Clinical Pathology: General Principles, Microbiology

• Nontuberculous mycobacterial (NTM) are ubiquitous in the environment (e.g., soil and water). Their clinical significance on isolation from a patient specimen depends on their known pathogenicity, site of isolation, as well as the patient’s immune status and clinical presentation.

• The isolation of an NTM species is more likely to be considered clinically relevant if it is recovered from a sterile body site or from multiple specimens.

Mycobacterium avium complex (MAC) is a slow-growing NTM that is most commonly identified in the United States using the AccuProbe system. This test uses targeted DNA probes that hybridize with complex-specific 16s rRNA.

• MAC is associated with pulmonary infections in patients with preexisting respiratory disease (e.g., chronic obstructive pulmonary disease, cervical lymphadenitis, and disseminated disease).

• The pathogenesis of MAC infections is not clearly understood, but it is known to be acquired by inhalation or ingestion and not by human-to-human transmission. MAC is known to thrive in hot water, resulting in pulmonary illness in healthy individuals who frequently use hot tubs. It is postulated that the forced air jets in these tubs aerosolize the organism, which is subsequently inhaled.

Griffith DE, Aksamit T, Brown-Elliott BA, et al: An Official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases. Am J Respir Crit Care Med. 2007;175:367-416.

Brown-Elliott BA, et al: Antimicrobial susceptibility testing, drug resistance mechanisms, and therapy of infections with nontuberculous mycobacteria. Clin Microbiol Rev. 2012;25:545-582.

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