Distinct role of outer membrane protein A in the intrinsic resistance of Acinetobacter baumannii and Acinetobacter nosocomialis.

Acinetobacter baumannii outer membrane protein A (AbOmpA) contributes to the intrinsic resistance of A. baumannii through the OmpA-like domain. The present study investigated the role of Acinetobacter nosocomialis OmpA (AnOmpA) in the intrinsic resistance of A. nosocomialis and compared it with the role of AbOmpA. The minimal inhibitory concentrations (MICs) of antimicrobial agents against wild-type A. nosocomialis ATCC 17903, ∆AnompA mutant, and single-copy AnompA-complemented strains were determined by performing E-test or agar dilution. Single-copy ompA cross-complemented strains were constructed by cross-inserting AnompA and AbompA open reading frames (ORFs) along with their native promoters into ∆AbompA and ∆AnompA mutant strains, respectively, and the MICs of antimicrobial agents against these strains were determined. The ∆AnompA mutant of A. nosocomialis was more susceptible to colistin (20.0-fold) and gentamicin (4.8-fold) than the wild-type strain. The MICs of gentamicin and tetracycline against the ∆AnompA mutant did not decrease in the presence of an efflux pump inhibitor. The MIC of trimethoprim against the ∆AnompA mutant harbouring PAbompA and AbompA ORF increased by >4.0-fold compared with that against the wild-type strain. However, the MICs of all the tested antimicrobial agents were similar against the wild-type A. baumannii ATCC 17978 and ∆AbompA mutant harbouring PAnompA and AnompA ORF. These results indicate that AnOmpA contributes to the intrinsic resistance of A. nosocomialis similar to AbOmpA. However, AbOmpA and AnOmpA perform different roles in the intrinsic resistance of trimethoprim.