Conditions at the time of inoculation influence survival of attenuated Escherichia coli O157:H7 on field-inoculated lettuce

Anne-Laure Moyne; Tyann Blessington; Thomas R Williams; Steven T Koike; Michael D Cahn; Maria L Marco; Linda J Harris

doi:10.1016/j.fm.2019.103274

Conditions at the time of inoculation influence survival of attenuated Escherichia coli O157:H7 on field-inoculated lettuce

Food Microbiol. 2020 Feb:85:103274. doi: 10.1016/j.fm.2019.103274. Epub 2019 Jul 16.

Authors

Anne-Laure Moyne¹, Tyann Blessington², Thomas R Williams³, Steven T Koike⁴, Michael D Cahn⁵, Maria L Marco⁶, Linda J Harris⁷

Affiliations

¹ Department of Food Science and Technology, One Shields Avenue, University of California, Davis, CA, 95616-5270, USA; Western Center for Food Safety, University of California, Davis, 1477 Drew Ave., Suite 101, Davis, CA, 95618, USA. Electronic address: amoyne@ucdavis.edu.
² Department of Food Science and Technology, One Shields Avenue, University of California, Davis, CA, 95616-5270, USA; Western Center for Food Safety, University of California, Davis, 1477 Drew Ave., Suite 101, Davis, CA, 95618, USA. Electronic address: tyblessington@gmail.com.
³ Department of Food Science and Technology, One Shields Avenue, University of California, Davis, CA, 95616-5270, USA. Electronic address: trwilliams1985@gmail.com.
⁴ University of California Cooperative Extension, 1432 Abbott Street, Salinas, CA, 93901, USA. Electronic address: skoike@trical.com.
⁵ University of California Cooperative Extension, 1432 Abbott Street, Salinas, CA, 93901, USA. Electronic address: mdcahn@ucanr.edu.
⁶ Department of Food Science and Technology, One Shields Avenue, University of California, Davis, CA, 95616-5270, USA. Electronic address: mmarco@ucdavis.edu.
⁷ Department of Food Science and Technology, One Shields Avenue, University of California, Davis, CA, 95616-5270, USA; Western Center for Food Safety, University of California, Davis, 1477 Drew Ave., Suite 101, Davis, CA, 95618, USA. Electronic address: ljharris@ucdavis.edu.

PMID: 31500714
DOI: 10.1016/j.fm.2019.103274

Abstract

The impact of plant development, environmental conditions at the time of inoculation, and inoculum concentration on survival of attenuated BSL1 Escherichia coli O157:H7 strain ATCC 700728 on field-grown romaine lettuce was evaluated over 3 years. E. coli 700728 was inoculated onto 4- and 6-week-old romaine lettuce plants in the Salinas Valley, CA, at night or the next morning with either low (5 log) or high (7 log) cell numbers per plant to simulate a single aqueous contamination event. At night, when leaf wetness and humidity levels were high, E. coli cell numbers declined by 0.5 log CFU/plant over the first 8-10 h. When applied in the morning, E. coli populations declined up to 2 log CFU/plant within 2 h. However, similar numbers of E. coli were retrieved from lettuce plants at 2 and 7 days. E. coli cell numbers per plant were significantly lower (P < 0.05) 7 days after application onto 4-week-old compared to 6-week-old plants. E. coli 700728 could be recovered by plating or enrichment from a greater proportion of plants for longer times when inoculated at high compared with low initial concentrations and after inoculation of 6-week-old plants compared with 4-week-old plants, even at the low initial inoculum. A contamination event near harvest or when leaf wetness and humidity levels are high may enhance survivability, even when low numbers of E. coli are introduced.

Keywords: Inoculum concentration; Leaf wetness; Plant age; Romaine lettuce.

MeSH terms

Colony Count, Microbial
Consumer Product Safety
Escherichia coli O157 / growth & development*
Food Microbiology
Humidity
Lactuca / microbiology*
Microbial Viability*
Plant Leaves / microbiology*
Time Factors