Comparison of methods to determine the microbial quality of alternative irrigation waters

The availability of water for crop irrigation is decreasing due to droughts, population growth, and pollution. Implementation of the Food Safety and Modernization Act (FSMA) for irrigation water standards will discourage growers to use poor microbial quality water for produce crop irrigation. We evaluated the applicability of a novel concentrator method for assessment of microbiological quality of alternative waters including secondary-treated wastewater (STWW), roof-harvest rainwater (RHW), and creek water (CW) in comparison to the standard membrane filtration method. Water samples of 100 ml were filtered through a 0.45 μm membrane filter using a vacuum manifold or concentrated to ∼250 μl using the innovative concentrator. Then they were directly enumerated on specific agars, or enriched to monitor the populations of fecal bacterial indicators (Escherichia coli, enterococci, total and fecal coliforms) and bacterial pathogens (Salmonella, Listeria monocytogenes, and E. coli O157:H7). Presumptive pathogens were confirmed by real-time quantitative PCR. In total, 25 samples of alternative water were analyzed including 7 STWW, 9 RHW, and 9 CW. No significant differences between both detection methods were observed when enumerating indicator bacterial populations and detecting the presence of pathogens in RHW and CW samples. Recovery of fecal coliforms in STWW samples by concentrator analysis was significantly lower than the membrane filtration technique. Results suggest that performance of the concentrator method is equivalent to membrane filtration method in determining the microbiological quality of CW and RHW waters; the type of the water sources may influence the accuracy and sensitivity of the concentrator analysis.