In vitro activity of orally administered antimicrobial agents against Haemophilus influenzae recovered from children monitored longitudinally in a group day-care center.

To determine whether the prevalence of resistance to commonly used oral antimicrobial agents varied over time, we compared the in vitro susceptibilities of 217 strains of Haemophilus influenzae recovered from nasopharyngeal secretions of children in a day-care center studied longitudinally between 1979 and 1987. The overall rate of beta-lactamase production in these strains was 18%, with rates of 57% in type b isolates (n = 21) and 14% in non-type b isolates (n = 196). The percentages of isolates for which MICs were less than or equal to 1.0 micrograms/ml for amoxicillin alone, amoxicillin in combination with clavulanic acid, and cefuroxime alone were 82, 92, and 93%, respectively. The percentage of strains for which cefaclor MICs were less than or equal to 1.0 micrograms/ml was only 0.5%. Isolates for which chloramphenicol MICs were greater than 2.0 micrograms/ml or for which trimethoprim-sulfamethoxazole MICs were greater than 0.5/9.5 micrograms/ml were uncommon: 1 and less than 1%, respectively. High concentrations of erythromycin alone and in combination with sulfisoxazole were required to inhibit the majority of test strains; there was no evidence of erythromycin-sulfisoxazole synergy. In vitro susceptibility to commonly used oral antimicrobial agents remained at a constant level when H. influenzae isolates collected from children in a day-care center during 1979 through 1983 were compared with strains collected during 1984 through 1987.