Influence of Copper Dose on <i>Mycobacterium avium</i> and <i>Legionella pneumophila</i> Growth in Premise Plumbing

Effects of copper at 0, 4, 30, 250, or 2000 μg/L on microbial communities were examined over an 11 month dosing period using triplicate 120 mL water heater microcosms with PEX-b pipes containing mature biofilms to simulate premise plumbing. Effluent total cell counts (TCCs) and <i>Mycobacterium avium</i> peaked at 250 μg/L, reflecting the dual role of copper as a nutrient and antimicrobial. TCCs and <i>M. avium</i> were relatively consistent among replicate microcosms at each dose, but <i>Legionella pneumophila</i> (<i>Lp</i>) diverged among biological triplicates at 250 μg/L, consistently producing high culturable <i>Lp</i> (average 2.5 log MPN/mL) in one microcosm and low/nondetectable levels in the other two. Repeated cross-inoculations and a reinoculation failed to normalize the microbial community composition across 250 μg/L and other triplicate microcosms. 16S rRNA gene amplicon sequencing revealed that the 250 μg/L replicate with a high <i>Lp</i> was characterized by a distinct microbial community composition relative to the two replicates. At 2000 μg/L copper, microbial diversity and TCCs initially decreased, but then TCCs subsequently increased and ultimately were not statistically different from the 250 μg/L microcosms. This study provides insight into mechanisms underlying nonlinear effects of copper dosing when applied as a disinfectant to premise plumbing for opportunistic pathogen control.