Submitted to: Journal of Food Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/18/2008
Publication Date: 4/1/2008
Citation: Chua, D., Goh, K., Saftner, R.A., Bhagwat, A.A. 2008. Fresh-cut Lettuce in Modified Atmosphere Packages Stored at Improper Temperatures Support Enterohemorrhagic E. coli Isolates to Survive Gastric Acid Challenge. Journal of Food Science. 73:M148-M153. Interpretive Summary: Incidences of food-borne outbreaks involving enterohemorrhagic Escherichia coli strains on leafy green produce have been reported. Producers rely mainly on refrigeration temperatures, and more recently on modified atmosphere packaging to extend shelf-life and to reduce the microbial load. This study examines how the extension of shelf-life may be creating opportunities for human pathogens to grow, survive and produce toxins. We investigated the effect of packaging conditions of leafy green vegetables on the ability of pathogenic E. coli strains to breach the gastric stomach barrier and increase the risk of disease. The study highlights some of the impending dangers of abusive storage temperatures especially with regards to produce packaged for extended shelf-life. The research will benefit the fresh produce industry, as well as increase the microbial food safety of America’s food supply.
Technical Abstract: Incidences of food-borne outbreaks involving enterohemorrhagic Escherichia coli strains with mutations in a key regulatory gene, rpoS, have been reported. Incentives, if any, for losing this regulatory function are not clear since the RpoS regulator is required for the expression of several environmental stress tolerance genes. RpoS also positively regulates two of the three acid-resistance systems of E. coli under aerobic growth conditions and enables the pathogen to survive gastric acid challenge. We selected six enterohemorrhagic E. coli isolates known to carry defective rpoS gene and analyzed resistance to the synthetic gastric juice after the strains were inoculated on fresh-cut lettuce and stored under modified atmosphere packaging (MAP) conditions. Sub-atmospheric oxygen partial pressures in MAP enabled all 6 isolates to induce acid-resistance over the 8-day storage period if the temperature was greater than or equal to 15 ºC. No acid-resistance was induced for MAP-lettuce left at temperatures less than or equal to 10 ºC or for lettuce packed and stored under aerobic conditions. The data underscore the impending danger of abusive storage temperatures especially with regards to the application of MAP to extend the shelf-life of fresh produce. The results also highlight the biological significance of having multiple acid-resistance pathways and the complex regulatory network of enterohemorrhagic E. coli strains.