SmaI typeability and tetracycline susceptibility/resistance in Streptococcus pyogenes isolates with efflux-mediated erythromycin resistance.

These findings contribute to conclusively clarifying why, among S. pyogenes isolates with the M phenotype, the tetracycline- resistant ones are, as a rule, SmaI typeable, while the tetracycline-susceptible ones are not. Previous studies have shown that, while the two related genetic elements (Tn1207.3 and 10394.4) typical of tetracycline-susceptible isolates are inserted into the same prophage, the tet(O)-mef(A) element typical of tetracycline-resistant isolates is inserted into a different prophage (6). The present results show that the R-M cassette encoding, in Tn1207.3-10394.4, the methyltransferase responsible for DNA resistance to cleavage by SmaI is absent in the tet(O)-mef(A) element where, together with an adjacent ORF, it is replaced by three new ORFs flanked, both upstream and downstream, by homologous regions.