Comparative activities of colistin, rifampicin, imipenem and sulbactam/ampicillin alone or in combination against epidemic multidrug-resistant Acinetobacter baumannii isolates producing OXA-58 carbapenemases

This study evaluated the activity of colistin, rifampicin, imipenem and sulbactam/ampicillin alone or in combination against nine epidemic multidrug-resistant Acinetobacter baumannii isolates producing OXA-58 carbapenemase in Naples, Italy. The isolates were susceptible to colistin but differed in their resistance to imipenem and rifampicin. Time–kill studies showed a bactericidal effect for colistin but not for imipenem, rifampicin or sulbactam/ampicillin used as single agents. Synergism was observed with combinations of rifampicin + imipenem or sulbactam/ampicillin for all isolates and with colistin + rifampicin for isolates showing higher minimum inhibitory concentrations for rifampicin. Combined use of the antimicrobials tested may provide good therapeutic options for OXA-58 carbapenemase-producing A. baumannii infections.