Lettuce rinse with cheese whey can reduce pathogens

Cheese whey fermented by an industrial starter consortium of lactic acid bacteria was evaluated for its antibacterial capacity to control a selection of pathogenic bacteria. For their relevance on outbreak reports related to vegetable consumption, this selection included Listeria monocytogenes, serotype 4b, Escherichia coli O157:H7, and Salmonella Goldcoast.

cheese.wheyOrganically grown lettuce was inoculated with an inoculum level of ∼107 colony-forming unit (CFU)/mL and was left for about 1 h in a safety cabinet before washing with a perceptual solution of 75:25 (v/v) fermented whey in water, for 1 and 10 min. Cells of pathogens recovered were then counted and their number compared with that obtained for a similar treatment, but using a chlorine solution at 110 ppm.

Results show that both treatments, either with chlorine or fermented whey, were able to significantly reduce (p < 0.05) the number of bacteria, in a range of 1.15–2.00 and 1.59–2.34 CFU/g, respectively, regarding the bacteria tested. Results suggest that the use of fermented whey may be as effective as the solution of chlorine used in industrial processes in reducing the pathogens under study (best efficacy shown for Salmonella), with the advantage of avoiding health risks arising from the formation of carcinogenic toxic chlorine derivates.

Preliminary study on the effect of fermented cheese whey on Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella Goldcoast populations inoculated onto fresh organic lettuce

Maria I.S. Santos,1,2,3,4 Ana I. Lima,4 Sara A.V.S. Monteiro,4 Ricardo M.S.B. Ferreira,4 Laurentina Pedroso,3 Isabel Sousa,2 and Maria A.S.S. Ferreira1

1Microbiology Laboratory, Department of Natural Resources, Environment and Territory, DRAT, LEAF, Instituto Superior de Agronomia, Universidade de Lisboa, Lisbon, Portugal.

2Eco-Processing of Food and Feed, CEE, LEAF, Instituto Superior de Agronomia, Universidade de Lisboa, Lisbon, Portugal.

3Faculty of Veterinary Medicine, Universidade Lusofona de Humanidades e Tecnologias, Lisbon, Portugal.

4Disease & Stress Biology, DRAT, LEAF, Instituto Superior de Agronomia, Universidade de Lisboa, Lisbon, Portugal.

Foodborne Pathogens and Disease, doi:10.1089/fpd.2015.2079

http://online.liebertpub.com/doi/abs/10.1089/fpd.2015.2079

Are ready-to-eat salads ready to eat?

We investigated a nationwide outbreak of Salmonella Coeln in Norway, including 26 cases identified between 20 October 2013 and 4 January 2014. We performed a matched case-control study, environmental investigation and detailed traceback of food purchases to identify the source of the outbreak.

lettuce.skull.noroIn the case-control study, cases were found to be more likely than controls to have consumed a ready-to-eat salad mix (matched odds ratio 20, 95% confidence interval 2·7–∞). By traceback of purchases one brand of ready-to-eat salad was indicated, but all environmental samples were negative for Salmonella.

This outbreak underlines that pre-washed and bagged salads carry a risk of infection despite thorough cleaning procedures by the importer. To further reduce the risk of infection by consumption of ready-to-eat salads product quality should be ensured by importers.

Outbreaks linked to salads reinforce the importance of implementation of appropriate food safety management systems, including good practices in lettuce production.

Are ready-to-eat salads ready to eat? An outbreak of Salmonella Coeln linked to imported, mixed, pre-washed and bagged salad, Norway, November 2013

F. Vestrheima1a2 c1, H. Langea1a3, K. Nygårda1, K. Borgena1, A. L. Westera1, M. L. Kvarmea4 and L. Volda1

Epidemiology and Infection, Volume 144, Issue 8, June 2016, pages 1756-1760, DOI: http://dx.doi.org/10.1017/S0950268815002769

http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=10299069&utm_source=Issue_Alert&utm_medium=RSS&utm_campaign=HYG

4 dead, 33 ill from Listeria in lettuce: Of course Dole knew

Beginning August 2, 1998, over 80 Americans fell ill, 15 were killed, and at least six women miscarried due to listerosis. On Dec. 19, 1998, the outbreak strain was found in an open package of hot dogs partially consumed by a victim. The manufacturer of the hot dogs, Sara Lee subsidiary Bil Mar Foods, Inc., quickly issued a recall of what would become 35 million pounds of hot dogs and other packaged meats produced at the company’s only plant in Michigan. By Christmas, testing of unopened packages of hot dogs from Bil Mar detected the same genetically unique L. monocytegenes bacteria, and production at the plant was halted.

