Molecular basis of resistance to macrolides, lincosamides and streptogramins in Staphylococcus saprophyticus clinical isolates

The aim of this study was to evaluate the prevalence of resistance to macrolide-lincosamide-streptogramin (MLS) antibiotics as well as to assess the molecular basis of this resistance amongst 72 Staphylococcus saprophyticus urinary isolates collected from 2005 to 2009 in University Hospital of Caen (France). Of the 72 strains studied, 33 (45.8%) were resistant to at least one MLS antibiotic, including 24 (72.7%) with an M phenotype, 5 (15.2%) with an inducible MLSB phenotype, 3 (9.1%) with a combined M + L phenotype and 1 (3.0%) with an L phenotype. All isolates were susceptible to the combination of streptogramins A and B. The resistance genes erm(A), erm(B), erm(C), msr(A) and lnu(A) were detected alone in 0, 0, 5 (15.2%), 24 (72.7%) and 1 (3.0%) of the 33 MLS-resistant isolates, respectively, whereas 2 strains (6.1%) were positive for both msr(A) and lnu(A). All msr(A)-positive isolates exhibited an M phenotype, whereas all five erm(C)-positive and all three lnu(A)-positive strains displayed, respectively, an inducible MLSB phenotype and an L phenotype with a positive Hodge test. Plasmid analysis indicated that erm(C) and lnu(A) genes were borne by small-size plasmids (ca. 2.5 kb), whereas larger plasmids (30-90 kb) harboured msr(A). In conclusion, these findings show a high prevalence of MLS resistance in S. saprophyticus, which was mainly associated with the presence of the msr(A) gene. Since S. saprophyticus colonises the gastrointestinal tract, it may constitute an unexpected reservoir for MLS resistance genes, in particular msr(A), amongst coagulase-negative staphylococci.