Salmonellosis Outbreaks in the United States Due to Fresh
Produce: Sources and Potential Intervention Measures
Irene B. Hanning,1J.D. Nutt,2,* and Steven C. Ricke1,3
Foodborne Salmonella spp. is a leading cause of foodborne illness in the United States each year. Traditionally,
most cases of salmonellosis were thought to originate from meat and poultry products. However, an increasing
number of salmonellosis outbreaks are occurring as a result of contaminated produce. Several produce items
specifically have been identified in outbreaks, and the ability of Salmonella to attach or internalize into vegetables
and fruits may be factors that make these produce items more likely to be sources of Salmonella. In addition,
environmental factors including contaminated water sources used to irrigate and wash produce crops have been
implicated in a large number of outbreaks. Salmonella is carried by both domesticated and wild animals and can
contaminate freshwater by direct or indirect contact. In some cases, direct contact of produce or seeds with
contaminated manure or animal wastes can lead to contaminated crops. This review examines outbreaks of
Salmonella due to contaminated produce, the potential sources of Salmonella, and possible control measures to
prevent contamination of produce.
vegetable industry to consistently supply consumers with a
wide array of products year round in much greater quantities.
Produce consumption in the United States has risen in recent
years due to the increase of public health interests and diet
trends. The sales of salad mixes alone increased from $167.5
Fresh Cut Produce Association, Arlington, VA, personal com-
productionvaluesoflettuce from$1.3billion dollarsin 1999 to
$2 billion in 2004 (USDA-ERS, 2005). It is generally perceived
that consumption of raw fresh produce products is safe and
that most foodborne disease outbreaks are caused by foods
primarily of animal origin (Sivapalasingam et al., 2004). How-
ever, the potential for foodborne pathogen contamination is
apparent in many aspects of vegetable production practices.
Accordingly, fresh fruit and vegetables have been implicated
ecent advances in processing techniques, preserva-
tion, and packaging have enabled the fresh fruit and
in a myriad of foodborne outbreaks and as sources of food-
borne pathogens (Doan and Davidson, 2000; Sewell and Far-
ber, 2001; Castillo and Rodrı ´guez-Garcı ´a, 2004; Mandrell and
Brandal, 2004; Sivapalasingam et al., 2004).
Fresh fruits and vegetables may be contaminated with ei-
ther spoilage or pathogenic microorganisms through culti-
vation practices, handling, and processing (Tournas, 2005).
Because the addition of preservation methods is often absent
in vegetable processing, there is a potentially greater risk for
microbial populations to contaminate and deteriorate the
product (Thomas and O’Beirne, 2000). Recent documented
foodborne disease outbreaks associated with fresh produce
have been on the rise. Bean and Griffin (1990) reported that
fruits and vegetables were the cause of 2% of the foodborne
diseaseoutbreaksin theUnited Statesbetween 1973and 1987.
By the 1990s, this had risen to 6% of all reported foodborne
outbreaks with over 16,000 cases of illness identified from
fresh produce and produce dishes in the United States
(Sivapalasingam et al., 2004; Mandrell and Brandal, 2004). The
most recent data indicate that 13% of outbreaks in the United
1Center for Food Safety–IFSE and Department of Food Science, University of Arkansas, Fayetteville, Arkansas.
2Poultry Science Department, Texas A&M University, College Station, Texas.
3Poultry Science Department, University of Arkansas, Fayetteville, Arkansas.
*Current address: Yum! Brands Inc., Dallas, Texas.
FOODBORNE PATHOGENS AND DISEASE
Volume 6, Number 6, 2009
ª Mary Ann Liebert, Inc.
States may be attributed to produce contaminated with
foodborne pathogens (Doyle and Erickson, 2007).
the need for freshwater sources to provide irrigation also in-
creases. This can be especially problematic for freshwater
sources that come in contact with regions where there are
large confined animal operations or large numbers of grazing
animals. In the western United States animal grazing is sub-
stantial enough to impact water quality (Larsen et al., 1994). In
addition, the application of animal wastes to agricultural soils
can increase levels of foodborne pathogens originating from
the manure of animal carriers (Crane et al., 1983; Edwards and
Daniels, 1992; Solomon et al., 2002; Hutchison et al., 2004). In
areas that are more densely populated or are closer to large
municipalities, raw sewage waste also can be a source of
human pathogens (Ait Melloul et al., 2001).
The globalization and importation of the food supply has
resulted in unique food safety challenges. The cooperation of
international agencies, formation of worldwide regulations,
and enhanced surveillance systems are indispensable for a
safe food supply (Kaferstein, 2003). The United States De-
partment of Agriculture-Economic Research Service (USDA-
ERS, 2008) estimates 13% of vegetables and 32% of fruits and
from 16.4% total fruits and vegetables in 1996. The burden of
inspecting these imported food items is placed on regula-
tory agencies that only inspect about 1% of imported food
(Buzby et al., 2008). Because greater volumes of fresh fruits
and vegetables are being imported along with greater vol-
umes being consumed, it is expected that the number of
foodborne illnesses due to produce will increase (Harris et al.,
Salmonella is a leading cause of foodborne illness in the
United States (CDC, 2008a). The USDA-ERS estimated 1.4
million cases of salmonellosis occurred in 2007 costing 2.5
billion dollars in terms of lost productivity and medical costs
(USDA-ERS, 2007). Salmonella has been shown to survive or
grow in a wide variety of produce. This review will focus on
produce that has been implicated in foodborne outbreaks in
the United States. Although outbreaks can also be attributed
to multiple sources including food preparers and the food
preparation environment, contamination of produce early in
the production chain such as the field and processing envi-
ronment will be the focus of this review. The environmental
aquatic sources of Salmonella contamination of produce also
are examined. Finally, control measures for limiting Salmo-
nella contamination in produce are discussed.
