Food Protection Trends - September/October 2022 - 370

overhead irrigation. Most study findings indicate that pathogen
transfer may occur from splashing under certain circumstances,
but the effect of splashing on gross in-field contamination is
still not completely understood. When the in-field contamination
levels are low, pathogen transfer propagated by rainfall
or irrigation (i.e., splashing) alone may not pose a major risk
of gross in-field contamination but may play a more significant
role in spreading contamination during harvest operations or
when the product is further trimmed, comingled, or processed
during or after harvest.
Plant characteristics and conditions
Plant characteristics and conditions also affect survival and
persistence of pathogens after they are deposited onto specialty
crops. Plant wounds and injuries may provide pathogens with
access to nutrients that may enhance survival on or in a plant.
When plants are wounded or injured by farm equipment, plant
pathogens, insects, and/or adverse weather conditions such
as frost or intense wind or heat, pathogens on plants may be
better able to survive and persist (5, 6, 17, 57, 95, 121, 123). In
growth chamber and greenhouse studies, USDA researchers
found that the susceptibility of lettuce damaged by downy
mildew (caused by the oomycete Bremia lactucae) to E. coli
O157:H7 and Salmonella increased 105
-fold compared with
102-fold on plants without downy mildew (123). Plant age
and leaf topography (e.g., roughness and stoma density) also
affected E. coli O157:H7 and Salmonella survival on leafy green
plants (18, 39, 61, 82, 85, 142).
A plant's genetic traits (genotype) also play a role in susceptibility
to human pathogens. Erickson et al. (43) tested five
lettuce cultivars (Green Star, Muir, New Red Fire, Gabriella,
and the romaine lettuce cultivar Coastal Star) in field trials to
evaluate differences among these cultivars in vulnerability to
E. coli O157:H7 and Salmonella contamination. Up to 5 days
post-inoculation, significant differences were found among
the cultivars' responses to Salmonella and E. coli O157:H7
contamination, but the differences disappeared by day 9. After
5 days post-inoculation, pathogen surface levels were so low
that no difference in pathogen survival was measurable, and the
study authors concluded that cultivar selection has minimal
impact on managing microbiological risk. Jacob and Melotto
(65) further explored the natural variability in the response
of 11 lettuce cultivars to the human pathogens S. enterica
Typhimurium and E. coli O157:H7 and the differences in the
immune responses of these lettuce cultivars. In contrast to the
findings of Erickson et al. (43), Jacob and Melotto found differences
in cultivar susceptibility to pathogen colonization and
in the plants' natural defenses against pathogen colonization.
Differences in the results of the two studies may be due to the
lettuce varieties tested and experimental conditions related to
how pathogens were inoculated onto plants. However, similar
to Erickson et al., Jacob and Melotto also observed a significant
decrease in internalized pathogen populations in 9 of the 11
lettuce cultivars 10 days after surface inoculation.
370 Food Protection Trends September/October
Other studies have been conducted on the native microbial
communities (the microbiota) of specialty crops and the role
they may play in pathogen survival by either protecting the
plants from adverse conditions or minimizing or excluding
pathogens (30, 108, 110, 142). In their 2-year field study,
Williams et al. (142) identified bacterial communities on
romaine lettuce leaves grown in the Salinas Valley, California
and reported that lettuce plants with low levels of persistent
attenuated E. coli O157:H7 also had lower levels of native
bacteria. The bacterial communities differed among all four
plantings and were strongly associated with the planting
season. Bacterial diversity increased as plants grew and was
affected by the irrigation method (drip or overhead irrigation).
When Poza-Carrion et al. (108) studied S. enterica survival
on romaine lettuce and cilantro leaves, they found that the
pathogen survived better on a wet leaf surface when it was
associated with precolonized clusters of native bacteria. Cooley
et al. (30) also reported a differential response of native
bacterial species (i.e., some protective and others competitive)
to E. coli O157:H7 on lettuce. Theofel et al. (130) sampled
leaves from an almond orchard next to and downwind from
a poultry operation. The microbiota of almond trees closest
to the poultry operation was different from the microbiota of
leaves on trees in the orchard collected further away from the
poultry operation.
Compounding effects
Major changes in environmental conditions such as those
caused by wildfires, hurricanes, and other natural disasters
also have an impact on the ecosystem in produce-growing
regions. For example, wildfires in California destroyed large
swaths of insect habitat, which may impact surviving insect
populations in various ways including shifting populations
from burned areas to nearby crop production areas. Other
than the effects of wildfires on insects in forest ecosystems,
changes in more widespread insect behavior have not been
extensively studied (106). Coupled with any type of crop
damage, a higher insect pest burden could result in a higher
food safety risk if contamination occurs. These adverse events
and weather conditions may not be a high food safety risk on
their own, but when they intersect with additional adverse
circumstances (e.g., crop damage, increased insect burden,
and changes in wildlife populations and habitat), they may
create conditions under which crops are more susceptible to
pathogen contamination.
PREVENTION: WHAT PRACTICES SHOULD YOU
CONSIDER INCLUDING IN YOUR FOOD SAFETY
PROGRAM AND WHAT OTHER MITIGATION
MEASURES ARE BEING DEVELOPED TO
REDUCE THE RISK OF ANIMAL-RELATED
CONTAMINATION?
In this section we explore various research-backed practices
that minimize the food safety risks associated with growing
produce in areas where animal agriculture such as concentrated
Food Protection Trends - September/October 2022

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