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: