Antimicrobial resistance among lower respiratory tract isolates of Haemophilus influenzae: Results of a 1992-93 Western Europe and USA collaborative surveillance study

During 1992 and 1993, 2718 respiratory tract isolates of Haemophilus influenzae were obtained from two study centres in each of five West European countries and five study centres in the USA. beta-Lactamase production was assessed and MICs of 14 antimicrobial agents determined in a single co-ordinating laboratory using a broth microdilution method in Mueller-Hinton-lysed horse blood medium. The prevalence of strains producing beta-lactamase varied between 0 and 37.9%. In general, the highest prevalence was in study centres from Spain and the USA with slightly lower rates observed in France and the UK. Only a single confirmed beta-lactamase-negative, ampicillin resistant strain was recovered during the entire study. Erythromycin resistance, defined as an MIC of greater than or equal to 4.0 mg/L, was noted in 57.5% of isolates. Among the other antimicrobials tested, resistance rates greater than or equal to 1.0% were observed only with cefaclor (3.7%), chloramphenicol (1.4%) and co-trimoxazole (2.5%). In no case, was the prevalence of resistance or beta-lactamase production significantly greater in 1993 than in 1992.