In vitro activity of second and third generation cephalosporins against ampicillin susceptible and resistant haemophilus influenzae.

One hundred seventy-five clinical isolates of Haemophilus influenzae (including 74 beta-lactamase-producing strains) were examined for susceptibility to ampicillin, cefonicid, cefamandole, cefuroxime and cefotaxime. Cefonicid and cefamandole exhibited similar activity against ampicillin-susceptible strains (MIC90 = 0.2 mg/l for both antibiotics); cefuroxime was slightly less active (MIC90 = 0.01 mg/l). However, cefonicid, cefuroxime and cefotaxime were all more active against beta-lactamase-producing H. influenzae than cefamandole (MIC90 = 1.0 mg/l for cefonicid, MIC90 = 2.0 mg/l for cefuroxime, MIC90 = 0.01 mg/l for cefotaxime, MIC90 = 5.0 mg/l for cefamandole). One hundred twenty-five of the 175 isolates were also tested for susceptibility to cefonicid and cefamandole by disc diffusion technique and a plot of zone diameter vs. MIC was analyzed for the beta-lactamase-producing strains.