Specimen cross-contamination by a strain of Mycobacterium tuberculosis lacking nitrate reductase activity.

An unusual strain of Mycobacterium tuberculosis lacking nitrate reductase activity was isolated from specimens from five patients over a 2-week period. Two patients, a father (patient 1) and his son (patient 5), had pulmonary tuberculosis. Specimens from three other patients (patients 2, 3, and 4) processed on the same 2 days as those from patient 1 yielded only sparse growth (approximately five colonies each) of M. tuberculosis. These and other data strongly suggest that specimens from the latter three patients were contaminated during processing with the strain of M. tuberculosis from patient 1.