E. coli O157 contamination of lettuce during overhead irrigation
U.S. Food and Drug Administration, U01-003-572
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.
Chase, J. A., E. R. Atwill, M. L. Partyka, R. F. Bond, and D. Oryang. 2017. Inactivation of E. coli O157:H7 in a fecal slurry matrix when inoculated onto romaine lettuce grown in the Salinas Valley, CA. J Food Prot, in press.
Atwill, E. R., J. A. Chase, D. Oryang, R. F. Bond, S. T. Koike, M. D. Cahn, M. Anderson, A. Mokhtari, and S. Dennis. 2015. Transfer of Escherichia coli O157:H7 from simulated wildlife scat onto romaine lettuce during foliar irrigation. J Food Prot 78:240-247.
“Survival of E. coli O157:H7 in fecal slurry on lettuce, 2011 and 2012. Technical Forum on Produce Safety.” JIFSAN & FDA. College Park, MD. February 8 – 9, 2017.
“Irrigation mediated transfer of E. coli O157:H7 from feces to lettuce, 2011 & 2012. Technical Forum on Produce Safety.” JIFSAN & FDA. College Park, MD. February 8 – 9, 2017.