title: Heat and drought stress during growth of lettuce (Lactuca sativa L.) does not promote internalization of Escherichia coli O157:H7.
abstract: Studies were done to determine the effect of heat stress on internalization of Escherichia coli O157:H7 in lettuce subjected to different watering practices during growth. Iceberg and romaine lettuce were grown in sandy soil in an environmental chamber at 23 degrees C during the day and 7 degrees C at night, with a 12-h photoperiod. Thirty days after transplanting seedlings, potting soil was inoculated with a five-strain mixture of green fluorescent protein-labeled E. coli O157:H7 at populations of 4 and 6 log CFU/g of soil. Lettuce plants were exposed to one of two temperature stress regimes: 36 degrees C during the day and 15 degrees C at night for 2 days, or 32 degrees C during the day and 15 degrees C at night for 3 days, both with a 12-h photoperiod. Control plants were held at 23 degrees C during the day and 7 degrees C at night for 3 days. Plants were either watered daily or not watered during the heat stress and control treatments. E. coli O157:H7 was detected by enrichment in all inoculated soil and rhizosphere samples from plants grown in inoculated soil. Less E. coli O157:H7 was detected in inoculated heat-stressed soil than in control soil. From inoculated pots, all leaf surfaces and macerated leaves that had been surface sanitized were negative for E. coli O157:H7. All surface-sanitized macerated roots from control samples and from 143 of 144 samples of inoculated samples were negative for E. coli O157:H7. Heat stress during growth of lettuce did not promote or enhance internalization of E. coli O157:H7, regardless of the moisture content in the soil.
authors.affiliation: Center for Food Safety, University of Georgia, Griffin, Georgia 30223-1797, USA.
authors.forename: Guodong
authors.initials: G
authors.lastname: Zhang
authors.forename: Li
authors.initials: L
authors.lastname: Ma
authors.forename: Larry R
authors.initials: LR
authors.lastname: Beuchat
authors.forename: Marilyn C
authors.initials: MC
authors.lastname: Erickson
authors.forename: Vanessa H
authors.initials: VH
authors.lastname: Phelan
authors.forename: Michael P
authors.initials: MP
authors.lastname: Doyle
mesh_terms: D055864:Droughts; D019453:Escherichia coli O157; D005516:Food Microbiology; D006358:Hot Temperature; D018545:Lactuca / Q000382:microbiology* / Q000502:physiology*; D013312:Stress, Physiological / Q000502:physiology*