Tn2009, a Tn916-like element containing mef(E) in Streptococcus pneumoniae

The association between the macrolide efflux gene mef(E) and the tet(M) gene was studied in two clinical strains of Streptococcus pneumoniae that belonged to serotypes 19F and 6A, respectively, and that were resistant to both tetracycline and erythromycin. The mef(E)-carrying element mega (macrolide efflux genetic assembly; 5,511 bp) was found to be inserted into a Tn916-like genetic element present in the chromosomes of the two pneumococcal strains. In both strains, mega was integrated at the same site, an open reading frame identical to orf6 of Tn916. The new composite element, Tn2009, was about 23.5 kb and, with the exception of the tet(M)-coding sequence, appeared to be identical in both strains. By sequencing of the junction fragments of Tn2009 at the site of insertion into the chromosome, it was possible to show that (i) the insertion site was identical in the two clinical strains and (ii) the integration of Tn2009 caused a 9.5 kb-deletion in the pneumococcal chromosome. It was not possible to detect the conjugal transfer of Tn2009 to a recipient pneumococcal strain; however, transfer of the whole element by transformation was shown to occur. It is possible to hypothesize that Tn2009 relies on transformation for its spread among clinical strains of S. pneumoniae.