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Background  Several biotic and abiotic factors have been reported to influence the proliferation of microbes, including  Legionella pneumophila , in hot water premise plumbing systems, but their combined effects have not been systematically evaluated. Here, we utilize simulated household water heaters to examine the effects of stepwise increases in temperature (32–53 °C), pipe material (copper vs. cross-linked polyethylene (PEX)), and influent assimilable organic carbon (0–700 μg/L) on opportunistic pathogen gene copy numbers and the microbiota composition, as determined by quantitative polymerase chain reaction and 16S rRNA gene amplicon sequencing.    Results  Temperature had an overarching influence on both the microbiota composition and  L. pneumophila  numbers.  L. pneumophila  peaked at 41 °C in the presence of PEX (1.58 × 10 5  gene copies/mL). At 53 °C,  L. pneumophila  was not detected. Several operational taxonomic units (OTUs) persisted across all conditions, accounting for 50% of the microbiota composition from 32 to 49 °C and 20% at 53 °C. Pipe material most strongly influenced microbiota composition at lower temperatures, driven by five to six OTUs enriched with each material. Copper pipes supported less  L. pneumophila  than PEX pipes (mean 2.5 log 10  lower) at temperatures ≤ 41 °C, but showed no difference in total bacterial numbers. Differences between pipe materials diminished with elevated temperature, probably resulting from decreased release of copper ions. At temperatures ≤ 45 °C, influent assimilable organic carbon correlated well with total bacterial numbers, but not with  L. pneumophila  numbers. At 53 °C, PEX pipes leached organic carbon, reducing the importance of dosed organic carbon.  L. pneumophila  numbers correlated with a  Legionella  OTU and a  Methylophilus  OTU identified by amplicon sequencing.    Conclusions  Temperature was the most effective factor for the control of  L. pneumophila , while microbiota composition shifted with each stepwise temperature increase. While copper pipe may also help shape the microbiota composition and limit  L. pneumophila  proliferation, its benefits might be constrained at higher temperatures. Influent assimilable organic carbon affected total bacterial numbers, but had minimal influence on opportunistic pathogen gene numbers or microbiota composition. These findings provide guidance among multiple control measures for the growth of opportunistic pathogens in hot water plumbing and insight into the mediating role of microbial ecological factors.    Electronic supplementary material  The online version of this article (10.1186/s40168-017-0348-5) contains supplementary material, which is available to authorized users.

microbiome

Interactive effects of temperature, organic carbon, and pipe material on microbiota composition and Legionella pneumophila in hot water plumbing systems