Repeated in vitro exposure of Clostridium difficile to metronidazole, vancomycin and linezolid does not result in resistance

Abstract number: 1135_27

Schmidt C., Ackermann G., Rodloff A.C.

Objective:  

Clostridium difficile remains the major cause of nosocomial diarrhoea. First line therapeutics for the treatment of C. difficile associated diarrhoea are metronidazole and vancomycin. Meanwhile impaired susceptibility was observed for both substances, however not directly linked to treatment failures. On the other hand, as many as 20% of patients had at least one recurrence of CDAD after the initial therapy was discontinued. Routine susceptibility testing is performed in few and specialized laboratories only. Therefore data on activity of both substances are limited. The potency of resistance development against vancomycin and metronidazole is not known. Linezolid is a new antimicrobal agent active against gram-positive bacteria.

Methods:  

Selection of resistant mutants of the ATCC strain 43255 (MIC in mg/l for metronidazole, vancomycin, linezolid: 0.064, 0.5, 0.75) was performed on supplemented agar plates containing increasing amounts of the antimicrobials. Bacteria collected from five agar plates (after 48 h growth) were collected and centrifuged. The pellet was re-suspended and exposed to the next higher antimicrobial concentration on agar plates.

Results:  

Repeating this procedure several times 7, 2 and 3 generations of bacteria for metronidazole, vancomycin and linezolid were generated, respectively. The bacteria were finally exposed to 5 mg/l metronidazole 2.0 mg/l vancomycin and 5 mg/l linezolid. Using Etest MHC values of 1.5 (23 × MHC), 2 (4 × MHC) and 2 (2.7 × MHC) mg/l could be confirmed.

Conclusion:  

Higher MHC values but not resistance could be selected in vitro in C. difficile against metronidazole, vancomycin and linezolid. This confirms the clinical observation that C. difficile bacteria resistant to metronidazole or vancomycin do not attribute to therapeutic failures in the treatment of CDAD.