Objectives: To analyse the sequence diversity of the tetracycline resistance gene tet(M) and its location on mobile elements in Enterococcus faecium and Enterococcus faecalis from humans, pigs and poultry in Denmark.
Methods: A total of 76 isolates were screened for Tn916/Tn1545-like and Tn5397-like transposons using PCR. tet(M) was sequenced in 15 of the isolates and compared with tet(M) sequences submitted to GenBank (phylogenetic analysis and signs of recombination). Plasmids were extracted, filter-mating experiments were performed and Tn5397-like transposons were further characterized in selected isolates.
Results: In 8 of 13 isolates of E. faecium from broilers, tet(M) was present on Tn5397-like transposons, whereas tet(M) was predominantly associated with Tn916/Tn1545-like transposons in E. faecium from pigs and humans, as well as in E. faecalis from humans, pigs and broilers (50 of 63 isolates). The tet(M) genes were divided into three major subgroups according to the phylogenetic analysis. Subgroup I consisted of tet(M) from Clostridium difficile and E. faecium associated with Tn5397-like elements, subgroup II consisted of tet(M) located on Tn916/Tn1545 family transposons and subgroup III consisted of tet(M) associated with composite elements containing several resistance genes. We found evidence of recombination both within and between these groups. Moreover, we identified an E. faecium isolate with both Tn916/Tn1545-like and Tn5397-like elements.
Conclusions: This study showed that enterococci contain diverse tet(M) genes present on different mobile elements, which may suggest that enterococci play an important role in the evolution and horizontal spread of mobile elements carrying tet(M). This is the first report of Tn5397-like elements in enterococci.