Urban flooding can dramatically affect the local microbial landscape and increase the risk of waterborne infection in flooded areas. Hurricane Harvey, the most destructive hurricane since Katrina in 2005, damaged more than 100000 homes in Houston and flooded numerous wastewater treatment plants. Here we surveyed microbial communities in floodwater inside and outside residences, bayou water, and residual bayou sediment collected immediately postflood. Levels of Escherichia coli, a fecal indicator organism, were elevated in bayou water samples as compared to historical levels, as were relative abundances of key indicator genes of anthropogenic sources of antibiotic resistance (sul1/16S rRNA and intI1/16S rRNA) based on 6 month postflood monitoring. Quantitative polymerase chain reaction measurements showed that gene markers corresponding to putative pathogenic bacteria were more abundant in indoor floodwater than in street floodwater and bayou water. Higher abundances of 16S rRNA and sul1 genes were also observed in indoor stagnant waters. Sediments mobilized by floodwater exhibited an increased abundance of putative pathogens postflood in both residential areas and public parks. Overall, this study demonstrates that extreme flooding can increase the level of exposure to pathogens and associated risks.

Unique water chemistry and operational conditions of reclaimed water distribution systems facilitated accumulation of sediment which resulted in depletion of disinfectants and microbial regrowth.

Unregulated private wells are understudied potential sources of community‐acquired Legionnaires’ disease. Here we conducted a comprehensive survey of 44 homes supplied by private wells in Wake County, North Carolina, quantifying Legionella spp. DNA, Legionella pneumophila DNA, and total bacterial 16S rRNA genes via real‐time polymerase chain reaction in hot and cold drinking water samples, along with culturable L. pneumophila via IDEXX Legiolert in cold drinking water samples. Legionella spp. DNA, L. pneumophila DNA and culturable L. pneumophila were detected in 100, 65·5 and 15·9% of the 44 homes, respectively, and culturable levels were comparable to some municipal surveys applying the same methods. Total coliforms and Escherichia coli were monitored as representative faecal indicators and were found in 20·4 and 0·0% of homes. Within certain sample types, Legionella spp. and L. pneumophila gene copy numbers were positively associated with total bacteria (i.e. total 16S rRNA genes) and water softener use, but were not associated with faecal indicator bacteria, inorganic water parameters or other well characteristics. These findings confirm that occurrence of Legionella and L. pneumophila is highly variable in private wells. Significance and Impact of the Study Legionella is the leading identified cause of waterborne disease outbreaks associated with US municipal water systems. While Legionella is known to occur naturally in groundwater, prior efforts to characterize its occurrence in unregulated private wells are limited to sampling at the wellhead and not in the home plumbing where Legionella can thrive. This work documents much higher levels of Legionella in home plumbing versus water directly from private wells and examines factors associated with higher Legionella occurrence.

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