Escherichia coli are Gram-negative rod-shaped bacteria and part of the normal bacterial flora in the gastrointestinal tract, while diarrhoeagenic E. colipathotypes such as Shiga toxin-producing E. coli (STEC) and enteropathogenic E. coli (EPEC) are able to cause gastrointestinal infections . STEC can lead to a severe disease, such as haemolytic-uraemic syndrome (HUS) . The risk of HUS has been related especially to children under 5 years and to elderly people. HUS is characterised by acute onset of microangiopathic haemolytic anaemia, renal injury and low platelet count.
More than 400 STEC serotypes have been recognised, of which the best-known serotype is O157:H7 . The most common non-O157:H7 serotypes causing human infections are O26, O103, O111 and O145 . The virulence of STEC is largely based on the production of Shiga toxin 1 or 2 and is identified by detecting the presence of stx1 or stx2 genes [1,4]. The virulence of EPEC is caused by its capability to form attaching and effacing (A/E) lesions in the small intestine. This capability requires the presence of virulence genes called the locus of enterocyte effacement (LEE) in a pathogenity island (PAI) that encodes intimin . Unlike STEC, EPEC do not produce Shiga toxin. EPEC are divided into two distinct groups by the presence of EPEC adherence factor plasmid (pEAF) expressing bundle-forming pili (BFP), which is a virulence determinant of typical EPEC (tEPEC) . Thus atypical EPEC (aEPEC) are defined as E. coli that produce A/E lesions but do not express BFP. Typical EPEC are best known as a cause of infantile diarrhoea, especially in developing countries . Diarrhoea-causing aEPEC have been shown to be separate group without a close relation to tEPEC, but some serotypes are genetically related to STEC . The pathogenity of aEPEC has been questioned but their involvement with diarrhoeal outbreaks supports the idea that certain strains are diarrhoeagenic [1,7].
Both STEC and EPEC are transmitted through the faecal-oral route, and outbreaks caused by STEC and aEPEC have been described after ingestion of contaminated food or water [7,8]. STEC is common in ruminants and can be found in foods contaminated by ruminant faeces . Most studies on STEC have focused on the serotype O157:H7, but infections and outbreaks caused by non-O157 strains are increasingly reported in Europe and elsewhere [10–13]. Atypical EPEC strains are found in animals used for food production, such as cattle, sheep, goat, pig and poultry, in contrast to tEPEC that has been found only in humans [1,14].
Since 1995, clinicians and clinical microbiology laboratories have been obliged to report culture-confirmed STEC infections to the Finnish Infectious Disease Registry (FIDR) maintained by the National Institute for Health and Welfare (THL) in Finland. EPEC infections are not reportable. Since PCR instead of culture became the standard for screening of diarrhoeal patients in 2013, the incidence of reported STEC infections has increased in Finland to 1.2–1.8 per 100,000 population between 2013 and 2015 compared with 0.2–0.6 per 100,000 between 2000 and 2012. From 1997 to 2015, six food- or waterborne STEC outbreaks were detected in Finland (Table 1).
Outbreak of multiple strains of non-O157 Shiga toxin-producing and enteropathogenic Escherichia coli associated with rocket salad, Finland, autumn 2016
Elizabeth Shogren and Susie Neilson of Reveal write that William Whitt suffered violent diarrhea for days. But once he began vomiting blood, he knew it was time to rush to the hospital. His body swelled up so much that his wife thought he looked like the Michelin Man, and on the inside, his intestines were inflamed and bleeding.
For four days last spring, doctors struggled to control the infection that was ravaging Whitt, a father of three in western Idaho. The pain was excruciating, even though he was given opioid painkillers intravenously every 10 minutes for days.
His family feared they would lose him.
“I was terrified. I wouldn’t leave the hospital because I wasn’t sure he was still going to be there when I got back,” said Whitt’s wife, Melinda.
Whitt and his family were baffled: How could a healthy 37-year-old suddenly get so sick? While he was fighting for his life, the U.S. Centers for Disease Control and Prevention quizzed Whitt, seeking information about what had sickened him.
Finally, the agency’s second call offered a clue: “They kept drilling me about salad,” Whitt recalled. Before he fell ill, he had eaten two salads from a pizza shop.
William Whitt and wife Melinda say it is irresponsible for the Food and Drug Administration to postpone water-testing requirements for produce growers. “People should be able to know that the food they’re buying is not going to harm them and their loved ones,” Melinda Whitt said.
The culprit turned out to be E. coli, a powerful pathogen that had contaminated romaine lettuce grown in Yuma, Arizona, and distributed nationwide. At least 210 people in 36 states were sickened. Five died and 27 suffered kidney failure. The same strain of E. coli that sickened them was detected in a Yuma canal used to irrigate some crops.
