title: Comparison of minimum inhibitory concentration by broth microdilution testing versus standard disc diffusion testing in the detection of penicillin, erythromycin and ciprofloxacin resistance in viridans group streptococci.
abstract: The aim of this study was to investigate the reliability of disc diffusion testing with penicillin, erythromycin and ciprofloxacin within the viridans group streptococci (VGS). In total, the antibiotic susceptibilities of 167 VGS isolates were compared by standard disc diffusion and broth microdilution methods, and these phenotypic data were compared to the carriage of the respective gene resistance determinants [ermB and mefA/E (macrolides); QRDR, gyrA, gyrB, parC and parE (quinolones)]. Overall, there were 35 discrepancies [resistant by MIC and susceptible by zone diameter (21.0%)] between MIC and disc diameter when penicillin susceptibility was interpreted by Clinical and Laboratory Standards Institute criteria. Scattergrams showed a bimodal distribution between non-susceptible and susceptible strains when erythromycin susceptibility was tested by both methods. Thirty-four (20.4%) isolates were categorized as resistant by MIC breakpoints, while disc diameter defined these as having intermediate resistance. With ciprofloxacin, three isolates (1.8%) showed minor discrepancies between MIC breakpoints and disc diameter. Isolates non-susceptible to all three antimicrobial agents tested were reliably distinguished from susceptible isolates by disc diffusion testing, except for the detection of low-level resistance to penicillin, where broth microdilution or an alternative quantitative MIC method should be used. Otherwise, we conclude that disc diffusion testing is a reliable method to detect strains of VGS non-susceptible to penicillin, erythromycin and ciprofloxacin, as demonstrated with their concordance to their gene resistance characteristics.
authors.affiliation: School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland BT52 1SA, UK.|Northern Ireland Public Health Laboratory, Department of Bacteriology, Belfast City Hospital, Lisburn Road, Belfast, Northern Ireland BT9 7AD, UK.
authors.forename: Yasunori
authors.initials: Y
authors.lastname: Maeda
authors.affiliation: Northern Ireland Public Health Laboratory, Department of Bacteriology, Belfast City Hospital, Lisburn Road, Belfast, Northern Ireland BT9 7AD, UK.
authors.forename: Colin E
authors.initials: CE
authors.lastname: Goldsmith
authors.affiliation: School of Dentistry, Queen's University of Belfast, Royal Group of Hospitals, Belfast, Northern Ireland, UK.
authors.forename: Wilson A
authors.initials: WA
authors.lastname: Coulter
authors.affiliation: School of Dentistry, Queen's University of Belfast, Royal Group of Hospitals, Belfast, Northern Ireland, UK.
authors.forename: Charlene
authors.initials: C
authors.lastname: Mason
authors.affiliation: School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland BT52 1SA, UK.
authors.forename: James S G
authors.initials: JSG
authors.lastname: Dooley
authors.affiliation: School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland BT52 1SA, UK.
authors.forename: Colm J
authors.initials: CJ
authors.lastname: Lowery
authors.affiliation: Northern Ireland Public Health Laboratory, Department of Bacteriology, Belfast City Hospital, Lisburn Road, Belfast, Northern Ireland BT9 7AD, UK.
authors.forename: B Cherie
authors.initials: BC
authors.lastname: Millar
authors.affiliation: School of Biomedical Sciences, University of Ulster, Coleraine, Northern Ireland BT52 1SA, UK.|Northern Ireland Public Health Laboratory, Department of Bacteriology, Belfast City Hospital, Lisburn Road, Belfast, Northern Ireland BT9 7AD, UK.
authors.forename: John E
authors.initials: JE
authors.lastname: Moore
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002939:Ciprofloxacin / Q000494:pharmacology; D024881:Drug Resistance, Bacterial / Q000235:genetics; D004917:Erythromycin / Q000494:pharmacology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods*; D010403:Penicillin Resistance; D010406:Penicillins / Q000494:pharmacology; D015203:Reproducibility of Results; D013290:Streptococcal Infections / Q000188:drug therapy / Q000382:microbiology; D034363:Viridans Streptococci / Q000187:drug effects* / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D010406:Penicillins; D002939:Ciprofloxacin; D004917:Erythromycin