Improved antimicrobial activity of colistin in combination with putative ArnT inhibitors in Pseudomonas aeruginosa biofilms

Biofilm is an adaptive strategy by which bacterial cells become embedded within an extracellular matrix that mediates surface adhesion and decreases drug penetration. In this environment tolerance to many antibiotics, including colistin, is favored. Colistin, which is the last-resort therapeutic option in infections by multi-drug resistant pathogens Gram-negative bacteria, interacts electrostatically with their outer membrane (OM) leading to membrane destabilization, increased permeability, and cell death. Colistin resistance is mediated by OM remodeling, which in Pseudomonas aeruginosa is mainly mediated lipid A aminoarabinosylation, whose last step is catalyzed by the aminoarabinosyl transferase ArnT. We have previously demonstrated that putative inhibitors of ArnT potentiate the activity of colistin against colistin resistant P. aeruginosa strains (doi:10.1093/jac/dkaa200; doi:10.1021/acs.joc.0c01459). Taking into consideration that the activity of colistin is impaired in biofilms, we have analyzed the expression of ArnT in biofilms, as compared to planktonic cells, and the colistin adjuvant activity of two putative inhibitors of ArnT, FDO and its derivative FDO-H. Biofilms were developed from two different colistin resistant strains, one in vitro evolved reference strain and one clinical isolate. Interestingly, ArnT expression appeared higher in biofilms respect to planktonic cells. Additionally, biofilms treated with colistin in combination with FDO or FDO-H showed reduced metabolic activity and viable cells counting respect to samples treated with colistin-only. Collectively, our data suggest that upregulation of ArnT may contribute to the reduced colistin activity in biofilms and that FDO and FDO-H potentiate colistin activity against P. aeruginosa biofilms.