Evidence for increased cost, morbidity and mortality caused by infections with bacteria resistant to antimicrobial compounds is accumulating. In hospitals, Enterococcus faecalis and E. faecium represent promiscuous reservoirs of antimicrobial resistance d eterminants. They are considered “difficult-to-treat” pathogens with a propensity to acquire “pan-resistance” that may spread to other bacteria, which makes them medically important targets for comparative and functional genomic studies. Characterization of strains from different environments suggests that multi-resistant clinical isolates belong to virulent lineages that are distinct from commensals and environmental strains, but the virulence determinants behind epidemic lineages remain largely unidenti fied. The construction of genome-wide microarrays based on the genome sequences of two multi-resistant strains (E. faecium DO and E. faecalis V583) by one of the partners, offers an unparalleled opportunity to dissect and compare the genomic composition a mong strains. This project is expected to reveal evolutionary processes and mechanisms behind acquired drug resistance and altered virulence in enterococci, and to contribute to the development of strategies for prevention and control of multi-resistant s trains. This will be obtained through: 1) use of microarrays to explore genomic diversity among strains from diverse environments and thereby the possibility to define the core enterococcal genome; 2) determination of the basic and stress-induced transcri ptome of E. faecalis and E. faecium; 3) examination of microevolution in antibiotic exposed enterococci; 4) examination of the relation between the putative integrative and conjugative element Tnvamp and the emergence of ecologically successful multi-resi stant lineages of E. faecium. The project takes the advantages of the long experience in enterococcal research in two complementary Norwegian groups, with national and international partners.