Influence of Copper Dose on Mycobacterium avium and Legionella pneumophila Growth in Premise Plumbing

Effects of copper at 0, 4, 30, 250, or 2000 µg/L on microbial communities were examined in an 11-month study using triplicate 120-mL water heater microcosms with PEX-b pipes containing mature biofilms to simulate premise plumbing. Effluent total cell counts (TCCs) and Mycobacterium avium peaked at 250 µg/L, reflecting the dual role of copper as a nutrient and antimicrobial. TCCs and M. avium were relatively consistent among replicate microcosms at each dose, but Legionella pneumophila (Lp) diverged among biological triplicates at 250 µg/L, consistently producing high culturable Lp (average 2.5 log MPN/mL) in one microcosm and low/non-detectable 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 high Lp was characterized by a distinct microbial community composition relative to the two replicates. At 2,000 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 non-linear effects of copper dosing when applied as a disinfectant to premise plumbing for opportunistic pathogen control.