Produce Outbreaks Due to Salmonella
Salmonellae are commonly found in the gastrointestinal
tract of numerous animals such as birds, reptiles, farm ani-
mals, and humans. Most cases of salmonellosis were previ-
ously thought to be attributed to consuming contaminated
increasing number of produce-associated foodborne out-
breaks in the United States associated with bacterial con-
tamination are primarily from Salmonella (Tauxe et al., 1997;
Harris et al., 2003). In fact, from 2002 to 2003, 31 produce-
associated outbreaks of Salmonella were reported while only
29 outbreaks were poultry related (CDC, 2008a). A wide va-
riety of produce has been shown to be a source of Salmonella
outbreaks including tomatoes, sprouts, watermelon, canta-
loupe, lettuce, and mangoes (Tauxe et al., 1997). Outbreak
investigations of Salmonella infection implicated watermelons
as early as 1950 (Harris et al., 2003).
Produce-linked outbreaks of Salmonella are summarized in
Table 1. Typically in larger outbreaks, the source of Salmonella
that contaminated the produce is eventually identified. How-
ever, trace-back investigations can be hindered due to the
complexity of the food supply. Information such as brand
name, date of purchase, Universal Product Code (UPC) code,
food item and may not be available. Furthermore, trace-back
investigations rely on consumers that have become ill for in-
formation and may only provide limited information because
1) only a limited number of cases are available; 2) people that
are interviewed typically have a limited recall about the foods
longer available for testing. As demonstrated by the recent
2008 outbreak of salmonellosis due to Serrano peppers, an in-
in the 2008 outbreak, initial questionnaires by investigating
(CDC, 2008b). Furthermore, it is estimated that only 1 case of
salmonellosis out of 38 may be reported to the public health
authorities and this also limits outbreak investigations
(Voetsch et al., 2004). Outbreaks due to specific produce items
the larger outbreaks were available and are given.
Melons have been associated with multiple salmonellosis
that sickened more than 400 people occurred due to con-
taminated cantaloupe originating from Mexico and Texas
(CDC, 1991). Severalpastoutbreaks havealso beenassociated
with watermelon but comparatively fewer cases were re-
ported (Gayler et al., 1955; CDC, 1979). If the outside of the
melon is contaminated, cutting the melon can transfer Sal-
monella to the edible portions (Gayler et al., 1955; Ukuku and
Sapers, 2007). Since melons are rich in sugars and nutrients
and have a nearly neutral pH (Gagliardi et al., 2003), it is not
surprising that when contaminated melon pieces are not
stored properly (i.e., room temperature) Salmonella popula-
tions can increase (Ukuku and Sapers, 2007).
The contamination of melons can be from multiple sour-
ces including contaminated manure, irrigation water, fertil-
izers, wildlife, wash water, processing equipment, and=or
packaging (Kaneko et al., 1999). However, a survey of farm
and processing facilities found that a majority of Salmonella
contamination resulted from the rind being inoculated dur-
ing immersion in contaminated wash water in post-harvest
facilities (Gagliardi et al., 2003). Water in the wash tank or
coolers that becomes contaminated can inoculate large
numbers of melons subsequently washed in the same water.
Cantaloupes, in particular, are difficult to clean once they are
inoculated because the surface of the melon has lenticles
(netting) that provide areas for attachment and protection
from sanitizers (Lester, 1988; Gagliardi et al., 2003). Vigorous
scrubbing with multiple detergents and disinfectants is
necessary to remove contamination and may not remove all
of the pathogens (Barak et al., 2003; Parnella et al., 2005).
636 HANNING ET AL.
Therefore, preventing the initial contamination of melons in
the wash tank is considered a critical control point.
The voluntary Melon Quality Control Program was initi-
ated by the produce industry in 1992 because of the increasing
number of Salmonella-contaminated melon products (unpub-
lished discussion, Melon Quality Program meeting, Dallas,
TX, October 14, 1991; Mohle-Boetani et al., 1999). Recom-
mendations coming out of this program include the use of
hyperchlorinated water in all processing steps of fruit and
vegetable production to help reduce bacterial contamination
(Tauxe et al., 1997). However, this program is voluntary for
U.S. producers and imported melons may not follow these
guidelines. As a result, outbreaks of salmonellosis from im-
demonstrated by the 2008 outbreak due to cantaloupe im-
ported from Honduras (FDA, 2008a).
Careful monitoring of chlorine levels in wash water is
critical because chlorine levels can be rapidly neutralized by
organic materials washed from the melons. In addition,
chlorine may produce off-flavors and undesirable appearance
of foods, and may be harmful to the environment. Alter-
natives to chlorination have been studied. Bacteriophage
treatment was reported to effectively reduce Salmonella pop-
ulations on cut melon by 3.5 log10(Leverentz et al., 2001).
