Evidence for infection by two distinct strains of Mycobacterium tuberculosis in pulmonary tuberculosis: report of 9 cases.

The sputum cultures of 26 patients in whom bacteriologic relapse occurred during or after an initial course of treatment with antimicrobial drugs were compared by phage typing with cultures isolated previously from the same patients. Nine (34 per cent) were different in phage type. Whether this indicates exogenous reinfection as the mechanism for the relapse, the presence of two distinct phage types present from the outset, or change in phage type of the original single strain has not been determined. These observations may have implications relating to epidemiology and control of tuberculosis.