Interspecies recombination between enterococci: Genetic and phenotypic diversity of vancomycin-resistant transconjugants

Handwerger and colleagues demonstrated that a particular clinical isolate o f Enterococcus faecium, designated GUC, and here redesignated as GUCR, can conjugatively transfer vancomycin resistance. The vancomycin resistance is encoded by a chromosomally born linked set of genes in the donor, designate d the vanA cluster, to the chromosome of an E. faecalis recipient, JH2-2. H ere it is reported that an earlier isolate of E; faecium from the same pati ent who later harbored the vancomycin-resistant E. faecium GUCR lacks the v anA gene cluster but is otherwise similar (by SmaI chromosomal fingerprint and metabolic fingerprinting) to the vancomycin-resistant GUCR. Therefore, "GUCS" is a strong suspect as the base strain for the clinical acquisition of the vanA cluster present in GUCR. Thirteen laboratory-generated vanA tra nsconjugants derived from conjugation between GUCR and JH2-2 were subjected to further analysis, allowing a comparison between transfer in the laborat ory and transfer that occurred in the clinical setting. Surprisingly, each JH2-2 transconjugant had a unique constellation of abilities to oxidize var ious members of a panel of potential carbon sources. This pattern was stabl e for each transconjugant, and it was not changed by growing the strains wi th or without vancomycin. Transconjugants had pulsed-field gel electrophore tic (PFGE) patterns largely consistent with that of JH2-2, the recipient in conjugation experiments. However, PFGE analysis showed that a large but va riable amount of DNA, between 145 kb and 277 kb, was transferred into diffe rent transconjugants. The mechanism appears to be conjugative transposition in which new DNA is added to the pre-existing genome rather than substitut ing for a segment in the recipient. Mapping and hybridization studies of se veral transconjugants showed that each received similar, but not exactly th e same, DNA fragment of at least 30 kb from the donor. Sequencing of 16S ri bosomal genes was used to confirm that the recipient and donor strains used in transconjugation experiments were different species. Sequence analysis was also used to consider the possibility that rRNA operons might be mobili zed in conjugation, but no evidence for the transfer of rDNA operons was fo und. An apparent insertion sequence in E. faecium almost identical to IS 14 85 and 57% sequence identity to IS 199 of Streptococcus mutans was found in the region of DNA transferred. The results imply new consequences of conju gative transfer and interspecies recombination.