A total of 92 genes that confer resistance to MLS antibiotics have been described to date. They can be roughly divided into three groups, depending on the mechanisms by which they confer resistance to one or all of these groups of antibiotics. Three main mechanisms of resistance to MLS antibiotics have been described: methylation of rRNA (target modification), active efflux and inactivation of the antibiotic. Target modification is achieved through the action of the protein product of one of more than 42 different erm (erythromycin rRNA methylase) genes. They confer cross resistance between macrolides, lincosamides and streptogramin B (so-called MLSB resistance) and evoke most concerns. Active efflux and inactivating enzymes (M and L) represent two additional mechanisms of resistance that are targeted only to particular antibiotics or antibiotic classes. Based on the mechanisms of resistance, various resistant phenotypes are expressed. The most prevalent phenotypes are ΜLSB (constitutive or inducible), which is associated with the presence mainly of ermA and ermC genes, followed by the MSB phenotype due to the presence of msrA gene. In livestock S. aureus strains, such as CC 398, other genes such as ermT, lnuA, lsaE and mphC genes are detected.
Part of the book: Staphylococcus Aureus