Prevalence and Mechanisms of Erythromycin Resistance in Streptococcus agalactiae from Healthy Pregnant Women

We sought to determine the resistance phenotypes for erythromycin and clindamycin and the mechanisms implicated in 93 Streptococcus agalactiae isolates recovered from healthy pregnant women. Susceptibility testing for erythromycin, clindamycin, penicillin, cefotaxime, vancomycin, quinupristin-dalfopristin, choramphenicol, ofloxacin, and meropenen was carried out by disc-diffusion test, and the E-test was also applied for erythromycin and clindamycin. The constitutive MLSB resistance (cMLS(B)) and inducible MLSB resistance (iMLS(B)) phenotypes, respectively, as well as the M resistance phenotype were determined by the erythromycin-clindamycin double-disc test. The presence of ermA, ermB, ermC, msrA, and mef(A/E) macrolide resistance genes was studied by PCR. Resistance to erythromycin and clindamycin was found in 15% and 9.6% of the isolates, respectively. The resistance phenotypes detected among the 14 erythromycin-resistant isolates were as follows (number of isolates): cMLSB (9), iMLSB (3), and M (2). The MICs for erythromycin and clindamycin were as follows: cMLSB isolates (128-256 and >= 32 mg/L, respectively), iMLSB isolates (16-256 and 1 mg/L), and M isolates (2-8 and 1 mg/L). The following combination of genes were detected among isolates with cMLSB or iMLSB phenotypes: erm(B) (6 isolates), ermA + ermTR (3), ermA + ermB + ermTR (1), and none of these genes (2). The two isolates with M phenotype harbored the mef(A/E), and msrA gene was also found in one of them.