Evaluation of pathogen survival in fresh water sediments and potential impact on irrigation water quality sampling programs.

The potential contamination of fresh produce with pathogens of human health concern via irrigation water has long been recognized. The variety of irrigation water sources—groundwater wells, ponds, rivers, streams, municipal water, and reclaimed (treated wastewater) water—combined with the diversity of potential waste stream inputs have resulted in a complex issue when trying to understand and address the fate and transport of both fecal indicator bacteria (FIB) and pathogens in agricultural water resources. An area of uncertainty involves the transport of pathogens in fresh water sediments. Therefore, the objective of this study is to evaluate the relationship between pathogens and FIB in fresh water sources over time and the role that fresh water sediments may play in the harboring and distribution of pathogens in water sources used for irrigation. In order to complete this objective, aquatic mesocosms (i.e. an experimental water enclosure) and an indoor fluvarium system that simulates flowing fresh water environments will be used in this study to evaluate the survival and behavior of 14 different pathogens including Listeria, Salmonella, and shiga toxin-producing Escherichia coli, 5 surrogates (non pathogens), and 2 FIB.

The potential contamination of fresh produce with pathogens of human health concern via irrigation water has long been recognized. The variety of irrigation water sources—groundwater wells, ponds, rivers, streams, municipal water, and reclaimed (treated wastewater) water—combined with the diversity of potential waste stream inputs have resulted in a complex issue when trying to understand and address the fate and transport of both fecal indicator bacteria (FIB) and pathogens in agricultural water resources. Even though FIB have been relied upon to predict microbial water quality for the last 4 decades, there is still no consensus on the best FIB nor whether adherence to FIB exceedance levels actual protect public health. In addition, sediments have been identified as important reservoirs for FIB and thus potentially for human pathogens; however, the role of sediments in the resuspension of pathogens in the water column has not been thoroughly evaluated. Therefore, in this proof of concept study, relationships (i.e., seasonal variations in survival and attachment) between FIB, human pathogens, and surrogate microorganisms within fresh water sediments will be investigated through the use of aquatic mesocosms. To further investigate the role of sediments in water quality, an indoor fluvarium will be utilized to assess the resuspension and subsequent partitioning of FIB and pathogen surrogates from surface sediments during high flow events in surface water sources. The results of this study will provide a scientific basis for a more in depth study on FIB – pathogen relationships in fresh surface water sources. Through completion of this study, these data could better inform the implementation of risk-based sampling programs and the formation of pathogen fate and transport models that better predict potential health risk.

Objective 1: Determine the distribution and survival of FIB (E. coli and enterococci), pathogens, and pathogen surrogates in sediments of aquatic mesocosms exposed to seasonal variations. 

Objective 2: Investigate the resuspension and subsequent partitioning of FIB and pathogen surrogates from surface sediments using an indoor fluvarium to simulate high flow events in surface water sources.

As indicated previously, there is still a tremendous amount of data analysis to perform, and thus, any summary of findings should be considered preliminary. Overall, the data suggests that Salmonella is more persistent than Listeria spp. (more specifically, L. monocytogenes) and STECs in both the water and sediments of the mesocosms, though there seems to be some strain dependent factors at play. In addition, the FIB E. coli seems to be a fairly conservative indicator for predicting microbial water quality and pathogen behavior although enterococci trended with L.monocytogenes which may be useful in future studies of this pathogen in the environment. With respect to pathogen surrogates, Salmonella Typhimurium (ATCC 53647) seemed to follow the same survival/persistence patterns as the pathogenic Salmonella serovars indicating its potential usefulness as a surrogate for studying pathogenic Salmonella survival. Surrogates for L.monocytogenes—L. innocua and L. seeligeri—were overall more stable than the L. monocytogenes strains used in the present study. Based on these preliminary data and observations, my recommendation would be that E. coli is suitable FIB for predicting microbial water quality. In addition, I would suggest that future studies focus on evaluation of microorganisms within real world freshwater systems using novel passive sampling techniques for detection of FIB and pathogens over time.