May 2022 - 14

FOOD SAFETY
Fall seasonal effects linked to E. coli outbreaks
By Kim Kaplan
USDA ARS
USDA Agricultural Research Service
(ARS) scientists have begun to uncover
details underlying a pattern of seasonal E.
coli O157:H7 outbreaks linked to bagged
romaine lettuce.
That E. coli O157:H7 infection
outbreaks connected to romaine are
more frequently associated with lettuce
commercially grown and harvested at the
end of the growing seasons in California
and Arizona has been recognized for
several years. 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.
Most of these outbreaks involved
romaine lettuce harvested in the fall on
the California Central Coast such as in
Salinas, and in late winter in Southern
California and Arizona. These two states
are the major lettuce growing areas in
the United States with farm production
valued at nearly $2.7 billion in 2021.
" To begin unraveling 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
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 the
microbial quality of fresh-cut bagged
lettuce.
The researchers were a little surprised
grew in Salinas, and processed and
cold-stored in modified atmosphere
packaging as is done commercially, " said
ARS microbiologist Maria Brandl, with
the Produce Safety and Microbiology
Research Unit of the ARS Western
Regional Research Center in Albany,
California, and leader of the study.
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
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of the variety with long shelf life was
significantly greater when harvested in
the fall than in spring. 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, " said study co-author ARS
geneticist Ivan Simko, with the Crop
Improvement and Protection Research
Unit in Salinas, California.
In addition, the research team, which
included biologist Susan Leonard
and others at the U.S. Food and Drug
Administration, demonstrated that the
bacterial community present on bagged
to find such differences in E. coli survival
in fall-harvested lettuces compared to
spring-harvested ones because many
current hypotheses 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
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. 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, "
Brandl said. VGN
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May 2022

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