Nosocomial Outbreak-Derived Serratia marcescens : In Vitro Susceptibility to Antimicrobial Agents in Comparison with Carbapenem-Resistant Strains

Serratia marcescens strains derived from an outbreak of nosocomial septicaemia in Tokyo were tested for their in vitro susceptibilities to 43 antimicrobial agents, in comparison with carbapenemresistant strains isolated from other hospitals in Tokyo. The outbreak-derived strains were highly susceptible to 4th-generation cephems, oxacephems, monobactams, carbapenems, newer fluoroquinolones and trimethoprim, but highly resistant to penicillins, 1st- and 2nd- generation cephems and macrolides, and moderately resistant to many of 3rd-generation cephems, penems, aminoglycosides and sulfamethoxazole. These data markedly contrasted to those of carbapenem-resistant strains isolated in Tokyo that exhibited resistance (or lower susceptibility) to most of the antimicrobial agents except for monobactams and trimethoprim. Effects of β-lactamase inhibitors, and glucose-6-phosphate on the minimum inhibitory concentrations (MICs) of fosfomycin were also examined. The data indicate that the nosocomial outbreak of S. marcescens was not primarily due to bacterial multiple drug resistance, and highlights the importance of control with susceptible antimicrobial agents in the early stages of a blood stream infection.