OBJECTIVES: Potential intra- and inter-species transfers of vancomycin resistance genes (vanA gene cluster) between Enterococcus strains were evaluated in the gut of heteroxenic mice harbouring a human microbiota. METHODS: Mice colonized with a stable population of E. faecium 64/3 or E. faecalis JH2-2 recipient strain and harbouring an enterococci-free human microbiota were obtained. Donor strain E. faecium HC-VI2 of clinical origin was administered orogastrically to these mice and transfers were evaluated over time in faecal samples. RESULTS: Only intraspecies transfers were detected in the digestive tract (DT) of mice harbouring a human microbiota. E. faecium 64/3 transconjugants were detected at several sampling times over the 60 day experiment to levels up to 10(3) cfu/g of faeces, but they did not steadily colonize the DT. CONCLUSIONS: Here, we show for the first time that transfer of the vanA gene cluster can occur between Enterococcus strains in the DT colonized with a human microbiota and in the absence of selective pressure. The colonization properties of other enterococci transconjugants and the influence of vancomycin intake should be further investigated since transfers in the DT of animals and humans might contribute to emergence and dissemination of new vancomycin-resistant bacteria.