## 4 Mycobacteria Ecological Niches
### Water  
The ability of NTM to grow in water environments has been under the focus of numerous studies in natural sources, such as lakes, rivers, streams, and seawater [181], but also artificial environments, such as drinking water pipelines [182], hot tubs [183], foot-baths [184], residential faucets [185], hospital faucets and ice machines [186], diagnostic laboratories [187], bottled and municipal water [185], showerheads [31], brook waters [188], cooling towers [189], and wastewater treatment plants [190]. The concomitant presence of NTM and _Legionellae_ in water distribution systems has been previously reported [191], often attributed to several biological similarities between both microbes. Due to their hydrophobicity, NTM cells become attached to air bubbles that rise in the water column, reach the surface and burst, forming craters that collapse, leading to water droplets formation [192]. These droplets are enriched in NTM up to 10,000 times when compared with the water column, and can posteriorly be inhaled, reaching mammal lungs [192]. A great concentration of organic materials occurs at the air-water interface, together with the accumulation of NTM-containing bubbles, leading to a favorable environment for NTM enrichment [193]. The fact that NTM hold highly-hydrophobic cell walls facilitating aerosolization and surface adherence may explain their highly-infectious behavior. In marine waters, NTM are present in lower burden due to the high concentration of NaCl, a growth inhibitor for most species [194]. Therefore, freshwater tanks show a greater burden of NTM than those tanks supplied with seawater [194]. NTM possess a natural capacity of colonizing and persisting in several human-engineered environments, such as water systems, being able to enter these systems through several routes, namely cross-connections and backflows, equipment or personnel before entry, finished water storage vessels,improper treatment of materials, inadequate distribution system security, joints and seals, leaking pipes, valves, and water treatment breakthroughs [195]. In surface water, NTM enter drinking water treatment systems by soil particle attachment, so the reduction of water turbidity leads to a reduction of NTM burden entering the water system [82]. Additionally, NTM naturally aggregate in water, a characteristic that favors increased resistance to disinfection in wastewater treatment plants, with inactivation being particularly compromised by aggregates larger than 41 um[196]. Their intrinsic capacity to resist the action of several disinfectants, such as chlorine, contrary to the majority of other environmental bacteria, lead to a competitive advantage resulting in nutrient consumption and biofilm formation in the absence of competition, also reducing the likelihood of wash out [197]. NTM concentration increases in pipe systems far distant from the treatment plant [73]. In additoin, NTM cells growing in biofilms are more resistant to disinfection [31, 69]. However, cells released from the biofilm would be transient of intermediately resistant to disinfectants and antibiotics, returning to similar susceptibility phenotypic profiles of suspension-grown cells [31, 69]. Thus, the reduction of water particulate content reduces NTM burden in treated water. Nowadays, a shift from chlorine to chloramine has been occurring in the drinking water industry to avoid the production of halogenated carcinogens and to increase the microbicide effect over biofilms [191]. However, the frequency of recovery of NTM increased [191]. Further studies are thus needed to fully understand this issue and evaluate the public health impact of this measure. Moreover, households with lower water temperatures (<55 \({}^{\circ}\)C) are more likely to harbor NTM that those with higher water temperatures (>55 \({}^{\circ}\)C) [73]. _M. avium_, _M. intracellular_, _M. kansasii_, and _M. xenopi_ have been isolated from hot water systems (>50 \({}^{\circ}\)C) [195].