Interpretive criteria for susceptibilities of Haemophilus influenzae to ampicillin, amoxicillin, and amoxicillin-clavulanic acid.
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.
Fuchs
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PC
Department of Pathology, St. Vincent Hospital & Medical Center, Portland, OR 97225.
Barry
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AL
D000658:Amoxicillin / Q000494:pharmacology*; D019980:Amoxicillin-Potassium Clavulanate Combination; D000667:Ampicillin / Q000494:pharmacology*; D000668:Ampicillin Resistance; D002969:Clavulanic Acids / Q000494:pharmacology; D004359:Drug Therapy, Combination / Q000494:pharmacology*; D006193:Haemophilus influenzae / Q000187:drug effects*; D008826:Microbial Sensitivity Tests; D001618:beta-Lactamases / Q000032:analysis
D002969:Clavulanic Acids; D019980:Amoxicillin-Potassium Clavulanate Combination; D000667:Ampicillin; D000658:Amoxicillin; D001618:beta-Lactamases