Shiga toxin-producing Escherichia coli (STEC) outbreaks are often linked to processed lettuce, which undergoes substantial physiological changes during storage. To better understand the storage and seasonality factors that may contribute to STEC outbreaks from bagged romaine lettuce, scientists from the U.S. Food and Drug Administration’s Center for Food Safety and Applied Nutrition (FDA’s CFSAN) and the U.S. Department of Agriculture’s Agricultural Research Service (USDA ARS) used whole genome sequencing to investigate the bacterial microbiome and STEC O157:H7 colonization of fresh-cut lettuce. Two cultivars, with either long or short shelf life, were observed, and were subject to different variables: whether the lettuce was harvested in the spring or fall, and whether the lettuce was stored in cold or warm temperatures.

The results of this study suggest that seasonality, shelf life, and storage atmosphere are all main factors in the prevalence of STEC in lettuce. The microbiomes of bagged romaine lettuce samples were also affected by the lettuce’s season of harvesting, lettuce deterioration state, and survivability of STEC on the lettuce. Notable findings include: 

• In the cultivar with short shelf life, STEC O157:H7 survived much better when stored in a cold atmosphere
• STEC O157:H7 in both cultivars multiplied rapidly at high storage temperatures 
• Fall harvesting and lettuce deterioration were both factors associated with high STEC O157:H7 survivability, even at low storage temperatures
• Elevated carbon dioxide levels in packaging were correlated with STEC O157:H7 multiplication at high storage temperatures
• STEC O157:H7 population changes occurred at low storage temperatures
• Fall and spring microbiomes differed before and during storage at both temperatures, with fall microbiomes supporting the greatest STEC O157:H7 survivability in both cultivars.

This study fills knowledge gaps that were identified in FDA’s Leafy Greens STEC Action Plan. FDA hopes that the new insight gained about the relationship between lettuce microbiomes and storage atmospheres will help the agency combat foodborne illness outbreaks caused by leafy greens.