Colistin Resistance in Environmental Isolates of Acinetobacter baumannii

Abstract

Although the molecular mechanisms of carbapenem resistance of environmental isolates of Acinetobacter baumannii are well described, data on the mechanisms of colistin resistance are scarce. In this study, we report the molecular mechanisms of colistin resistance in environmental isolates of A. baumannii. Seven clinically relevant isolates of A. baumannii belonging to ST-2Pasteur were recovered from hospital wastewater and wastewater treatment plant. The phenotypic resistance to colistin was confirmed by broth microdilution with minimum inhibitory concentration values ranging from 20 to 160 mg/L. Colistin sulfate and colistimethate sodium showed bactericidal activity against two colistin-heteroresistant isolates in vitro, but substantially recovery of population was observed after prolonged incubation. In silico genome analysis revealed nucleotide variations resulting in amino acid changes in LpxC (N286D), LpxD (E117K), PmrB (A138T, R263S, L267W, Q309P, and A444V), and EptA (F166L, I228V, R348K, A370S, and K531T). According to reverse transcription quantitative PCR, all isolates had increased levels of eptA mRNA and decreased levels of lpxA and lpxD mRNA. Isolates expressed low hydrophobicity, biofilm, and pellicle formation, but showed excellent survival in river water during 50 days of monitoring. Colistin- and pandrug-resistant A. baumannii disseminated in the environment could represent the source for the occurrence of serious community-acquired infections.