Vertical stratification of the water microbiome in an electric water heater tank: implications for premise plumbing opportunistic pathogens

Hot water systems are the most frequent environment associated with the prevalence and growth of opportunistic premise plumbing pathogens (OPPPs). Previous studies identified water heaters as a source of waterborne diseases and concluded that design variables may contribute to their prevalence. A multifaceted approach was used to investigate the vertical stratification of the microbiome and selected OPPPs in an electric water heater tank connected to a home plumbing system simulator. Results show that the microbiome is highly diverse with evidence of temperature stratification and temporal structuring influenced by the partial drainage of the tank. Representatives of the <i>Mycobacterium</i> spp. were the most prevalent taxa, followed by <i>Legionella</i> spp., and a relatively low prevalence of <i>free-living amoeba Vermamoeba vermiformis</i>. Higher concentrations of <i>Legionella pneumophila</i> at the bottom of the tank indicated the potential growth and protection of this opportunistic pathogen at this location. Overall, partial drainage of the water tank (60% of the tank capacity) did not significantly mitigate the microbiome and selected OPPPs. The outcome of this study sheds light on the role of vertical stratification on water quality and demonstrates the resilience of the microbial community residing in an electric water heater tank and the implications for public health.