Evaluation of factors that influence cross contamination of Escherichia coli between gloves and lettuce during harvesting and fate of Escherichia coli O157:H7 and Salmonella on fresh-cut lettuce under conditions of consumer handling and storage.

摘要

The increase in consumption of lettuce over the past two decades has been coupled with an increase in outbreaks of foodborne illness, particularly Escherichia coli O157:H7 gastroenteritis linked to these products. Risks associated with consumption of lettuce include contamination along production and distribution chain from preharvest to consumption as well as amplification at points in the chain where conditions are conducive to microbial growth. The objectives of this study were to evaluate two risk nodes: the use of gloves for harvesting lettuce and storage and handling of fresh-cut lettuce in the home. The impact of glove type on the transfer of E. coli (generic E. coli or attenuated E. coli O157:H7) was evaluated using a model agar system or lettuce leaves in a controlled laboratory setting and during stimulated lettuce harvesting in the field. In the model system, glove type impacted both the survival of the inoculated microorganisms during drying and the number of cells transferred to sequential agar plates. Survival was better for generic E. coli in all cases. For both E. coli strains, survival was better on disposable gloves and transfer was higher from smooth glove types. A single inoculated glove could contaminate 10% of the 20 sequentially harvested heads of lettuce. Transfer from contaminated heads of lettuce to clean gloves and subsequently harvested lettuce was also observed. The role of glove type was less clear but generally higher rates of transfer were observed with disposable smooth gloves. In the second part of the study the influence of consumer handling practices on the survival or growth of E. coli O157:H7 and Salmonella was evaluated on fresh-cut lettuce. E. coli O157:H7 or Salmonella were inoculated onto fresh-cut Romaine lettuce at a level of 3 log CFU/g. The inoculated lettuce was placed in containers and stored at 4 or 23°C. Slow (less than 1 log CFU/g) declines were observed at 4°C over 6 days of storage. At 23ºC, lag times of 5.6 and 7.9 h were predicted for E. coli O157:H7 and Salmonella with population increases of 1.3 and 1.2 log CFU/g in 24 h, respectively. Antimicrobial effects of olive oil, vinegar or combination used as dressings on inoculated lettuce was investigated, only weak antimicrobial effect was observed for over 4 h of storage at 23°C at common dressing levels. In conclusion, glove type may influence survival of E. coli during desiccation and the amount of bacteria transferred at each touch point. In the home, exposing fresh-cut lettuce to ambient temperatures; current consumer recommendations to limit ambient exposure to no more than 2 h is appropriate.