Understanding Escherichia coli O157:H7 presence, pervasiveness, and persistence in constructed treatment wetland systems

Abstract

Treatment wetlands (TW) are a wastewater remediation technology that relies on the natural ability of wetland plant species and the associated microbial consortia to remove pollutants and improve water quality. Although there is substantial research on chemical pollutant remediation by TW, the removal of bacterial pathogens is much more varied and limited in scope. Escherichia coli O157:H7 is a bacterial pathogen that has caused numerous outbreaks and infections in the United States alone and is closely associated with improper water treatment. Understanding how E. coli O157:H7 could potentially persist and survive through a TW process is important in order to appropriately determine the efficacy of TW for treating water and protecting human health. This work used epifluorescent microscopy and qPCR relative DNA abundance to track E. coli O157:H7 tagged with a fluorescent DsRed protein in various environments pertaining to a TW. Two high performing wetland plant species, Carex utriculata and Schoenoplectus acutus, were used in hydroponic and simulated TW columns to better understand how the bacteria localize and persist. Teflon nylon strings (diameter 0.71-1.02 mm), cleaned and with established biofilm, were run hydroponically as control inert surfaces. Unplanted gravel columns were used as a nonplanted control for column experiments. E. coli O157:H7-DsRed were observed by microscopy on root surfaces both in hydroponic reactors and lab scale TW columns. The organisms persisted, forming microcolonies shortly after initial inoculation on both root and nylon surfaces. In the lab scale columns, cells persisted for three weeks, although strong biofilm formation was not observed. qPCR also provided evidence that E. coli O157:H7 was able to persist on the tested surfaces of plant roots, nylon inert surfaces, and gravel, showing higher abundance S. acutus roots than on the inert surface and gravel, however higher in unplanted gravel overall. For the plant types, C. utriculata was statistically lower for E. coli O157:H7 abundance than S. acutus over time. This work provides evidence that E. coli O157:H7 is able to colonize and persist in a TW environment, and plant surfaces may offer a higher inactivation than an inert matrix.