Aquatic ecosystems are impacted significantly by urbanization, but our knowledge of microbial assemblages in urban environments and their dissemination and interaction with connected natural aquatic environments is not well developed. For example we have a limited understanding of what microorganisms are favored by landscapes dominated by built infrastructure, how microbial assemblages are distributed and move through urban ecosystems, and what consequences may arise from continuous flushing of urban-derived microorganisms into waterways. Here we aim to extend our understanding of the microorganisms associated with one critical component of urban areas, sewer infrastructure. Sewer infrastructure is a collector of runoff and household microbial assemblages originating in an urban landscape and a primary connector between cities and recreational surface waters. To further our understanding of sewerage microbial communities, we collected sewage treatment plant influent from 71 U.S. cities on three occasions in one year. We then used massively parallel sequencing of 16S rRNA genes to characterize the bacterial assemblages in those samples. In concert with this characterization, we quantified and tracked the dissemination of sewage-associated microorganisms into an urban harbor during various weather driven environmental scenarios.
We found the most abundant bacteria in sewage were non-fecal genera such as Arcobacter, Acinetobacter, and Aeromonas, and these taxa typically represented >25% of the community. Sewage samples across all samples had roughly the same bacterial community structure, but geographic and recurrent seasonal distribution abundance patterns were present for populations of the dominant genera. Our data indicate sewer pipe systems are a unique and dynamic microbial ecosystem with a persistent resident community. In an urbanized estuary, we found the sewer pipe associated bacteria were consistently present, but increased dramatically during rain events. Event loading estimates indicated that up to 10^15 cells of the most abundant sewage taxa could be delivered into the estuary in a 24-hour period following heavy rain. It is clear urban sewer pipes select for a unique microbial community whose members are regularly disseminated into surface waters, but the fate of these organisms, contribution to natural ecosystems, and impact on human health remain relatively unknown.