However, like antibiotics, Salmonella can become resistant to
bacteriophages, which eventually renders this treatment in-
effective (Santander and Robeson, 2007). Hydrogen peroxide
application has also been demonstrated to be effective, but
only if treated within 24 hours of contamination (Ukuku and
Fett, 2002). Similarly, commercial sanitizers were only effec-
tive if used within 1 hour of inoculating melons (Ukuku and
The need for effective treatments that can disinfect con-
taminated melons remains. The genetic basis for Salmonella
attachment to plants has been discovered and this informa-
tion could be used to develop effective treatments. It has been
demonstrated that Salmonella regulates production of cellu-
lose using the bcsA gene and the O-antigen capsule gene yihO
in order to attach to and colonize plants (Barak et al., 2007). A
key regulator of the O-antigen capsule and cellulose produc-
tion is the positive regulator AgfD (Zogaj et al., 2001; Barak
et al., 2005). In addition to regulation of these genes, AgfD is
also suggested to be a key regulator for survival of Salmonella
outside the host (Barak et al., 2007). Thin aggregative fimbrae
may also be used for attachment and genes required for ex-
pression of fimbrae include rpoS and agfD (Romling et al.,
2005). Importantly, these genes are also necessary for viru-
lence expression during human infection (Barak et al., 2005).
Using this information, potential treatment options could
be explored by monitoring Salmonella genes after exposure to
treatments to determine any effect on attachment and viru-
lence. For example, organic acids have been explored as an-
timicrobial treatments and the organic acids butyrate,
propionate, and acetate all have biocidal effects against
Salmonella (Ricke, 2003; Van Immerseel et al., 2006). Butyrate
and propionate decreased expression of hilA (virulence gene
regulator), invF (cell invasion factor), and sipC (cell invasion
protein), but acetate led to an increase of hilA and invF
(Durant et al., 2000; Lawhon et al., 2002). Therefore, acetate
may not be a good choice as an antimicrobial due to in-
creases in virulence expression. Furthermore, any Salmonella
surviving exposure to an organic acid may be more likely to
survive further acid treatment due to the induction of an acid
tolerance response and produce cross-protection to other
antimicrobials (Kwon and Ricke, 1998; Kwon et al., 2000;
In 1999, the U.S. Food and Drug Administration (FDA)
of sprout irrigation waters (FDA, 1999). In three outbreaks of
Salmonella Enteritidis from 2000 to 2001, it was determined
that the contamination was a result of these guidelines not
being followed (Mohle-Boetani et al., 2009). Specifically, the
seeds were being treated with 11,000ppm or less sodium
hypochlorite rather than the FDA-recommended level of
20,000ppm (Winthrop et al., 2003; Mohle-Boetani et al., 2009).
Additionally, sprout irrigation water at the producer’s farm
tested positive for Salmonella. A survey of California sprout
growers confirmed that most alfalfa sprout growers were
achieving the 20,000ppm disinfection guidelines and if fol-
lowed, disinfection was achieved (Thomas et al., 2003).
However, the same survey found that the mung bean sprouts
sampled were not achieving the disinfection guidelines and
thus mung bean sprouts may represent a larger risk. It was
noted in this survey that seeds and calcium hypochlorite were
not measured prior to mixing with water which may partially
explain why treatment was not fully effective.
In the outbreaks of Salmonella Enteritidis due to contami-
nated mung sprouts, growers and private laboratories, which
noted positive results, did not inform public health officials
(Mohle-Boetani et al., 2009). Furthermore, the grower contin-
ued to use seeds from a contaminated lot. These two points
perpetuated the outbreak which could have been interrupted
with help from public health officials. However, it is not
clearly recommended that growers report positive results to
public health officials, which represents a serious gap in food
From 1995 to 2003, no fewer than 10 outbreaks of salmo-
nellosis occurred due to contaminated sprouts. Alfalfa
sprouts were the source of two large outbreaks in 1995 and
1996, in which more than 700 people were made ill and one
death was reported. Mung sprouts were also a source of
Salmonella that resulted in 45 reported illnesses in 2000
(Mohle-Boetani et al., 2009). One potential source of contam-
ination to seeds may be contact with birds or rodents during
storage or shipping (Mahon et al., 1997). In addition, some
seeds may be set or scored in fields upon which animals
directly graze and these seeds can become contaminated from
animal feces (Brooks et al., 2001). This is the scenario of an
outbreak in September of 2008 in which 13 illnesses in Wa-
shington and Oregon of Salmonella Typhimurium were di-
rectly linked to consuming alfalfa sprouts (FDA, 2008b).
