Genetic elements responsible for erythromycin resistance in streptococci

Until a decade ago, knowledge about the genetic elements responsible for erythromycin resistance in streptococci was virtually confined to a few plasmids or transposons carrying erm(B), then called ermAM or simply erm (56, 65). Such transposons mainly included Tn917, detected in Enterococcus faecalis when it was still regarded as a Streptococcus species (94, 101, 102), and Tn1545, detected in S. pneumoniae and also encoding resistance, besides tetracycline, to erythromycin and kanamycin (31, 32). Remarkably, Tn1545 was related to Tn916 (47), the prototype of a family of broad-host-range conjugative transposons conferring tetracycline resistance via the tet(M) gene (24, 81). Other Tn916- related erm(B)-carrying transposons early described in Streptococcus species (81) ceased to be reported in later studies. During the last decade, the discovery of the above-mentioned variety of erythromycin resistance genes in streptococci has been closely followed by the identification and characterization of a variety of genetic elements responsible for the resistance and its possible spread via intra- and interspecific transfer. Different erythromycin resistance genes are carried by different elements: in the case of mef genes, such close gene-element association was a major argument for recommending that mef(A), mef(E), and any future mef variants continue to be discriminated and kept apart (60) as opposed to being collected in a single class, mef(A), due to their high degree of similarity (84). This minireview is aimed at presenting such new knowledge about the genetic elements responsible for erythromycin resistance in streptococci.