In-vitro synergistic activity between ethambutol and fluorinated quinolones against Mycobacterium avium complex

Ciprofloxacin, ofloxacin and norfloxacin were ineffective at clinically relevant concentrations against the Mycobacterium avium complex (MAC) in vitro as measured by radiometric respirometry. Only two of 30 clinical isolates of MAC were susceptible to any of the tested quinolones. By contrast good antibacterial activity was obtained when any of the quinolones was combined with ethambutol. The synergistic effect was most pronounced for the combination of ethambutol and ciprofloxacin, to which 76 of 100 strains were susceptible. It is suggested that the synergism is based on an enhanced penetration of the quinolones by ethambutol.