Comparative susceptibilities of ampicillin and chloramphenicol resistant Haemophilus influenzae to fifteen antibiotics.

Eighty-three isolates of ampicillin and chloramphenicol resistant Haemophilus influenzae were tested for susceptibility to fifteen antibiotics by the agar dilution method. Fifty-four were from paediatric patients with H. influenzae disease and 29 from nasopharyngeal carriers (pre-school children). Twenty-five strains belonged to serotype b, one to serotype a, one to serotype c and the rest were non-typable. All strains produced beta-lactamase and inactivated chloramphenicol in a rapid bioassay, suggesting the production of chloramphenicol-acetyltransferase. The most active drugs were ceftriaxone, cefotaxime, latamoxef, aztreonam and desacetyl-cefotaxime (MIC90: 0.03, 0.06, 0.12, 0.25 and 0.25 mg/l, respectively). Cefuroxime, rifampicin and imipenem (MIC90 1 mg/l), and the combination of amoxycillin and clavulanic acid (MIC90 2:1 mg/l), also showed good activity. Cefaclor, erythromycin, tetracycline, trimethoprim, sulfamethoxazole and cotrimoxazole were the least active of the drugs studied. The excellent in-vitro activity of the new beta-lactam agents against H. influenzae resistant to ampicillin and chloramphenicol offers a therapeutic alternative in the treatment of serious infections caused by these micro-organisms.