[Extra and intracellular activity of dirithromycin against Mycobacterium avium]

The in vitro activity of dirithromycin alone and in combination with clofazimin, ethambutol and rifabutin was tested against thirty strains of Mycobacterium avium isolated from patients. Extracellular activity of dirithromycin was assessed by determining MICs using the radiometric methodology in 7H12 broth at two pHs 6.8 and 7.4. The MICs obtained at pH 7.4 were 3 to 4 more dilutions lower than those obtained at pH 6.8. Activity of pairs of antibiotics was measured using the FIC indices. Dirithromycin-clofazimin combination demonstrated the most important additive effects and even produced synergic effect against 5 of 30 strains. Studies of intracellular bacteria showed that the most effective bactericidal combination was dirithromycin, clofazimin and ethambutol together.