For more than a decade, it’s been clear that there’s a gaping hole in American food safety: Growers aren’t required to test their irrigation water for pathogens such as E. coli. As a result, contaminated water can end up on fruits and vegetables.
After several high-profile disease outbreaks linked to food, Congress in 2011 ordered a fix, and produce growers this year would have begun testing their water under rules crafted by the Obama administration’s Food and Drug Administration.
But six months before people were sickened by the contaminated romaine, President Donald Trump’s FDA – responding to pressure from the farm industry and Trump’s order to eliminate regulations – shelved the water-testing rules for at least four years.
Despite this deadly outbreak, the FDA has shown no sign of reconsidering its plan to postpone the rules. The agency also is considering major changes, such as allowing some produce growers to test less frequently or find alternatives to water testing to ensure the safety of their crops.
“Mystifying, isn’t it?” said Trevor Suslow, a food safety expert at the University of California, Davis. “If the risk factor associated with agricultural water use is that closely tied to contamination and outbreaks, there needs to be something now. … I can’t think of a reason to justify waiting four to six to eight years to get started.”
The deadly Yuma outbreak underscores that irrigation water is a prime source of foodborne illnesses. In some cases, the feces of livestock or wild animals flow into a creek. Then the tainted water seeps into wells or is sprayed onto produce, which is then harvested, processed and sold at stores and restaurants. Salad greens are particularly vulnerable because they often are eaten raw and can harbor bacteria when torn.
After an E. coli outbreak killed three people who ate spinach grown in California’s Salinas Valley in 2006, most California and Arizona growers of leafy greens signed agreements to voluntarily test their irrigation water.
Whitt’s lettuce would have been covered by those agreements. But his story illustrates the limits of a voluntary safety program and how lethal E. coli can be even when precautions are taken by farms and processors.
Farm groups contend that water testing is too expensive and should not apply to produce such as apples or onions, which are less likely to carry pathogens.
“I think the whole thing is an overblown attempt to exert government power over us,” said Bob Allen, a Washington state apple farmer.
While postponing the water-testing rules would save growers $12 million per year, it also would cost consumers $108 million per year in medical expenses, according to an FDA analysis.
“The Yuma outbreak does indeed emphasize the urgency of putting agricultural water standards in place, but it is important that they be the right standards, ones that both meet our public health mission and are feasible for growers to meet,” FDA spokeswoman Juli Putnam said in response to written questions.
In addition, the FDA did not sample water in a Yuma irrigation canal until seven weeks after the area’s lettuce was identified as the cause of last spring’s outbreak. And university scientists trying to learn from the outbreak say farmers have not shared water data with them as they try to figure out how it occurred and avoid future ones.
My wife does a better Minnosotan accent, spending her yute in Albert Lea, eh?
He didn’t like the photo, right, made by the creative couple of Heather and Christian, who used to work in my lab, and opened the conversation with, “How could you print that?”
I said it was an accurate description of what had been publically known about the leafy greens folks since the E. coli O157 spinach outbreak of 2006 (I’m old, waiting for news on the birth of my third grandson).
I said great, make it public, so people can judge on their own.
Fresh Express has now been linked to 395 cases of Cyclospora through their lettuce served at McDonalds.
U.S. Rep. Rosa DeLauro, D-Conn., is pressing Food and Drug Administration Commissioner Scott Gottlieb for specifics about the investigation of the cyclosporaoutbreak linked to product sold by Fresh Express.
In an Aug. 3 letter her office released to the media, DeLauro said she wrote the letter “out of concern about the current outbreak of cyclosporiasis as well as the transparency and timeliness of your agency’s ongoing investigation.”
“Although once rare in the United States, parasitic outbreaks caused by cyclospora have become more common over the last several decades,” she said in the letter. “Many of these outbreaks have continually been found to be associated with imported fruits and vegetables.”
The recent outbreak is currently responsible for 395 infections — including 16 hospitalizations — across 15 states.
The parasite was first found when the FDA conducted testing on an unused package of Fresh Express salad mix, distributed to a McDonald’s restaurant, containing romaine lettuce and carrots.
The FDA states as of July 13, McDonald’s decided to stop selling the salads at restaurants impacted in Illinois, Iowa, Indiana, Wisconsin, Michigan. Ohio, Minnesota, Nebraska, South Dakota. Montana, North Dakota, Kentucky, West Virginia and Missouri.
In a July 20, statement, McDonald’s said the health and safety of their customers is their top priority.
“The health and safety of our customers and the people who work in McDonald’s restaurants is always our top priority. The additional states identified by the FDA and CDC are among the same states where a week ago we proactively decided to remove our lettuce blend in impacted restaurants and replace it through a different supplier. McDonald’s is committed to the highest standards of food safety and quality and we continue to cooperate and support regulatory and public health officials in their investigations. For those seeking additional information about Cyclospora, we encourage them to visit the CDC and FDA websites.”
