Abstract
Vancomycin-resistant enterococci (VRE) have frequently been isolated from Norwegian poultry production following the prohibition of the glycopeptide growth promoter avoparcin since 1995. In the present
study, a close genetic linkage between the
vanA
and
erm
(B) determinants in an
Enterococcus hirae
isolate of poultry origin is demonstrated, a result that indicates a mechanism for co-selection
and maintenance of vancomycin resistance in absence of selective pressure from avoparcin. A total of 36
vanA
-positive enterococci of poultry origin, also phenotypically resistant to erythromycin
and/or tetracycline, were analyzed by PCR for identification of
erm
and
tet
resistance determinants. An
E. hirae
isolate harbored
erm
(B) and
tet
(K), and in this isolate
vanA
and
erm
(B) were located on a
Bam
HI fragment of an ~50-kb plasmid. Approximately 3 kb of this fragment was amplified by PCR with
vanA
and
erm
(B) primers. Sequence
analysis of the region between
erm
(B) and
vanZ
of Tn
1546
showed a truncated IS
1216V
inserted downstream of the
erm
(B) stop codon, aligned with a conserved copy of the
3′-inverted terminal repeat of Tn
1546
. Mating experiments with the
E. hirae
isolate as donor and
E. faecalis
JH2-2 as recipient did not result in any transconjugants, indicating
that the
vanA
/
erm
(B)-carrying plasmid was nonconjugative under the given experimental conditions.