Fast detection of Mycobacterium tuberculosis in culture-positive sputum samples by nitrate reductase activity

Microscopy and bacterial culture are the main tools in the diagnosis of tuberculosis. Since the slow growth of Mycobacterium tuberculosis impairs rapid diagnosis strategies, especially in countries where the latter are the only available resources, the ongoing development of new and inexpensive tools based on mycobacterial metabolism optimizing growth detection with preliminary identification is greatly welcome. When compared to the other species from the M. tuberculosis complex, M. tuberculosis is a strong nitrate reducer. Current assay compares the nitrate reductase activity of M. tuberculosis from pulmonary specimens cultivated in nitrate-supplemented media. Fifty-five sputum samples were decontaminated and inoculated in conventional (Middlebrook 7H9. Ogawa Kudoh-OK) and in nitrate-supplemented media (Middlebrook 7H9-N, Ogawa Kudoh-N). An aliquot from the media directly reacted with Griess reagent (7H9-N and OK-N) every five days, or transferred to a nitrate substrate solution (7H9, OK). Nitrate to nitrite reduction was considered positive, revealed by the pink color, indicating bacterial growth. As reference method, the Mycobacteria Growth Indicator Tube (MGIT) was used for sensitivity calculations and statistical analysis. 7H9-N and OK-N assays proved to perform better in detecting M tuberculosis than conventional assays (7H9 and OK). Indeed, broth nitrate-supplemented medium (7H9-N) was comparable to MGIT to detect M. tuberculosis, except in growth detection time. Results show that 7H9-N may be used as an alternative tool particularly in low-income countries since it is a simple and cheap technique, and does not restrict diagnosis to single-source products.