Comparison of agar dilution, broth dilution and disk diffusion testing of Ampicillin against Haemophilus spp. by using in house and commercially prepared media

An evaluation to determine the optimal method for the in vitro susceptibility testing ofHaemophilus strains to ampicillin was undertaken. In our hands, in-house-prepared Haemophilus Test Medium used by either the broth macrodilution or agar dilution method produced the most consistent results, especially for I-lactamase-negative, ampicillin-resistant H. influenzae strains. In 1985, the National Committee for Clinical Laboratory Standards (NCCLS) published disk diffusion (DD) and dilu-tion susceptibility testing criteria for the in vitro susceptibility testing of Haemophilus influenzae (M2-A3 [12] and M7-A [13], respectively). These standards applied to the testing of ampi-cillin, amoxicillin-clavulanate, ampicillin-sulbactam, and chlor-amphenicol when H. infiuenzae was tested with chocolatized Mueller-Hinton agar for DD testing and Mueller-Hinton broth supplemented with lysed horse blood for dilution susceptibility testing. In January 1990, NCCLS published revised guidelines for H