April 18, 2022
Scientists from the U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition and the U.S. Department of Agriculture’s Agricultural Research Service (USDA ARS) have been working collaboratively to understand the seasonal effects and other factors that may be contributing to outbreaks of E.coli O157:H7 linked to bagged romaine lettuce – and they are making significant progress. Leafy greens, including bagged romaine lettuce, have been implicated in outbreaks of foodborne illness caused by Shiga toxin-producing E. coli (STEC), the most common of which is E. coli O157:H7.
In a study recently published in the BMC Environmental Microbiome, FDA and USDA scientists presented findings which reveal that season, and lettuce shelf life, can influence the bacterial communities and behavior of E. coli O157:H7 on cut lettuce stored in modified atmosphere packaging. The scientists found that E. coli O157:H7 survived significantly better in cold-stored packaged romaine harvested in the fall than on the same varieties harvested in late spring. The research team also demonstrated that the microbiome present on bagged romaine differed by season, lettuce deterioration state, and whether the survival of E. coli O157:H7 on the lettuce was high or low. For example, the team found that E.coli O157:H7 survived better in lettuce that was harvested in the fall than lettuce harvested in the spring during cold storage. The analysis demonstrated how trends in E. coli survival correlated with trends in microbiome composition.
A microbiome is a community of microorganisms (such as bacteria, fungi, and viruses) that inhabit a particular environment. To conduct this work, USDA ARS cultivated two romaine lettuce types with different shelf life in Salinas, CA. The lettuce was grown and harvested in the spring and fall and then processed as fresh cut product. The product was then inoculated with E. coli O157:H7 before all of it was packaged in modified atmospheric packaging (MAP) and stored at either cold or warm temperatures. The FDA then contributed cutting-edge genomic sequencing tools to characterize the microbiome composition present on the various lettuce samples.
This study provided insight into the relationship between the lettuce microbial ecology and the potential for E. coli O157:H7 survival during cold storage. This is a significant step toward closing the knowledge gaps identified in the FDA’s Leafy Greens STEC Action Plan and helping the agency and its partners to reduce foodborne illness linked to the consumption of leafy greens.