title: Antimicrobial susceptibility testing of enterococci: results of a survey conducted by the United Kingdom National External Quality Assessment Scheme for Microbiology.
abstract: Six strains of enterococci were distributed to participants in the United Kingdom National External Quality Assessment Scheme for Microbiology with a request that they be tested for susceptibility to ampicillin/penicillin and vancomycin and for high-level resistance to gentamicin. More than 98% of laboratories correctly reported the resistant strains as resistant to penicillin and ampicillin. However, there was a poorer correlation for the susceptible strains, with only 1-6% and 69-83% of laboratories reporting the strains as susceptible to penicillin and ampicillin respectively. Those laboratories which used 5-25 micrograms ampicillin discs or breakpoint methods reported proportionally more results correctly for the susceptible strains than those using 2 micrograms discs. Participants using Escherichia coli or an enterococcus as a control and those not using a control at all were more likely to report ampicillin-susceptible strains correctly than those using Staphylococcus aureus NCTC 6571 (Oxford strain). For vancomycin-susceptible and high-level vancomycin-resistant strains, 93-96% and 96-99% of reports respectively were correct. For the low-level vancomycin-resistant strains, only 50-54% of reports were correct. Participants using a 5 micrograms vancomycin disc reported proportionally more of the low-level resistant strains correctly than those using 10, 20, 25, 30 or 100 micrograms discs. Participants not using controls were proportionally more likely to report the low-level resistant strains as susceptible than those using an enterococcus or S. aureus NCTC 6571 as a control. For the high-level gentamicin-resistant (HLGR) strains, 96-98% of reports were correct. Depending on the strain, between 12% and 41% of participants also reported high-level resistance for the strains which were not HLGR. Those using low-content discs (< or = 30 micrograms) and breakpoint or MIC methods were more likely to report strains which were not HLGR as HLGR than those using high-content discs. Those using an enterococcus as a control were less likely to report strains which were not HLGR as HLGR than those using either S. aureus NCTC 6571 or E. coli as a control and those not using a control.
authors.affiliation: Quality Assurance Laboratory, Central Public Health Laboratory, Colindale, London, UK.
authors.forename: J J
authors.initials: JJ
authors.lastname: Snell
authors.forename: D F
authors.initials: DF
authors.lastname: Brown
authors.forename: S F
authors.initials: SF
authors.lastname: Perry
authors.forename: R
authors.initials: R
authors.lastname: George
mesh_terms: D000668:Ampicillin Resistance; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016983:Enterococcus / Q000187:drug effects*; D013293:Enterococcus faecalis / Q000187:drug effects; D016984:Enterococcus faecium / Q000187:drug effects; D005839:Gentamicins / Q000494:pharmacology; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000592:standards*; D010403:Penicillin Resistance; D011785:Quality Assurance, Health Care; D006113:United Kingdom; D014640:Vancomycin / Q000494:pharmacology
chemical_list: D000900:Anti-Bacterial Agents; D005839:Gentamicins; D014640:Vancomycin