VanB-type Resistance Research Articles

Characterization of Tn 1547, a composite transposon flanked by the IS 16 and IS 256-like elements, that confers vancomycin resistance in Enterococcus faecalis BM4281

A 64-kb genetic element harboring a vanB vancomycin-resistance (Vm R) gene cluster was shown to translocate from the chromosome of Enterococcus faecalis BM4281 into the hemolysin (Hly) plasmid, pIP964. Sequence analysis of the resulting junction fragments indicated that the Vm R genes were carried by a composite transposon, Tn 1547, bounded by two distantly related insertion sequences (IS), designated IS 256-like and IS 16, in a direct orientation. IS 256-like (1324 bp) was identical to IS 256, except for two nucleotide (nt) transitions in the putative transposase gene. IS 16 (1466 bp) contained a large open reading frame (ORF) that was 61% identical to the gene encoding the putative transposase of IS 256. There was 58% identity between the deduced amino acid (aa) sequences of the putative transposases of IS 256-like (390 aa) and IS 16 (395 aa). IS 16 was delineated by imperfect inverted repeats (IR) (18 out of 26 bp) which were related ( 23 26 bp identity) to their respective imperfect IR counterparts in IS 256. The difference between the IS was not a barrier for transposition of Tn 1547 which generated an 8-bp duplication at the target site. Dissemination of VanB-type resistance among enterococci results from two mechanisms: (i) large conjugative elements translocate from chromosome to chromosome following inter-strain transfer; and ( ii) as described in this report, the Vm R genes transpose from replicon to replicon within the same strain as part of composite transposons that are internal to the conjugative elements.

Efficacy of vancomycin and teicoplanin alone and in combination with streptomycin in experimental, low-level vancomycin-resistant, VanB-type Enterococcus faecalis endocarditis.

The efficacy of vancomycin (VM) and teicoplanin (TE), alone and in combination with streptomycin (SM), against enterococci that express low-level VanB-type VM resistance was investigated in experimental endocarditis using isogenic strains of Enterococcus faecalis susceptible to glycopeptides and aminoglycosides or inducibly resistant to low levels of VM (MIC = 16 micrograms/ml). VM was significantly less active against the resistant strain than against the susceptible strain, establishing that low-level VanB-type VM resistance can influence therapeutic efficacy. By contrast, TE had equally good activity against both strains. VM or TE combined with SM was synergistic and bactericidal against the resistant strain in vitro. While both combinations were efficient in reducing bacterial density in vivo, TE plus SM was significantly superior to VM plus SM if valve sterilization was considered. These data suggest that despite the presence of low-level VanB-type resistance, combination therapy with a glycopeptide and SM (and presumably other aminoglycosides to which there is not high-level resistance) will nevertheless provide effective bactericidal activity.

Conjugal transfer of the vancomycin resistance determinant vanB between enterococci involves the movement of large genetic elements from chromosome to chromosome.

Resistance to various levels of vancomycin and susceptibility to teicoplanin in enterococci (VanB phenotype) is mediated by the vanB gene cluster. VanB-type resistance was transferred by intra- and inter-specific conjugation between different strains of Enterococcus. Analysis of SfiI-digested genomic DNA by zero integrated-field electrophoresis followed by Southern hybridization revealed vanB-containing chromosomal insertions of approximately 90-250 kb in the transconjugants. Thus, transfer of VanB-type resistance is associated with the movement of large genetic elements from chromosome to chromosome.