The susceptibility testing of 13 strains of Mycobacterium leprae to rifampicin and the determination of minimal effective dosage.

By use of the mouse footpad technique, the susceptibility testing of 13 strains of Mycobacterium leprae to rifampicin (RFP) and the determination of minimal effective dosage (MED) were carried out. Among these strains of M. leprae, 8 were obtained from previously untreated multibacillary leprosy patients and 5 from relapsed leprosy patients without using RFP previously. The results showed that the MED of all strains to RFP were less than or equal to 0.001% FRP in the diet, 5 strains being equal to 0.001%, 5 less than or equal to 0.0001%, 2 greater than or equal to 0.0003% and 1 less than or equal to 0.0003%. The results indicated that the MED value of RFP could be lower than that of other reports. Because the critical concentration of RFP for assessment of RFP-resistant strains is not well established a further study would be worthwhile. The results of the determination of sera RFP concentrations in mice administered the RFP diet were identical with that of Holmes' report. Five of the 13 strains also showed that the growth of bacilli were suppressed by 10 mg/kg RFP using the gavage method.