Drinking water microbiology in a water-efficient building: Stagnation, seasonality, and physiochemical effects on opportunistic pathogen and total bacteria proliferation

The rising trend in water conservation awareness has given rise to the use of water-efficient appliances and fixtures for residential potable water systems. This study characterizes the microbial dynamics at a water-efficient residential building over the course of one year (58 sampling events) and examines the effects of water stagnation, season, and changes in physicochemical properties on the occurrence of opportunistic pathogen markers. Mean heterotrophic plate counts (HPC) were typically lowest upon entering the building at the service line, but increased by several orders of magnitude at the furthest location in the building plumbing. Legionella spp. and Mycobacterium spp. were detected in the plumbing, with the highest detection occurring in the summer months. Log-transformed HPC were significantly correlated with total cell counts (TCC) (rs = 0.714, p<0.01), Legionella spp. (rs = 0.534, p<0.01), and Mycobacterium spp. occurrence (rs= 0.458, p<0.01). Reduced water usage induced longer stagnation times and longer stagnation times were correlated with an increase in Legionella spp. (rs = 0.356, p<0.001), Mycobacterium spp. (rs = 0.287, p<0.001), TCC (rs = 0.216, p<0.001) and HPC (rs = 0.395 , p<0.001). Interrelationships between seasonal shifts in water chemistry and genus-level genetic markers for opportunistic pathogens were revealed. This study highlights how drinking water microbiology varies seasonally and spatially throughout a low-flow plumbing building and highlights the possible unintended consequences associated with increases in stagnation time.