title: Inactivation of Escherichia coli O157:H7 on Romaine Lettuce When Inoculated in a Fecal Slurry Matrix.
abstract: A field trial was conducted in July 2011 to quantify the inactivation rate of Escherichia coli O157:H7 when mixed with fecal slurry and applied to romaine lettuce leaves. Lettuce was grown under commercial conditions in Salinas Valley, CA. One-half milliliter of rabbit fecal slurry, containing 6.3 × 107 CFU of E. coli O157:H7, was inoculated onto the upper (adaxial) surface of a lower leaf on 240 heads of lettuce within 30 min after a 2.5-h irrigation event. Forty-eight romaine lettuce heads were collected per event at 2.5 h (day 0.1), 19.75 h (day 0.8), 43.25 h (day 1.8), 67.25 h (day 2.8), and 91.75 h (day 3.8) postinoculation and were analyzed for the concentration of E. coli O157:H7 (Ct). E. coli O157:H7 was detected on 100% of collected heads in concentrations ranging from 340 to 3.40 × 1010 most probable number (MPN) per head. Enumeration data indicate substantial growth of E. coli O157:H7 postinoculation (2.5 h), leading to elevated concentrations, 1 to 3 log above the starting inoculum concentration (Co). By the end of the 92-h trial, we observed a net 0.8-log mean reduction of E. coli O157:H7 compared with Co; however, after accounting for the substantial bacterial growth, there was an overall 2.3-log reduction by the final sampling event (92 h). On the basis of two different regression models that used either the raw data for Ct or log-transformed values of Ct/Co during the period 2.5 to 91.75 h postinoculation, there was an estimated 76 to 80% reduction per day in bacterial counts; however, more accurate predictions of MPN per head of lettuce were generated by using non-log-transformed values of Ct. This study provides insight into the survival of E. coli O157:H7 transferred via splash from a contaminated fecal source onto produce during irrigation. Moreover, these findings can help generate inactivation times following a potential contamination incident.
authors.affiliation: Western Center for Food Safety, University of California, Davis, 1477 Drew Avenue, Suite 101, Davis, California 95618.
authors.forename: Jennifer A
authors.initials: JA
authors.lastname: Chase
authors.affiliation: Western Center for Food Safety, University of California, Davis, 1477 Drew Avenue, Suite 101, Davis, California 95618.
authors.forename: Edward R
authors.initials: ER
authors.lastname: Atwill
authors.affiliation: Western Center for Food Safety, University of California, Davis, 1477 Drew Avenue, Suite 101, Davis, California 95618.
authors.forename: Melissa L
authors.initials: ML
authors.lastname: Partyka
authors.affiliation: Western Center for Food Safety, University of California, Davis, 1477 Drew Avenue, Suite 101, Davis, California 95618.
authors.forename: Ronald F
authors.initials: RF
authors.lastname: Bond
authors.affiliation: Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, 5100 Paint Branch Parkway, College Park, Maryland 20740, USA.
authors.forename: David
authors.initials: D
authors.lastname: Oryang
keywords: 
            Escherichia coli O157:H7; Fecal contamination; Field trial; Produce; Slurry; Survival