Enterococcus faecium produces membrane vesicles containing virulence factors and antimicrobial resistance related proteins.

Enterococcus faecium is a commensal but also a bacteremia causing pathogen, which is inherently resistant to several antimicrobials and has a great ability to acquire new traits. Bacterial membrane vesicles (MVs) are increasingly recognized as a mode of cell-free communication and a way to deliver virulence factors and/or antimicrobial resistance determinants. These features make MVs interesting research targets in research on critical hospital pathogens. This study describes for the first time that E. faecium strains produce MVs. It presents a morphological as well as a proteomic analysis of MVs isolated from four different, clinically relevant E. faecium strains grown under two different conditions and identifies MV-associated proteins in all of them. Interestingly, 11 virulence factors are found among the MV-associated proteins, including biofilm-promoting proteins and extracellular matrix-binding proteins, which may aid in enterococcal colonization. Additionally, 11 antimicrobial resistance-related proteins were MV-associated. Among those, all proteins encoded by the vanA-cluster of a vancomycin resistant strain were found to be MV-associated. This implies that E. faecium MVs may be utilized by the bacterium to release proteins promoting virulence, pathogenicity and antimicrobial resistance. SIGNIFICANCE: Enterococcal infections, especially bacteremia and endocarditis, are challenging to treat because E. faecium have acquired resistance to multiple classes of antimicrobials, including ampicillin, aminoglycosides, and glycopeptides. Thus, research on different modes of enterococcal pathogenicity is warranted. This study utilized a proteomic approach to identify MV-associated proteins of different nosocomial E. faecium strains representing four clinically relevant sequence types (STs), namely ST17, ST18, ST78, and ST192. The presented data suggest that E. faecium MVs are involved in virulence and antimicrobial resistance.