Mechanisms of ampicillin resistance in Haemophilus influenzae from respiratory tract.

13 of 100 consecutive isolates of Haemophilus influenza obtained from respiratory specimens over the six months to June, 1979, had diminished sensitivity to ampicillin. 6 of the 13 strains produced beta-lactamase, whilst the remaining 7 had no evidence of this enzyme, either in whole cells or in extracts prepared by sonication. The minimum inhibitory concentration of ampicillin for the penicillinase-negative strains ranged from 1 mg/l to 8 mg/l on repeated testing with a carefully controlled agar-dilution technique. The findings contrast strongly with those of earlier surveys of the sensitivity of respiratory strains of H. influenzae to ampicillin and confirm the existence of two mechanisms of resistance to ampicillin in the species.