Pathophysiology of Tuberculosis and Microbiological Diagnosis

Abstract

Tuberculosis (TB) is an infectious disease of epidemic proportions, fired not only by poverty and human immunodeficiency virus (HIV) infection but also by incomplete understanding of its pathogenesis and lack of access to accurate and rapid diagnosis. Mycobacterium tuberculosis is a highly successful type of bacteria because it produces two distinct disease entities, namely primary TB that mediates protective immunity to disseminated infection; and post-primary TB that causes tissue damage leading to formation of cavities necessary to bacterial transmission. Following exposure to a person with pulmonary TB, the risk of developing infection depends on the capacity of the person infected to transmit the disease and the susceptibility of the person exposed to infection. From exposure to infection to disease, there is a pathogenetic continuum. Individuals may advance or reverse states within the spectrum of infection. The dissemination of M. tuberculosis out of the lungs happens during all infections and results in secondary lesions. The microbiological diagnosis of active TB is based on acid-fast bacillus (AFB) smear microscopy, nucleic acid amplification tests (NAAT), and culture. Culture remains the gold standard for TB diagnosis. It increases the potential of diagnosing TB at early stages of the disease, allows extrapulmonary TB diagnosis, species identification, and drug susceptibility testing. However, it takes weeks before results are available. NAAT have significantly reduced this delay. The identification of individuals with latent TB infection is based on imperfect tests, namely tuberculin skin test and interferon-γ release assay.