Because Salmonella can survive for months on seeds
(Beuchat and Scouten, 2002) and has the ability to grow dur-
ing the sprouting process, even a low level of seed contami-
nation can be considered a risk. The growth of Salmonella
during the sprouting process has been well documented
(Jacquette et al., 1996; Gandhi et al., 2001). In fact, under cer-
tain sprouting conditions, Salmonella can grow as much as 4
advances in equipment used and in practices during cultiva-
tion have been implemented to reduce growth during
SALMONELLA IN PRODUCE 637
Table 1. Examples of Human Outbreaks of Salmonellosis Due to Contaminated Produce
No. of illnesses
Gayler et al. 1955
CDC 1991; FDA, 2001
Mohle-Boetani et al. 1999
Ill food handler
Environment at sprouter
Mohle-Boetani et al. 2009
Environment at sprouter
Mohle-Boetani et al. 2009
Environment at sprouter
Mohle-Boetani et al. 2009
Mahon et al., 1997
Van Beneden et al., 1999
Jaquette et al., 1996
Jaquette et al., 1996
Taormina et al. 1999
Taormina et al. 1999
Taormina et al. 1999
Backer et al., 2000
Taormina et al. 1999
Harris et al., 2003
Proctor et al., 2001
Winthrop et al., 2003
Brooks et al., 2001
Sivapalasingam et al., 2004
Contaminated field by
Cummings et al., 2001
Gupta and Crowe, 2001
water used for irrigation
Greene et al., 2008
Gupta et al., 2007
Gupta et al., 2007
water used for irrigation
(related to 2002 outbreak)
Greene et al., 2005
Red Chili peppers
Sivapalasingam et al., 2003
Beatty et al., 2004
Contamination at grower?
Campbell et al., 2001
74 total (11 in US)
Contamination at grower?
Pezzoli et al., 2008
typhimurium var. Copenhagen
Salmonella Group B
Food preparation environment
aData source http:= =www.cdc.gov=foodborneoutbreaks=outbreak_data.htm. If no source is given, no source was determined for that outbreak.
sprouting. Among these improvements are using a rotary
drum sprouting process and employing frequent irrigation
with unrestricted drainage, which has been shown to aid in
washing away any pathogen contamination (Fu et al., 2008).
In addition, cool water used for irrigation and cool air blow-
ing over the sprouts has also been demonstrated to reduce the
rate of pathogen growth.
Even with these improvements, the risk of contamination
to products still exists which warrants a need for effective
post-harvest treatments. For example, Gandhi and Matthews
(2003) demonstrated that combined treatment of seeds with
20,000ppm calcium hypochlorite followed by 100ppm chlo-
rine or calcinated calcium during germination and sprout
growth did not eliminate Salmonella from alfalfa seeds and
sprouts. Low doses of irradiation (1 to 2kGy) were effective at
eliminating Salmonella and did not adversely affect sensory or
nutritional properties of the sprouts (Saroj et al., 2007), but
public perception of irradiated food remains suspicious at
best (O’Bryan et al., 2008). Each treatment option has some
drawbacks, therefore, before any commercial application can
be pursued, additional treatments need to be evaluated to
determine the optimal treatment to render sprouts safe.
Salmonella outbreaks due to contaminated produce are
usually widely dispersed, which suggests that contamination
is occurring early in production, such as in the field or in a
processing plant. Contamination of tomatoes early in pro-
(1995). Theresearchers demonstrated that if dilated plant cells
on the surface of a warm tomato are exposed to cold water
contaminated with Salmonella, the cells of the tomato will
rapidly contract and take in the Salmonella through openings
such as the stem scar. It has also been demonstrated that
to the exterior of the tomato and populations can increase
with time, depending on the environmental conditions (Itur-
riaga et al., 2007). Shi et al. (2007) reported that many serovars
of Salmonella can colonize and survive on tomatoes, but that
growth was serovar dependent. Serovars Enteritidis, Typhi-
murium, and Dublin (group D serovars more commonly as-
sociated with poultry) were less adapted to grow and persist
than Hadar, Montevideo, or Newport (group C serovars). The
ability of only some serovars to grow in tomatoes may be a
reason why only a few serovars of Salmonella are linked to
3.4 to 4.8 depending on the ripeness of the tomato (Wolf et al.,
1979). However, even at this pH, Zhuang et al. (1995) dem-
onstrated that Salmonella Montevideo populations in chopped
tomatoes can increase by 1.5 and 2.5 log units when stored for
22 hours at temperatures of 208C and 308C, respectively.
Investigations of Salmonella outbreaks due to contaminated
tomatoes have pinpointed sources including contaminated
irrigation water and contaminated wash water (Table 1). One
particular outbreak in 2005 was eventually linked to an out-
break that occurred 3 years earlier that was thought to have
affected as many as 2500 people (Greene et al., 2008). Through
use of pulsed-field gel electrophoresis, isolates from these two
outbreaks were shown to be genotypically identical to each
other and to Salmonella isolated from pond water that was
being used for irrigation purposes. No source of Salmonella to
thepond waterwasisolated,but investigatorsspeculatedthat
wild birds, reptiles, or amphibians were a likely source.
Once contaminated, tomatoes may be difficult to clean
because the efficacy of chlorine to remove Salmonella on to-
matoes depends on the location of Salmonella on the tomato.
Salmonella on stem scars and cracks in the skin survive better
than Salmonella on the smooth skin (Cummings et al., 2001).
Furthermore, the ability of Salmonella and other foodborne
pathogens to internalize into tomatoes and other produce
creates a significant challenge of eliminating the bacteria from
these foods. Sanitizing treatments are generally ineffective
against internalized pathogens because they are physically
protected from the chemicals. Like chlorine, treatments such
as electrolyzed water or ozonated water were shown to
eliminate Salmonella from the surfaces of the tomato (Chaidez
et al., 2007; Park et al., 2008) but are ineffective at reaching any
internalized Salmonella. Currently, irradiation is the only ef-
in tomatoes or any other produce because of its ability to
penetrate tissues of produce to eliminate internalized patho-
gens (Saroj et al., 2007). Furthermore, Gram-negative bacteria
are very susceptible to even low doses (O’Bryan et al., 2008).
