Environmental Sources of Nontuberculous Mycobacteria

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Key points

  • Nontuberculous mycobacteria (NTM) are opportunistic pathogens; sources include drinking water, natural water, and soils. NTM are not contaminants, but normal inhabitants of those habitats.

  • Human infection occurs because NTM are in the same habitats that humans encounter and are thereby exposed to waters, aerosols, or dusts that can be inhaled or swallowed.

  • NTM are more resistant to common water disinfectants, such as chlorine, and their use selects NTM persistence in drinking water distribution

Nontuberculous mycobacteria

The NTM group currently encompasses over 150 individual species.3 All, with few exceptions, are opportunistic pathogens. The dramatic increase in new Mycobacterium species is owing to:

  • 1.

    Increased susceptibility of individuals infected with the human immunodeficiency virus to NTM infection,

  • 2.

    Improved techniques for primary culture of NTM, and

  • 3.

    Detection of infection by direct DNA isolation and sequencing.

Because the ecology, sources, physiology, virulence, and antibiotic susceptibility of the NTM

Sources of nontuberculous mycobacteria

NTM are natural inhabitants of soils, lakes, rivers, and streams. Box 1 lists NTM sources for human infection. The origin of NTM is soils, from whence it enters surface waters. Both northern hemisphere pine (boreal) forest soils12 and southeastern US coastal acidic, brown water swamps have high numbers of NTM (106 per gram).13 That habitat predilection is owing, in part, to NTM’s preference for growth at acidic pH10 and growth stimulation by humic and fulvic acids, whose concentration is high

Why do nontuberculous mycobacteria inhabit the human environment?

The physiologic characteristics of the NTM are major determinants of its habitat occupancy. The surface hydrophobicity of NTM, owing to the lipid-rich outer membrane,4 drives the attachment of cells to surfaces in both natural and human-engineered environments.6 Attached cells, no matter how slowly growing, will not be washed out of a flowing system (eg, river or pipe). Hydrophobicity also drives the enrichment of NTM in aerosols above bodies of water5 and is responsible, in part, for NTM’s

Transmission of nontuberculous mycobacteria

Routes that have been proposed for NTM transmission include (1) aerosolization and inhalation, (2) swallowing and aspiration, and (3) introduction into wounds, either through injury or surgical intervention (or both). To prove the existence of a transmission route, it is insufficient to simply isolate an NTM species or subspecies from both a patient and some environmental compartment (water or soil). Rather, to prove that one of those sources or routes was involved in infection requires the use

Avoiding nontuberculous mycobacteria

The driver for identifying measures to avoid NTM exposure and infection is the fact that a substantial number of patients become reinfected with NTM, even after successful antibiotic therapy.30 This is a consequence of the patient’s continuing and increased susceptibility to NTM disease. To identify exposure-reduction measures, the approach in the Falkinham laboratory has been to identify factors that are associated with an absence of NTM in premises plumbing. That, in turn, is based on the

Summary

NTM are opportunistic pathogens whose sources include drinking water, natural water, soils, and dusts. The NTM are not contaminants, but rather normal inhabitants of those habitats. Human infection occurs because NTM are in the same habitats that humans occupy and humans are thereby exposed to NTM in waters, aerosols, or dusts that can be inhaled or swallowed. Human-engineered habitats, such as a drinking water distribution system and premises plumbing select for and are ideal habitats for NTM.

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