Abstract

The majority of species of Mycobacterium are opportunistic pathogens (e.g., M. avium, M. marinum, and M. chelonae) of animals, birds, and humans. They are normal inhabitants of natural waters, drinking waters, and soils. High numbers are recovered from estuaries and peat-rich soils. Mycobacteria are hydrophobic and as a result prefer attachment to surfaces, rather than suspension in water. In drinking water systems, most mycobacteria are found in biofilms on the surfaces of pipes.

Common characteristics of mycobacteria include: hydrophobicity, slow growth, growth over a wide range of temperatures (15°-42°C) and oxygen levels (6%-21% oxygen). They are also markedly disinfectant- and antibiotic-resistant. Growth rate distinguishes two groups of mycobacteria: the slow growers require greater than 1 week (e.g., 2-4 weeks) to form colonies, and the rapid growers, able to form colonies in less than 1 week. The rapidly growing mycobacteria are more susceptible to antibiotics and disinfectants than are the slow growers, but still considerably more resistant than most bacteria. Most are oligotrophic, able to grow in natural and drinking waters provided low concentrations of organic matter (> 50 µg/L) are present.

Although mycobacteria are poor competitors against other microorganisms because of their relatively slow growth, their disinfectant-resistance (e.g., chlorine) leads to their proliferation in disinfected water systems. Further, mycobacteria in biofilms are more resistant to disinfectants than cells in suspension. As a consequence, mycobacteria in a water system can rapidly repopulate the system from the biofilm following disinfection. Fortunately, mycobacteria are as susceptible to ultraviolet (UV) irradiation as are other microorganisms, supporting the use of UV to keep mycobacterial numbers low.