Quantitative antibiotic susceptibility testing: Haemophilus influenzae type B.

Twelve strains of Haemophilus influenzae were tested for susceptibility to gentamicin, ampicillin, chloramphenicol and cephalothin using two methods, agar dilution and microdilution (broth). Although inocula and incubation conditions were standardized, significant differences in the minimal inhibitory concentration (MIC) were seen as a result of growth media. Method dependent differences were also observed for some antibiotics. Notwithstanding such variation, high level resistance of H. influenzae to ampicillin was readily detected by either broth or agar dilution tests.