Studies on the resistance of Clostridium difficile to antimicrobial agents.

The susceptibility of C. difficile isolated at the Department of Medical Microbiology of the University of Zurich to a wide selection of antibacterial, antimycobacterial and antifungal agents was tested in vitro. Great differences in susceptibility were found against chloramphenicol, clindamycin, erythromycin, rifamycin, and tetracycline. Resistance to clindamycin and erythromycin could always be transferred jointly to a susceptible C. difficile strain by mixed culture on filters at low frequencies (1 X 10(-8) to 4 X 10(-8) per donor cell). Transfer of tetracycline resistance occurred at frequencies of 3 X 10(-7) to 5 X 10(-7). Chloramphenicol and rifamycin resistance could not be transferred in the system used (frequencies less than 10(-8)). Although a total of 38,000 colonies was screened by various methods known to affect plasmid replication, resistance to chloramphenicol, clindamycin, erythromycin, rifamycin, and tetracycline could not be eliminated. No connection between plasmid DNA and antimicrobial resistance could be established. Especially, no plasmid DNA was involved in the transfer of resistance determinants from resistant to susceptible strains.