Detection of N526K-substituted penicillin-binding protein 3 conferring low-level mutational resistance to b-lactam antibiotics in Haemophilus influenzae by disc diffusion testing on Mueller-Hinton agar according to EUCAST guidelines

Results: Epidemiological cut-off values of 19/20 mm for ampicillin (2 mg) and 11/12 mm for benzylpenicillin (1 U) accurately categorized 96% of the study strains, and outperformed cephalosporin-containing discs in the discrimination of mutational resistance in b-lactamase-non-producing isolates. Current EUCAST interpretative criteria for the categorization of clinical resistance showed concordance between resistance rates based on MIC and zone diameter breakpoints for both ampicillin and cefuroxime, but categorization of individual isolates was not consistent.