four.monkeysA decade later, the deaths of two Toronto nursing home residents in the summer of 2008 were attributed to listeriosis infections. These illnesses eventually prompted an August 17, 2008 advisory to consumers by the Canadian Food Inspection Agency (CFIA) and Maple Leaf Foods, Inc. to avoid serving or consuming certain brands of deli meat as the products could be contaminated with L. monocytogenes. When genetic testing determined a match between contaminated meat products and listeriosis patients, all products manufactured at a Toronto Maple Leaf Foods plant were recalled and the facility closed. An investigation by the company determined that organic material trapped deep inside the plant’s meat slicing equipment harbored L. monocytogenes, despite routine sanitization that met specifications of the equipment manufacturer. In total, 57 cases of listeriosis as well as 22 deaths were definitively connected to the consumption of the plant’s contaminated deli meats.

As far back as 2013, Blue Bell ice cream was finding Listeria in places like floors, catwalks and cleaning tubs. Blue Bell had positive listeria findings from at least 11 swabs of plant surfaces between March 2013 and November 2014. Each time, it vigorously cleaned the area, and moved on without testing the equipment that touches the ice cream. At the same time, Blue Bell had problems with the layout of its plants, with condensation dripping all over the place. After federal officials linked an illness outbreak to Blue Bell in 2015, they tested the company’s food processing equipment and found LM. Three people died and 10 were sickened.

In all three Listeria outbreaks, the companies had data that showed an increase in Listeria-positive samples.

But rather than pay attention, they ignored the safety.

Those who study engineering failures –the BP oil well in the Gulf, the space shuttle Challenger, Bhopal – say the same thing: human behavior can mess things up.

listeria4In most cases, an attitude prevails that is, “things didn’t go bad yesterday, so the chances are, things won’t go bad today.”

And those in charge begin to ignore the safety systems.

Or hope the problem will just go away.

Kellogg’s was taking Salmonella-contaminated peanut paste based on paperwork in 2009? Pay attention, Nestle did.

In 2009, the operator of a yakiniku barbecue restaurant chain linked to four deaths and 70 illnesses from E. coli O111 in raw beef in Japan admitted it had not tested raw meat served at its outlets for bacteria, as required by the health ministry.

“We’d never had a positive result [from a bacteria test], not once. So we assumed our meat would always be bacteria-free.”

Chipotle Mexican Grill was aware of a norovirus outbreak among people who had eaten in one of its restaurants in Simi Valley, Calif., but did not tell public health officials there until after it had closed and cleaned the restaurant. More than 200 people were sickened.

So it’s no surprise that officials at Dole’s Springfield, Ohio plant, which bags lettuce and other supposedly healthy meals, knew about Listeria in its facility for 18 months before shutting down and issuing a recall.

Four people have died and 33 sickened in Canada and the U.S. from Listeria in the Dole products.

Kudos to Bill Marler and his Food Safety News, as well as Food Poisoning Bulletin, for filing the Freedom of Information request on U.S. Food and Drug Administration inspections at the Dole plant and putting together a preliminary picture of who knew what when.

Inspection reports (483) obtained by Food Safety News revealed the timeline of positive Listeria results and inaction. Dole Fresh Vegetables Inc. finally suspended production at its salad plant in Springfield, OH, on Jan. 21 this year after a random test by state officials showed a bagged salad contained Listeria monocytogenes.

Dole restarted production at the plant in Springfield, OH, on April 21. Company officials won’t say what was done to clean the plant or how they plan to prevent future contamination there.

powell_soli_AUG2Inspectors from FDA checked the production plant three times in January and twice in February after genetic fingerprinting showed the undeniable link between the sick people and salads from the facility. They collected swab samples, unfinished product samples, testing records and other documents and information.

According to the FDA’s inspection reports, in July 2014 Dole did swab tests of surfaces in the Springfield plant. The tests returned positive results for Listeria, but the facility kept producing salads, shipping them to dozens of states and at least five Canadian provinces.

At least five more times in 2014 and three times in late 2015 Dole’s internal tests showed Listeria contamination, but Dole kept the salad lines kept rolling until January this year.

The FDA inspection report states that Dole’s vice president for quality assurance and food safety, as well as the company’s quality assurance manager, were aware internal tests on Jan. 5 and 7 this year showed Listeria on equipment and other surfaces in the plant. But Dole continued to produce and ship salads.

