Two distinct genetic elements are responsible for erm(TR)-mediated erythromycin resistance in tetracycline-susceptible and tetracycline-resistant strains of Streptococcus pyogenes.

In Streptococcus pyogenes, inducible erythromycin (ERY) resistance is due to posttranscriptional methylation of an adenine residue in 23S rRNA that can be encoded either by the erm(B) gene or by the more recently described erm(TR) gene. Two erm(TR)-carrying genetic elements, showing extensive DNA identities, have thus far been sequenced: ICE10750-RD.2 (∼49 kb) and Tn1806 (∼54 kb), from tetracycline (TET)-susceptible strains of S. pyogenes and Streptococcus pneumoniae, respectively. However, TET resistance, commonly mediated by the tet(O) gene, is widespread in erm(TR)-positive S. pyogenes. In this study, 23 S. pyogenes clinical strains with erm(TR)-mediated ERY resistance-3 TET susceptible and 20 TET resistant-were investigated. Two erm(TR)-carrying elements sharing only a short, high-identity erm(TR)-containing core sequence were comprehensively characterized: ICESp1108 (45,456 bp) from the TET-susceptible strain C1 and ICESp2905 (65,575 bp) from the TET-resistant strain iB21. While ICESp1108 exhibited extensive identities to ICE10750-RD.2 and Tn1806, ICESp2905 showed a previously unreported genetic organization resulting from the insertion of separate erm(TR)- and tet(O)-containing fragments in a scaffold of clostridial origin. Transferability by conjugation of the erm(TR) elements from the same strains used in this study had been demonstrated in earlier investigations. Unlike ICE10750-RD.2 and Tn1806, which are integrated into an hsdM chromosomal gene, both ICESp1108 and ICESp2905 shared the chromosomal integration site at the 3' end of the conserved rum gene, which is an integration hot spot for several mobile streptococcal elements. By using PCR-mapping assays, erm(TR)-carrying elements closely resembling ICESp1108 and ICESp2905 were shown in the other TET-susceptible and TET-resistant test strains, respectively.