Bac 32, a novel bacteriocin widely disseminated among clinical isolates of Enterococcus faecium

A total of 636 vancomycin-resistant Enterococcus faecium (VRE) isolates that had been obtained between 1994 and 1999 from the Medical School Hospital of the University of Michigan, Ann Arbor, were tested for bacteriocin production. Two hundred seventy-seven (44%) of the strains were bacteriocinogenic; and 193 of these exhibited activity against Enterococcus faecium, Enterococcus hirae, and Enterococcus durans. Strain VRE200 harbors the highly efficient conjugative gentamicin resistance plasmid pG200 (70 kb) and bacteriocin plasmid pTI1 (12.5 kb). The bacteriocin encoded on pTI1 was designated bacteriocin 32 (Bac 32). Bacteriocin 32 was active against E. faecium, E. hirae, and E. durans but showed no activity against Listeria monocytogenes. The Bac 32 genetic locus consists of a bacteriocin gene (bacA) and an immunity gene (bacB). Neither of these genes showed significant homology to any known bacteriocin determinants. The deduced bacA product is 89 amino acids in length, with a putative signal peptide of 19 amino acids at the N terminus. The bacB gene encodes a deduced 55-amino-acid protein without a signal sequence. One hundred eighty-nine strains (97.9%) of the 193 strains with activity against the 3 test enterococcal strains gave rise to the expected specific PCR product with a primer specific for bacA, indicating that there is a high incidence of Bac 32 production among VRE clinical isolates. Data from Southern analyses of plasmid DNA from 189 of the Bac 32-producing strains with a plasmid pTI1-specific probe suggested that 137 (72.5%) of the strains harbored a pTI1-type plasmid. Bac 32 or Bac 32-type bacteriocin activity and the determinant genes were also identified in 22 (39.3%) of a total of 56 vancomycin-sensitive E. faecium clinical isolates, which suggests that this bacteriocin is widely disseminated among E. faecium strains.