Articles on Medical Diseases and Conditions

There are other mycobacteria besides Mycobacterium tuberculosis, some of which are frequently pathogenic for humans and some of which rarely cause human infection. The nontuberculous mycobacteria were originally called “atypical mycobacteria.” The first useful classification was that of Runyon, who subdivided the nontuberculous mycobacteria into four groups, depending on growth speed and colony characteristics (Table 14-2). These groups have some clinical value as rough guides to the type of organism present while awaiting more definitive identification and speciation. It is desirable to place the organisms in the correct species, since some members (species) of any of the Runyon groups may not be pathogenic very often or may differ in degree of pathogenicity. Mycobacterium intracellulare or Mycobacterium avium-intracellulare complex (formerly known as the Battey Mycobacterium), a member of Runyon group III, and Mycobacterium kansasii, from Runyon group I, together cause the majority of significant nontuberculous mycobacterial infections in about equal proportions. These produce a disease similar to pulmonary tuberculosis (although often milder or more indolent) that is much more frequent in adults. M. avium-intracellulare infections are very frequent in persons with acquired immunodeficiency syndrome (AIDS) or AIDS-related conditions. The mycobacterial infection frequently becomes bacteremic or disseminated due to compromise of the immune system by HIV-1 causing AIDS. Runyon group II organisms are more frequent in children and clinically tend to cause cervical lymphadenopathy. Diagnosis of the nontuberculous mycobacteria is essentially the same as for M. tuberculosis. Skin tests (old tuberculin or PPD) for M. tuberculosis will also cross-react with the nontuberculous mycobacteria. In general, the nontuberculous mycobacteria tend to produce less reaction to standard tuberculin skin tests than M. tuberculosis. In fact, several studies claim that the majority of positive intermediate-strength tuberculin skin test results that produce a reaction of less than 10 mm diameter are due to nontuberculous mycobacterial infection rather than TB. Skin test antigens are available for each of the nontuberculous mycobacterial groups, although some reports challenge the specificity of these preparations. The main clinical importance of these nontuberculous organisms is resistance that many have toward one or more of the standard antituberculous chemotherapeutic agents.

Table 14-2 Classification of the atypical mycobacteria

Several reports have linked one of the non-MTB organisms, M. paratuberculosis, to Crohn’s disease (regional ileitis).