In vitro conjugative transfer of VanA vancomycin resistance betweenEnterococci andListeriae of different species

In a study designed to gain data on the in vitro transferability of vancomycin resistance from enterococci of the VanA phenotype to listeriae of different species, three clinicalEnterococcus isolates —Enterococcus faecium LS10,Enterococcus faecalis LS4, andEnterococcus faecalis A3208, all harboring a plasmid that strongly hybridized with avanA probe — were used as donors in transfer experiments. Strains of fiveListeria species were used as recipients. FromEnterococcus faecium LS10, glycopeptide resistance was transferred toListeria monocytogenes, Listeria ivanovii, andListeria welshimeri recipients, whereas no transfer occurred toListeria seeligeri orListeria innocua strains. From the twoEnterococcus faecalis isolates, no transfer occurred to anyListeria recipient. MICs of both vancomycin and teicoplanin were 256 mg/l for all transconjugants tested. Furthermore, all transconjugants harbored a plasmid that strongly hybridized with thevanA probe, withvanA consistently located in anEcoRI fragment of about 4 kb. Exposure ofListeria transconjugants to vancomycin resulted in synthesis of a membrane protein similar in size (39 kDa) to a vancomycin-induced membrane protein ofEnterococcus faecium LS10. In retransfer experiments withListeria transconjugants used as donors, glycopeptide resistance was transferred to allListeria recipients tested, including strains ofListeria innocua andListeria seeligeri, which were unable to receive the resistance fromEnterococcus faecium LS10. The frequency ofvanA transfer to listerial recipients was greater in retransfer experiments than in the primary matings. These findings suggest that thevanA resistance determinant might spread to the established pathogenListeria monocytogenes, both directly from a resistant enterococcus and through strains of nonpathogenicListeria species acting as intermediate resistance vehicles.