[Study of the minimal inhibitory concentration of rifabutine (Ansamycin LM 427) for Mycobacterium tuberculosis, Mycobacterium xenopi and Mycobacterium avium-intracellulare].

: Ansamycin LM 427 (or rifabutine) is a spiropiperidyl-rifamycin which has been proposed as a treatment for tuberculosis and mycobacterial infections resistant to rifampicin. The minimal inhibitory concentration (MCI) of rifabutine and of rifampicin for 10 strains of M. tuberculosis sensitive to rifampicin, 15 strains of M. tuberculosis resistant to rifampicin, 10 strains of M. xenopi and 15 of M. avium-intracellulare have been measured in a comparative fashion on gel DIFCO 7H11 medium and also on a Loewenstein-Jensen medium. The MCI of rifampicin and of rifabutine inhibiting 90% of the strains (MCI 90) of M. tuberculosis sensitive to rifampicin have been 0.06 mg/l on 7H11 media and 1 mg/l on Loewenstein-Jensen media respectively. For strains of M. tuberculosis resistant to rifampicin they were respectively 256 and 8 mg/l on 7H11 and of 1024 and 128 Loewenstein-Jensen, for the strains of M. xenopi they were 8 and 0.5 on 7H11 and 64 and 4 mg/l on Loewenstein-Jensen respectively, and finally for strains of M. avium-intracellulare they were 64 and 2 mg on 7H11 and 256 and 16 on Loewenstein-Jensen media. With rare exceptions for proven mycobacterial infections there was a link of the order of 20:1 between the values for CMI of rifampicin and rifabutine measured on Loewenstein-Jensen medium and on 7H11; the MCIs were ten times more elevated on the first than on the second and the liaison between the MCI of rifampicin and the MCI of rifabutine tended to show that these two antibiotics have the same mode of action.(ABSTRACT TRUNCATED AT 250 WORDS)