Analysis of Regulatory Region of ermAM Genes in Streptococci and Enterococci Highly Resistant to Macrolides and Lincosamides

Target modification is the major mechanism of resistance to macrolide antibiotics in streptococci and enterococci and is due to the synthesis of a methylase which modifies the ribosome (5). This modification confers cross-resistance to macrolides, lincosamides and streptogramins B, the so-called MLSB phenotype. In streptococci and enterococci, MLSB resistance is encoded by genes related to the ermAM gene from the plasmid pAM77 of Streptococcus sanguis (6) and often borne by transposons Tn 1545 (4), Tn 917 (3), or by plasmids. Expression of the resistance may be inducible by macrolides or constitutive depending upon regulatory region located upstream the ermAM gene. In the case of plasmid pAMβ1 from E. faecalis, constitutive expression of resistance was related to a deletion of the 5′ regulatory region, including leader peptide sequences, of the ermAM gene (1). We have studied the expression of macrolide resistance in clinical isolates of streptococci and enterococci and analysed the regulatory sequences of the ermAM-related genes.