Heteroresistance to Meropenem in Carbapenem-Susceptible Acinetobacter baumannii

The characteristics of carbapenem heteroresistance were studied in 14 apparently carbapenem-susceptible Acinetobacter baumannii isolates. The MICs for carbapenems were determined, and the isolates were genotyped by pulsed-field gel electrophoresis (PFGE) and sequence typing (ST). Population analysis, testing of the stability of the heteroresistant subpopulations, and time-killing assays were performed. The agar dilution MICs of both imipenem and meropenem for the native isolates ranged from 0.25 to 4 mg/liter. The isolates belonged to nine PFGE types and exhibited seven ST allelic profiles. Population analysis revealed subpopulations that grew in the presence of imipenem at concentrations of up to 8 mg/liter and meropenem at concentrations of up to 32 mg/liter. The meropenem-heteroresistant subpopulations of 11 isolates exhibited stable resistance with MICs that ranged from 16 to >32 mg/liter; their PFGE profiles were identical to those of the native isolates. Time-kill assays with meropenem revealed less pronounced killing for 10 isolates. These findings indicate that meropenem pressure can produce meropenem-heteroresistant subpopulations that might subsequently select for highly resistant strains.Acinetobacter baumannii is an opportunistic pathogen associated with severe hospital infections (1, 16), which often need the use of carbapenems as the treatment of last resort. However, reduced susceptibility or resistance to carbapenems is increasingly being observed among A. baumannii clinical isolates (2, 5, 25).In a preliminary work, we reported on the growth of distinct colonies within the inhibition halo around carbapenem disks or Etest strips and assessed these colonies as having the phenotypic manifestation of carbapenem heteroresistance (17). Likewise, a recent study (12) has described colistin-heteroresistant subpopulations among apparently colistin-susceptible A. baumannii clinical isolates, implying an intrinsic potential of the species to overcome drug pressure. These observations suggest that the use of carbapenems or colistin to treat severe multidrug-resistant A. baumannii infections may lead to the development of resistance.The present study aimed to evaluate in vitro the incidence of carbapenem heteroresistance, to characterize the traits of the heteroresistant subpopulations, and to assess the efficacies of carbapenems against heteroresistant A. baumannii isolates. The study included carbapenem-susceptible A. baumannii clinical isolates that cause infections; such infections are commonly treated with carbapenems, although it is recognized that the heterogeneity of resistance might affect the outcome. The study also determined how the heterogeneity of resistance might affect the treatment outcome.