Molecular epidemiology, antimicrobial resistance, and virulence determinants of Enterococcus faecium bloodstream...

Enterococcus faecium is a major cause of hospital-acquired infections, particularly bloodstream infections, which are associated with high mortality rates. This study aimed to characterize the molecular epidemiology, antimicrobial resistance profiles, and virulence determinants of E. faecium isolates from bloodstream infections in a tertiary hospital in Shandong, China, over a seven-year period (2017-2023).

Among 91 E. faecium isolates, 4 were vancomycin-resistant (VREfm), and 87 were vancomycin-susceptible (VSEfm). High resistance rates (> 87.9%) were observed for ciprofloxacin, levofloxacin, penicillin, ampicillin, and erythromycin. Multilocus sequence typing (MLST) identified 16 sequence types (STs), dominated by ST78 (41.8%) and ST555 (22.0%), with a novel ST designated ST2760. Notably, the prevalence of ST555 increased significantly from 18.2% in 2021 to 43.8% in 2023. Thirteen of the 16 STs (95.6% of isolates) were assigned to Clonal Complex 17 (CC17), while ST775, ST922, and the novel ST2760 were phylogenetically distinct from this predominant lineage. Virulence gene analysis revealed high detection rates (> 73%) of adhesion-associated genes (pilA, pilB, acm, sgrA, scm, esp, and ecbA). The pilA gene was significantly more prevalent in ST78 (92.1%) than in ST555 (40.0%), conferring higher biofilm-forming capacity. The absence of pilA in ST555 strains prior to 2021 correlated with lower detection rates, while its emergence post-2021 coincided with enhanced environmental fitness and increased clinical prevalence.

This study highlights the predominance of ST78 and ST555 in E. faecium bloodstream infections, with their increasing prevalence linked to acquisition of the pilA gene and enhanced environmental survival through biofilm formation. The identification of a novel ST2760 underscores the ongoing genetic diversity of E. faecium. These findings provide critical insights for optimizing infection control strategies and guiding empirical therapy for E. faecium bloodstream infections.