Microbial Quality of Aquaculture Water Used for Produce Irrigation

Abstract

The use of recycled water from an aquaculture fish tank to irrigate produce, or aquaponics, has grown rapidly in the past decades, with a large diversity of system designs. Since the water is reused from the fish tank, there are concerns about the produce grown becoming contaminated by foodborne pathogens such as Escherichia coli, Salmonella, Listeria monocytogenes, etc. that might be present in the fish waste. This study evaluated the microbial quality of the water from a tilapia production tank for irrigation and the soil used for tomato and cucumber growth. The possibility of Listeria monocytogenes and Salmonella contamination in the tomatoes and cucumbers was also examined. Populations of generic E. coli and coliforms, commonly used as indicators of fecal microbial contamination and water quality, were monitored every two weeks. Water effluents from the tilapia fish tank and the plant soils were collected, with 5 replicates for water samples and 15 replicates for soil samples. E. coli and coliform populations were detected using 3M Petrifilm E. coli/coliform plates, and the data was analyzed using ANOVA. L. monocytogenes and Salmonella were monitored on tomatoes and cucumbers by plating methods, and confirmed by PCR. The E. coli population in the tank effluent had a geometric mean (GM) of 49 CFU/100mL and a statistical threshold value (STV) of 62 CFU/100 mL in November 2016, decreased to an undetectable level during winter, and rose to a GM of 30 CFU/100 mL and a STV of 127 CFU/100 mL in May 2017. Coliform populations followed a similar trend, with the highest and lowest populations having a GM of 1,820 and 3 CFU/100 mL, respectively. Populations of E. coli and coliforms in soil were typically higher than in water, with the highest at 2940 CFU/g for coliforms and 293 CFU/g for E. coli. L. monocytogenes was detected in five cucumber and one tomato samples; Salmonella wasn’t found in any produce samples. The generic E. coli population in irrigation water is lower than the regulation limits of 126 CFU/100mL (GM) and 410 CFU/mL (STV) set by the U.S. FDA's Produce Safety Rule.