July 2022 - 16

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
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 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
of the variety with long shelf life was
significantly greater when harvested
in the fall than in spring, said study
co-author and ARS geneticist Ivan
Simko, with the Crop Improvement and
Protection Research Unit in Salinas.
" I have observed this in previous
studies as well but the significance to
E. coli on lettuce had not been tested, "
Simko said. " Here we showed that greater
deterioration in the fall lettuce was
associated with better pathogen survival. "
In addition, the research team, which
included biologist Susan Leonard
and others at the Food and Drug
Administration, demonstrated that
the bacterial community present on
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 freshcut
product, " Brandl said. " 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? " she said.
" 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. " VGN
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July 2022

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