The increasing use of polymyxins1 in addition to the dissemination of plasmid-borne colistin resistance threatens to cause a serious breach in our last line of defence against multidrug-resistant Gram-negative pathogens, and heralds the emergence of truly pan-resistant infections. Colistin resistance often arises through covalent modification of lipid A with cationic residues such as phosphoethanolamine—as is mediated by Mcr-1 (ref. 2)—which reduce the affinity of polymyxins for lipopolysaccharide3. Thus, new strategies are needed to address the rapidly diminishing number of treatment options for Gram-negative infections4. The difficulty in eradicating Gram-negative bacteria is largely due to their highly impermeable outer membrane, which serves as a barrier to many otherwise effective antibiotics5. Here, we describe an unconventional screening platform designed to enrich for non-lethal, outer-membrane-active compounds with potential as adjuvants for conventional antibiotics. This approach identified the antiprotozoal drug pentamidine6 as an effective perturbant of the Gram-negative outer membrane through its interaction with lipopolysaccharide. Pentamidine displayed synergy with antibiotics typically restricted to Gram-positive bacteria, yielding effective drug combinations with activity against a wide range of Gram-negative pathogens in vitro, and against systemic Acinetobacter baumannii infections in mice. Notably, the adjuvant activity of pentamidine persisted in polymyxin-resistant bacteria in vitro and in vivo. Overall, pentamidine and its structural analogues represent unexploited molecules for the treatment of Gram-negative infections, particularly those having acquired polymyxin resistance determinants.
The authors thank K. Iyer and L. Carfrae for assistance with mouse infection experiments, and M. Mulvey from the University of Manitoba for providing the environmental mcr-1-positive E. coli isolates. This work was supported by Discovery and Foundation grants from the Natural Sciences and Engineering Research Council and the Canadian Institutes of Health Research (FDN-143215) to E.D.B., by grants from Cystic Fibrosis Canada and the Ontario Research Fund to E.D.B., by a grant from the Michael G. DeGroote Institute for Infectious Disease Research to E.D.B. and B.K.C., by an operating grant from the Canadian Institutes of Health Research to B.K.C. (MOP-82704), by a Foundation grant from the Canadian Institutes of Health Research to C.W. (FDN-CEHA-26119), by salary awards to E.D.B., B.K.C. and C.W. from the Canada Research Chairs Program, by a fellowship from the Fonds de reserche en santé du Québec to J.-P.C., by a fellowship from the Canadian Institutes of Health Research DSECT Program to S.F., by a scholarship from the Ontario Graduate Scholarships Program to C.R.M. and by scholarships to J.M.S. from the Canadian Institutes of Health Research and the Ontario Graduate Scholarships Program.
E. coli Keio collection gene deletion mutants that displayed sensitivity to novobiocin, rifampicin, and/or erythromycin at 37 °C, and/or resistance to vancomycin at 15 °C.
Gene ontology, biosynthetic pathway andpromoter activation enrichment by the vancomycin suppression screen of the E. coli Keio collection at 15 °C.
Screen of 1,440 previously approved drugs against E. coli BW25113 at 15 °C in the presence of 16 μg/ml vancomycin.
FIC indices of pentamidine/rifampicin combinations against Gram-negative clinical isolates from the Wright Clinical Collection.
Activity of polymyxin B against naturally resistant clinical isolates.
Characterization of spontaneous pentamidine/rifampicin suppressor mutants.