Prevalence and analysis of microbiological factors associated with phenotypic heterogeneous resistance to carbapenems in Acinetobacter baumannii.

The objectives of this study were to determine the prevalence of Acinetobacter baumannii with phenotypic heterogeneous resistance (PHR) to carbapenems (colonies inside the halo of inhibition) and to analyse its association with several microbiological variables. Acinetobacter baumannii isolates collected in Spain were used to analyse: (i) minimum inhibitory concentrations (MICs) of carbapenems; (ii) heteroresistance to carbapenems; (iii) genes encoding β-lactamases (bla genes); (iv) insertion sequences; and (v) inactivation of genes encoding porins (CarO, OprD and Omp33-36) and genes associated with the AdeABC efflux system (adeB, adeR and adeS). Polymerase chain reaction (PCR) amplification was used for gene detection. The rate of PHR was 20% to imipenem and 24% to meropenem. Susceptibility to imipenem was observed in 39% of PHR isolates. MICs of carbapenems for colonies were similar (± 1 log(2) dilution) to those of their parental isolates. These colonies growing inside the inhibition halo also reproduced the PHR to carbapenems. Differences observed between PHR isolates and non-PHR isolates were: bla(OXA-58-like), 57% vs. 0%; oprD-like, 96% vs. 56%; adeB, 89% vs. 94%; adeR, 82% vs. 94%; adeS, 82% vs. 94%; ISAba2, 61% vs. 31%; and ISAba3, 57% vs. 0%. No interruption of genes encoding porins or the efflux-related genes (adeB, adeR and adeS) was observed. In conclusion, A. baumannii strains with PHR to carbapenems are widespread in Spain. This phenotype is present in carbapenem-susceptible isolates as well as those that are not susceptible to carbapenems. Heteroresistance cannot explain the PHR to carbapenems, which appears to relate more to persistence or tolerance to carbapenems. bla(OXA-58-like), bla(OXA-51-like), ISAba2 and ISAba3 are associated with PHR to carbapenems. Inactivation of genes encoding porins or genes related to AdeABC is infrequent.