Nontuberculous mycobacteria (NTM) are ubiquitous in our environment so that exposure to these organisms is universal and unavoidable.1 Unlike traditional tuberculosis, most of the approximately 150 currently identified NTM species are either nonvirulent or have low virulence for humans. The prevalence of NTM is rising in the United States, with many epidemiologic reports suggesting that NTM lung disease has surpassed tuberculosis.2,3 Clinicians are encountering NTM infections with increasing frequency and a general knowledge of common pathogens and diagnostic criteria are essential for the care and successful treatment of these individuals.
Prior to the AIDS epidemic, most NTM cases presented as indolent, cavitating pulmonary infections in persons with underlying lung diseases such as chronic obstructive pulmonary disease (COPD) or previous tuberculosis. During the 1980s, infections due to the more common NTM (Mycobacterium avium, some M. kansasii, some unclassified) emerged as complications of acquired immunodeficiency syndrome (AIDS). M. avium and M. intracellulare were termed M. avium complex or MAC infections.4,5 Subsequently, a syndrome of predominantly mid–lung zone bronchiectasis with MAC pulmonary infection was described in otherwise healthy middle-aged women, many with distinct body morphotypes including scoliosis, pectus excavatum, and mitral valve prolapse.6,7 This syndrome is often referred to as Lady Windermere syndrome.8 Mycobacterial infections after solid-organ and hematopoietic transplantation have also increased in frequency, reflecting both increased exposure and/or improved diagnostic methods, and almost universal use of central venous access devices.9 In the absence of mandatory infection reporting, the true incidence of positive cultures or NTM disease in the general population and transplant recipients in the United States can only be estimated.
Organisms and Development of Disease
The microbiology, epidemiology, and pathogenesis of nontuberculous mycobacterial infections are considered below.
Almost 150 NTM species have now been identified and speciated. The increased number of species reflects improved microbiologic techniques for isolating NTM from clinical specimens and the use of 16 S rRNA gene sequencing as a standard for defining new species.10,11 To simplify understanding of these organisms, particularly as applied to clinical circumstances, they often are grouped into complexes of closely related species. The M. avium complex, for example, consists of multiple species, with the most frequent being M. avium and M. intracellulare. Certain species, such as M. kansasii are associated with human disease more often than others and are presumed to be more virulent while species such as M. gordonae, M. terrae complex, and M. mucogenicum most often represent contamination of the respiratory tract from exposure to tap water.12
A number of important considerations in the laboratory evaluation should be recognized:
Clinicians must be aware that these organisms may be present and request mycobacterial cultures in appropriate specimens. The Gram stain will not adequately detect mycobacteria. The ...