False amoxicillin/clavulanic acid susceptibility in Bacteroides fragilis using gradient strip tests

• Published in: Anaerobe 2021-06, Vol.69, p.102358-102358, Article 102358
• Main Authors: Rentenaar, Rob J., Bovo-Heijmans, Bianca, Diggle, Joanna, Fluit, Ad C., Wootton, Mandy
• Format: Article
• Language: English
• Subjects: Agar dilution; Amoxicillin/clavulanic acid; Bacteroides fragilis; Betalactamase; CfiA; Etest; Fixed concentration; Gradient strip test; Meropenem; MTS; Ratio
• ISSN: 1075-9964; 1095-8274
• DOI: 10.1016/j.anaerobe.2021.102358

Repeatedly, too low MIC results were obtained in Bacteroides fragilis quality assessment strains, using gradient strip tests with a ratio of amoxicillin:clavulanic acid of 2:1. We aimed to find the most accurate available gradient strip tests for susceptibility testing of amoxicillin/clavulanic acid in B. fragilis in comparison with agar dilution with EUCAST methodology and breakpoints. Twenty-seven clinical B. fragilis isolates were investigated using gold standard EUCAST amoxicillin/clavulanic acid agar dilution (fixed clavulanic acid concentration at 2 mg/L, with increasing amoxicillin concentrations) as well as three commercial gradient strip tests: XL (ratio), AUG (ratio) or AMC (fixed concentration). Using agar dilution (fixed concentration), 19 isolates were susceptible, 1 isolate was susceptible increased exposure (I) and 7 isolates were resistant. Categorical agreement of the gradient strip tests with agar dilution (fixed concentration) was 70% for XL (ratio), 71% for AUG (ratio) and 89% for AMC (fixed concentration). Very major error rates in comparison with agar dilution (fixed concentration) were 100%, 0%, and 0%, respectively. EUCAST breakpoint usage in amoxicillin/clavulanic acid susceptibility tests for B. fragilis should be accompanied by EUCAST methodology. When using alternative methods such as gradient strip tests, a higher degree of alignment with EUCAST methodology, such as using fixed clavulanic acid concentrations, improves precision. •Amoxicillin/clavulanic acid gradient strip test MIC results of Bacteroides fragilis using a fixed clavulanic acid concentration were acceptably concordant with EUCAST gold standard agar dilution MIC results.•Gradient strip testing of Bacteroides fragilis using a ratio of amoxicillin to clavulanic acid resulted in smaller MIC ranges and low categorical agreement with EUCAST gold standard agar dilution.•Some meropenem resistant division II B. fragilis isolates were amoxicillin/clavulanic acid susceptible.