Food Safety

A field trial in Salinas Valley, California, was conducted during July 2011, and again in July and October of 2012 to quantify the microbial load that transfers from wildlife feces onto nearby lettuce during foliar irrigation. Romaine lettuce was grown using standard commercial practices and irrigated using an impact sprinkler design. Five grams of rabbit feces was spiked with 1.29X10⁸ CFU of Escherichia coli O157:H7 and placed 23, 22, and 21 days and immediately before a 2 hr irrigation event. Immediately after irrigation, 168 heads of lettuce ranging from 23 to 69 cm (from 9 to 27 in.) from the fecal deposits were collected, and the concentration of E. coli O157:H7 was determined. Thirty-eight percent of the collected lettuce heads had detectable E. coli O157:H7, ranging from 1 MPN to 2.30X10⁵ MPN per head and a mean concentration of 7.37X10³ MPN per head. Based on this weighted arithmetic mean concentration of 7.37X10³ MPN of bacteria per positive head, only 0.00573% of the original 5 g of scat with its mean load of 1.29X10⁸ CFU was transferred to the positive heads of lettuce. Bacterial contamination was limited to the outer leaves of lettuce. In addition, factors associated with the transfer of E. coli O157:H7 from scat to lettuce were distance between the scat and lettuce, age of scat before irrigation, and mean distance between scat and the irrigation sprinkler heads. This study quantified the transfer coefficient between scat and adjacent heads of lettuce as a function of irrigation. The data can be used to populate a quantitative produce risk assessment model for E. coli O157:H7 in romaine lettuce to inform risk management and food safety policies.