However, treatment with irradiation can produce off flavors,
colors, and odors and destroy some nutrients (Fan and
Sokorai, 2008). As previously discussed, a lack of public
approval of irradiation hinders the employment of this
method for treating foods (O’Bryan et al., 2008).
Contaminated mangoes have been responsible for several
outbreaks of salmonellosis in the United States. Like toma-
toes, mangoes can internalize Salmonella to the fruit portion
via the stem scar (Bordini et al., 2007). However, unlike to-
matoes, mangoes have a nearly neutral pH and high sugar
content that is conducive to bacterial growth. Bordini et al.
(2007) showed that any internalized Salmonella survives dur-
ing cold storage (88C) and replicates at room temperature
Imported mangoes typically receive a hot water immersion
treatment (46.18C for 70 to 90 minutes) followed by a cold
water treatment (21.18C for 30 minutes) to control any fly
on the outer surfaces and the inner edible portion. The inter-
nalization mechanism of Salmonella into the mangoes can be
explained by air spaces present in the warm fruit contracting
into the fruit (Merker et al., 1999). Salmonella internalized into
mango was thought to be the source of a salmonellosis out-
break in 1999. In this outbreak, the contamination was traced
and toads, wild birds, and bird feces were found around the
tanks, which were the most likely sources of Salmonella
(Sivapalasingam et al., 2003).
Like tomatoes, no effective treatment exists to eliminate
Salmonella from mangoes once the bacteria are internalized.
This emphasizes the need for effective treatments of wash
water in tanks to remove any Salmonella contaminating the
outside of the fruit and to kill any Salmonella that might be
introduced into the wash water. In fact, inadequately chlori-
nated wash water is thought to be a large contributing factor
HANNING ET AL.
to the 1999 and 2001 outbreaks of Salmonella in mangoes
(Beatty et al., 2004). Research supports the effectiveness of
chlorine treatment of the wash water. Soto et al. (2007) dem-
onstrated that either chlorine or copper ion treatment of wash
water was effective at preventing internalization of Salmonella
in mangoes. Until research leads to treatments that can con-
sistently eliminate internalized Salmonella, effective treatment
and control of Salmonella in the wash water will be crucial to
preventing salmonellosis due to contaminated produce.
In April of 2008, an outbreak of Salmonella serotype Saint-
in Texas and New Mexico with 17 hospitalizations and 30
more cases being investigated in seven other states. Pre-
liminary investigations suggested Roma, plum, or round to-
matoes were the source of Salmonella. By early June, 145 cases
were reported and 23 people were hospitalized across 16
states. Tomatoes remained the suspected cause at this point,
but the Salmonella causing the outbreak had not been isolated
from tomatoes or growers’ fields. In mid-July, after a lengthy
investigation the FDA determined that tomatoes were not the
source of the outbreak. However, by this time 1220 people
were sickened in 42 states, the District of Columbia, and
Canada. The FDA shifted the warning from consuming raw
tomatoes to jalapeno and Serrano peppers. The source of the
outbreak was finally identified and reported on July 30, 2008,
to be contaminated irrigation water used at a farm in Mexico
to water crops of jalapeno and Serrano peppers.
largest outbreak of salmonellosis due to fresh produce in the
United States. Besides losses incurred due to medical costs
and loss of productivity, this particular outbreak included
additional costs that tomato growers incurred. Tomato sales
plummeted due to the FDA-issued warning against the con-
sumption of tomatoes. Florida growers alone estimated losses
of 100 million dollars and were requesting disaster relief
funds from the government. The United Fresh Produce As-
sociation claims federal agencies caused more than 300 mil-
lion dollars in losses nationwide. Since tomatoes had been the
source of multiple outbreaks in the past and no outbreaks
have been recorded from Serrano peppers, tomatoes seemed
like the most likely source. However, this outbreak demon-
Minor outbreaks and vegetables as indirect sources
Less extensive outbreaks due to mushrooms, carrots, let-
tuce, and other produce have been reported. There remains
very little information concerning the relatively less promi-
Outbreaks Due To Salmonella. In addition, since these vege-
tables are usually consumed together with other vegetables in
salads, it is difficult to distinguish the single vegetable source
of the pathogen. Herbs, in particularly cilantro and basil,have
been implicated in outbreaks. In 1999, cilantro was identified
as a probable source of salmonellosis, but the point of con-
tamination was never firmly identified (Campbell et al., 1999).
Contaminated basil caused an international outbreak that
sickened 19 people in the United Kingdom (Pezzoli et al.,
2008). There were also 11 cases in the United States with the
In the case of potatoes, potato salads containing mayon-
naise are typically thought to be the primary source of infec-
tion since mayonnaise contains eggs and eggs are a leading
source of Salmonella (CDC, 2008a). Therefore, some prepared
potato salads that are improperly stored have led to food-
borne salmonellosis. If eggs containing Salmonella are used
and acidic vinegar is not added, the Salmonella remains viable
in the mayonnaise (Xiong et al., 1999; Morgan et al., 2007).
