Susceptibility testing to
sulfadimethoxine of various mycobacteria was made using Ogawa egg medium containing various concentrations of the
drug. Each medium was inoculated by a 0.02 ml-sample of bacterial
suspensions (10 mg wet weight per ml) prepared from 10 day-old (M.
tuberculosis, 14 day-old) cultures growing on Ogawa egg medium after homogenizing the bacteria by shaking with glass beads. The media inoculated were incubated at 37 degrees C for 14 days (M. marinum, at 28 degrees C). The minimal inhibitory concentration (MIC) was determined as the lowest
drug concentration on which the growth of bacteria was completely inhibited. However, residual growth occurred often. This was regarded as negative growth, because control medium containing no
drug always exhibited abundant membraneous growth. Of the mycobacteria tested, M. kansasii (MICs, 0.8-3.2 micrograms/ml) and M. xenopi (MICs, 0.2-3.2 micrograms/ml) were most susceptible to this
drug. Other mycobacteria showed the MICs higher than 3.2 micrograms/ml. The
drug seemed to be useful in the treatment of
infections caused by M. kansasii and M. xenopi. Furthermore, the susceptibility testing to
sulfadimethoxine was considered to be useful for differentiation between two photochromogens, M. kansasii and M. marinum and for differentiation between two scotochromogens, M. scrofulaceum and M. gordonae (Fig. 3 and 4).