Mycobacterium avium complex develop resistance to synergistically active drug combinations during infection.

Isolates of Mycobacterium avium complex from five patients on longterm (3-5 yrs) anti-mycobacterial drug treatment were collected during the early and late phase of disease, and studied in vitro for their susceptibility to anti-mycobacterial drugs and drug-combinations. All isolates were resistant or moderately resistant to ethambutol, rifampicin and streptomycin when given singly; however, all strains isolated early in the disease were susceptible to the combination of ethambutol with either rifampicin or streptomycin. All late isolates had developed resistance to one or both of these combinations. Three of the patients died within a year after the last isolation of M. avium complex, and the two remaining patients still have severe chronic disease. It is concluded that the susceptibility of M. avium strains to combinations of drugs should be monitored during the course of treatment, in order to guide the selection of effective drug-combinations throughout the infection.