Effect of prior antibiotics on the susceptibility of Haemophilus influenzae type b to ampicillin

A prospective study of the influence of prior antibiotics on ampicillin susceptibility of H influenzae type b from children with invasive disease was performed from July 1978 to March 1980. One hundred seventy-eight unrelated strains of H influenzae type b were isolated from children with invasive disease. Forty-eight (27%) isolates were resistant to ampicillin. The likelihood of isolating an ampicillin-resistant H influenzae type b was significantly greater in children who had received either any antibiotic or a β-lactam antibiotic than in children who did not receive prior antibiotics. Invasive disease due to H influenzae type b that was ampicillin-resistant occurred almost twice as frequently in children who had received antibiotics within one month of admission to the hospital compared to children studied concomitantly who had not received prior antibiotic therapy.