title: Identification of a novel keyhole phenotype in double-disk diffusion assays of clindamycin-resistant erythromycin-sensitive strains of Streptococcus agalactiae.
abstract: Our objective was to characterize 46 unique, erythromycin-sensitive, and clindamycin-resistant Streptococcus agalactiae strains from S. Korea that displayed a novel phenotype in double-disk diffusion assay. We used polymerase chain reaction to determine presence of erythromycin and clindamycin resistance genes, disc diffusion assays to determine resistance phenotype, and microbroth dilution to determine minimal inhibitory concentration. We detected a novel phenotype in the double-disk diffusion assay for inducible resistance among 46 S. agalactiae strains that were both erythromycin sensitive and clindamycin resistant. Thirty-two strains with the novel phenotype tested positive for erm(B) by DNA-DNA hybridization; sequencing of the erm(B) gene revealed mutations in the ribosomal binding site region in the erm(B) open reading frame, which is consistent with a lack of erythromycin resistance phenotype. Although identified from patients at multiple hospitals, genotyping suggested that the strains are closely related. The new phenotype shows increased sensitivity to clindamycin in the presence of erythromycin.
authors.affiliation: Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, Michigan 48109, USA. usha@umich.edu
authors.forename: Usha
authors.initials: U
authors.lastname: Srinivasan
authors.forename: Brady
authors.initials: B
authors.lastname: Miller
authors.forename: Joan
authors.initials: J
authors.lastname: Debusscher
authors.forename: Carl F
authors.initials: CF
authors.lastname: Marrs
authors.forename: Lixin
authors.initials: L
authors.lastname: Zhang
authors.forename: Yong Soo
authors.initials: YS
authors.lastname: Seo
authors.forename: Kwan-Young
authors.initials: KY
authors.lastname: Oh
authors.forename: Moon Young
authors.initials: MY
authors.lastname: Kim
authors.forename: Hye-Ryung
authors.initials: HR
authors.lastname: Yoon
authors.forename: Moran
authors.initials: M
authors.lastname: Kl
authors.forename: Betsy
authors.initials: B
authors.lastname: Foxman
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002981:Clindamycin / Q000494:pharmacology; D024881:Drug Resistance, Bacterial; D004917:Erythromycin / Q000494:pharmacology; D005260:Female; D006801:Humans; D008826:Microbial Sensitivity Tests; D009154:Mutation; D016366:Open Reading Frames; D010641:Phenotype; D016133:Polymerase Chain Reaction; D011247:Pregnancy; D056910:Republic of Korea / Q000453:epidemiology; D013290:Streptococcal Infections / Q000188:drug therapy* / Q000382:microbiology; D013292:Streptococcus agalactiae / Q000187:drug effects* / Q000235:genetics / Q000302:isolation & purification
chemical_list: D000900:Anti-Bacterial Agents; D002981:Clindamycin; D004917:Erythromycin