The anthelmintic oxyclozanide restores the activity of colistin against colistin-resistant Gram-negative bacilli

ABSTRACT Due to the significant increase in antimicrobial resistance in Gram-negative bacilli (GNB), development of non-antimicrobial therapeutic alternatives, which can be used together with the few and non-optimal available antimicrobial agents such as colistin, has become an urgent need. In this context, dysregulation of the bacterial cell wall could be a therapeutic adjuvant to the activity of colistin. The aim of this study was to analyse the activity of oxyclozanide, an anthelmintic drug, in combination with colistin against colistin-susceptible (Col-S) and colistin-resistant (Col-R) GNB. Three Col-S reference strains and 13 clinical isolates (1 Col-S, 12 Col-R) of Acinetobacter baumannii, Pseudomonas aeruginosa and Klebsiella pneumoniae were studied. Microdilution assays and time–kill curves were performed to examine the activity of oxyclozanide in combination with colistin. The outer membrane protein (OMP) profile, membrane permeability and cell wall structure of Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide were assessed by SDS-PAGE, fluorescence microscopy and transmission electron microscopy, respectively. Oxyclozanide in combination with colistin increased the activity of colistin against Col-S and Col-R A. baumannii, P. aeruginosa and K. pneumoniae. Time–kill curves showed synergistic activity between oxyclozanide and colistin against these bacterial isolates. Moreover, Col-R A. baumannii, P. aeruginosa and K. pneumoniae in the presence of oxyclozanide presented greater permeability and disruption of their cell wall than Col-S strains, without modification of their OMP profile. These data suggest that combination of oxyclozanide and colistin may be a new alternative for the treatment of Col-R GNB infections.