Cyclospora sucks. My aunt, my mom’s sister, got it in Florida from basil, about a decade ago.
(Doesn’t she look amazing at 80, left.)
Cyclospora isn’t one of those things doctors routinely check for. Then you’re sick for about six weeks until some bright doc figures it out.
The U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) issued an alert to the public on “beef, pork and poultry salad and wrap products potentially contaminated with Cyclospora that were distributed by Caito Foods LLC, of Indianapolis,” Indiana.
USDA also released a public health alert after Indianapolis-based food distributor Caito Foods “received notification from their lettuce supplier, Fresh Express, that the chopped romaine that is used to manufacture some of their salads and wraps was being recalled.”
“Fresh Express follows rigid food safety requirements and preventive controls throughout our supply chain that are carefully designed to mitigate against potential health risks. Working together with public health officials, we are hopeful a definitive source of the outbreak clusters will be identified soon.”
Still here, Mike. You can call me in Australia through Google voice 785-532-1925 and tell me what Fresh Express is doing, and why they are importing lettuce in the middle of North American summer.
Lettuce has been implicated in human norovirus (HuNoV) outbreaks. The virus is stable on the leaf surface for at least 2 weeks; however, the dynamics of virus internalization have not been fully investigated. The purpose of this study was to assess the internalization and distribution of HuNoV and two surrogate viruses, porcine sapovirus (SaV) and Tulane virus (TV), in lettuce and spinach.
Viral inoculations through the roots of seedlings and the petiole of leaves from mature plants were performed, and the viruses were tracked on days 1 and 6 post-root inoculation and at 16 h and 72 h post-petiole inoculation. Confocal microscopy was used to visualize root-internalized HuNoV.
In both lettuce and spinach, (i) HuNoV was internalized into the roots and leaves at similar RNA titers, whereas surrogate viruses were more restricted to the roots, (ii) all three viruses were stable inside the roots and leaves for at least 6 days, and (iii) HuNoV disseminated similarly inside the central veins and leaf lamina, whereas surrogate viruses were more restricted to the central veins. Infectious TV, but not SaV, was detectable in all tissues, suggesting that TV has greater stability than SaV. HuNoV was visualized inside the roots’ vascular bundle and the leaf mesophyll of both plants.
In conclusion, using surrogate viruses may underestimate the level of HuNoV internalization into edible leaves. The internalization of HuNoV through roots and cut leaves and the dissemination into various spinach and lettuce tissues raise concerns of internal contamination through irrigation and/or wash water.
IMPORTANCE Human noroviruses are the leading cause of foodborne outbreaks, with lettuce being implicated in the majority of outbreaks. The virus causes acute gastroenteritis in all age groups, with more severe symptoms in children, the elderly, and immunocompromised patients, contributing to over 200,000 deaths worldwide annually. The majority of deaths due to HuNoV occur in the developing world, where limited sanitation exists along with poor wastewater treatment facilities, resulting in the contamination of water resources that are often used for irrigation.
Our study confirms the ability of lettuce and spinach to internalize HuNoV from contaminated water through the roots into the edible leaves. Since these leafy greens are consumed with minimal processing that targets only surface pathogens, the internalized HuNoV presents an added risk to consumers. Thus, preventive measures should be in place to limit the contamination of irrigation water. In addition, better processing technologies are needed to inactivate internalized viral pathogens.
Tissue distribution and visualization of internalized norovirus in leafy greens
In October, 1996, a 16-month-old Denver girl drank Smoothie juice manufactured by Odwalla Inc. of Half Moon Bay, California. She died several weeks later; 64 others became ill in several western U.S. states and British Columbia after drinking the same juices, which contained unpasteurized apple cider — and E. coli O157:H7. Investigators believed that some of the apples used to make the cider might have been insufficiently washed after falling to the ground and coming into contact with deer feces (Powell and Leiss, 1997) not that washing would do much.
Almost 10 years later, on Sept. 14, 2006, the U.S. Food and Drug Administration announced that an outbreak of E. coli O157: H7 had killed a 77-year-old woman and sickened 49 others (United States Food and Drug Administration, 2006). The outbreak ultimately killed four and sickened at least 200 across the U.S. This was documented-outbreak 29 linked to leafy greens, but also apparently the tipping point for growers to finally get religion about commodity-wide food safety, following the way of their farmer friends in California, 10 years later.