The plant kept operating until Jan. 21. The following day Dole posted a recall notice with the FDA and the Canadian Food Inspection Agency (CFIA) for salads produced at the Springfield facility. Dole branded salads and house brands for Walmart, Kroger, Loblaws and Aldi were included in the recall.

Marler, a prominent food safety lawyer who represents one of the victims in a lawsuit against Dole told Stephanie Strom of the N.Y. Times, “If the government inspectors hadn’t showed up, who knows when or if they were going to tell anyone.”

“They’d been having positive tests for listeria for some time,” said William Goldfield, a spokesman for Dole. “We understand these recent news reports may raise questions among our consumers and customers. They should be assured, however, that we have worked in conjunction with the F.D.A. to address those observations and ensure that Dole products are safe.”

Lauren Sucher, a spokeswoman at the F.D.A., said that companies must notify the agency when they find a food has a “reasonable probability” of causing serious adverse health consequences.

But, Ms. Sucher said, not all strains of listeria cause disease. “When listeria is found in the manufacturing environment, rather than on the food itself, it is not uncommon for a company to immediately take corrective action rather than test further to see if the strain of listeria poses a threat,” she wrote in an email.

Food companies that find listeria during periodic testing are not required to run further tests to determine whether the pathogen is of a toxic variety.

In Dole’s case, it was swabbing various locations in its plant in Springfield, Ohio, not necessarily testing the finished products, according to the F.D.A. inspection. Rather, Canadian public health officials investigating an outbreak of listeriosis dating to summer 2015, tested bagged Dole salads and found four varieties that were contaminated.

More than 200 now sick with Salmonella linked to Australian lettuce

I’ve been sitting on this for a week now, naively hoping there would be further information from lettuce producers, processors and health types.

lettuceAnd then I remembered this is Australia, where information on foodborne outbreaks disappears into a black hole, maybe to reappear in court a couple of years later, maybe not.

At last count, the number of people struck down by a Salmonella outbreak linked to a Bacchus Marsh salad supplier had passed 200.

Nothing since.

The outbreak, which has been traced back to Tripod Farmers salad ingredients, has generated lots of discussion about supporting farmers and almost nothing about the sick people.

Bulmer Farms managing director Andrew Bulmer, who farms at Lindenow in East Gippsland, said he had seen demand drop back by 30 to 40 per cent, even though his business was not linked to the outbreak and that, “They’re 100 per cent safe and people should have confidence.” Bulmer Farms managing director Andrew Bulmer, who farms at Lindenow in East Gippsland, said he had seen demand drop back by 30 to 40 per cent, even though his business was not linked to the outbreak.

Mr. Bulmer said it had been a big hit during their busiest season.

“We supply a lot of raw product into processing companies that then put bagged product on shelves in supermarkets and we’ve seen a 30 to 40 per cent downturn in that business with demand for raw products into those processors,” he said.

“I’d expect it’d be another couple of weeks until it dies down a little bit and consumers regain the confidence to go back out and buy the washed salads which, by the way are ready to go as of now.

“They’re 100 per cent safe and people should have confidence.”

Why?

lettuce.skull_.noro_-1What are the safety protocols on Victorian lettuce farms? How often is irrigation water tested? What kind of soil amendments are used (poop)? Is there any end-product testing to verify systems are working? Is there a rigorous employee handwashing and sanitation program? Are all of these steps verified? Does management instill a culture of food safety first?

There is no such thing as 100% safe.

But reasonable steps can be taken to reduce risk.

And producers, don’t leave it to health types to inform the public. They’ll still have their jobs and supers after the outbreak. If you, as a producer, are doing the right thing, go and brag about it.

54 sick: Salmonella in lettuce spreads in Australia

This is how bad public reporting of foodborne illness is in Australia.

lettuce.skull.noroRetailers, even with crappy Internet, we have cameras, and you’ll be found out.

An increasing number of Queenslanders claim they’ve been made ill from supermarket-bought salads in the wake of salmonella outbreak, but Woolworths and Coles insist there’s no problem with Queensland supply.

“The supplier in question does not supply into Queensland so there is no need to worry,” a Woolworths spokesman told The Courier-Mail.

While a Cole statement confirmed: “None of the recalled products are sold in Queensland, there is no cause for concern.”

It is believed Coles and Woolworths have stopped taking supplies from Tripod indefinitely.

Chelsea Bienke is just one of Queensland consumers who believe a bout of extreme vomiting and diarrhea was sparked after eating Woolworths lettuce.

