Temporal stability of Salmonella enterica and Listeria monocytogenes levels across irrigation water types

Enteric bacterial pathogens in irrigation water can be a public health and food safety issue when contaminating produce. Microbial water quality varies depending on the location and type of water source. We hypothesis temporally stable spatial patterns in levels of Salmonella enterica and Listeria monocytogenes exist in individual water sources in the same region. To test this hypothesis, samples were collected from six water sources in the mid-Atlantic U.S over two years, twice every month during growing season (May to September) and once a month during non-growing seasons (October to April). The sampling sites represented four rivers and two ponds. Several environmental covariates (conductivity, ORP, pH, salinity, dissolved oxygen, turbidity, cumulative rainfall for one and seven day prior to sampling dates) were measured in conjunction with quantitative pathogen recovery. Temporal stability was quantified by computing the mean relative difference (MRD) of pathogen levels from the average across the monitoring location at the same sampling dates. The Spearman rank correlation coefficient between each pathogen level on successive sampling dates was calculated. Levels of both pathogens at each location demonstrated temporally stable spatial patterns. The overall MRD values of pathogens in river water were higher than MRD values of pathogens in pond water. The MRD values for S. enterica were similar to the MRD values of cumulative 7-day rainfall. An inverse relationship between the MRD values of L. monocytogenes and water temperature were found.  The Spearman rank correlation for both pathogens indicated moderate to strong relationship between consecutive sampling events in approximately 80% of cases. These can be used to improve regional monitoring of microbial quality of irrigation water.