Activity of cephalosporin antibiotics against Haemophilus influenzae.

Minimum inhibitory concentrations (MICs) of nine cephalosporins were determined for 90 isolates of Haemophilus influenzae including 70 amoxycillin-resistant strains. Cefotaxime was the most active of the parenteral antibiotics tested, with a modal MIC of 0.03 mg/l, compared with ceftazidime 0.12 mg/l, cefamandole 0.25 mg/l, cefuroxime 0.5 mg/l and cefathiamidine 4 mg/l. Cefaclor was the most active of the oral cephalosporins tested, with a modal MIC of 2 mg/l compared with cephalexin 8 mg/l and cephradine 32 mg/l. Amoxycillin-sensitive and beta-lactamase-producing haemophili had similar MICs for all cephalosporins. Twenty amoxycillin-resistant beta-lactamase non-producing strains showed high MICs for all beta-lactams tested. A cell permeability barrier is postulated as a mechanism of resistance in these strains.