Comparative bactericidal activity of cefixime, carumonam, enoxacin and roxithromycin with those of other antibiotics against resistant Haemophilus influenzae including beta-lactam tolerant strains.

One hundred isolates of Haemophilus influenzae including 50 beta-lactamase producing, five ampicillin-resistant non-beta-lactamase producing and five beta-lactam tolerant strains were tested for susceptibility (MICs and MBCs) to ampicillin, aztreonam, carumonam, cefixime, cefaclor, cefamandole, cefotaxime, imipenem, enoxacin, ciprofloxacin, roxithromycin, erythromycin, chloramphenicol, and co-trimoxazole, by a microdilution broth method. Cefotaxime, enoxacin and ciprofloxacin with MIC90 and MBC90 of less than 0.03 mg/l) were the most active antimicrobial agents tested. Cefixime, carumonam, aztreonam, and co-trimoxazole (MIC90 and MBC90 less than 0.25 mg/l) showed good activity against most strains. Roxithromycin and erythromycin had limited antibacterial activity (MIC90, 8 and 4 mg/l respectively). There were no chloramphenicol-resistant strains. Five beta-lactamase-negative strains were resistant to ampicillin, cefaclor and cefamandole but susceptible to other beta-lactams tested. Different patterns of tolerance were observed: four of five tolerant strains were tolerant to ampicillin and cefamandole, three to cefixime, cefaclor and cefotaxime, one to aztreonam. One tolerant strain was a beta-lactamase producer. Two other strains were tolerant only to co-trimoxazole.