Penetration of Escherichia coli O157:H7 into lettuce as influenced by modified atmosphere and temperature

The effects of temperature and atmospheric oxygen concentration on the respiration rate of iceberg lettuce and Escherichia coli O157:H7 cells attachment to and penetration into damaged lettuce tissues were evaluated. Respiration rate of lettuce decreased as the temperature was reduced from 37 to 10 degreesC. Reducing the temperature further to 4 degreesC did not affect the respiration rate of lettuce. Respiration rate was also reduced by lowering the atmospheric oxygen concentration. Lettuce was submerged in E. coli O157:H7 inoculum at 4, 10, 22, or 37 degreesC under 21 or 2.7% oxygen. Attachment and penetration of E. coli O157:H7 were not related to the respiration rate. The greatest numbers of E. coli O157:H7 cells attached to damaged lettuce tissues at 22 degreesC at both oxygen concentrations. More cells were attached under 21% oxygen than under 2.7% oxygen at each temperature, but this difference was small. Penetration of E. coli O157:H7 into lettuce tissue was determined by immunostaining with a fluorescein isothiocyanate-labeled antibody. Under 21% oxygen. E. coli O157:H7 cells showed greatest penetration when lettuce was held at 4 degreesC, compared to 10, 22, or 37 degreesC, and were detected at an average of 101 mum below the surfaces of cut tissues. However, under 2.7% oxygen, there were no differences in degree of penetration among four incubation temperatures. The degree of E. coli O157:H7 penetration into lettuce tissue at 4 or 22 degreesC was greater under 21% oxygen than under 2.7% oxygen; however, no difference was observed at 37 degreesC. Conditions that promote pathogen penetration into tissue could decrease the effectiveness of decontamination treatments.