Opportunistic pathogens (OPs) are of special concern for immunocompromised populations and are known to grow in both drinking water and reclaimed water (i.e., non-potable recycled water) distribution systems, with aerosol inhalation and other non-ingestion exposures that are not addressed by existing regulatory frameworks. Factors enabling the growth of OPs in water distribution and premise (i.e., building) plumbing systems distributing reclaimed and other water sources systems are poorly understood especially for the emerging OP, Naegleria fowleri (i.e. brain-eating amoeba). Three phases of investigation were carried out to identify factors that facilitate the growth of OPs in main distribution and premise plumbing systems, with particular attention on reclaimed water systems, aging water mains, and private well systems. Phase one examined the role of biological treatment to remove organic carbon and disinfectant type on the occurrence of OPs during distribution of reclaimed water. Laboratory-scale simulated reclaimed water distribution systems were employed to systematically examine the effects of prior granular activated carbon (GAC) biofiltration of the water; chlorine, chloramines, or no disinfectant, and water ages ranging up to 5 days. The second and third phases of research explored the role of nitrification, iron corrosion, and disinfectant on the growth of N. fowleri both in municipal drinking water from a city grappling with aging water infrastructure and untreated private well water.

Results from the simulated reclaimed water distribution systems suggested that biologically-active GAC filtration may unintentionally select for specific OPs, contrary to expectations and experiences with oligotrophic conditions in potable water systems. While GAC biofiltration was associated with lower total bacteria and Legionella spp. gene markers, there were no apparent benefits in terms of other OPs analyzed. Similarly, disinfectant treatments successful for controlling OPs in potable water were either ineffective or associated with increased levels of OPs, such as Mycobacterium spp. and Acanthamoeba spp., in the reclaimed water examined.

In the potable water study, it was possible to recreate conditions associated with growth of N. fowleri in the aged main distribution system from where the water for the experiment was collected; including corroding iron mains, nitrification, and disinfectant decay. While the effects of nitrification could not be confirmed, there was a clear association of iron corrosion with N. fowleri proliferation. The role of iron was explored further in what, to the author's knowledge, was the first study of N. fowleri in private wells. Analysis of 40 wells found correlations between N. fowleri and stagnant iron levels, further supporting the hypothesis that iron corrosion or iron encourages the growth of N. fowleri, and, because wells are not routinely disinfected, not necessarily as a result of promoting disinfectant decay. As this study took place following a major flooding event, it provided insight not only into how surface water contamination may influence private well water microbial communities, but also added to the understanding that current recommendations for disinfecting private wells are inadequate and standards should be implemented to aid homeowners in the event of flooding.

This exploratory research illuminated several factors influencing the OP growth in a range of water systems. Identifying key variables that control growth is crucial to improving the safety of these systems.