The Brisbane woman and her sister were extremely ill for days after eating a meal with mixed salad.

“We had diarrhea for days and I was vomiting” Ms Bienke said.

“I felt like complete crap. I couldn’t go to work this week.”

She said she doubted claims that the product wouldn’t affect any customers.

“Well it’s strange how we bought the product last week and by Tuesday we were vomiting and had diarrhea,” Ms Bienke said.

“My niece who doesn’t eat the product is perfectly fine.”

Another Queenslander wrote on Coles’ Facebook page: “Are you sure Qld products are not affected. I bought spinach and rocket mix. My child has been unwell for three days.”

Another Brisbane mum posted on the same page: “You say Qld isn’t affected but my kids have been sick with headache and nausea. We eat your salads all the time and five of my six kids have been sick.

28 sick: Salmonella outbreak linked to Coles and Woolworths lettuce in Australia

An urgent national recall has been issued for pre-packaged lettuce and salad linked to a salmonella outbreak that has hospitalized two people.

lettuce.skull.noroThe Victorian Department of Health and Human Services said it had identified a number of cases of the infection linked to lettuce grown and packaged by Victorian company Tripod Farmers.

It is sold at Coles, Woolworths, Bi-Lo and other grocers as Coles 4 Leaf Mix, Woolworths salad mix, SupaSalad Supamix and Wash N Toss salad mix.

The affected products have best before dates leading up to and including February 14 and are in all states and territories except for Tasmania and Western Australia.

The health department identified the lettuce after recording an unusually high level of Salmonella Anatum strain infections and traced a number of those back to the products.

“Normally we only see a handful of cases of this strain each year, but so far this year there have been 28 adult cases of salmonella anatum – mostly adults – notified to the department,” the department’s senior medical advisor Dr Finn Romanes said.

“As a result of following up the food histories of a number of people we have discovered a common source – the Tripod Farmers lettuce.

“Tests of three products from two batches have also tested positive for Salmonella Anatum bacterium.”

“We do expect to see more cases,” he said.

“We are working with the company to understand what may have occurred… They have instituted a comprehensive clean-up to make sure any risk is minimized.”

salm.salad.aust.feb.16

Lettuce is overrated: Bad bugs in leafy greens

Microbial pathogen infiltration in fresh leafy greens is a significant food safety risk factor.

lettuce.tomato.skullIn various postharvest operations, vacuum cooling is a critical process for maintaining the quality of fresh produce. The overall goal of this study was to evaluate the risk of vacuum cooling-induced infiltration of Escherichia coliO157:H7 into lettuce using multiphoton microscopy.

Multiphoton imaging was chosen as the method to locate E. coli O157:H7 within an intact lettuce leaf due to its high spatial resolution, low background fluorescence, and near-infrared (NIR) excitation source compared to those of conventional confocal microscopy. The variables vacuum cooling, surface moisture, and leaf side were evaluated in a three-way factorial study with E. coli O157:H7 on lettuce. A total of 188 image stacks were collected. The images were analyzed for E. coli O157:H7 association with stomata and E. coli O157:H7 infiltration. The quantitative imaging data were statistically analyzed using analysis of variance (ANOVA).

The results indicate that the low-moisture condition led to an increased risk of microbial association with stomata (P < 0.05). Additionally, the interaction between vacuum cooling levels and moisture levels led to an increased risk of infiltration (P < 0.05). This study also demonstrates the potential of multiphoton imaging for improving sensitivity and resolution of imaging-based measurements of microbial interactions with intact leaf structures, including infiltration.

 Influence of vacuum cooling on Escherichia coli O157:H7 infiltration infresh leafy greens via a multiphoton-imaging approach

Appl. Environ. Microbiol. January 2016 82:106-115; Accepted manuscript posted online 16 October 2015, doi:10.1128/AEM.02327-15

Erica Vonasek and Nitin Nitin

http://aem.asm.org/content/82/1/106.abstract?etoc

Not much E. coli on Romaine at retail

A total of 720 whole, romaine lettuce heads were purchased from retail locations in the Upper Midwest and assessed for coliform and Escherichia coli contamination and for the presence of E. coli O157:H7.

