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

Phenotypes of resistance to the macrolide-lincosamide-ketolide-streptogrami n (MLKS) group of antibiotics have been determined in 540 clinical isolates of staphylococci (210 Staphylococcus aureus and 330 coagulase-negative spe cies). Results of disc diffusion tests using erythromycin A, oleandomycin, rokitamycin, clindamycin, telithromycin, quinupristin and dalfopristin deli neated four main groups corresponding to those defined classically using er ythromycin and clindamycin only, but with sub-divisions. Resistance to eryt hromycin was more common in coagulase-negative strains (56%) than in S. aur eus (16%); telithromycin, clindamycin, quinupristin-dalfopristin and rokita mycin were active against >97% of S. aureus strains and >88% of the coagula se-negative strains. The commonest resistance phenotype was 'inducible MLSB ' (12% in S. aureus, 31% in coagulase-negative strains); this group could b e divided in terms of the different inducing abilities of erythromycin and oleandomycin. 'Constitutive MLSB' 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 constituti vely resistant mutants from inducible strains, and of resistant mutants fro m 'MS' strains. This extended phenotyping scheme has revealed further compl exities and evolutionary possibilities in patterns of resistance to this gr oup of antibiotics.