Large-volume sampling for improved characterization of microbial contamination in agricultural waters: a case study from southeastern U.S. produce farms

Fresh produce growers in the United States are required to conduct agricultural water assessments to help mitigate the risks associated with potentially contaminated irrigation water. Microbial water quality is typically assessed using the fecal indicator bacteria, <i>Escherichia coli</i>, collected from small-volume grab samples. Testing irrigation water for alternative fecal indicators, microbial source tracking (MST) markers, or pathogens may provide additional insight into sources of fecal contamination, risk of pathogen presence, or hazards associated with the agricultural watershed. In this study, we conducted laboratory recovery experiments and a field study to evaluate sample collection methods for agricultural water assessments. Dead-end ultrafiltration (DEUF) was used to collect large-volume (50 L) irrigation water samples alongside 1-L grab samples monthly from three farms in southern Georgia, USA, for 1 year. DEUF and grab samples were analyzed for fecal indicators, <i>Salmonella</i>, <i>E. coli</i> O157:H7, and MST markers. DEUF resulted in higher detection rates than grab sampling for <i>Salmonella</i> and most fecal indicators. DEUF also revealed associations between <i>Salmonella</i> and fecal indicators that grab sampling did not. This study demonstrates the applicability and benefit of DEUF for the sensitive detection of pathogens, fecal indicators, and MST markers in produce irrigation water.