Romaine-Lettuce-PhotosDuring a 16-month period (August 2010 through December 2011), coliform and E. coli counts were enumerated on Petrifilm, and the presence of E. coli O157:H7 and the virulence gene eae was evaluated by real-time PCR (qPCR). Over half (400 of 720) of the lettuce samples were processed with an immunomagnetic separation step before the qPCR assay. All retail lettuce samples were negative for E. coli O157:H7 when tested with the R.A.P.I.D. LT qPCR targeting a region of the O-antigen, and only two (0.28%) were positive for the eae gene when tested with LightCycler qPCR.

On Petrifilm, coliform counts of most lettuce samples (96.4%) were between <101 and 103 CFU/g, and E. coli counts for nearly all lettuce samples (98.2%) were <101 CFU/g. No seasonal trend in coliform and E. coli counts was observed throughout the examination period nor was a difference in coliform counts observed between packaged and nonpackaged lettuce heads.

These results contribute to the limited recorded data and understanding of microbial contamination of whole romaine lettuce heads purchased from retail locations, specifically revealing the absence of E. coli O157:H7 and low levels of contamination with coliforms and other E. coli strains.

 Occurrence of coliform and Escherichia coli contamination and absence of Escherichia coli O157:H7 on romaine lettuce from retail stores in the Upper Midwest

Journal of Food Protection®, Number 9, September 2015, pp. 1624-1769

Greve, Josephine D.; Zietlow, Mark S.; Miller, Kevin M.; Ellingson, Jay L. E.

http://www.ingentaconnect.com/content/iafp/jfp/2015/00000078/00000009/art00013

Leave vegetation alone: Clearing habitat surrounding farm fields fails to reduce pathogens

The effort to improve food safety by clearing wild vegetation surrounding crops is not helping, and in some cases may even backfire, according to a new study led by researchers at the University of California, Berkeley.

cow.poop.spinachThe findings, to be reported Monday, Aug. 10, in the journal Proceedings of the National Academy of Sciences, call into question the effectiveness of removing non-crop vegetation as a way to reduce field contamination of fresh produce by disease-causing pathogens. This practice led to extensive loss of habitat in a region that is globally important for food production and natural resources.

The practice was implemented largely in response to a 2006 outbreak of pathogenic E. coli in packaged spinach that killed three people and sickened hundreds of others in the United States. That outbreak was traced to a farm in California’s Central Coast, a region that supplies more than 70 percent of the country’s salad vegetables. The disease-causing E. coli strain was found throughout the farm environment — including in the feces of nearby cattle and wild pigs — but the cause of the outbreak has never been officially determined.

“Wildlife took much of the blame for that outbreak, even though rates of E. coli in wildlife are generally very low,” said study lead author Daniel Karp, a NatureNet postdoctoral research fellow in UC Berkeley’s Department of Environmental Science, Policy and Management and The Nature Conservancy. “Now, growers are pressured by buyers to implement practices meant to discourage wildlife from approaching fields of produce. This includes clearing bushes, plants and trees that might serve as habitat or food sources for wild animals. Our study found that this practice has not led to the reductions in E. coli and Salmonella that people were hoping for.”

Instead, the study authors noted that the presence of diverse habitats bordering food crops can actually provide a number of agricultural benefits.

“There is strong evidence that natural habitats surrounding crop fields encourage wild bee populations and help the production of pollinated food crops,” said study senior author Claire Kremen, a UC Berkeley professor of environmental science, policy and management. “There have also been studies that suggest that a landscape with diverse plant life can filter out agrichemical runoff and even bacteria. Changing this dynamic shouldn’t be taken lightly.”

The researchers analyzed about 250,000 tests of produce, irrigation waters and rodents conducted by industry and academics from 2007 through 2013. The tests were conducted on samples from 295 farms in the United States, Mexico and Chile, and targeted the presence of pathogenic E. coli, Salmonella and generic strains of E. coli. The researchers combined the test data with a fine-scale land-use map to identify characteristics of the landscape surrounding the agricultural fields.

The researchers found that the removal of riparian or other vegetation did not result in lower detection of pathogens in produce, water or rodents. Overall, the prevalence of pathogenic E. coli in leafy green vegetables had increased since the outbreak, even as growers removed non-crop flora. In fact, the growers who removed the most vegetation experienced the greatest increase in pathogenic E. coli and Salmonella in their vegetables over time.

lettuce“Clearing surrounding vegetation is a costly, labor-intensive practice that threatens wildlife habitat,” said Karp. “Since it does not improve food safety, there is no reason to continue this practice.”

The study did find, however, that the likelihood of detecting pathogenic E. coli was greater when fields were within 1.5 kilometers of grazeable land than when they were farther away.

