Exogenous reinfection with Mycobacterium tuberculosis confirmed by phage typing.

Cultures of sputum of a patient with his third clinical episode of pulmonary tuberculosis yielded a strain of Mycobacterium tuberculosis that was distinctly different in drug susceptibility, urease activity, and phage type from the strain that had caused his second episode. A potential source of exogenous reinfection was identified. This occurrence in no way controverts existing epidemiologic evidence that endogenous reactivation is the pathogenetic mechanism for most initial episodes of tuberculosis, especially in areas of low infectivity.