Studies uncover E. coli outbreak patterns in lettuce Agricultural Research Service sci- ent i s t s at the U. S. Depar tment of Agriculture have begun to uncover de- tails underlying a pattern of seasonal E. coli outbreaks linked to bagged ro- maine lettuce. seasons in California and Arizona. Although contamination of lettuce products is rare, between 1998 and 2019, 36 outbreaks that traced back to lettuce were recorded by the Centers for Disease Control and Prevention. in romaine lettuce harvested in the fall on the Central Coast such as in Salinas, and in late winter in Southern California and Arizona.
tors 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 com- mercially,” said USDA microbiologist Maria Brandl. She leads a study on E. coli outbreak patterns andworks at the Produce Safety and Microbiology Research Unit of the ARS Western Regional Research Center in Albany. One of the most significant findings of this study is that E. coli survived on average 5.6 times better in cold-stored packaged romaine harvested in the fall than on the same varieties 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 varietywith long shelf lifewas signifi- cantly greater when harvested in the fall than in spring,” saidUSDAgeneticist Ivan Simko, a study co-author with the Crop Improvement and Protection Research Unit in Salinas. “I have observed this in previous stud- ies 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,” Simko said. In addition, the research teamdemon- strated that the bacterial community present on bagged romaine differed by season, lettuce deterioration state and whether survival of E. coli on the lettuce was high or low. This suggests a potential for using the microbiome as an indicator of themicro- bial quality of fresh-cut bagged lettuce. The research team included biologist Susan Leonard and others at the U.S. Food and Drug Administration. The researchers reported they were a little surprised to find such differenc- es in E. coli survival in fall-harvested lettuces compared to spring-harvested crops, because many current hypothe- ses about the seasonality of outbreaks linked to lettuce focus on differences in the prevalence of the E. coli pathogen in the environment. “While prevalence may also be in- volved, 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 said. “These are the next areas we want to work on,” Brandl added. “For example, is this something we could try to ma- nipulate in the long run through plant physiology and breeding, and microbi- ome 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.” The Agricultural Research Service is USDA’s chief scient i f ic in-house research agency.
The two states aremajor lettuce-grow- ing areas in the United States, with farm production valued at nearly $2.7 billion in 2021. “To begin unravel l ing the causes of these seasonal outbreaks, our re- search team looked at var ious fac-
It has been recognized for several years that E. coli infection outbreaks connect- ed to romaine are more frequently asso- ciated with lettuce commercially grown and harvested at the end of the growing
Most of those outbreaks involved E. coli O157:H7, the most common strain of the bacteria. The E. coli detections were reported
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12 Ag Alert April 27, 2022
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