In the decade between these two watershed outbreaks, almost 500 outbreaks of foodborne illness involving fresh produce were documented, publicized and led to some changes within the industry, yet what author Malcolm Gladwell would call a tipping point — “a point at which a slow gradual change becomes irreversible and then proceeds with gathering pace” — in public awareness about produce-associated risks) did not happen until the spinach E. coli O157:H7 outbreak in the fall of 2006. At what point did sufficient evidence exist to compel the fresh produce industry to embrace the kind of change the sector has heralded since 2007? And at what point will future evidence be deemed sufficient to initiate change within an industry?
The 1993 outbreak of E. coli O157:H7 associated with undercooked hamburgers at the Jack-in-the-Box fast food chain propelled microbial food safety to the forefront of public awareness, at least in the U.S. (Powell and Leiss, 1997). In 1996, following extensive public and political discussions about microbial food safety in meat, the focus shifted to fresh fruits and vegetables, following an outbreak of Cyclospora cayetanesis ultimately linked to Guatemalan raspberries that sickened 1,465 in 21 U.S. states and two Canadian provinces (U.S. Centers for Disease Control and Prevention, 1997). That same year, Beuchat (1996) published a review on pathogenic microorganisms in fresh fruits and vegetables and identified numerous pathways of contamination.
By 1997, researchers at CDC were stating that pathogens could contaminate at any point along the fresh produce food chain — at the farm, processing plant, transportation vehicle, retail store or foodservice operation and the home — and that by understanding where potential problems existed, it was possible to develop strategies to reduce risks of contamination (Tauxe et al., 1997). Researchers also reported that the use of pathogen-free water for washing would minimize risk of contamination (Suslow, 1997; Beuchat, 1998).
Beuchat and Ryu (1997) reported in a review that sources of pathogenic microorganisms for produce included:
Water used to apply fungicides, insecticides
Green or inadequately composted manure
Wild and domestic animals (including fowl and reptiles)
Human handling (workers, consumers)
Transport containers (field to packing shed)
Wild and domestic animals (including fowl and reptiles)
Wash and rinse water
Sorting, packing, cutting, and further processing equipment
Improper packaging (including new packaging technologies)
Cross-contamination (other foods in storage, preparation, and display areas)
Improper display temperature.
kFresh fruits and vegetables were identified as the source of several outbreaks of foodborne illness in the early 1990s, especially leafy greens (Table 1).
E. coli O157:H7
E. coli O157:H7
E. coli O157:H7
E. coli O153:H48
E. coli O153:H47
E. coli O157:H7
E. coli O153:H46
E. coli O157:H10
E. coli O153:H49
Outbreaks of foodborne illness related to leafy greens, 1992-1996.
Dave Gombas told an International Association for Food Protection symposium on leafy green safety on Oct. 6, 2006 in Washington, D.C. that if growers did everything they were supposed to do — in the form of good agricultural practices — and it was verified, there may be fewer outbreaks. He then said government needs to spend a lot more on research.
Wow. The same person who has vacillated between the Produce Marketing Association and the U.S. Food and Drug Administration for the past couple of decades (all you critics who complain about folks jumping back-and-forth-and-back as part of a genetically-engineered conspiracy may want to look at the all-natural, all-good-for-ya produce sector) pronounced on grower verification in which nothing has been done.
Since we were on the same panel in Washington, in 2006, I asked Gombas, why is the industry calling for more investment in research about the alleged unknowns of microbial contamination of produce when the real issue seems to be on-farm delivery and verification? Hiding behind the unknown is easy, working on verifying what is being done is much harder.
More calls for research.
Nothing on human behavior in a fresh produce environment.
It’s just another case of saying the right things in public, but failing to acknowledge what happens on individual farms. Verification is tough. Auditing may not work, because many of these outbreaks happened on third -party audited operations. Putting growers in a classroom doesn’t work, and there’s no evidence that begging for government oversight yields a product that results in fewer sick people.
In 1999, several more outbreaks of Shiga-toxin producing E. coli (STEC) were linked to leafy greens (Table 2), and the U.S. group, the United Fresh Fruit and Vegetable Association, developed and published HACCP-based food safety guidelines for industry (United Fresh Fruit and Vegetable Association, 1999).
E. coli O157:H9
E. coli O111:H8
E. coli O157:H11
E. coli O157:H7
E. coli O157:H7
E. coli O157:H7
Table 2. 1999 U.S. outbreaks of STEC linked to leafy greens
By 2000, Rafferty and colleagues demonstrated that E. coli could spread on-farm in plant production cuttings from one contaminated source, magnifying an outbreak to a whole farm (Rafferty et al., 2000). A 2001 outbreak of Shigella flexneri (886 ill) in tomatoes further focused public and scientific attention onto fresh produce.
