Roxithromycin alone and in combination with sulphamethoxazole against Haemophilus influenzae

Haemophilus influenzae is only moderately susceptible to erythromycin but previous studies in vitro and in vivo have shown that it is fully susceptible when erythromycin is combined with sulphonamides. The purpose of this study was to evaluate whether the activity of the new macrolide antibiotic roxithromycin was improved after combination with sulphamethoxazole. One hundred and eighty fresh clinical isolates of Haemophilus influenzae, comprising 74 ampicillin-susceptible, beta-lactamase negative, and 106 ampicillin-resistant, beta-lactamase positive, strains, were tested for their susceptibility to erythromycin, roxithromycin, sulphamethoxazole, erythromycin-sulphamethoxazole and roxithromycin-sulphamethoxazole in a fixed ratio of 1:19. Bacteriostatic and bactericidal synergy was found between roxithromycin and sulphamethoxazole more commonly than between erythromycin and sulphamethoxazole. Antagonism, was less common between roxithromycin and sulphamethoxazole than erythromycin and sulphamethoxazole. These results suggest that the in-vitro activity of roxithromycin is improved by combination with sulphamethoxazole, making this combination more attractive in the oral therapy of infections due to H. influenzae.