Tuberculosis (TB) is caused by Mycobacterium tuberculosis, an acid-fast bacillus (AFB). TB is one of the most common diseases worldwide. This organism infects both compromised (eg, those with HIV infection) and noncompromised hosts. Rapid and accurate detection is important to prevent spread of the disease and for initiation of appropriate chemotherapy. Typically, TB infections produce a wide range of tissue changes including granulomatous inflammation. This inflammation may present as caseous necrosis, aggregates of epithelioid cells, and frequently multinucleated Langhans’ giant cells.
Normally, culture is essential for definitive diagnosis of AFB infections, including those caused by M tuberculosis, M avium complex, or M marinum. Sometimes, clinical lesions are biopsied and all the tissue is embedded in paraffin. When subsequent histological review reveals the presence of an AFB infection (AFB smear-positive tissue and/or presence of granulomatous changes), culture is no longer possible. But, identification of the AFB to the genus/species level can be performed directly from the fixed, paraffin-embedded tissue using pyrosequencing.