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Interpretive criteria for susceptibilities of Haemophilus influenzae to ampicillin, amoxicillin, and amoxicillin-clavulanic acid.

Department of Pathology, St. Vincent Hospital & Medical Center, Portland, OR 97225.

ABSTRACT

One hundred fifty isolates of Haemophilus influenzae (including 30 beta-lactamase-positive strains and 23 beta-lactamase-negative, ampicillin-resistant strains) were tested for susceptibilities to ampicillin, amoxicillin, and amoxicillin-clavulanic acid (A/C) by the broth microdilution method in Haemophilus test medium (HTM) and in Mueller-Hinton medium with lysed horse blood and by the disk diffusion method on HTM agar. Our results support the use of HTM for susceptibility testing of H. influenzae but raise a number of questions regarding the interpretive criteria currently in use, particularly with respect to the fourfold difference in MIC susceptibility breakpoints for ampicillin and A/C and a resulting high proportion of A/C-susceptible beta-lactamase-negative, ampicillin-resistant strains.

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