Previously the author observed that
sulfadimethoxine is most effective among sulfa drugs against Mycobacterium avium complex strains (Tsukamura, M.: Kekkaku 58: 247-250, 1983) and used this
drug in the treatment of pulmonary
infection caused by M. avium complex (Tsukamura, M.: Kekkaku 59: 33-37, 1983). In the present study, some supplemental observations were carried out on the in vitro activity of this
drug. In vitro susceptibility testing of
sulfadimethoxine by the use of Ogawa egg medium was influenced by the number of viable bacteria (colony-forming units) used in the susceptibility testing. By the use of small inocula (50 to 100 colony-forming units), minimal inhibitory concentration (MIC) was determined as 0.8 to 3.13 micrograms/ml, whereas, by the use of large inocula, the MIC was determined as 6.25 to 25 micrograms/ml. Furthermore, the prolongation of incubation time resulted in the elevation of the MIC. The finding shows that the activity of this
drug is only to delay the growth. Comparing various combinations with low concentrations of other drugs, combinations with p-aminosalicylate,
isoniazid and
pyrazinamide seemed to be antagonistic, and combinations with the other drugs seemed to be additive. The influence of the inoculation size on the reading of MICs were compared in Mycobacterium tuberculosis and M. avium complex strains. The susceptibility testings to most drugs were influenced by the inoculation size. However, the testings to
ethionamide,
ethambutol and
isoniazid in M.
tuberculosis strains were less influenced by the inoculation size. In contrast, the susceptibility testings to
ethionamide and
isoniazid in M. avium complex strains seemed to be more influenced.(ABSTRACT TRUNCATED AT 250 WORDS)