcdA©2 (green bars), the toxin B mutants are tcdB©1 and tcdB©2 (blue bars). Toxin A and toxin B cytotoxicity assays using HT-29 (A) and Vero (B) cells, respectively. Data represent the mean ± s.e.m.; n = 3. Figure modified from ref. .

Figure 4

Kaplan-Meier survival curve demonstrating days from colonization with C. difficile to hamster death. Figure modified from ref. .