Leafy greens, lettuce, cantaloupes, mangoes and strawberries. These are just some of the foods that have sickened or even killed people when they were contaminated with foodborne pathogens such as E. coli, Listeria and Salmonella.
Amidst the confusing swirl of information about these and other produce outbreaks, the question arises: Were some of these pathogens inside the produce? Could it be — in some cases, anyway — that the plant’s roots sucked the pathogens up out of the soil and transferred them through the plant into its edible parts?
Could this happen when uncomposted manure is applied to a field, or when water contaminated with livestock waste is used to irrigate a crop, or when deer wandering through a field or geese flying over it leave behind some fecal droppings? Could some of the pathogens from the manure, polluted irrigation water, or droppings from wildlife soak down into the ground and become available to the roots of the plants?
No idle question, this. In fact, it’s been asked enough times for serious research to be done to try and answer it. The main reason, of course, is the need to come up with information that food producers and handlers can use to prevent their crops from becoming contaminated — without being sidetracked by possibilities that don’t hold water.
Just glancing through the newly proposed food safety rules for fruits and vegetables, it becomes readily apparent that a good deal of attention is paid to preventing the surfaces of fruits and vegetables, especially those that are eaten raw, from being contaminated with foodborne pathogens. Strategies cited in the proposed rules include taking steps to ensure that irrigation and wash water is free from pathogens, that farm and processing equipment doesn’t harbor pathogens and that farmworkers have proper handwashing facilities.
But the question persists: If plants can suck contaminants up out of the soil, what in the world can a producer or processor do if livestock or wildlife pollutes nearby irrigation water or if wandering wildlife contaminates the soil with fecal droppings?
None of this is far-fetched. After all, it was E. coli O157:H7 in droppings from deer wandering through some strawberry fields in Oregon that contaminated berries that would eventually kill an elderly woman and send seven people to the hospital, three of whom suffered kidney failure, according to public health officials.
And it was the same potentially fatal type of E. coli that killed three people and sickened more than 200 in the 2006 outbreak linked to baby spinach grown in California. The likely culprits in that outbreak were wild pigs or livestock that wandered through the field or perhaps nearby irrigation water that had been contaminated by livestock or wildlife. But even with matching DNA samples, the exact way the pathogen actually spread to the spinach remains unknown.
Some mysteries brought to light
It was the 2006 baby spinach E. coli outbreak that led to a research project, funded by Fresh Express, that looked at the question of whether spinach roots could suck pathogens up from soil and transport them through the stems into the leaves.
The research on whether this sort of internalization of E. coli into is possible is also relevant to other salad greens.
The project tracked the pathogen’s journey from the field to harvest. By placing a fluorescence gene at a specific location in the E. coli bacterium, the researchers could use a specialized microscope to see where the pathogen was going.
It turned out that not only could the E. coli survive in the soil for up to 28 days, but the E. coli cells were also able to migrate from the soil into the roots of the spinach plants.
However, after looking at whether any of the E. coli strains traveled past the roots and up into the plant’s interior structures, the researchers concluded that that sort of internalization appeared to be unlikely.
“We wanted to investigate this, because it was one of the questions out there,” USDA microbiologist Manan Sharma told Food Safety News in an earlier interview. “We’ve taken something that has been of concern for eight or nine years and put it to rest.”
He said that thanks to the results of this research project, spinach growers and processors could focus on more likely routes of contamination so they can prevent that from happening.
He also told Food Safety News that if the plants didn’t have a way to keep the pathogens from traveling up into the plants’ leaves, there would probably be a lot more contaminated produce and a lot more sick people.
Even so, Sharma said that the results from this research couldn’t be translated into other crops.
“Each crop system should receive its own evaluation of the risk of the uptake of foodborne pathogens through root systems,” he said.
So, what about other crops?
That’s what scientists wanted to know when they did a review last year of the literature about research projects that focused on whether the edible parts of crops could be contaminated from pathogens or viruses that get into the plants’ roots and then travel up into the edible parts of the plant. Does that happen? they wanted to know.
The paper about this literature review, published May 2012 in Foodborne Pathogens and Disease, starts right out by saying that root uptake of enteric pathogens — those relating to or affecting the intestines — and subsequent internalization into the plants has been a large area of research with results varying due to differences in experimental design, systems tested and pathogens and crops used.
Referring to the 2006 E. coli outbreak linked to baby spinach contaminated with E. coli O157:H7 as the catalyst for more research on whether or how root uptake can contaminate produce, the paper explains that the debate on this topic has led to the need to review the literature. According to the paper, outbreaks of foodborne illnesses have been increasingly linked to the consumption of fruits and vegetables, with the source of some of these outbreaks traced back to the farm. Even so, in many outbreaks, the way the produce was actually contaminated remains unknown.
According to data from the Center for Science in the Public Interest, produce outbreaks accounted for 13 percent of foodborne outbreaks during 1990-2005.
Kalmia E. Kniel, Department of Animal and Food Sciences at the University of Delaware, and one of the project’s researchers, told Food Safety News that, when all is said and done, the results of the literature review show that it’s “very unlikely” that contamination of produce occurs in the field through root uptake.
“There’s enough literature to say that,” she said.
When asked about root crops such as carrots, Kniel said there’s no evidence to show that they’re at risk when it comes to internalizing pathogens from the soil.
That’s not to say, of course, that root crops grown in soil contaminated with pathogens won’t have some pathogens on their surfaces when they’re harvested.
The paper’s conclusion says that “generally, the presence of internalized pathogens in roots of plants does not directly correlate with internalized pathogens in the edible or foliar tissues of crops.”
Even so, the researchers say that any future research about root uptake should include realistic growing conditions, along with realistic pathogen contamination levels.
