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 0.25¾MIC 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.

The ability of fidaxomicin to suppress early exotoxin production and endospore formation by historical and epidemic strains of C. difficile may explain its clinical success in treating severe and recurrent cases of CDI disease.