Susceptibility of Haemophilus influenzae type b to ampicillin-sulbactam

Seventy-five strains of Haemophilus influenzae type b, including 45 beta-lactamase-positive strains, were tested by MIC and time kill studies for susceptibility to ampicillin-sulbactam at various ratios. beta-Lactamase-negative strains were inhibited by lower concentrations of ampicillin-sulbactam than beta-lactamase-positive organisms. beta-Lactamase-negative strains showed a decrease in CFU per milliliter by a factor of 10(4) after a 24-h incubation with ampicillin-sulbactam. beta-Lactamase-positive isolates showed an initial decrease by a factor of up to 10(2) CFU per milliliter, but by the end of incubation these isolates grew to approximately the same cell density as the antibiotic-free control regardless of ampicillin-sulbactam ratios. Caution should be exercised in the use of this combination in treatment of meningitis, in which a high bacterial density is commonly encountered.