In-vitro activity of clarithromycin combined with its 14-hydroxy metabolite A-62671 against Haemophilus influenzae.

Clarithromycin and its 14-hydroxy metabolite (A-62671), were tested against 20 strains of Haemophilus influenzae. Minimum inhibitory and bactericidal concentrations of the two compounds alone and in combination were determined in a microbroth system which allowed continuous turbidimetric measurement for the construction of growth curves. MICs of clarithromycin were 2-8 and 1-4 mg/l for A-62671 for the majority of strains. MBCs were identical to or one dilution higher than the respective MICs. Combinations of the two compounds were inhibitory or bactericidal at concentrations that were lower than the MICs or MBCs. On the basis of fractional inhibitory (FIC) or fractional bactericidal (FBC) concentration indices, the interactions between clarithromycin and its metabolite were defined as additive.