D-Ala-D-Ser VanN-Type Transferable Vancomycin Resistance in Enterococcus faecium

Enterococcus faecium UCN71, isolated from a blood culture, was resistant to low levels of vancomycin (MIC, 16 mu g/ml) but susceptible to teicoplanin (MIC, 0.5 mu g/ml). No amplification was observed with primers specific for the previously described glycopeptide resistance ligase genes, but a PCR product corresponding to a gene called vanN was obtained using degenerate primers and was sequenced. The deduced VanN protein was related (65% identity) to the D-alanine: D-serine VanL ligase. The organization of the vanN gene cluster, determined using degenerate primers and by thermal asymmetric interlaced (TAIL)-PCR, was similar to that of the vanC operons. A single promoter upstream from the resistance operon was identified by rapid amplification of cDNA ends (RACE)-PCR. The presence of peptidoglycan precursors ending in D-serine and D, D-peptidase activities in the absence of vancomycin indicated constitutive expression of the resistance operon. VanN-type resistance was transferable by conjugation to E. faecium. This is the first report of transferable D-Ala-D-Ser-type resistance in E. faecium.