Solomon and colleagues (2002a) discovered that the transmission of E. coli O157:H7 to lettuce was possible through both spray and drip irrigation. They also found that the pathogen persisted on the plants for 20 days following application and submerging the lettuce in a solution of 200ppm chlorine did not eliminate all viable E.coli O157:H7 cells, suggesting that irrigation water of unknown microbial quality should be avoided in lettuce production (Solomon et al., 2002a). In a follow-up experiment, Solomon and colleagues (2002b) explored the transmission of E. coli O157:H7 from manure-contaminated soil and irrigation water to lettuce plants. The researchers recovered viable cells from the inner tissues of the lettuce plants and found that the cells migrated to internal locations in plant tissue and were thus protected from the action of sanitizing agents. These experiments demonstrated that E. coli O157:H7 could enter the lettuce plant through the root system and migrate throughout the edible portion of the plant (Solomon et al., 2002b). Such results were widely reported in general media.
During this time, several outbreaks of E. coli were again linked to lettuce and salad (Table 3).
E. coli O157:H7
E. coli O157:H7
E. coli O157:H8
E. coli O157:H7
E. coli O157:H7
Table 3: Leafy green outbreaks of STEC, 2000 — 2002.
In 2003, according to Mexican growers, the market impact of an outbreak of hepatitis A traced to exported green onions lasted up to 4 months while prices fell 72 per cent (Calvin et al., 2004). Roma tomatoes were identified as the source of a salmonellosis outbreak that resulted in over 560 cases in both Canada and the US (CDC 2005).
During 2003-2005, several additional outbreaks of E. coli O157:H7 were linked to fresh leafy greens, including one multi-state outbreak involving Dole bagged lettuce (Table 4).
E. coli O157:H7
E. coli O157:H7
E. coli O157:H7
E. coli O157:H7
Table 4: Leafy green STEC outbreaks, 2003 — 2005.
During 2005–2006, four large multistate outbreaks of Salmonella infections associated with eating raw tomatoes at restaurants occurred in the U.S., resulting in 459 culture-confirmed cases of salmonellosis in 21 states. Investigations determined that the tomatoes had been supplied to restaurants either whole or precut from tomato fields in Florida, Ohio, and Virginia (CDC, 2006).
Allwood and colleagues (2004) examined 40 items of fresh produce taken from a retail setting in the U.S. that had been preprocessed (including cut, shredded, chopped or peeled) at or before the point of purchase. They found fecal contamination indicators (E. coli, F-specific coliphages, and noroviruses) were present in 48 per cent of samples.
Researchers in Minnesota conducted a small-scale comparative study of organic versus conventionally grown produce. They found that while all samples were virtually free of pathogens, E. coli was 19 times more prevalent on produce acquired from the organic farms (Mukherjee et al., 2004). They estimated that this was due to the common use of manure aged for less than a year. Use of cattle manure was found to be of higher risk as E. coli was found 2.4 times more often on farms using it than other animal manures (Mukherjee et al., 2004).
On Sept. 14, 2006, the U.S. Food and Drug Administration (2006) issued a public statement warning against the consumption of bagged fresh spinach.
“Given the severity of this illness and the seriousness of the outbreak,” stated Dr. Robert Brackett, Director of FDA’s Center for Food Safety and Applied Nutrition (CFSAN), “FDA believes that a warning to consumers is needed (United States Food and Drug Administration, 2006).”
That is no different from the sometimes conflicting messages coming from FDA today about the E. coli O157:H7 outbreak on lettuce that originated in Yuma, Arizona: these public health folks are figuring it out on the go.
Sean Rossman of USA Today reports today that in the current E. coli O157:H7 outbreak linked to Yuma lettuce, 70% of those who’ve gotten sick are female.
Similarly, when leafy greens were the culprit of an E. coli outbreak last year, 67% of those infected were women or girls. In 2016, females were 73% of those ill from an outbreak in alfalfa sprouts, notes the U.S. Centers for Disease Control and Prevention.
The first line of defense is the farm, not the consumer.
All ruminants — cows, sheep, goats, deer — can carry dangerous E. coli like the O157:H7 strain that sickened people in the spinach outbreak, as well as the Taco Bell and Taco Johns outbreaks ultimately traced to lettuce.
Any commodity is only as good as its worst grower.
We’ve had a few peer-reviewed thoughts on these topics:
Actually, it was the U.S. Centers for Disease Control and Prevention (CDC) and the Food and Drug Administration (FDA) who jointly declared an end to the E coli O157 outbreak after nearly two months of investigation. At least 66 people across the U.S and Canada became ill, 22 were hospitalized, and 2 died during November and December, all linked to consumption of romaine lettuce.
What’s been missing is any response from the leafy greens marketing agency types.
Silence – the LGMA cone of silence — is golden, I guess.
CDC announced on January 25, 2018, that this outbreak appears to be over, because the last case became ill on December 12, 2017. This indicates that the food causing illness is no longer available in the marketplace or consumers’ homes.
