Loss of LPS is involved in the virulence and resistance to colistin of colistin-resistant Acinetobacter nosocomialis mutants selected in vitro

Acinetobacter nosocomialis has increasingly been reported as an opportunistic pathogen causing nosocomial infections. Although it is more susceptible to all antimicrobial agents than Acinetobacter baumannii, MDR clinical isolates have also been described. In addition, several studies have shown a high percentage of resistance to colistin. Therefore, in the present study we investigated the mechanism of resistance to colistin in this microorganism. Colistin-resistant strains were selected from the original colistin-susceptible A. nosocomialis strain following multi-step mutant selection. Comparative genomic and proteomic analyses of both colistin-susceptible and colistin-resistant A. nosocomialis strains were performed. In addition, virulence was investigated using the Caenorhabditis elegans assay. The colistin-resistant mutants selected showed a lower resistance profile for other types of antibacterial agents together with a significant decrease in virulence. The LT50 (i.e. time required to kill 50% of the nematodes) for the colistin-susceptible strain (WT) was 7 days compared with 9 days for the colistin-resistant strain (256) (P