In Vitro Activity of Solithromycin against Erythromycin-Resistant Streptococcus agalactiae

The in vitro antibacterial activity of solithromycin (CEM-101) against macrolide-resistant isolates (n = 62) of Streptococcus agalactiae (group B streptococcus GBS]) was determined. Phenotypic characterization of macrolide-resistant strains was performed by double-disc diffusion testing. A multiplex PCR was used to identify the erm(B), erm(TR), and mef(A/E) genes, capsular genotypes, and alpha-like (Alp) protein genes from the GBS strains. Determination of MIC was carried out using the microdilution broth method. The Etest method was used for penicillin, azithromycin, clarithromycin, and erythromycin. Solithromycin had a MIC₅₀ of ≤0.008 μg/ml and a MIC₉₀ of 0.015 μg/ml against macrolide-susceptible S. agalactiae. These MICs were lower than those displayed by penicillin (MIC₅₀ of 0.032 μg/ml and MIC₉₀ of 0.047 μg/ml), the antibiotic agent of choice for prophylaxis and treatment of GBS infections. Against macrolide-resistant S. agalactiae, solithromycin had a MIC₅₀ of 0.03 μg/ml and a MIC₉₀ of 0.125 μg/ml. Against erm(B) strains, solithromycin had a MIC₅₀ of 0.03 μg/ml and a MIC₉₀ of 0.06 μg/ml, while against mef(A) strains, it had a MIC₅₀ of 0.03 μg/ml and a MIC₉₀ of 0.125 μg/ml. Most erythromycin-resistant GBS strains were of serotype V (64.5%) and associated significantly with alp2-3. Moreover, a statistically significant association was observed between the constitutive macrolide-lincosamide-streptogramin B resistance (cMLS_B) phenotype and the erm(B) gene-carrying strains, the alp2-3 gene and the M phenotype, and the mef(A/E) gene and epsilon. Overall, our results show that solithromycin had lower or similar MICs than penicillin and potent activity against macrolide-resistant strains independent of their genotype or phenotype, representing a valid therapeutic alternative where β-lactams cannot be used.