Tuberculosis

Tuberculosis (commonly abbreviated as TB) is an infection caused by the bacterium Mycobacterium tuberculosis, which most commonly affects the lungs (pulmonary TB) but can also affect the central nervous system (meningitis), lymphatic system, circulatory system (Miliary tuberculosis), genitourinary system, bones and joints.

Tuberculosis is one of the deadliest and common major infectious diseases today. As of 2004, 14.6 million people have active TB disease with nine million new cases of the disease and nearly two million deaths, [1] mostly in developing countries. However, developing countries are not the only places with tuberculosis. There is a rising number of people in the developed world who contract tuberculosis because they have compromised immune systems, typically as a result of immunosupressive drugs or HIV/AIDS. These people are at particular risk of tuberculosis infection and active tuberculosis disease.

Most of those infected (90%) have asymptomatic latent TB infection (LTBI). There is a 10% lifetime chance that LTBI will progress to TB disease which, if left untreated, will kill more than 50% of its victims. TB is one of the top four infectious killing diseases in the world: TB kills 1.7 million, and malaria kills 2-3 million.

HIV/AIDS, the neglect of TB control programs, and immigration have caused a resurgence of tuberculosis. Multidrug-resistant strains of TB (MDR-TB) and Extreme Drug-Resistance in Tuberculosis (XDR-TB) are emerging. The World Health Organization declared TB a global health emergency in 1993, and the Stop TB Partnership proposed a Global Plan to Stop Tuberculosis which aims to save an additional 14 million lives between 2006 and 2015.

The cause of tuberculosis, Mycobacterium tuberculosis (MTB), is a slow-growing aerobic bacterium that divides every 16 to 20 hours. This is extremely slow compared to other bacteria (although not the slowest), which tend to have division times measured in minutes (among the fastest growing bacteria is a strain of E. coli that can divide roughly every 20 minutes; by contrast, Mycobacterium leprae divides every 20 days). MTB is not classified as either Gram-positive or Gram-negative because it does not have the chemical characteristics of either. If a Gram stain is performed, it stains very weakly Gram-positive or not at all (ghost cells). It is a small rod-like bacillus which can withstand weak disinfectants and can survive in a dry state for weeks but, spontaneously, can only grow within a host organism (in vitro culture of M. tuberculosis took a long time to be achieved, but is nowadays a routine laboratory procedure).

MTB is identified microscopically by its staining characteristics: it retains certain stains after being treated with acidic solution, and is thus classified as an "acid-fast bacillus" or AFB. In the most common staining technique, the Ziehl-Neelsen stain, AFB are stained a bright red which stands out clearly against a blue background. Acid-fast bacilli can also be visualized by fluorescent microscopy, and by an auramine-rhodamine stain.

The M. tuberculosis complex includes 3 other mycobacteria which can cause tuberculosis: M. bovis, M. africanum and M. microti. The first two are very rare causes of disease and the last one does not cause human disease.

Nontuberculous mycobacteria (NTM) are other mycobacteria (besides M. leprae which causes leprosy) which may cause pulmonary disease resembling TB, lymphadenitis, skin disease, or disseminated disease. These include Mycobacterium avium, M. kansasii, and others.

Acid-fast bacilli (AFB) (shown in red) are tubercle bacilli Mycobacterium tuberculosis.