Effects of ciprofloxacin on the expression and production of exotoxins by Clostridium difficile.

Hypervirulent BI/NAP1/027 strains of Clostridium difficile have been associated with increased mortality of C. difficile infection (CDI). The emergence of highly fluoroquinolone (FLQ)-resistant BI/NAP1/027 strains suggests that FLQ exposure may be a risk factor for CDI development. However, the mechanism for this is not clear. We compared the effects of subinhibitory concentrations of ciprofloxacin on Toxin A and B gene expression and protein production in recent (strain 039) and historical (strain 5325) BI/NAP1/027 clinical isolates with high- and low-level ciprofloxacin resistance, respectively. In the highly ciprofloxacin-resistant isolate (strain 039), ciprofloxacin significantly and dose-dependently increased Toxin A gene expression and shifted its expression to earlier in its growth cycle; TcdB gene expression also increased but was less sensitive to low-dose ciprofloxacin. Maximal Toxin A/B production (4 ng ml(-1)) was increased twofold and occurred significantly earlier than in the untreated control. In strain 5325, ciprofloxacin at 0.25×MIC markedly increased both tcdA and tcdB expression but their temporal dynamics were unchanged. Maximal toxin production (250 ng ml(-1)) was reduced approximately threefold compared with that of the untreated control. These results demonstrate significant differences in ciprofloxacin-induced toxin gene expression and protein production among BI/NAP1/027 isolates, and offer a new paradigm for FLQ-associated CDI caused by recent, highly antibiotic-resistant strains.