Insights into the mechanisms underlying cell wall-active agents and gentamicin bactericidal synergism against Enterococcus faecalis

BACKGROUND: Enterococcus faecalis is a common cause of healthcare-associated infections. They characteristically exhibit reduced susceptibility to penicillins and elevated MICs for aminoglycosides, limiting these drugs as single-agent therapies. Combinations of cell wall synthesis inhibitors and aminoglycosides have a synergistic effect, resulting in bactericidal activity. The mechanism behind this synergism is not fully understood. OBJECTIVES: The present study was performed to explore the relationship between synergistic activity between cell wall-active agents/aminoglycoside combinations and cell membrane energetics in E. faecalis. METHODS: Analysis was performed using reference broth microdilution MIC testing, checkerboard assays, time-kill assays and fluorescent microscopy. RESULTS: We observed that cell wall remodelling agents promoted aminoglycoside uptake facilitated by antibiotic-induced decreases in the intracellular pH (pHi). CONCLUSIONS: Drugs that inhibit cell wall synthesis induce a decrease in the pHi, suggesting that cell wall remodelling is linked to ion transport and cytoplasmic acidification. Our study supports a model in which inhibition of peptidoglycan synthesis produces membrane stress-associated intracellular acidification that promotes ΔpH-dependent aminoglycoside uptake in E. faecalis.