Commercially prepared mayonnaise has a water activity, pH,
and salt content that creates a very hostile environment for
Salmonella. In fact, research has shown that the addition of
commercially produced mayonnaise actually inhibits the
growth of Salmonella in some foods (Doyle, 2005).
Besides the produce linked to outbreaks, Salmonella has
been identified from a number of produce items purchased at
retail. Salmonella has been isolated from oranges, chilis, cab-
bage, artichokes, cauliflower, celery, eggplant, spinach, and
zucchini (Harris et al., 2003). This may be due to differences in
processing of the produce item post-harvest or the fact that
some of the produce items are consumed after cooking. In
addition, there may be some intrinsic factors associated with
these items that prevents Salmonella from growing in the
product or modifies virulence properties of Salmonella. It has
been suggested that the food matrix influences the virulence
of pathogens because differences in stresses associated with
food matrices can have varying affects on bacterial growth
physiology and gene expression (Buchanan and Bagi, 2000;
Nutt et al., 2003a). Comparing these produce items with
produce that has been implicated in outbreaks may provide
valuable information that could be used to develop control
strategies for Salmonella in produce.
The outbreaks from tomatoes, peppers, and melons that
affected hundreds of people across multiple states demon-
strate that clean irrigation water is critical to preventing
be difficult to trace and sources such as domestic animal
manure can pollute irrigation water (Uhlich et al., 2008). If not
properly addressed, these sources of contamination can per-
sist and cause continuous outbreaks such as the tomato out-
breaks of 2002 and 2005, which were both linked to the same
water source but occurred over a 3-year period (Greene et al.,
2008). The importance of environmental sources of Salmonella
and the potential route of contamination to produce are dis-
cussed in the following sections.
Environmental Sources of Vegetable
Contamination from the environment
Contamination of fresh produce can occur virtually any-
where throughout the production process. Preharvest sources
of contamination are likely to be present before the fruits or
vegetables are even removed from their natural environ-
ments. Many spore-forming bacteria such as Clostridium
botulinum, C. perfringens, and enterotoxigenic Bacillus cereus
are found in the soil and may be naturally present on some
fruits and vegetables (Beuchat and Ryu, 1997). Other types of
bacteria or viruses present on fresh produce may be due to
direct contact with fecal material or exposure to untreated
wastewater used for irrigation. The use of rawanimal manure
SALMONELLA IN PRODUCE641
as fertilizer can increase the presence of microorganisms on
fruits and vegetables as well (Crane et al., 1983; Edwards and
Daniels, 1992; Solomon et al., 2002; Hutchison et al., 2004).
Another source of preharvest contamination that is often
overlooked is contact with wild or domestic animals. For ex-
ample, wild birds are known to potentially harbor bacterial
foodborne pathogens including Campylobacter and Salmonella
(Luechtefeld et al., 1980; Keener et al., 2004; Saleha, 2004).
Contact with fecalmaterial fromthese animalscanpotentially
result in the contamination of vegetable crops.
Specific agricultural practices such as the use of animal
manure as opposed to chemical fertilizers may be a factor in
introducing these foodborne pathogens to fresh produce
(Tauxe et al., 1997). Also, the use of contaminated irrigation
water is a potential vehicle for transferring Salmonella to the
surfaces of fresh fruits and vegetables (Steele and Odumeru,
2004). Given the incidence of Salmonella as a foodborne
pathogen in general and its distribution in a variety of envi-
ronments, Salmonella species causing foodborne disease are of
particular concern as produce contaminants.
Foodborne Salmonella in animal sources
A majority of Salmonella infections are zoonotic and the
type of animal reservoir is a vital factor in understanding the
epidemiological pattern of the disease. While poultry is con-
sidered the most prevalent and important source of salmo-
this bacterial species as well. Reptiles are known to carry a
variety of Salmonella serovars including S. enterica (Geue and
Lo ¨schner, 2002). In a report issued by the Centers for Disease
Control and Prevention (CDC), it was reported that there
were over 50,000 cases of reptile-associated salmonellosis in
1996 (CDC, 1999). Cattle have also been documented to be a
source of Salmonella as well as Escherichia coli (Himathongk-
ham et al., 1999). Cattle infected with Salmonella can excrete
symptoms (Losinger et al., 1995). Cattle recovering from a
clinical salmonellosis infection may shed the organism typi-
cally from 2 to 12 weeks (Galland et al., 2000).
The use of animal manure to fertilize soil is both econom-
ically and environmentally beneficial in maintaining soil fer-
tility and quality. However, animal manure used to fertilize
grazing grasses of pastures can introduce pathogens that can
eventually colonize the gastrointestinal tracts of other food
animals and animals from which manure is produced (Holley
et al., 2008). Manures used to fertilize crops that contain
pathogens can eventually contaminate produce products.
There is some competition with soil organisms as well as
adverse conditions that can lead to a reduction of pathogens;
suggested that pathogen movement through the soil is likely
and they could reach aquifers or surface waters. If these wa-
ters are used for irrigation, crop contamination may occur.
An additional factor to consider is the ability of pathogens
to become endophytes. Contaminated irrigation water and
subsequent internalization of E. coli O157:H7 into spinach
resulted in a large outbreak causing 204 people to become ill
(Anonymous, 2006). The simple recycling of animal wastes
can lead to large outbreaks and highly complex epidemio-
logical investigations. Therefore, it is critical to control and
eliminate pathogens from manure regardless of where the
manure is applied.
