| Summary: | Premise plumbing conditions can contribute to low chlorine or chloramine disinfectant residuals and reactions that encourage opportunistic pathogen growth and create risk of Legionnaires’ Disease outbreaks. This bench-scale study investigated the growth of <i>Legionella</i> spp. and <i>Acanthamoeba</i> in direct contact with premise plumbing materials—glass-only control, cross-linked polyethylene (PEX) pipe, magnesium anode rods, iron pipe, iron oxide, pH 10, or a combination of factors. Simulated glass water heaters (SGWHs) were colonized by <i>Legionella pneumophila</i> and exposed to a sequence of 0, 0.1, 0.25, and 0.5 mg/L chlorine or chloramine, at two levels of total organic carbon (TOC), over 8 weeks. <i>Legionella pneumophila</i> thrived in the presence of the magnesium anode by itself and or combination with other factors. In most cases, 0.5 mg/L Cl<sub>2</sub> caused a significant rapid reduction of <i>L. pneumophila</i>, <i>Legionella</i> spp., or total bacteria (16S rRNA) gene copy numbers, but at higher TOC (>1.0 mg C/L), a chlorine residual of 0.5 mg/L Cl<sub>2</sub> was not effective. Notably, <i>Acanthamoeba</i> was not significantly reduced by the 0.5 mg/L chlorine dose.
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