Microbial composition of purified waters and implications for regrowth control in municipal water systems

The limits of water treatment to control microbial regrowth were examined using highly purified waters. Measurable microbial genetic material was detected in the product water in a survey of thirteen laboratory pure water systems. Illumina 16S rRNA gene amplicon sequencing revealed surprisingly diverse microbial assemblages, confirmed to be active in bioassays, with no direct relationship to quality or maintenance of the systems. With storage under both light and dark conditions, a 2-log increase in bacterial genetic markers was observed within 10 days, indicating viable oligotrophic communities despite rigorous treatment steps. With growth, microbial communities shifted concurrent with enrichment of Proteobacteria groups capable of nitrogen fixation (Bradyrhizobium) and H2 oxidation (Comamonadaceae). This study has implications not only for laboratory studies, which rely on highly purified waters, but also for municipal drinking water, which depends on treatment to reduce nutrients sufficiently to limit downstream regrowth of microorganisms.