The Novel Macrolide-Lincosamide-Streptogramin B Resistance Gene erm(44) Is Associated with a Prophage in Staphylococcus xylosus

A novel erythromycin ribosomemethylase gene, erm(44), that confers resistance to macrolide, lincosamide, and streptogramin B (MLSB) antibiotics was identified by whole-genome sequencing of the chromosome of Staphylococcus xylosus isolated from bo-vine mastitis milk. The erm(44) gene is preceded by a regulatory sequence that encodes two leader peptides responsible for the inducible expression of the methylase gene, as demonstrated by cloning in Staphylococcus aureus. The erm(44) gene is located on a 53-kb putative prophage designatedJW4341-pro. The 56 predicted open reading frames ofJW4341-pro are structurally organized into the five functional modules found in members of the family Siphoviridae.JW4341-pro is site-specifically inte-grated into the S. xylosus chromosome, where it is flanked by two perfect 19-bp direct repeats, and exhibits the ability to circu-larize. The presence of erm(44) in three additional S. xylosus strains suggests that this putative prophage has the potential to dis-seminate MLSB resistance. Staphylococcus xylosus is a ubiquitous bacterium and a com-mensal of human and animal skin (1). It is also used in the fermentation and production of cheeses and raw meat products (2, 3). Although rarely associated with human infections (4–6), S. xylosus plays a major role in the pathogenesis of subclinical bovine mastitis, in which it is in frequent contact with intramammary antibiotics used for mastitis treatment, such as macrolides and lincosamides (7–10). Simultaneous resistance to both of these drugs is due mainly to the acquisition of erythromycin ribosome methylase (erm) genes (11). Erm methylases are enzymes that add one or two methyl groups to adenine A2058 of the 23S rRNA, preventing the binding of macrolide, lincosamide, and strepto-gramin B (MLSB) antibiotics. To date, 35 Erm methylases have been identified in bacterial species (12, 13), 10 of which [Erm(A)