Enterococcus faecium colonization and persistence in a model of diabetic wound infection

Chronic wound infections are a common comorbidity of diabetes mellitus and can progress to amputation if untreated, yet effective strategies to manage these infections are limited. Commensals such as Enterococcus faecium and Staphylococcus epidermidis can transition into opportunistic pathogens when host defenses are compromised, underscoring the complexity of chronic wound microbiology. E. faecium, particularly vancomycin-resistant strains, are an understudied, clinically important cause of chronic diabetic wound infections. Using a low-dose streptozocin-induced diabetic mouse model, we characterized E. faecium wound infection dynamics and identified differences in colonization and clearance compared to non-diabetic animals. At eight hours post infection (hpi), control mice exhibited higher E. faecium wound colony forming units (CFU) than diabetic mice, but cleared the infection more efficiently, resulting in similar CFU by 24 hpi. By contrast, diabetic mice showed impaired clearance, with elevated CFU persisting through 72 hpi. In mixed species infection with S. epidermidis, S. epidermidis CFU increased at 72 hpi while E. faecium CFU remained comparable to single species infection. Despite strong initial cytokine and neutrophil responses, E. faecium persisted in all wounds. Sustained neutrophil recruitment at 72 hpi occurred only in diabetic mice, whereas macrophage accumulation increased from 24 to 72 hpi in all wounds, including sterile controls. Histological analysis showed epithelial hyper-thickening in both groups, indicating that diabetes and E. faecium each contribute to impaired wound healing. This study establishes a diabetic mouse model of E. faecium wound infection and suggests that E. faecium modulates innate immune responses to persist in the wound bed.