Ampicillin-resistant Haemophilus influenzae colonizing ambulatory children.

The prevalence of ampicillin sodium-resistant Haemophilus influenzae was determined from throat cultures of 305 ambulatory children. Resistant strains were detected in 3% of children, amounting to 12% of Haemophilus isolates. Factors associated with carriage of ampicillin-resistant strains were sought: only ampicillin exposure was significant. Among children who had received ampicillin or amoxicillin trihydrate within six months, 9% harbored ampicillin-resistant strains compared with 1.4% among those not exposed. Colonization with H influenzae was more frequent following the use of ampicillin, and a higher proportion of isolates was resistant rates was observed with other antibiotics or with factors such as age, sex, otitis history, or day-care center exposure. The association between ampicillin/amoxicillin usage and carriage of ampicillin-resistant strains is increasingly clear. It remains to be determined whether the use of newer antibiotics for otitis also will encourage the development of resistance in H influenzae.