Enterococcus faecalis-derived lactic acid suppresses macrophage activation to facilitate persistent and polymicrobial wound infections

Macrophage activation is essential for innate immunity and antimicrobial defense. We show that Enterococcus faecalis suppresses macrophage activation through lactic-acid-mediated acidification of the extracellular environment, enabling pathogen persistence. E. faecalis-derived lactic acid acts via the lactate transporter monocarboxylate transporter 1 (MCT-1) and the sensor GPR81 to initiate complementary mechanisms that collaboratively reduce nuclear factor κB (NF-κB) activity. Lactic acid acts through MCT-1 to inhibit extracellular signal-regulated kinase and STAT3 phosphorylation, leading to reduced levels of the adaptor MyD88 involved in NF-κB activation. Lactic acid signaling to GPR81 induces phosphorylation of the transcription factor YAP, ultimately attenuating NF-κB signaling. A bacterial mutant lacking lactate dehydrogenase is unable to acidify the environment and thus fails to inhibit NF-κB. In a murine wound infection model, lactic-acid-driven immunosuppression enables prolonged E. faecalis persistence and enhances the fitness of co-infecting bacteria such as Escherichia coli. These findings reveal how bacterial lactic acid subverts innate immunity to support chronic and polymicrobial infections.