Assessment of Potential Zoonotic Risks in Aquaponic Lettuce Production: A Prototype for Experimental Greenhouse Trials

Abstract

Aquaponics is the integration of aquaculture and hydroponics that is now being used as a model for sustainable food production. Since fresh vegetables are usually consumed raw, especially leafy greens, there are concerns about food safety and zoonotic risks from fish waste for this emerging industry. The purpose of this study was to develop an experimental recirculating aquaponic system (RAS) prototype to then investigate persistence and transfer via root uptake of an attenuated Salmonella strain in a RAS used for leafy green production. Initially, the infective dose of Salmonella enterica serovar Typhimurium strain to naïve tilapia (Oreochromis spp.) fingerlings was determined by intragastric challenge. Then, using two nonlethal doses (10⁵ and 10⁹ colony-forming units [CFU] per fish), a second group of fish was challenged and used in the laboratory-controlled RAS utilizing hydroponic lettuce. Salmonella was quantified in the system components (tanks, tubing, plant bed substrate), fish waste, and lettuce plants (roots and plants) using microbiological and molecular analysis. From the first challenge, only the three highest inoculums (10⁸–10¹⁰ CFU) yielded positive isolation of Salmonella from fish. In the second challenge, at least one of the RAS-holding Salmonella-infected fish was positive for the presence of Salmonella at the tested time points. No clinical signs or mortality were observed. On Day 42, two fish from one RAS, inoculated with a high dose (10⁹ CFU), had positive stomach and intestinal tissues with concentrations ranging from 0.36 to 160 most probable number/g of tissue. No plants or other tank system components were positive. This study describes a potential system to investigate foodborne diseases in RAS and identifies potential strategies to reduce the risk of bacterial contamination.