Thiostrepton hijacks pyoverdine receptors to inhibit growth ofPseudomonas aeruginosa

Abstract

ABSTRACTPseudomonas aeruginosais a biofilm-forming opportunistic pathogen and intrinsically resistant to many antibiotics. In a high-throughput screen for molecules that modulate biofilm formation, we discovered that the thiopeptide antibiotic, thiostrepton (TS) - considered inactive against Gram-negative bacteria - stimulatedP. aeruginosabiofilm formation in a dose-dependent manner. This phenotype is characteristic of exposure to antimicrobial compounds at sub-inhibitory concentrations, suggesting that TS was active againstP. aeruginosa. Supporting this observation, TS inhibited growth of a panel of 96 multidrug-resistant (MDR)P. aeruginosaclinical isolates at low micromolar concentrations. TS also had activity againstAcinetobacter baumanniiclinical isolates. Expression of Tsr - a 23S rRNA-modifying methyltransferase - in trans conferred TS resistance, confirming that the drug acted via its canonical mode of action, inhibition of ribosome function. Deletion of oligopeptide permease systems used by other peptide antibiotics for uptake failed confer TS resistance. TS susceptibility was inversely proportional to iron availability, suggesting that TS exploits uptake pathways whose expression is increased under iron starvation. Consistent with this finding, TS activity againstP. aeruginosaandA. baumanniiwas potentiated by FDA-approved iron chelators deferiprone and deferasirox. Screening ofP. aeruginosamutants for TS resistance revealed that it exploits pyoverdine receptors FpvA and FpvB to cross the outer membrane. Our data show that the biofilm stimulation phenotype can reveal cryptic sub-inhibitory antibiotic activity, and that TS has activity against select multidrug resistant Gram-negative pathogens under iron-limited growth conditions, similar to those encountered at sites of infection.