Patterns of phenotypic resistance to the macrolide-lincosamide-ketolide-streptogramin group of antibiotics in staphylococci.

Phenotypes of resistance to the macrolide-lincosamide-ketolide-streptogramin (MLKS) group of antibiotics have been determined in 540 clinical isolates of staphylococci (210 Staphylococcus aureus and 330 coagulase-negative species). Results of disc diffusion tests using erythromycin A, oleandomycin, rokitamycin, clindamycin, telithromycin, quinupristin and dalfopristin delineated four main groups corresponding to those defined classically using erythromycin and clindamycin only, but with sub-divisions. Resistance to erythromycin was more common in coagulase-negative strains (56%) than in S. aureus (16%); telithromycin, clindamycin, quinupristin-dalfopristin and rokitamycin were active against >97% of S. aureus strains and >88% of the coagulase-negative strains. The commonest resistance phenotype was 'inducible MLS(B)' (12% in S. aureus, 31% in coagulase-negative strains); this group could be divided in terms of the different inducing abilities of erythromycin and oleandomycin. 'Constitutive MLS(B)' and 'MS' phenotypes were more often found in coagulase-negative strains (11 and 13%, respectively) than in S. aureus (2 and 1%). Novel phenotypes were found during the isolation of constitutively resistant mutants from inducible strains, and of resistant mutants from 'MS' strains. This extended phenotyping scheme has revealed further complexities and evolutionary possibilities in patterns of resistance to this group of antibiotics.