Macrolide–lincosamide–streptogramin B resistance phenotypes in clinical staphylococcal isolates

Abstract

The prevalence of macrolide–lincosamide–streptogramin B (MLS B) resistance as well as the MLS B resistance phenotypes were investigated by the double-disk diffusion test among 532 clinical staphylococci isolates in a Turkish university hospital. The activity of other antimicrobials, including trimethoprim/sulfamethoxazole, telithromycin, quinupristin/dalfopristin, linezolid, gentamicin, chloramphenicol, ciprofloxacin and vancomycin, was also evaluated. Of 532 isolates, 38.5% were resistant to MLS B antibiotics; 63.9% of the resistant isolates exhibited a constitutive phenotype (cMLS B) whereas 36.1% expressed an inducible resistance phenotype (iMLS B). MLS B resistance was more prevalent among coagulase-negative staphylococci (CoNS) strains. Oxacillin-resistant strains exhibited significantly higher MLS B resistance rates compared with oxacillin-susceptible strains ( P < 0.0001). The most frequently detected resistance phenotype among the total staphylococcal isolates was the constitutive type and this phenotype was more frequently encountered among oxacillin-resistant strains. With the exception of the fully active agents such as vancomycin, linezolid and quinupristin/dalfopristin, the most effective antibiotics were telithromycin and chloramphenicol among all isolates. Susceptibility rates to other antibiotics tested were higher among isolates without MLS B resistance than isolates with MLS B resistance. The detection of a considerable rate (43.5%) of iMLS B resistance among erythromycin-resistant/clindamycin-susceptible strains suggests that the true percentage of clindamycin resistance may be underestimated if testing for inducible resistance is not performed.