Cross-talk between the WalKR system and vancomycin resistance in Enterococcus faecium

Enterococcus faecium is an important antibiotic-resistant pathogenic bacterium. The antibiotic vancomycin is widely used for the treatment of infections caused by E. faecium, but is not always effective, even if strains causing infections are phenotypically susceptible to vancomycin. To understand how E. faecium responds to exposure to vancomycin, we performed in vitro evolution experiments, coupled to whole-genome sequencing, to map the evolutionary trajectory upon exposure to vancomycin. Through these experiments, we identified mutations in the genes encoding the essential bacterial two-component system WalKR were associated with higher minimum inhibitory concentrations of vancomycin. We characterised mutants in walKR and found they had larger cell walls which may act as a 'sponge' to absorb vancomycin. We also observed a chaining phenotype, which indicates differences in the cell wall of the mutants in walKR and the wild-type strain. Subsequent studies have implemented a variety of functional genomic and biochemical techniques to map the impact of walKR mutations on E. faecium physiology and elucidate the genes that are regulated by the WalKR system in E. faecium. The project highlights the pathways through which E. faecium can adapt to the presence of antibiotics in its environment, which contributes to its recent emergence as an important cause of hospital-acquired infections.