Nontuberculous mycobacteria

Nontuberculous mycobacteria (NTM), or atypical mycobacteria or mycobacteria other than tuberculosis (MOTT), are mycobacteria which do not cause tuberculosis or Hansen's disease (also known as leprosy).

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Introduction

Mycobacteria are a family of small, rod-shaped bacilli that can be classified into 3 main groups for the purpose of diagnosis and treatment:

• Mycobacterium tuberculosis complex which can cause tuberculosis: M. tuberculosis, M. bovis, M. africanum, , M. microti and M. canetti.
• M. leprae which causes Hansen's disease or leprosy.
• Nontuberculous mycobacteria (NTM) are all the other mycobacteria which can cause pulmonary disease resembling tuberculosis, lymphadenitis, skin disease, or disseminated disease.

Taxonomy

Nontuberculous mycobacteria are often called anonymous mycobacteria as this group of bacteria are not properly classified 50 years ago. In 1959, botanist Ernest Runyon put these human disease-associated bacteria into four groups:^[1]

• Photochromogens, which develop pigments in or after being exposed to light. Examples include M. kansasii, M. simiae and M. marinum.^[1]
• Scotochromogens, which become pigmented in darkness. Examples include M. scrofulaceum and M. szulgai.^[1]
• Non-chromogens, which includes a group of prevalent opportunistic pathogens called M. avium complex (MAC). Other examples are M. ulcerans, M. xenopi, M. malmoense, M. terrae, M. haemophilum and M. genavense.^[2]
• Rapid growers include four well recognized pathogenic rapidly growing non-chromogenic species: M. chelonae, M. abscessus, M. fortuitum and M. peregrinum. Other examples cause disease rarely, such as M. smegmatis and M. flavescens.^[2]

The number of identified and cataloged NTM species has been increasing rapidly, from about 50 in 1997 to over 125 by January 2007. The surge is mainly due to improved isolation and identification technique.^[3]

Epidemiology

NTM are widely distributed in the environment, particularly in wet soil, marshland, streams, rivers and estuaries. Different spieces of NTM prefer different types of environment.^[4] Human disease is always acquired from environmental exposures, and unlike tuberculosis and leprosy, there has been no evidence of animal-to-human or human-to-human transmission of NTM, hence its alternative label "environmental bacteria".^[5]

NTM diseases have been seen in most industrialized countries, which incidence rates vary from 1.0 to 1.8 cases per 100,000 persons. Most of the NTM disease cases involve the species MAC, M. fortuitum and M. kansasii. Although there are reports suggesting that the incidence of NTM diseases has increased over the past few decades, this observation has not been conclusively established due to the lack of comprehensive surveillance efforts.^[5]

Pathogenesis

The most common clinical manifestation of NTM disease is lung disease, but lymphatic, skin/soft tissue, and disseminated disease are also important.^[5]

Pulmonary disease caused by NTM is most often seen in middle-aged or elderly men with lung damage. It can also be found in individuals with AIDS, cystic fibrosis and malignant disease. It can be caused by many NTM species which depends on region, but most frequently MAC and M. kansasii.^[6]

Lymphadenitis can be caused by various species that is different from one place to another; but again, MAC is the main cause worldwide. Most patient are aged less than 5 years, but the incidence is rare for children having BCG vaccine. The disease has a high curability.^[7]

Soft tissue disease due to NTM infection include post-traumatic abscesses (caused by rapid growers), swimming pool granuloma (caused by M. marinum) and Buruli ulcer (caused by M. ulcerans or M. shinshuense). Post-traumatic abscesses most commonly occur after injection.^[7]

Disseminated mycobacterial disease is common in US and Europe in AIDS patient in the 1980s and early 1990s, though the incidence has declined in developed nations since the introduction of highly active antiretroviral therapy. It can also happen in individuals after having renal transplantation.^[6]

Diagnosis

Diagnosis of opportunist mycobacterial is made by repeated isolation and identification of the pathogen with compatible clinical and radiological features. Similar to M. tuberculosis, most of nontuberculous mycobacteria can be detected microscopically and grow on Löwenstein-Jensen medium.^[6] Many reference centres now use nucleic acid-based method such as sequence differences detection in the gene coding for 16S ribosomal RNA to identify the species.^[8]

Notes

1. ^ ^{a b c} Grange, p. 221
2. ^ ^{a b} Grange, p. 221
3. ^ American Thoracic Society, p.369
4. ^ Grange, p. 226
5. ^ ^{a b c} American Thoracic Society, p. 370
6. ^ ^{a b c} Grange, p. 225
7. ^ ^{a b} Grange, p. 223
8. ^ Grange, p. 226

References

• Griffith, David E.; Aksamit, Timothy; & A. Brown-Elliott, Barbara et al. (2007). American Thoracic Society Guidelines: Diagnosis, Treatment and Prevention of Nontuberculous Mycobacterial Diseases. Am. J. Respiratory and Critical Care Medicine, Vol. 175, pp. 367-417.

• Grange, J. M. (2007). "Environmental mycobacteria". In Greenwood, David; Slack, Richard; Peitherer, John; & Barer, Mike (Eds.), Medical Microbiology (17th ed.), pp. 221-227. Elsevier. ISBN 978-0-443-10209-7.