Objectives: To examine the development of rifampicin resistance in multidrug-resistant Acinetobacter baumannii exposed to rifampicin and the prevention of the appearance of rifampicin-resistant mutants when rifampicin is used in association with imipenem or sulbactam.
Methods: A clinical strain of multidrug-resistant A. baumannii was used to examine the frequency of resistance to rifampicin in vivo, in a pneumonia model in immunocompetent C57BL/6 mice. The in vitro and in vivo prevention of the development of resistance to rifampicin was analysed using rifampicin alone or in association with imipenem or sulbactam, in time-kill studies and in the experimental murine pneumonia, respectively.
Results: Rifampicin-resistant mutants were found at 48 and 72 h, both in vitro and in vivo, when rifampicin was used alone, with the MIC increasing from 4 to > or =128 mg/L. The in vivo frequency of rifampicin-resistant mutants was 3 x 10(-6). On the contrary, no resistant mutants appeared after 72 h, in vitro or in vivo, when rifampicin was employed in association with imipenem or sulbactam. After six daily passages in rifampicin-free agar plates the resistant mutants maintained the high resistance to rifampicin (> or =128 mg/L).
Conclusions: These results suggest that rifampicin must not be used alone in the treatment of infections caused by multidrug-resistant A. baumannii. In these cases, rifampicin may be used in combination with imipenem or sulbactam, which prevent the development of resistance to rifampicin.