**Abstract**

Horizontal gene transfer constitutes a key driving force in bacterial evolution. The ability to acquire mobile genetic elements encoding antimicrobial resistance has contributed to the emergence of *Enterococcus faecium* as one of the main human nosocomial opportunistic pathogens. The deep analysis of the vancomycin-resistant *E. faecium* (VREfm) population's mobilome, as the architecture and evolution of the core genome enables to observe VREfm plasticity and power of adaptation in animals, plants, environment and food. The persistence of VREfm is facilitated by the exchange of plasmids, phages and conjugative transposons that have allowed them to achieve a rapid adaptation to changes in environmental conditions. They can acquire resistance determinants from several species and transfer resistance genes to other potentially pathogenic bacteria such as methicillin-resistant *Staphylococcus aureus* strains.

**Keywords:** *E. faecium*, vancomycin, resistance, mobile genetic elements
