Polymyxins are amongst the most important
antibiotics in modern medicine, in recent times their clinical utility has been overshadowed by nosocomial outbreaks of
polymyxin resistant MDR Gram-negative 'superbugs'. An effective strategy to surmount
polymyxin resistance is combination
therapy with FDA-approved non-
antibiotic drugs. Herein we used untargeted metabolomics to investigate the mechanism(s) of synergy between
polymyxin B and the
selective estrogen receptor modulator (
SERM)
tamoxifen against a
polymyxin-resistant MDR
cystic fibrosis (CF) Pseudomonas aeruginosa FADDI-PA006 isolate (
polymyxin B MIC=8 mg/L , it is an MDR
polymyxin resistant P. aeruginosa isolated from the lungs of a CF patient). The metabolome of FADDI-PA006 was profiled at 15 min, 1 and 4 h following treatment with
polymyxin B (2 mg/L),
tamoxifen (8 mg/L) either as monotherapy or in combination. At 15 min, the combination treatment induced a marked decrease in
lipids, primarily
fatty acid and
glycerophospholipid metabolites that are involved in the biosynthesis of bacterial membranes. In line with the
polymyxin-resistant status of this strain, at 1 h, both
polymyxin B and
tamoxifen monotherapies produced little effect on bacterial metabolism. In contrast to the combination which induced extensive reduction (≥ 1.0-log2-fold, p ≤ 0.05; FDR ≤ 0.05) in the levels of essential intermediates involved in cell envelope biosynthesis. Overall, these novel findings demonstrate that the primary mechanisms underlying the synergistic bactericidal effect of the combination against the
polymyxin-resistant P. aeruginosa CF isolate FADDI-PA006 involves a disruption of the cell envelope biogenesis and an inhibition of
aminoarabinose LPS modifications that confer
polymyxin resistance.