Although this outbreak appears to be over, the FDA’s outbreak investigation team is continuing to work with federal, state and local partners to determine what leafy greens made people ill, what people ate, where they bought it, and identify the distribution chain — all with the goal of identifying any common food or points where the food might have become contaminated. To date, no common link has been identified.
Because whole genome sequencing showed that the E. coli O157:H7 strain that resulted in the U.S. illnesses was closely related genetically to the strain that caused illnesses in Canada, the FDA and CDC have been in contact with Canadian food safety authorities throughout this outbreak.
That’s my response to people who ask about the proportionally high rates of foodborne illness in lettuce and other leafy greens eaten raw.
I like spinach – in a lasagna or stir-fry – but not raw.
Raw is risky.
There’s a bunch of new findings on foodborne pathogens and leafy greens which are summarized below.
In the sphere of public conversation, it is notable the Leafy Greens Marketing Agreement, the group formed after the 2006 E. coli-in-spinach outbreak that killed four and sickened at least 200 in the U.S. – has been once again silent on any research or outbreaks that associate risk with greens.
The scientists have discovered that juices released from damaged leaves also had the effect of enhancing the virulence of the pathogen, potentially increasing its ability to cause infection in the consumer.
The research is led by Dr Primrose Freestone of the University’s Department of Infection, Immunity and Inflammation and PhD student Giannis Koukkidis, who has been funded by a Biotechnology and Biological Sciences Research Council (BBSRC) i-case Studentship.
Their research investigates novel methods of preventing food poisoning pathogens from attaching to the surface of salad leaves to help producers improve food safety for consumers.
This latest study, published in Applied and Environmental Microbiology, found that juices from damaged leaves in bagged spinach and mixed salad increased Salmonella pathogen growth 2400-fold over a control group and also enhanced their adherence to surfaces and overall virulence, or capacity to cause disease.
Dr Freestone said: “Salad leaves are cut during harvesting and we found that even microliters of the juices (less than 1/200th of a teaspoon) which leach from the cut-ends of the leaves enabled Salmonella to grow in water, even when it was refrigerated. These juices also helped the Salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria, and even enabled the pathogen to attach to the salad bag container.
“This strongly emphasizes the need for salad leaf growers to maintain high food safety standards as even a few Salmonella cells in a salad bag at the time of purchase could become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated. Even small traces of juices released from damaged leaves can make the pathogen grow better and become more able to cause disease.
“It also serves as a reminder to consume a bagged salad as soon as possible after it is opened. We found that once opened, the bacteria naturally present on the leaves also grew much faster even when kept cold in the fridge.
“This research did not look for evidence of Salmonella in bagged salads. Instead, it examined how Salmonella grows on salad leaves when they are damaged.”
Leafy green and other salad vegetables are an important part of a healthy diet, providing vitamins, minerals, and dietary fiber. Ready to eat prepared salads are particularly popular, are widely consumed and so of significant economic importance. Over recent years there has however been a number of outbreaks associated with fresh salad produce contaminated with Salmonella and E. coli both in the USA and Europe.
This has triggered considerable interest in effective strategies for controls and interventions measures both in UK industry, the EU and key research funding bodies.
Despite a number of published reports on improving the microbiological safety of salad leaf production, very few studies have investigated the behavior of Salmonella once the leaves have been bagged.
Giannis said: “Anything which enhances adherence of foodborne pathogens to leaf surfaces also increases their persistence and ability to resist removal, such as during salad washing procedures. Even more worrying for those who might eat a Salmonella contaminated salad was the finding that proteins required for the virulence (capacity to cause infection) of the bacteria were increased when the Salmonella came into contact with the salad leaf juices. “Preventing enteric pathogen contamination of fresh salad produce would not only reassure consumers but will also benefit the economy due to fewer days lost through food poisoning. We are now working hard to find ways of preventing salad-based infections.”
No comment from the LGMA.
While this research may make it seem like pre-packaged salads pose a scary risk, the researchers themselves were quick to say they still eat bagged salads. But they make sure to look for packages that have appropriate use-by dates and crisp-looking leaves. They stay away from salads that have mushy, slimy-looking greens, or bags with accumulated salad juice at the bottom. And they make sure to eat the greens within one day of purchase.
“Our project does not indicate any increased risk to eating leafy salads, but it does provide a better understanding of the factors contributing to food poisoning risks,” said Freestone.
If you feel like it, you can wash greens that have already been pre-washed by manufacturers just before eating, but Freestone says this doesn’t have much of an effect on the salmonella bacteria that may already be attached or internalized by the leaves.
