Chapter 39

Mycobacteria are slender, rod-like bacilli. They belong to the order of Actinomycetales, microbes of the soil. The mycobacteria have complex cell walls rich in lipids and waxes. These cell walls are related to several distinctive features including their staining properties and growth rates.

Although classified as gram-positive, the mycobacteria usually do not take up gram stain and thus appear as “ghost” or gram-neutral forms. Classically the mycobacteria have been stained with the red dye, carbol fuchsin, enhanced by heating or alkalinity. So stained, the mycobacteria are resistant to decolorizing with the potent agent acid-alcohol. This has given rise to the designation “acid-fast bacilli,” or AFB. Modern laboratories may use the fluorochrome technique employing a dye, auramine-o, which fluoresces yellow when excited by ultraviolet light.

Most species of mycobacteria are characterized by very slow growth on culture media, replicating usually in the range of 18–24 h. Thus, laboratory isolation of mycobacteria usually requires 5–15 days in liquid media and 15–30 days on solid media.

The most common human pathogens are Mycobacteriumtuberculosis and M leprae (Hansen’s bacillus, which causes the cutaneous-neural disease leprosy). However, there is an array of related microbes found widely in the soil and water that cause lung and extrapulmonary disease in humans. These latter organisms have been referred to variably as atypical TB, mycobacteria other than TB (MOTT), nontuberculous mycobacteria (NTM) or environmental mycobacteria (EM). In this chapter, “environmental mycobacteria” will be used.

### Essentials of Diagnosis

• • Tuberculosis organisms are spread from human to human but are not exquisitely communicable.
• • Only 10% of people with normal immune function develop active disease following a primary infection.
• • Primary tuberculosis is most often a nondescript infiltrate with ipsilateral reactive hilar or mediastinal nodes.
• • Reactivation tuberculosis is characterized by respiratory symptoms with upper lobe fibronodular infiltrates with or without cavitation.
• • Tuberculosis is usually limited to the lungs, with lymph nodes and pleura the only common sites of extrapulmonary spread.
• • Identification of acid-fast organisms by staining or culture is diagnostic of disease.

Among the mycobacteria there is a group of closely related microbes that is referred to as the “tuberculosis complex.” The group includes M tuberculosis, M bovis, M canetti, M africanum, and M microti. Although the microbes are highly comparable by DNA analysis, they are generally distinguishable in terms of natural reservoirs, human pathogenicity, and transmission patterns. Thus, I believe it is appropriate clinically to preserve the term “tuberculosis” or TB for diseases caused by M tuberculosis. Disease caused by other mycobacteria should, I believe, be referred to as “mycobacteriosis” due to “M x.”

### General Considerations

TB is arguably the most “successful” parasite of humans. The World Health Organization (WHO) estimates that roughly one-third of the world’s population or 2 billion persons are infected with M tuberculosis. However, the patterns ...

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