Environmental inactivation and irrigation-mediated regrowth of Escherichia coli O157:H7 on romaine lettuce when inoculated in a fecal slurry matrix

Jennifer A. Chase, Melissa L. Partyka, Ronald F. Bond, Edward R Atwill

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


Field trials were conducted in July-August and October 2012 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, California. One-half milliliter of rabbit, chicken, or pig fecal slurry, containing an average of 4.05 10 7 CFU E. coli O157:H7 (C 0 ), was inoculated onto the upper (adaxial) surface of a lower leaf on 288 heads of lettuce per trial immediately following a 2.5 h irrigation event. To estimate the bacterial inactivation rate as a function of time, fecal matrix, irrigation and seasonal climate effects, sets of lettuce heads (n = 28) were sampled each day over 10 days and the concentration of E. coli O157:H7 (C t ) determined. E. coli O157:H7 was detected on 100% of heads during the 10-day duration, with concentrations ranging from 340 MPN/head (∼5-log reduction) to >3.45 10 12 MPN/head (∼5-log growth). Relative to C 0 , on day 10 (C t = 12 ) we observed an overall 2.6-log and 3.2-log mean reduction of E. coli O157:H7 in July and October, respectively. However, we observed relative maximum concentrations due to bacterial growth on day 6 (maximum C t = 8 ) apparently stimulated by foliar irrigation on day 5. From this maximum there was a mean 5.3-log and 5.1-log reduction by day 10 (C t = 12 ) for the July and October trials, respectively. This study provides insight into the inactivation and growth kinetics of E. coli O157:H7 on romaine lettuce leaves under natural field conditions. This study provides evidence that harvesting within 24 h post irrigation has the potential to increase the concentration of E. coli O157:H7 contamination, if present on heads of romaine lettuce; foliar irrigation can temporarily stimulate substantial regrowth of E. coli O157:H7.

Original languageEnglish (US)
Article numbere6591
Issue number3
StatePublished - Jan 1 2019


  • E. coli O157:H7
  • Enumeration
  • Fecal contamination
  • Field trial
  • Foliar irrigation
  • Food safety
  • Produce
  • Romaine lettuce
  • Slurry
  • Survival

ASJC Scopus subject areas

  • Neuroscience(all)
  • Biochemistry, Genetics and Molecular Biology(all)
  • Agricultural and Biological Sciences(all)


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