Foodborne disease outbreaks associated with fresh produce irrigated with contaminated water are a constant threat to consumer health. In this study, the impact of irrigation water on product safety from different food production systems (commercial to small-scale faming and homestead gardens) was assessed.
Hygiene indicators (total coliforms, Escherichia coli), and selected foodborne pathogens (Salmonella spp., Listeria monocytogenes, and Escherichia coli O157:H7) of water and leafy green vegetables were analyzed. Microbiological parameters of all irrigation water (except borehole) exceeded maximum limits set by the Department of Water Affairs for safe irrigation water. Microbial parameters for leafy greens ranged from 2.94 to 4.31 log CFU/g (aerobic plate counts) and 1 to 5.27 log MPN/100g (total coliforms and E. coli). Salmonella and E. coli O157:H7 were not detected in all samples tested but L. monocytogenes was present in irrigation water (commercial and small-scale farm, and homestead gardens).
This study highlights the potential riskiness of using polluted water for crop production in different agricultural settings.
No comment from LGMA.
Adaptive response of Listeria monocytogenes to heat, salinity and low pH, after habituation on cherry tomatoes and lettuce leaves
Sofia V. Poimenidou, Danai-Natalia Chatzithoma, George-John Nychas, Panagiotis N. Skandamis
Pathogens found on fresh produce may encounter low temperatures, high acidity and limited nutrient availability. The aim of this study was to evaluate the effect of habituation of Listeria monocytogenes on cherry tomatoes or lettuce leaves on its subsequent response to inhibitory levels of acid, osmotic and heat stress.
Habituation was performed by inoculating lettuce coupons, whole cherry tomatoes or tryptic soy broth (TSB) with a three-strains composite of L. monocytogenes, which were further incubated at 5°C for 24 hours or 5 days. Additionally, cells grown overnight in TSB supplemented with 0.6% yeast extract (TSBYE) at 30°C were used as control cells. Following habituation, L. monocytogenes cells were harvested and exposed to: (i) pH 3.5 adjusted with lactic acid, acetic acid or hydrochloric acid (HCl), and pH 1.5 (HCl) for 6 h; (ii) 20% NaCl and (iii) 60°C for 150 s.
Results showed that tomato-habituated L. monocytogenes cells were more tolerant (P < 0.05) to acid or osmotic stress than those habituated on lettuce, and habituation on both foods resulted in more stress resistant cells than prior growth in TSB. On the contrary, the highest resistance to heat stress (P < 0.05) was exhibited by the lettuce-habituated L. monocytogenes cells followed by TSB-grown cells at 5°C for 24 h, whereas tomato-habituated cells were highly sensitized. Prolonged starvation on fresh produce (5 days vs. 24 h) increased resistance to osmotic and acid stress, but reduced thermotolerance, regardless of the pre-exposure environment (i.e., tomatoes, lettuce or TSB).
These results indicate that L. monocytogenes cells habituated on fresh produce at low temperatures might acquire resistance to subsequent antimicrobial treatments raising important food safety implications.
No comment from LGMA.
Efficacy of post-harvest rinsing and bleach disinfection of E. coli O157:H7 on spinach leaf surfaces
Attachment and detachment kinetics of Escherichia coli O157:H7 from baby spinach leaf epicuticle layers were investigated using a parallel plate flow chamber. Mass transfer rate coefficients were used to determine the impact of water chemistry and common bleach disinfection rinses on the removal and inactivation of the pathogen. Attachment mass transfer rate coefficients generally increased with ionic strength. Detachment mass transfer rate coefficients were nearly the same in KCl and AGW rinses; however, the detachment phase lasted longer in KCl than AGW (18 ± 4 min and 4 ± 2 min, respectively), indicating that the ions present during attachment play a significant role in the cells’ ability to remain attached. Specifically, increasing bleach rinse concentration by two orders of magnitude was found to increase the detachment mass transfer rate coefficient by 20 times (from 5.7 ± 0.7 × 10−11 m/s to 112.1 ± 26.8 × 10−11 m/s for 10 ppb and 1000 ppb, respectively), and up to 88 ± 4% of attached cells remained alive.
The spinach leaf texture was incorporated within a COMSOL model of disinfectant concentration gradients, which revealed nearly 15% of the leaf surface is exposed to almost 1000 times lower concentration than the bulk rinse solution.
No comment from LGMA.
