Occurrence of macrolide-lincosamide-streptogramin resistances among staphylococcal clinical isolates at a university medical center. Is false susceptibility to new macrolides and clindamycin a contemporary clinical and in vitro testing problem?

A total of 2189 staphylococcal strains at the University of Iowa Hospitals and Clinics (Iowa City, IA) were initially screened to determine the incidence of constitutive (29.8%) and potential inducible macrolide-lincosamide-streptogramin (MLS) resistance (11.3%). Staphylococcus haemolyticus and S. epidermidis (62.5% and 55.3%) showed the highest incidence of constitutive resistance. Staphylococcus hominis had the highest incidence of inducible resistance (40.6%), while S. aureus had the lowest rate for both resistance types. The overall ratio of constitutive-inducible MLS resistance was 4:1. Among strains initially speciated using the Vitek System GPI card, there was only a 69% species identification reproducibility, and 78% accuracy versus a reference identification method. A random sample of 105 Staphylococcus spp. isolates with discordant macrolide (erythromycin resistant) and lincosamide (clindamycin susceptible) susceptibility patterns were tested against 16 antimicrobial agents by using a reference broth microdilution method. All erythromycin-resistant Staphylococcus spp. were also resistant to other 14-member macrolides and azithromycin, while all organisms remained susceptible to clindamycin, rifampin, vancomycin, and the streptogramin compounds (RP59500 and virginiamycin). Resistance to teicoplanin was identified among some oxacillin-resistant S. haemolyticus strains. Of 105 isolates, 65 (62%) showed inducible MLS resistance, 28 (27%) were noninducible, and 12 (11%) were either fully susceptible or resistant to the MLS drugs (Vitek System interpretation errors). MLS disk induction tests revealed two inducible resistance phenotypes: ML and MLS. Staphylococcus aureus showed the highest inducible resistance rate at 95% with an MLS-predominant pattern. In contrast, endemic S. haemolyticus isolates did not demonstrate inducible resistance that is, efflux-mediated erythromycin resistance.(ABSTRACT TRUNCATED AT 250 WORDS)