Improving colistin activity against Pseudomonas aeruginosa biofilms

Colistin is the last-resort therapeutic option in infections by multi-drug resistant pathogens and for this it has been included by World Health Organization among the “highest priority critically important antimicrobials”. Colistin binding to outer membrane (OM) of Gram negative bacteria leads to membrane destabilization, increasing permeability and finally cell death. OM remodeling, by chemical groups that reduce its negative charge, induces resistance to colistin. In Pseudomonas aeruginosa addition of aminoarabinosyl residues (Ara4N) to the lipid-A is the major pathway leading to colistin resistance, whose last step is catalyzed by the aminoarabinosyl transferase ArnT. We have previously identified a natural compound, BBN149 (renamed FDO), as putative inhibitors of ArnT, which potentiates colistin activity against planktonic colistin-resistant P. aeruginosa strain (Ghirga et al., 2020, doi:10.1093/jac/dkaa200). As biofilms are relevant to the persistence of bacterial infection, we tested the colistin adjuvant activity of FDO, and its derivative FDO-H, on P. aeruginosa biofilms. First, biofilms were grown in medium containing colistin, and the resulting biofilms have been quantified by crystal violet, metabolic assay and viable cell counting. The resulting data showed a dose-dependent reduction in metabolic activity, total biomass and cell viability of biofilms treated with colistin. In biofilms treated with colistin and FDO or FDO-H, viable cells counting showed the adjuvant activity of both compounds with one log reduction of CFU in sample treated with colistin+compound respect to colistin only. Importantly, similar results were obtained with an in vitro evolved P. aeruginosa reference strain (PA14 colR5, doi:10.1128/AAC.01820-17) and with a clinical colR isolate. Collectively, our data suggest that FDO and FDO-H potentiate colistin activity against P. aeruginosa biofilms.