Scientists at the U.S. Department of Agriculture's Agricultural Research Service have linked seasonal E. coli outbreaks to bagged romaine lettuce.
For several years now, ARS researchers have recognized E. coli O157:H7 infection outbreaks are connected to romaine lettuce harvested in the fall in California and Arizona, according to an April 12 news release. There have been 36 outbreaks between 1998 and 2019, according to the release. These two states are the major lettuce growers in the US with production valued at nearly $2.7 billion in 2021.
"To begin unravelling the causes of these seasonal outbreaks, our research team looked at various factors to identify conditions that may increase E. coli survival on fresh-cut lettuce that we grew in Salinas, and processed and cold-stored in modified atmosphere packaging as is done commercially," said Maria Brandl, microbiologist at the ARS Western Regional Research Center in Albany, Calif., according to the news release.
The research team was able to discover E. coli survived 5.6 times better on romaine lettuce harvested in the fall than on lettuce harvested in late spring.
"We also found that among romaine varieties with longer and shorter shelf life in this study, the deterioration rate of the variety with long shelf life was significantly greater when harvested in the fall than in spring," Ivan Simko ARS, geneticist with the Crop Improvement and Protection Research Unit in Salinas, Calif., said. "I have observed this in previous studies as well but the significance to E. coli on lettuce had not been tested. Here we showed that greater deterioration in the fall lettuce was associated with better pathogen survival."
"While prevalence may also be involved, our results strongly indicate that fall-harvested romaine and the microbe communities it harbors have intrinsic characteristics that make them a better place for E. coli to survive in fresh-cut product," Brandl added. "These are the next areas we want to work on. For example, is this something we could try to manipulate in the long run through plant physiology and breeding and microbiome engineering? And would this trend also be observed for E. coli survival in the field? Our observations definitely open an entire new branch of inquiry about outbreak seasonality."
