Influence of Nutrients and the Native on E. Coli Survival in the Beach Environment

Abstract

E. coli is used as an indicator for water quality to determine if water poses a health risk for pathogens. Past research has shown that E. coli is present in high numbers in freshwater beach sands distinct from fecal pollution events, yet the precise mechanism for their persistence in not well understood. Persistent E. coli populations in sand can resuspend into adjacent water and lead to increased beach closures when no threat is present. This work identifies factors that influence the survival of E. coli in sand using laboratory microcosms to replicate beach conditions. Microcosms were deployed to examine the effect of genetic background, competition with autochthonous microbes, and increased nutrient sources on E. coli survival. E. coli strains can be characterized into phylogroups, genetic divergences through evolution of the strain. E. coli phylogroup B2 is commonly recovered from mammals, while phylogroup B1 is commonly recovered from the environment. Survival was comparable between the distinct phylotypes, however, having a deficient stress response greatly reduced survival. In the absence of the native community under nutrient conditions comparable to those observed in sand, E. coli cell densities remained within an order of magnitude of initial concentrations after 35 days of incubation. Increasing carbon and nitrogen concentrations resulted in higher E. coli levels over time, with increased nitrogen associated with higher levels of survival in the first two weeks, and increased carbon providing an advantage at later time points. However, the highest survival was found with the addition of both carbon and nitrogen. Native sand seeded with fresh Cladophora maintained higher concentrations of E. coli, compared to sand containing decayed Cladophora or no Cladophora. Our findings demonstrate persistent E. coli populations in sand can be affected by the availability of carbon and nitrogen, the ability to regulate stress, and the presence of algae mats (i.e., Cladophora). Further, native microbial communities may modulate survival by outcompeting E. coli for nutrients.