Abstract
Five genotypes of acquired glycopeptide resistance have been documented in enterococci, with
vanA
and
vanB
being the most globally widespread and prevalent. Resistance results from the production
of peptidoglycan precursors with reduced binding affinity for glycopeptides and is encoded by complex clusters of
van
genes. The prototype VanA element is Tn
1546
, a 10.8-kb transposon that
carries the
vanRSHAXYZ
genes. Diverse VanA elements exist, but all share the
vanRSHAX
cluster and are believed to be derived from a Tn
1546
progenitor. The sequences of these genes are
remarkably conserved, with only a few point mutations identified. VanA elements do however vary by the presence of deletions and insertion sequences (IS) in nonessential genes (
orf1, orf2, vanY,
and
vanZ
) and intergenic regions. IS transposition probably plays a key role in VanA element evolution. By contrast,
vanB
gene clusters show greater sequence divergence. Three
vanB
alleles have been reported, of which
vanB2
appears to be the most widespread, generally as part of Tn
5382
and related elements. To date, only four
Enterococcus faecium
strains with
VanD resistance have been reported, and each contained a distinct
vanD
allele. The VanE and VanG types have each been identified in single strains of
Enterococcus faecalis
. The existence of
distinct genotypes, together with the allelic nature of
vanB
and
vanD
, suggests that
van
clusters have transferred to enterococci on multiple occasions from undefined donor species,
with subsequent horizontal dissemination, particularly of VanA and VanB elements, among enterococci. Characterization of glycopeptide resistance elements yields information on their evolution and broadens
our insights into the epidemiology of resistant enterococci.