WGS study identifying laboratory mutations that encode colistin resistance in Acinetobacter baumannii. The induction and identification of novel Colistin resistance mutations in Acinetobacter baumannii and their potential implications

Acinetobaceter baumannii is an important cause of opportunistic hospital acquired infection. A. baumannii has been grouped with other Gram negative bacteria in the ESKAPE pathogens (Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa, and Enterobacter species) due to its importance and high levels of antimicrobial resistance. Multidrug resistant A. baumannii has rapidly risen in Vietnam, where colistin is becoming the drug of last resort for many infections. In this study we generated spontaneous colistin resistant daughter isolates (up to 2,048 µg/µl) from four colistin susceptible Vietnamese isolates and susceptible reference strains (MIC <1.5 µg/µl). Whole genome sequencing was used to identify single nucleotide mutations that could be attributed to the reduced colistin susceptibility. We identified six lpxACD and three pmrB mutations, the majority of which were novel. In addition we identified further mutations in six A. baumannii genes (vacJ, pldA, ttg2C, pheS and ABBFA_00135) that we hypothesise have a role in reduced colistin susceptibility. This study has expanded the compendium of known colistin resistance mutations and potentially alternative mechanisms of resistance. Further our work demonstrates how rapidly A. baumannii can generate resistance to what is often a drug of last chance and highlights the need for improved surveillance or risk the doomsday scenario of wholly drug resistant A. baumannii.