Genetic elements carrying macrolide efflux genes in streptococci.

Type M resistance to macrolides in Streptococcus pyogenes and Streptococcus pneumoniae is associated to the presence of macrolide efflux genes (mef). These genes are carried by 3 very conserved chromosomal genetic elements: (i) "mega", 5.5 kb, typical of S. pneumoniae, carries mef(E), can insert at different sites in the bacterial chromosome and in other genetic elements, does not have genes encoding putative recombinases or transposases, and it is not conjugative; (ii) Tn1207.3, 54, 52 kb, typical of S. pyogenes, carries mef(A), it integrates at a single specific site in the bacterial chromosome, carries 3 ORFs encoding for putative recombinases, it is transferred by conjugation among different streptococcal species, and its genome resembles that of a bacteriophage; (iii) Tn1207.1, 7.2 kb, is a defective form of Tn1207.3 found in a clonal population of S. pneumoniae, it carries mef(A), it integrates at a single specific site in the bacterial chromosome (the same of Tn1207.3), and it is not conjugative.