Determination of Haemophilus influenzae and Haemophilus parainfluenzae susceptibility to ampicillin and other antibiotics.

A sample comprising 40 H. influenzae and 74 H. parainfluenzae strains was used to verify methods for determining susceptibility to antibiotics. Modified Levinthal agar proved to be suitable for the agar dilution and agar diffusion method, while brain heart infusion with the thermally released components of sheep blood (X and V factor) and lysed horse blood performed well in the dilution micromethod. The iodometric method served well for beta-lactamase production. A substantial proportion of strains was resistant to penicillin, erythromycin, roxitromycin and sulfamethoxazole. Ampicillin susceptibility was of crucial importance. Resistance was largely due to beta-lactamase production. Since there are ampicillin-resistant strains which fail to produce beta-lactamase, it is necessary either to determine the MIC value or use a disk with 2 micrograms ampicillin. A disk containing 10 micrograms ampicillin may yield a false positive result.