Drug sensitivity of M. leprae isolated from leprosy patients administered DDS for long period of time.

Drug sensitivity was tested using liquid medium on three stains isolated from the subcutaneous nodules of L-type patients who have long been administered DDS alone. The results revealed that the first strain was resistant to DDS up to the concentration of 1.0 microgram/ml suggesting as if it were DDS dependent or enhanced strain, whereas the second strain was completely sensitive to DDS even at the lowest concentration of 0.01 microgram/ml suggesting possible inactivation of this drug in the host patient. The third strain was completely resistant to 0.1 microgram/ml, but sensitive to 1.0 microgram/ml of DDS, suggesting that the therapeutic effect can not be expected any more, when the strain becomes resistant to 0.1 microgram/ml of DDS. All of the three strains were sensitive to REP at the concentration of 0.01 microgram/ml, and the host patients of the former two strains showed rapid improvement of the clinical symptomes after REP administration. That the second strain was sensitive to INH at the concentratin of 0.01 microgram/ml suggested the availability of the combined use of INH in the chemotherapy of leprosy.