title: Agar dilution method for detection of inducible clindamycin resistance in Staphylococcus spp.
abstract: We describe the development and validation of an agar dilution method for the detection of inducible clindamycin resistance by using 227 previously characterized erythromycin-resistant, clindamycin-susceptible Staphylococcus sp. isolates. Mueller-Hinton agar with defibrinated horse blood containing a range of erythromycin concentrations (1 to 8 mg/liter) combined with clindamycin at 0.5 mg/liter was used to determine the optimal concentration that produced growth of inducible isolates while inhibiting that of isolates without the inducible phenotype. A concentration of clindamycin of 0.5 mg/liter with erythromycin at 1 mg/liter was the optimal combination for detection of inducible resistance and resulted in a sensitivity of 100% (95% confidence interval [CI], 97.9 to 100) and a specificity of 100% (95% CI, 93.0 to 100). Attention must be paid to ensuring that a sufficient inoculum has been used, since an inoculum below the standard 10(7) bacteria/ml may result in false-negative results. This method has been incorporated into routine use in our laboratory.
authors.affiliation: Department of Microbiology, Royal North Shore Hospital, Sydney, New South Wales, Australia.
authors.forename: Clarence J
authors.initials: CJ
authors.lastname: Fernandes
authors.forename: Matthew V N
authors.initials: MV
authors.lastname: O'Sullivan
authors.forename: Yongwei
authors.initials: Y
authors.lastname: Cai
authors.forename: Fanrong
authors.initials: F
authors.lastname: Kong
authors.forename: Xianyu
authors.initials: X
authors.lastname: Zeng
authors.forename: Gwendolyn L
authors.initials: GL
authors.lastname: Gilbert
authors.forename: George
authors.initials: G
authors.lastname: Kotsiou
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002981:Clindamycin / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry; D024881:Drug Resistance, Bacterial; D004917:Erythromycin / Q000494:pharmacology; D005786:Gene Expression Regulation / Q000187:drug effects; D008826:Microbial Sensitivity Tests / Q000379:methods*; D012680:Sensitivity and Specificity; D013210:Staphylococcus / Q000187:drug effects*
chemical_list: D000900:Anti-Bacterial Agents; D003470:Culture Media; D002981:Clindamycin; D004917:Erythromycin