“It is unclear whether it was the cattle or wildlife grazing on those lands that were responsible for the elevated pathogen levels, but there are a number of ways that farming and ranching can co-exist in a diversified system,” said Karp.

Some suggestions include:

  • Leaving strips of vegetation between the grazed areas and fresh produce areas
  • Fencing off upstream waterways from cattle to prevent waste from going downstream
  • Planting crops that are usually cooked before being eaten – such as corn, artichokes and wheat – between fresh produce fields and grazeable lands

After the 2006 E. coli outbreak in spinach, California’s agricultural industry implemented a voluntary certification program called the Leafy Green Products Handler Marketing Agreement. At the federal level, in 2011 President Obama signed the Food Safety Modernization Act, considered one of the most sweeping reforms in farming practices in decades. Both efforts shift the focus to preventing rather than responding to outbreaks.

Notably, neither the federal law nor the state program calls for the removal of wildlife habitat surrounding crops, but private buyers, keen on retaining consumer confidence in their products, may still require growers to take steps that go beyond government regulations.

“The real worry for me is that federal law will be interpreted as the floor rather than the ceiling of what farmers should do,” said Karp. “There is this misguided idea that agricultural fields should be a sanitized, sterilized environment, like a hospital, but nature doesn’t work that way.”

Other co-authors of the study are Sasha Gennet, senior scientist at The Nature Conservancy; Christopher Kilonzo, Melissa Partyka and Edward Atwill at UC Davis; and Nicolas Chaumont at Stanford University.

96863_web

A farming landscape can be co-managed for both produce safety and nature conservation. Promising practices include: (1) planting low-risk crops between leafy-green vegetables and pathogen sources, such as grazeable lands, (2) buffering farm fields with non-crop vegetation to filter pathogens from runoff (3) fencing upstream waterways from cattle and wildlife (4) attracting livestock away from upstream waterways with water troughs, supplement and feed (5) vaccinating cattle against foodborne pathogens (6) creating secondary treatment wetlands near feedlots and high-intensity grazing operations (7) reducing agro-chemical applications to bolster bacteria that depredate and compete with E. coli (8) exposing compost heaps to high temperatures through regular turning to enhance soil fertility without compromising food safety, and (9) maintaining diverse wildlife communities with fewer competent disease hosts.

Illustration by Mattias Lanas and Joseph Burg

E. coli O157 gets a boost from downy mildew in lettuce

Escherichia coli O157:H7 is more likely to contaminate lettuce when downy mildew is already present, according to U.S. Department of Agriculture (USDA) scientists.

downeymildew150723Downy mildew, a lettuce disease caused by the fungus-like water mold Bremia lactucae, is one of the biggest problems that lettuce growers must deal with.

But microbiologist Maria Brandl, with the USDA Agricultural Research Service’s (ARS) Produce Safety and Microbiology Research Unit in Albany, California, has been investigating why so many E. coli O157:H7 outbreaks can be traced back to lettuce fields when E. coli O157:H7 sources are as diverse as undercooked beef, sprouts, raw dairy, shelled walnuts, fruits and vegetables. ARS is USDA’s chief in-house research agency.

Lettuce leaves are actually a harsh place for microbes to survive. But the epidemiological evidence is indisputable about how often lettuce is the source of E. coli O157:H7 contamination.

In earlier research, Brandl found that E. coli O157:H7 preferred cut, injured and younger leaves to undamaged and older ones. Then, she collaborated with ARS geneticist and lettuce breeder Ivan Simko from the Crop Improvement and Protection Research Unit in Salinas, California.

They found that under warm temperature and on wet leaves, E. coli O157:H7 multiplied 1,000-fold more in downy mildew lesions than on healthy lettuce leaf tissue. Even on dry lettuce leaves, where most bacteria struggle to survive, E. coli O157:H7 persisted in greater numbers when downy mildew disease was present.

The researchers also found that E. coli O157:H7 did not grow as well in downy mildew lesions on the lettuce line RH08-0464, bred by Simko and a colleague to be less susceptible to the lettuce disease, as the bacteria did on Triple Threat, a commercial variety that is highly susceptible to downy mildew.

The exact factors that caused less growth of E. coli O157:H7 in the more resistant line still need to be carefully explored. But if a genetic hurdle to E. coli O157:H7 colonization could be bred into commercial lettuce varieties along with downy mildew resistance, it would add a new defensive line to contamination of lettuce, helping farmers to improve the microbial safety of their crop as well as control their number-one plant disease problem.