Susceptibility of ampicillin-resistant Haemophilus influenzae to seven penicillins.

Sixty-seven clinical isolates of Haemophilus influenzae from various sections of the United States, England, and Germany were tested for susceptibility to penicillin, ampicillin, amoxicillin, epicillin, carbenicillin, ticarcillin, and methicillin. Fifty-three of the strains had previously been judged to be ampicillin resistant and 14 had been determined to be ampicillin susceptible. Fifty-two of the 53 resistant strains produced beta-lactamase, but none of the susceptible strains produced it. On the basis of minimal inhibitory concentrations, the most active compounds were ticarcillin and carbenicillin. Whether this greater activity is useful clinically has not been established.