Bacteriostatic and bactericidal activities of rifampicin, ethambutol, enviomycin and streptomycin on
Mycobacterium avium-Mycobacterium intracellulare complex (MAI complex) strains were determined. The susceptibility testings were made in Ogawa egg medium, and minimal inhibitory concentrations (MICs) were determined as the lowest concentration of drugs, in which the growth of 20 to 100 colony-forming units was completely inhibited. The MICs correspond to the MICs that can inhibit the growth of 95 to 99% of bacterial population. Bactericidal activity was determined in a modified Dubos liquid medium (1.3g of Dubos TB Broth Base were dissolved in 180m
l of distilled water and this solution was supplemented by 20m
l of bovine serum). A one m
l-sample of bacterial suspensions (10mg wet weight per m
l) was added to 9m
l of the Dubos liquid medium, and the medium was incubated at 37°C for 0, 1, 3 and 7 days under shaking condition (56 strokes per minute 8cm amplitude). The bactericidal activity was measured as the number of colony-forming units contained in a 0.02m
l-sample of the medium.
The bactericidal activities of rifampicin and ethambutol were weak or absent even in strains 13008 and 13016, which were very susceptible to all four drugs. However, the bactericidal activities of streptomycin and enviomycin could be observed in these strains. In contrast, the bactericidal activities of all four drugs were absent or, if any, very weak in strains 13022, 13034 and 11004, which were naturally resistant to these drugs in the susceptibility testings, although the strains were isolated from patients who were untreated previously by any antituberculosis drugs or from fowl (strain 11004). On the other hand, the bactericidal activities of rifampicin, streptomycin and enviomycin were evident in
Mycobacterium tuberculosis strain H37Rv. Only ethambutol was not bactericidal on this
M. tuberculosis strain.
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