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

Copper can be released from pipes or added intentionally to drinking water as an antimicrobial, possibly limiting biofilm and pathogen growth in building (i.e., premise) plumbing systems, but longer-term effects of various doses are poorly defined. Here, triplicate 120-mL water heater microcosms equipped with PEX-b pipes containing mature biofilms were fed influent dosed with copper at 0, 4, 30, 250, or 2000 ppb (as total Cu) and monitored for 11 months. Effluent total cell counts and Mycobacterium avium peaked at 250-ppb copper dosage, reflecting the dual role of copper as a nutrient and antimicrobial. Total cell counts and Mycobacterium avium were relatively consistent in replicate microcosms at each dose. While Legionella pneumophila (Lp) responded similarly to copper and also peaked at 250 ppb, this dose also caused bifurcating behavior of biological replicates, with one consistently producing high culturable Lp (average of 2.5 log (MPN/mL)) and the other two consistently producing low or non-detect levels over an 11-month dosing period. We cross-inoculated the microcosms ten times periodically throughout the 6-month acclimation phase before copper dosing, and reinoculated the microcosms with Lp one time ∼ 3.5 months into dosing in an attempt to normalize the microbial community composition across replicate microcosms. However, Lp levels across the three 250-ppb microcosms remained bifurcated throughout the copper dosing period. 16S rRNA gene sequencing revealed that the 250-ppb replicate with high Lp was characterized by a distinct microbial community composition relative to the two replicates with low Lp . At the highest dose of 2,000 ppb copper total cell counts initially decreased by about 78%, but then steadily increased to a point where they were not statistically different from the 250-ppb microcosms. Dosing 2000 ppb copper also reduced microbial diversity.