Presence of Salmonella in water
The microbiological quality of irrigation water, regardless
of the source, is crucial in maintaining safe food products.
Non-wastewater sources that are not normally considered
contaminated with waste or fecal material could play a large
role in the presence of Salmonella and other microorganisms
on fresh produce. Major rivers have been used as irrigation
sources for many agricultural settings (Assadian et al., 1999;
Garcia et al., 2001). The annual bacterial loads of Salmonella in
these rivers and coastal areas are essential for assessing po-
tential risk (Baudart et al., 2000). Salmonella appear to possess
the mechanisms to retain viability and successfully survive in
these river environments as well. In samples of river water,
Salmonella cells were shown to be viable even after 31 days of
exposure as determined by reculturing and obtaining plate
counts (Santo Domingo et al., 2000). Furthermore, Salmonella
cells exposed to and surviving stresses associated with
et al., 2003b).
Not only do Salmonella cells survive in river-water samples
but this may be one of the larger reservoirs harboring viable
bacteria. Baudart et al. (2000) reported that Salmonella Typhi-
murium was dominant in river water and marine and fresh-
water sediments. They concluded that the presence of this
species in the marine sediments near the river discharge
supported the ability of Salmonella to survive long-term in a
natural environment. In a study by Polo et al. (1998), envi-
ronmental water samples were collected and plated on media
selective for Salmonella cells. River-water samples yielded the
highest proportion of Salmonella cells compared to other
sampled water sources from beaches and freshwater reser-
voirs. This raises a major concern given that currently many
rivers are used extensively as irrigation sources for fruits and
Strategies for Limiting Salmonella
Contamination of Produce
The produce industry faces unique challenges for elimi-
nating pathogen contamination when compared to other
types of foods due to three specific factors related to produce:
1) produceistypicallyconsumed withoutcooking; 2)produce
is usually not packaged; and 3) the ability of pathogens to
internalize into produce exists. Due to these factors, addres-
sing prevention of contamination at all stages of production is
nature, it is unrealistic to assume that pathogen-free produce
can always be achieved.
Contamination intervention strategies such as Hazard
Analysis and Critical Control Point (HACCP) that were first
implemented for meat products are being designed for pro-
duce production. The application of HACCP to minimally
processed crops has been reviewed (Hurst, 2006; Leifert et al.,
points in processing that might introduce hazards to the fin-
ished product. A determination ofthe criticalcontrol points in
any production system is a complex procedure that varies
with each product and the process being used (Sperber, 1992).
642HANNING ET AL.
The USDA guide to minimize microbial food safety hazards
for fresh fruits and vegetables suggests proactive monitoring
of potential critical control points such as microbial testing of
agricultural water (USDA, FDA=CFSAN, 1998). However,
even with the proper plan and systems in place, it may still be
possible for some microbial contamination to occur and it is at
that point that corrective actions need to be taken. Corrective
actions can involve two activities: 1) determining and fixing
the point in the production chain at which contamination was
introduced; and 2) determining what to do with the product.
Potentially contaminated products will need to be evalu-
ated to determine if a microbiological risk exists. After which
it can be determined if treatments will render the product safe
or if the product will have to be discarded. Treatment options
to eliminate bacteria from produce are being explored. Even
packaging is being designed with specific applications for
produce. These challenges producers face and options for
addressing problems are discussed in the next sections.
Elimination of pathogens
Treatment of produce poses a particular challenge because
consumers demand a fresh and minimally processed product.
Furthermore, not all treatments may be equally effective at
eliminating different types of bacteria. For these reasons,
multiple hurdle or sequential intervention strategies may ul-
timately be the best option for produce growers and proces-
sors. This type of strategy first described by Leistner (1985)
employs the use of several treatments in a sequential order to
sublethally injure bacteria to the point they are not allowed to
recover before the application of a subsequent treatment.
Since sublethally injured bacteria are more susceptible to
treatments than unstressed cells (Ray, 1986), sequential ap-
any single treatment. A sequential treatment plan was shown
to be effective for decontaminating raw beef (Delmore et al.,
1998). A combination of pre-eviseration washing, an acetic
acid rinse, and a final wash followed by a second acetic acid
resulted in a reduction of coliform counts of nearly 2 logs
while washing or acetic acid rinse alone resulted in 0.4 and 1.1
log reductions, respectively. Similarly, Kang et al. (2001) re-
ported a combination of hot water rinse, followed by hot air
treatment and sequential rinses with lactic acid were more
effective at reducing microbial contamination on beef trim
than any single treatment.
Using multiple treatments may allow producers to reduce
the severity of treatments, which could result in a higher
quality product. This approach could be particularly useful
for irradiation of foods since high levels of irradiation can
produce off flavors and colors, but lower levels may not be
fully effective ateliminatingbacteria(O’Bryanetal.,2008).For
example, a low dose of irradiation alone did not destroy
clostridia spores but was sufficient to sensitize spores to fur-
ther heat treatment (Patterson, 2001). Similarly, the combi-
nation of treating carrots with plant oils and packaging in
modified atmosphere lowered the minimum doses of irradi-
ation necessary to eliminate Listeria from 0.36 to 0.17kGy
(Caillet et al., 2006).
