Mycobacteria

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            They are virtually the only bacteria that are acid-fast.

The term "acid-fast" refers to an organism's ability to retain the carbolfuchsin stain despite subsequent treatment with an ethanol-hydrochloric acid mixture.

The major pathogens are Mycobacterium tuberculosis, the cause of tuberculosis, and Mycobacterium leprae, the cause of leprosy.

Atypical mycobacteria, such as Mycobacterium aviumintracellulare complex and Mycobacterium kansasii, can cause tuberculosislike disease but are less frequent pathogens.

Rapidly growing mycobacteria, such as Mycobacterium chelonei, are saprophytes that occasionally cause human disease in immunocompromised hosts (Table 21-1).

Approximately one third of the world's population is infected with this organism.

Important Properties M tuberculosis grows slowly (ie, it has a doubling time of 18 hours, in contrast to most bacteria, which can double in number in 1 hour or less).

Because growth is so slow, cultures of clinical specimens must be held for 6-8 weeks before being recorded as negative.

These proteins are the antigens in the PPD (purified protein derivative) skin test (also known as the tuberculin skin test).

NaOH is used to concentrate clinical specimens; it destroys unwanted bacteria, human cells, and mucus but not the organism.

Transmission & Epidemiology M tuberculosis is transmitted from person to person by respiratory aerosol, and its initial site of infection is the lung.

Patients defective in cellular immunity, such as AIDS patients, are at much higher risk of disseminated, life-threatening tuberculosis.

Infection with measles virus can suppress cell-mediated immunity, resulting in a loss of tuberculin skin test reactivity and, in some instances, reactivation of dormant organisms and clinical disease.

Because drug resistance, especially to isoniazid (see below), is a problem, susceptibility tests should be performed.

However, the organism grows very slowly and susceptibility tests usually take several weeks, which is too long to guide the initial choice of drugs.

It is prescribed for (1) asymptomatic patients whose PPD skin test reaction has recently converted to positive, (2) children exposed to patients with symptomatic pulmonary tuberculosis, and (3) patients with a positive PPD skin test response who undergo immunosuppression.

Important Properties M leprae has not been grown in the laboratory, either on artificial media or in cell culture.

It can be grown in the mouse footpad or in the armadillo.

The optimal temperature for growth (30 °C) is lower than body temperature; it therefore grows preferentially in the skin and superficial nerves.

Transmission Infection is acquired by prolonged contact with patients with lepromatous leprosy, who discharge M leprae in large numbers in nasal secretions and from skin lesions.

The disease occurs worldwide, with most cases in the tropical areas of Asia and Africa.

Pathogenesis The organism replicates intracellularly, typically within skin histiocytes, endothelial cells, and the Schwann cells of nerves.

There are two distinct forms of leprosy---tubercu-loid and lepromatous---with several intermediate forms between the two extremes (Table 21-3).

In tuberculoid leprosy, the cell-mediated immune response to the organism limits its growth, very few acid-fast bacilli are seen, granulomas containing giant cells form, and the lepromin skin test result is positive.

After the onset of therapy, patients with lepromatous leprosy often develop erythema nodosum leprosum (ENL), which is interpreted as a sign that cell-mediated immunity is being restored.

The disfiguring appearance of the disease results from several factors: (1) the skin anesthesia results in burns and other traumas, which often become infected; (2) resorption of bone leads to loss of features such as the nose and fingertips; and (3) infiltration of the skin and nerves leads to thickening and folding of the skin.

Many 'High Reduced or absent Laboratory Diagnosis In lepromatous leprosy, the bacilli are easily demonstrated by performing an acid-fast stain of skin lesions or nasal scrapings.

Treatment The mainstay of therapy is dapsone (diaminodiphenylsulfone), but because sufficient resistance to the drug has emerged, combination therapy is now recommended, eg, dapsche, rifampin, and clofazimine for lepromatous leprosy and dapsone and rifampin for the tuberculoid form.

Treatment is given for at least 2 years or until the lesions are free of organisms.

2. Mycobacterium tuberculosis grows very slowly and has complex nutritional requirements.

10. What are the predisposing factors to disease caused by (1) Mycobacterium avium-intracellulare complex and (2) Mycobacterium fortuitum-chelonei complex?