Evolution of polyamine resistance in Staphylococcus aureus through modulation of potassium transport

Microbes must adapt to diverse biotic and abiotic factors encountered in host environments. Polyamines are an abundant class of aliphatic molecules that play essential roles in fundamental cellular processes across the tree of life. Surprisingly, the bacterial pathogen Staphylococcus aureus is highly sensitive to polyamines encountered during infection, and acquisition of a polyamine resistance locus has been implicated in the spread of the prominent USA300 methicillin-resistant S. aureus lineage. At present, alternative pathways of polyamine resistance in staphylococci are largely unknown. Here, we applied experimental evolution to identify novel mechanisms and consequences of S. aureus adaptation when exposed to increasing concentrations of the polyamine spermine. Evolved populations of S. aureus exhibited striking evidence of parallel adaptation, accumulating independent mutations in the potassium transporter genes ktrA and ktrD. Mutations in either ktrA or ktrD are sufficient to confer polyamine resistance and function in an additive manner. Moreover, we find that ktr mutations provide protection against multiple classes of unrelated cationic antibiotics, suggesting a common mechanism of resistance. Consistent with this hypothesis, ktr mutants exhibit alterations in cell surface charge indicative of reduced affinity and uptake of cationic molecules. Finally, we observe that laboratory-evolved ktr mutations are also present in diverse natural S. aureus isolates, suggesting these mutations contribute to antimicrobial resistance during human infections. Collectively, this study identifies a new role for the S. aureus potassium transport system in resistance to both host-derived and clinically used antimicrobials.

Staphylococcus aureus is a leading cause of infectious disease-related deaths globally. Understanding factors that govern adaptation and survival of S. aureus and other pathogens in the host environment is critical for improving infection outcomes. It has been known for several years that S. aureus is highly sensitive to polyamines, a broadly produced class of molecules that play important cellular functions across bacteria and eukaryotes. How S. aureus is capable of adapting to polyamine toxicity remains largely mysterious. Using experimental evolution, our study reveals that changes in potassium transport are sufficient to confer high-level polyamine resistance in S. aureus while simultaneously increasing resistance to unrelated classes of clinically used antibiotics. Our results identify new roles for bacterial potassium transport in polyamine resistance as well as highlighting the utility of experimental evolution for identifying new genetic determinants of pathogen adaptation.