Mixed infection by Clostridium difficile in an in vitro model of the human gut.

OBJECTIVES Clostridium difficile infection (CDI) is still a major clinical challenge. Previous studies have demonstrated multiple distinct C. difficile strains in the faeces of patients with CDI; yet whether true mixed CDI occurs in vivo is unclear. In this study we evaluated whether two distinct C. difficile strains could co-germinate and co-proliferate in an in vitro human gut model. METHODS An in vitro triple-stage chemostat was used to study the responses of two PCR ribotype 001 C. difficile strains following exposure to ceftriaxone at concentrations observed in vivo (7 days). C. difficile viable counts (vegetative and spore forms), cytotoxin titres and indigenous microflora viable counts were monitored throughout the experiment. RESULTS Both C. difficile strains germinated and proliferated following exposure to ceftriaxone. Cytotoxin production was detected in the gut model following C. difficile spore germination and proliferation. Ceftriaxone elicited reduced viable counts of Bifidobacterium spp. and elevated viable counts of Enterococcus spp. CONCLUSIONS These data suggest that multiple C. difficile strains are able to proliferate concurrently in an in vitro model reflective of the human colon. Previous studies in the gut model have reflected clinical observations so clinicians should be mindful of the possibility that multiple C. difficile strains may infect patients. These observations augment recent human epidemiological studies in this area.