In vitro colistin susceptibility of pandrug-resistant Ac. baumannii is restored in the presence of selenium nanoparticles

Aims To investigate the synergistic activity of colistin and selenium nanoparticles (SeNPs) against pandrug-resistant (PDR) Ac. baumannii. Methods and Results Chequerboard and time-kill assays were employed to explore the potential synergistic interactions between colistin and SeNPs against Ac. baumannii isolates (8), previously determined as colistin-resistant (MIC range 16-256 mu g ml(-1)). Also, whole-genome sequencing (WGS) and gene expression analyses were used to elucidate the mechanisms of colistin resistance. Exceptionally strong synergistic activity (FICI range 0.004-0.035) of colistin and SeNPs against colistin-resistant isolates was revealed. Colistin (0.5 or 1 mu g ml(-1)) used in combination with SeNPs (0.5 mu g ml(-1)) was able to reduce initial inoculum during the first 4 h of incubation, in contrast to colistin (0.5, 1 or 2 mu g ml(-1)) alone. Conclusions These findings propose colistin/SeNPs combination as a new option to fight PDR Ac. baumannii, the therapeutic possibilities of which should be proved in future in vivo studies. Significance and Impact of Study Here we present the first evidence of synergy between colistin and selenium compounds against bacteria in general. Also, WGS and gene expression analyses provide some new insights into Ac. baumannii colistin resistance mechanisms.