Characterization of the chromosomal aac(6')-Ii gene specific for Enterococcus faecium

Chromosomal gene aac(6')-Ii of Enterococcus faecium CIP 54-32, encoding a 6'-N-aminoglycoside acetyltransferase was characterized. The gene was identified as a coding sequence of 549 bp corresponding to a protein with a calculated mass of 20,666 Da. Analysis of the sequence of the deduced protein suggested that it was the second member of a subfamily of AAC(6')-I enzymes. Insertional inactivation of aac(6')-Ii led to aminoglycoside susceptibility of CIP 54-32, suggesting that this gene plays a role in resistance to AAC(6')-I substrates. The gene was detected by DNA hybridization in all 26 strains of E. faecium tested but not in 44 other enterococci of 13 species. These data suggest that the aac(6')-Ii gene is species specific and may be used to identify E. faecium.