Some of the research projects used sterilized soil or extremely high levels of pathogens, for example.
Kniel also told Food Safety News that because pathogens are good at exchanging genetic material, especially when under pressure, scientists need to stay one step ahead of how that might affect root uptake and from there internalization into the plant.
According to the same research paper, the topic of viral pathogens is critically important to food safety. That’s because from 1973 to 2006, 60 percent of U.S. foodborne outbreaks associated with eating leafy greens were caused by noroviruses, while Salmonella and E. coli only accounted for 10 percent of the outbreaks.
While noroviruses — often referred to as “the stomach flu” because they produce gastrointestinal symptoms — typically involve food contaminated by food handlers, several outbreaks from fresh produce have been linked to environmental contamination (in the field, for example).
Then, too, one of the largest outbreaks of hepatitis A virus in the United States was linked to eating green onions contaminated by the virus. That outbreak sickened about 1,000 people and killed four. And even though polluted irrigation water and farmworkers were among the likely sources for the outbreak, the exact way the onions were contaminated remains unknown.
The research also revealed that while contaminated soil triggered little to no internalization of pathogens from the roots into the plants, that wasn’t always the case with plants grown hydroponically, especially in the case of viruses that can get people sick.
“Following good agricultural practices and using clean water is essential for hydroponics,” Kniel said.
How, then, do pathogens get inside the crops?
“Internalization” happens when pathogens get inside the edible parts of fresh produce. But if not from root uptake, then how?
In a paper titled Internalization of Fresh Produce by Pathogens, which appeared last year in the Annual Review of Food Science and Technology, Marilyn C. Erickson of the Center for Food Safety at the University of Georgia, shared some observations about internalization and root uptake in leafy greens.
She found that while pathogens can get into plants in a number of locations on the plant and in a number of different kinds of produce — both before and after harvest — it is unlikely they enter through roots or seeds when grown in soil under normal growing conditions.
However, some growing, harvesting and processing conditions can open the way for pathogens to get onto and into the produce.
For example, her research showed that a film of moisture on the leaves appears to be a critical factor in a pathogen’s ability to reside on leaf surfaces and then to migrate and infiltrate into the stomata of the plants.
Stomata are tiny openings, typically found on the outer skin of a leaf but also in other parts of the plant. These little “mouths” are made up of two cells, referred to as “guard cells” that surround a tiny pore called a stoma. The stomata’s main job is to allow gases such as carbon dioxide, water vapor and oxygen to quickly move into and out of the leaf.
Erickson said that enteric pathogens can lodge in the stomata or be trapped in crevices of leafy greens that are exposed to contaminated water after harvest.
In addition, surfaces of the greens that have been cut during harvest or during minimal processing furnish sites on the leaves that are especially vulnerable to penetration by the pathogens.
When looking at actual growing conditions, Erickson told Food Safety News that the microflora (organisms that are already in the soil) far outnumber any pathogens that might also be in the soil. In general, the indigenous organisms easily outcompete the pathogens in their search for nutrients, which they need to survive. She said that in “normal soil,” you’d need concentrations of about 10,000 E. coli bacteria per gram for root uptake to happen, and even more in moist soil.
Her research has led her to conjecture that plants have defenses against internalization when they’re growing. “Internalization is more likely to happen after harvest,” she said, referring to cuts in the surfaces of the leaves as an example.
Food safety scientists have pointed out that once a pathogen migrates to a cut surface where nutrients are oozing out, it’s almost impossible to dislodge them. Like any other hungry organism, they’ll hold on tight to a source of food. From there, they can migrate into the plant’s edible parts in search for yet more food.
The sweet nutrients inside cantaloupes can lure pathogens on the surface into a melon that has been nicked or cut through these openings. From there, they can travel into the melon itself in search of even more food.
In a research paper about Salmonella contamination in cantaloupes that appeared in the International Journal of Food Microbiology, Trevor Suslow, a food safety scientist at the University of California, Davis, and his colleagues concluded that the outcomes of the project strongly indicated that root uptake and the transportation of Salmonella from the soil due to contaminated irrigation water is “highly unlikely” to occur — even under “exaggerated worst-case” growing conditions.
However, any inputs, such as contaminated irrigation water, which can contain Salmonella, would have the potential to contaminate surfaces of the melons that come into contact with the pathogens — even at low levels.
According to the report’s conclusions, these pathogens, if on the ground’s surface where the cantaloupes are growing, could get onto the melons’ surfaces and from there be transferred to other melons by farmworkers, harvesting equipment or transportation vehicles, for example.
In an earlier interview, Suslow told Food Safety News that as far as he knows, foodborne pathogens can’t penetrate the surface of produce on their own. Generally it takes some kind of opening on the surface to provide a pathway to the subsurface of the produce. But when that happens, he said, even antibacterial solutions won’t be able to rid the produce of pathogens.
The research paper concludes by warning that contamination of the external rind of the melons from irrigation water carrying pathogens remains a concern in melon production. For that reason, it’s important to establish critical limits for melon irrigation in California and other growing regions with similar arid and semi-arid climates, soil texture and crop-management practices.
An industry giant, California provides 70 percent of the cantaloupes sold in this country. During the state’s five-month season, the industry typically packs and ships around 30 million cartons of cantaloupes. A carton contains 12 to 18 cantaloupes.
Cantaloupes stand out in the roll call of recent food poisoning outbreaks. In 2011, Listeria-contaminated cantaloupes from a farm in Colorado sickened more than 140 people and killed 33. And last year, Salmonella-contaminated cantaloupes from a farm in Indiana killed 2 people and sickened more than 175 in 21 states.© Food Safety News