Susceptibility of strains of Streptococcus agalactiae to macrolides and lincosamides, phenotype patterns and resistance genes.

The Group B streptococcus (Streptococcus agalactiae) is a pathogen of increasing importance in human disease. We therefore studied the susceptibility of clinical isolates of S. agalactiae to penicillin G, erythromycin, azithromycin and clindamycin using National Committee for Clinical Laboratory Standards methodology, and we also determined the phenotypes of macrolide-lincosamide susceptibility and the resistance genes implicated in a group of selected isolates of the different phenotypes. We used 221 isolates collected between 1997 and 1999 in two Health Authority Areas in Móstoles and Granada, Spain. The minimal concentration for 90% inhibition (MIC90) for penicillin G was 0.12 mg/L and all the isolates tested were susceptible. One hundred and eighty-five (83.7%) were susceptible to erythromycin and azithromycin and 191 (86.4%) were susceptible to miocamycin and clindamycin. Twenty-three isolates (10.4%) had a constitutive MLSB phenotype, seven (3.2%) an inducible phenotype, and six (2.7%) an M phenotype. All except one of the MLSB phenotype isolates tested (n = 23) carried erm genes; in two strains with the mef (A) gene, all the M phenotype (n = 6) isolates tested carried mef genes, while erm and mef (A) genes were absent in all the macrolide-lincosamide-susceptible (n = 12) isolates tested. In our environment, resistance to macrolide and lincosamide in S. agalactiae was present in 10-16% of the isolates. The majority of resistant strains had the MLSB phenotype.