Comparative genomic characterization and antimicrobial resistance of bacteremia-causing Enterococcus faecium and...

Enterococci are common commensals of the human gut and important opportunistic pathogens, with Enterococcus faecium and Enterococcus faecalis being the most clinically prevalent species. A significant epidemiological shift has emerged with an increasing clinical burden of E. faecium. To compare genomic evolution of E. faecium and E. faecalis, we performed whole-genome sequencing on 93 E. faecium and 32 E. faecalis isolates causing bloodstream infections at a single hospital (2022-2024). Analysis of patient demographics revealed that E. faecium infections originated from fewer sources than E. faecalis, with a higher proportion deriving from intra-abdominal infections. Multilocus sequence typing identified ST78 and ST789 as the predominant sequence types for E. faecium, whereas ST16 and ST179 were most common for E. faecalis. E. faecium carried more antimicrobial resistance genes and putative virulence marker (PVM)-type virulence genes than E. faecalis, with vancomycin resistance predominantly mediated by vanHAX (33/93, 35.5%) and a single E. faecalis isolate also carrying vanHAX (1/32, 3.1%); the structurally incomplete vanHMX gene cluster was detected in 11 E. faecium isolates. Pan-genome analysis indicated a larger core genome in E. faecalis compared to E. faecium, consistent with greater plasmid replicon diversity in the latter. Intra-host comparisons showed that two E. faecalis pairs from the same patient were clonally related, with one isolate acquiring a vanHAX plasmid conferring vancomycin resistance. In contrast, E. faecium isolates exhibited marked genomic diversity even among clonally related pairs. These findings suggest that E. faecium possesses greater genomic plasticity and adaptive potential to the clinical environment.IMPORTANCEThis study provides a detailed comparison of clinical and genomic features between Enterococcus faecium and Enterococcus faecalis from the same hospital setting. We show that E. faecium isolates, mainly ST78/ST789, carry more antimicrobial resistance genes and a higher number of putative virulence marker (PVM) genes than E. faecalis, reflecting their hospital-adapted nature. E. faecium also exhibits a smaller core genome and greater diversity of plasmid replicon types, indicating higher genomic plasticity and capacity for horizontal gene transfer. By contrast, E. faecalis retains a larger core genome and a set of classical virulence factors, and its within-host isolates are clonally related. These distinct genomic profiles help to understand how the two species adapt to clinical environments and may inform more targeted infection control strategies and resistance surveillance.