Gallium: an antimicrobial not prone to select for resistance?

Gallium-mediated siderophore quenching as an evolutionarily robust antibacterial treatment Adin Ross-Gillespie, Michael Weigert, Sam P. Brown, and Rolf Kümmerli EMPH
Published 30 January 2014, 10.1093/emph/eou003 

Background and objectives: Conventional antibiotics select strongly for resistance and are consequently losing efficacy worldwide. Extracellular quenching of shared virulence factors could represent a more promising strategy because (a) it reduces the available routes to resistance (since extracellular action precludes any mutations blocking a drug’s entry into cells or hastening its exit) and (b) it weakens selection for resistance, since fitness benefits to emergent mutants are diluted across all cells in a cooperative collective. Here, we tested this hypothesis empirically.

Methodology: We used gallium to quench the iron-scavenging siderophores secreted and shared among pathogenic Pseudomonas aeruginosa bacteria, and quantitatively monitored its effects on growth in vitro. We assayed virulence in acute infections of caterpillar hosts (Galleria mellonella), and tracked resistance emergence over time using experimental evolution.

Results: Gallium strongly inhibited bacterial growth in vitro, primarily via its siderophore quenching activity. Moreover, bacterial siderophore production peaked at intermediate gallium concentrations, indicating additional metabolic costs in this range. In vivo, gallium attenuated virulence and growth – even more so than in infections with siderophore-deficient strains. Crucially, while resistance soon evolved against conventional antibiotic treatments, gallium treatments retained their efficacy over time.

Conclusions: Extracellular quenching of bacterial public goods could offer an effective and evolutionarily robust control strategy.