Even if a completely pathogen-free product is achieved
after processing, a large problem to the produce industry is
the lack of packaging that can help prevent re-contamination
of products following any sanitizing steps. Produce can be
exposed to multiple contamination routes following proces-
sing during shipping and display in retail markets. Packaging
is typically not used for produce but some new technologies
are being developed.
Prevention of pathogen recontamination
and growth by packaging
Most produce items are sold without packaging, therefore
the possibility of contamination is present after any post-
harvest treatment. Akins et al. (2008) reported that cantaloupe
taken from the wash tank exhibited lower microbial counts
than cantaloupe taken after conveyor belts in the packaging
area of the processing plant. This suggested that the conveyor
belts may have contributed to recontamination of the canta-
loupe. The risk of contamination at the retail level also exists
simply due to the fact that most produce is not packaged and
can be subsequently contaminated by handling. Espinoza-
Medina et al. (2006) reported that 16.7% of workers han-
dling produce were polymerase chain reaction positive for
The use of packaging for cut produce not only reduces this
to extend shelf-life. The produce industry has employed
methods such as modified atmospheric packaging (MAP) to
help suppress microbial growth in their products and to ex-
tend shelf life. MAP is a commonly used packaging system to
suppress growth of microorganisms where produce is pack-
aged in a low level of O2that is replaced with N2or CO2
(typically 50% O2, 30% N2, and 20% CO2). This atmosphere
can result in a lower pH, which may inhibit the growth of
bacteria (Daniels et al., 1985). Evidence has also shown that
elevated levels of CO2in produce packaging extends the lag
phase of bacteria and can help slow its growth (Zagory, 1999).
The nature and abundance of the gasses used in MAP affect
certain types of organisms in different ways. Lower levels of
O2allow for the growth of microaerophilic organisms such as
lactic acid bacteria while elevated CO2levels favor Gram-
positive as opposed to Gram-negative bacteria (Brackett,
However, the capability of these MAP systems may not
always yield products that are safe for consumption and are
sometimes ineffective in reducing microbial growth. Abdul-
Raouf et al. (1993) determined that the use of MAP was not
effective in decreasing the survival capabilities of E. coli
O157:H7 on cucumber or shredded lettuce samples. Sy-
nergistic treatment approaches have been suggested such as
the use of MAP in combination with a disinfectant-containing
washandmaybemoreeffective. JinandLee(2007)found that
MAP alone had no effect on reducing the levels of Salmonella
on mung sprouts, but the combined treatments of chlorine
dioxide wash and MAP reduced Salmonella by 3.0 log CFU=g
and maintained a reduced level for 7 days.
Edible films have been explored for applications as pack-
aging of produce. The edible film can create a modified at-
mosphere around the fruit or vegetable that can aid in
extending shelf-life (Navarro-Tarazaga et al., 2008). Edible
coatings also offer a moisture barrier that can prevent weight
loss of the produce (Baldwin, 1997). In addition, coatings can
be impregnated with antimicrobials that could improve food
safety quality (Vargas et al., 2008). However, problems of
SALMONELLA IN PRODUCE 643
functionality and durability are associated with the use of
edible films for produce. Furthermore, any antimicrobials
would only be effective against the microorganisms located
on theproducesurface. Further engineering ofediblefilms for
produce application is currently being developed (Vargas
et al., 2008).
A considerable challenge remains to ensure safe produce
due to a lack of packaging and the possibility of the produce
becoming recontaminated after any post-harvest treatment.
Ultimately, a multiple hurdle approach may be the most ef-
fective strategy for eliminating Salmonella from produce. The
ability of pathogens to become internalized into fruit or en-
dophytes within plant tissues also poses a considerable chal-
lenge for producers. The internalization of pathogens in
produce has sparked extensive research efforts towards de-
veloping methods to remove pathogens from the tissues. As
plants is currently being investigated becomes better under-
stood this may provide valuable information to develop more
effective intervention strategies (Heaton and Jones, 2008).
Wastewater and freshwater contamination by foodborne
pathogens can contribute to the eventual contamination of
fruits and vegetables irrigated with water sources that come
in contact with these sources. The effect of aquatic environ-
ments on pathogen survival and incidence may have a sub-
sequent impact on the amount of contamination. The overall
focus of future research needs to be directed at environmental
points of bacterial contamination in vegetable production
including animal and water sources because of the ubiquitous
nature of their ecology. Foodborne salmonellae are an ideal
group of bacteria to understand how to differentiate the rel-
ative importance of these dissemination pathways and opti-
mal methods for control. This should provide considerable
insight because Salmonella have been extensively studied and
are sufficiently wellknowngenetically for molecularmethods
to be readily applicable.
This review and JDN were supported by the Texas Higher
Education Coordinating Board’s Advanced Technology Pro-
gram (#000517-0361-1999) and Hatch grant H8311 adminis-
tered by the Texas Agricultural Experiment Station. Authors
IH and SCRare supported by USDA-NRI grant# 2007- 35201-
No competing financial interests exist.
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Address correspondence to:
Irene B. Hanning, B.S., Ph.D.
Department of Food Science
University of Arkansas
2650 Young Ave.
Fayetteville, AR 72704
648HANNING ET AL.
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