Mycobacterium tuberculosis- susceptibility to infection
Overview
Mycobacterium tuberculosis (MTB) is a pathogenic bacterial species in the genus Mycobacterium and the causative agent of most cases of tuberculosis.[1] First discovered in 1882 by Robert Koch, M. tuberculosis has an unusual, waxy coating on the cell surface (primarily mycolic acid), which makes the cells impervious to Gram staining; acid-fast techniques are used instead. The physiology of M. tuberculosis is highly aerobic and requires high levels of oxygen. Primarily a pathogen of the mammalian respiratory system, MTB infects the lungs, causing pneumonia.[1] The M. tuberculosis genome was sequenced in 1998
Diagnosis
Sputum is taken on three successive mornings as the number of organisms could be low, and the specimen is treated with 3% KOH or NaOH for liquefaction and decontamination. Gram stain should never be performed, as the organism is an "acid-fast bacillus" (AFB), meaning that it retains certain stains after being treated with acidic solution. In the most common staining technique, the Ziehl-Neelsen stain, AFB are stained a bright red, which stands out clearly against a blue background; therefore, the bacteria are sometimes called red snappers.[11] The reason for the acid-fast staining is because of its thick waxy cell wall.[12] The waxy quality of the cell wall is mainly due to the presence of mycolic acids. This waxy cell wall also is responsible for the typical caseous granuloma formation in tuberculosis. The component responsible, trehalose dimycolate, is called the cord factor. A grading system exists for interpretation of the microscopic findings based on the number of organisms observed in each field. It should be noted that the Ziehl-Neelsen stain is positive in only 50% of cases, which means that, even if no organisms are observed, further investigation is still required. Acid-fast bacilli can also be visualized by fluorescent microscopy using auramine-rhodamine stain for screening, which makes them appear somewhat golden in color. Also, M. tuberculosis is grown on a selective medium known as Lowenstein-Jensen medium, which has traditionally been used for this purpose. However, this method is quite slow, as this organism requires 6-8 weeks to grow, which delays reporting of results. A faster results can now be obtained using Middlebrook medium. It should be taken into consideration that during an advanced stage of tuberculosis, the organism may infect almost any part of the body, which means that a specimen should appropriately be chosen (e.g. intestinal tuberculosis-stool). An immunochromatographic serological assay for the diagnosis of M. tuberculosis has also been developed.[13]