Development of growth and survival models for Salmonella and Listeria monocytogenes during non-isothermal time-temperature profiles in leafy greens
Leafy greens contaminated with Salmonella enterica have been linked to large number of illnesses in many countries in recent years. Listeria monocytogenes is also a pathogen of concern for leafy greens because of its prevalence in the growing and processing environment and its ability to grow at refrigeration temperatures. Experimental data for the growth and survival of S. enterica and L. monocytogenes under different conditions and storage temperatures were retrieved from published studies. Predictive models were developed using the three-phase linear model as a primary growth model and square-root model to calculate specific growth rate (ln CFU g−1 h−1) at different temperatures (°C). The square-root model for S. enterica was calculated as μ = (0.020(Temperature+0.57))2. The square-root model for L. monocytogenes was fitted as μ = (0.023(Temperature-0.60))2. The growth-survival model for S. enterica and growth model for L. monocytogenes were validated using several dynamic time-temperature profiles during the production and supply chain of leafy greens. The models from this study will be useful for future microbial risk assessments and predictions of behavior of S. enterica and L. monocytogenes in the leafy greens production and supply chain.
No comment from LGMA.
Is there a relation between the microscopic leaf morphology and the association of Salmonella and Escherichia coli O157:H7 with iceberg lettuce leaves?
Journal of Food Protection, Number 10, October 2016, pp. 1656-1662, pp. 1784-1788(5)
I Van der Linden, M Eriksson, M Uyttendaele, F Devlieghere
To prevent contamination of fresh produce with enteric pathogens, more insight into mechanisms that may influence the association of these pathogens with fresh produce is needed.
In this study, Escherichia coli O157:H7 and Salmonella were chosen as model pathogens, and fresh cut iceberg lettuce was chosen as a model fresh produce type. The morphological structure of iceberg lettuce leaves (stomatal density and length of cell margins per leaf area) was quantified by means of leaf peels and light microscopy of leaves at different stages of development (outer, middle, and inner leaves of the crop) on both leaf sides (abaxial and adxial) and in three leaf regions (top, center, and bottom). The morphology of the top region of the leaves was distinctly different from that of the center and base, with a significantly higher stomatal density (up to five times more stomata), different cell shape, and longer cell margins (two to three times longer). Morphological differences between the same regions of the leaves at different stages of development were smaller or nonsignificant. An attachment assay with two attenuated E. coli O157:H7 strains (84-24h11-GFP and BRMSID 188 GFP) and two Salmonella strains (serovars Thompson and Typhimurium) was performed on different regions of the middle leaves. Our results confirmed earlier reports that these pathogens have a higher affinity for the base of the lettuce leaf than the top. Differences of up to 2.12 log CFU/g were seen (E. coli O157:H7 86-24h11GFP). Intermediate attachment occurred in the central region.
The higher incidence of preferential bacterial attachment sites such as stomata and cell margins or grooves could not explain the differences observed in the association of the tested pathogens with different regions of iceberg lettuce leaves.
No comment from LGMA.
The N.Y Times reportsthe one place the one place the Salinas Valley’s bounty of antioxidants does not often appear is on the tables of the migrant workers who harvest it.
More than a third of the children in the Salinas City Elementary School District are homeless; overall diabetes rates are rising and projected to soar; and 85 percent of farmworkers in the valley are overweight or obese, partly because unhealthy food is less costly, said Marc B. Schenker, a professor at the University of California, Davis, who studies the health of farmworkers.
A case was defined as a person with laboratory-confirmed YE O9 infection with the outbreak multilocus variable-number tandem repeat analysis (MLVA)-profile (5-6-9-8-9-9). We conducted a case–control study in the military setting and calculated odds ratios (OR) using logistic regression. Traceback investigations were conducted to identify common suppliers and products in commercial kitchens frequented by cases. By 28 May, we identified 133 cases, of which 117 were linked to four military bases and 16 were civilians from geographically dispersed counties. Among foods consumed by cases, multivariable analysis pointed to mixed salad as a potential source of illness (OR 10.26; 95% confidence interval (CI): 0.85–123.57). The four military bases and cafeterias visited by 14/16 civilian cases received iceberg lettuce or radicchio rosso from the same supplier. Secondary transmission cannot be eliminated as a source of infection in the military camps.
The most likely source of the outbreak was salad mix containing imported radicchio rosso, due to its long shelf life. This outbreak is a reminder that fresh produce should not be discounted as a vehicle in prolonged outbreaks and that improvements are still required in the production and processing of fresh salad products.
National outbreak of Yersinia enterocolitica infections in military and civilian populations associated with consumption of mixed salad, Norway, 2014
Eurosurveillance, Volume 21, Issue 34, 25 August 2016, DOI: http://dx.doi.org/10.2807/1560-7917.ES.2016.21.34.30321
E MacDonald, M Einöder-Moreno, K Borgen, L Thorstensen Brandal, L Diab, Ø Fossli, B Guzman Herrador, AA Hassan, GS Johannessen, EJ Johansen, R Jørgensen Kimo, T Lier, BL Paulsen, R Popescu, C Tokle Schytte, K Sæbø Pattersen, L Vold, Ø Ørmen, AL Wester, M Wiklund, K Nygård