Hemophilus influenzae type B susceptibility to 17 antibiotics

One hundred and one strains of Hemophilus influenzae type b isolated at the Children's Hospital Medical Center from blood or CSF and 18 strains known to be resistant to ampicillin of which 17 were supplied to us by others were tested for their sensitivity to 17 antibiotics. Two groups were defined according to their sensitivity to ampicillin. When 10-3 bacteria were applied, 18 strains isolated from patients with ampicillin treatment failures had a median minimum inhibitory concentrations of 3.1 mug/ml. At 10-6 bacteria the median MIC for the resistant strains increased 512-fold, whereas the sensitive strains increased two-fold. The most active antibiotics against sensitive and resistant strains were rifampin, chloramphenicol, gentamicin, rosamycin, and erythromycin with MICs ranging from 0.2-0.8 mug/ml at 10-3 bacteria and from 0.2-1.6 mug/ml at 10-6.