Identification of β-Lactamase-Negative, Ampicillin-Resistant Strains of Haemophilus influenzae with Four Methods and Eight Media

A challenge set of 143 non-beta-lactamase-producing strains of Haemophilus influenzae was tested for ampicillin susceptibility on two broth media and six agar media, using broth microdilution, agar dilution, disk diffusion, and E-test procedures. When beta-lactamase-negative, ampicillin-resistant (BLNAR) strains were defined as those for which the ampicillin MIC was > or = 4.0 microg/ml, 5 to 44% of our selected strains were BLNAR depending on the medium and/or test method used. If nonsusceptible strains for which ampicillin MICs were intermediate were included in the BLNAR category, 32 to 50% of our isolates would be considered BLNAR. These data emphasize the need for a standardized testing procedure and a universal definition of BLNAR strains before the clinical relevance of such strains can be evaluated. NCCLS dilution tests with haemophilus test medium broth or agar are preferred for testing ampicillin against H. influenzae.