Emergence of rifampin-resistant Mycobacterium tuberculosis in Massachusetts.

Tests of sensitivity to rifampin of over 2,200 strains of Mycobacterium tuberculosis demonstrated a progressive increase in the number of rifampin-resistant isolates during the past four years in Massachusetts. No resistant strains were isolated in 1971, but two strains resistant to 1 mug of rifampin/ml were isolated in 1972. Nine rifampin-resistant strains were isolated in 1973 and 16 were isolated in 1974. All but two of the resistant strains were isolated from patients who had received therapy for tuberculosis and had demonstrated resistance to other antituberculous agents. Rifampin-resistant strains were isolated from two patients, however, who had not received prior chemotherapy. Both strains were fully susceptible in vitro to other antituberculous drugs.