Newswise — Washing fresh fruits and vegetables before eating may reduce the risk of food poisoning and those awful episodes of vomiting and diarrhea. But according to new research, described today at the 235th national meeting of the American Chemical Society, washing alone — even with chlorine disinfectants — may not be enough.
Studies show that certain disease-causing microbes are masters at playing hide-and-go seek with such chemical sanitizers. These bacteria can make their way inside the leaves of lettuce, spinach and other vegetables and fruit, where surface treatments cannot reach. In addition, microbes can organize themselves into tightly knit communities called biofilms that coat fruits and vegetables and protect the bacteria from harm. This kind of bacterial community can harbor multiple versions of infectious, disease-causing bacteria, such as Salmonella and E. coli.
Now, new findings from scientists at the U.S. Department of Agriculture suggest that irradiation, a food treatment currently being reviewed by the FDA, can effectively kill internalized pathogens that are beyond the reach of conventional chemical sanitizers.
Irradiation exposes food to a source of electron beams, creating positive and negative charges. It disrupts the genetic material of living cells, inactivating parasites and destroying pathogens and insects in food, including E. coli and Salmonella.
Using this technique on fresh and fresh-cut fruits and vegetables could provide a reliable way to reduce the numbers of foodborne illnesses reported each year in the United States, says Brendan A. Niemira, Ph.D., a microbiologist with the USDA's Agricultural Research Service in Wyndmoor, Pa., who directed the study.
"When bacteria are protected — whether they're inside a leaf or inside a biofilm — they're not going to be as easy to kill," Niemira says. "This is the first study to look at the use of irradiation on bacteria that reside inside the inner spaces of a leaf or buried within a biofilm."
The quantity of fresh fruits and vegetables in the United States has increased every year in the last decade. Unfortunately, the increase in consumption has been accompanied with an increase in the number of outbreaks and recalls due to contamination with human pathogens such as E. coli. Fresh fruits and vegetables carry the potential risk of contamination because they are generally grown in open fields with potential exposure to pathogens from soil, irrigation water, manure, wildlife or other sources.
"The spinach outbreak in the fall of 2006, in particular, raised questions about how these organisms survived the various treatments that are applied " the rinses and the washes and things," Niemira says.
At the time, research had already demonstrated that pathogens like Salmonella and E. coli can be drawn into fruits after they've formed, and can migrate into them during fruit growth and maturation if the plant is exposed to them during pollination or in the irrigation water. But questions remained as to whether a penetrating process such as irradiation could kill a pest located inside a leaf.
To see how internalized sources of bacteria responded to various treatments, Niemira and his colleagues devised a way to pull bacteria into the leaves of leafy green vegetables. The scientists cut leaves of romaine lettuce and baby spinach into pieces and submerged them in a cocktail mixture of E. coli. The bacteria was pushed inside the leaves with a vacuum perfusion process. The leaves were then treated with either a three-minute water wash, a three-minute chemical treatment or irradiation.
After treatment, the leaves were suspended in a neutral buffer solution and crushed to recover and count the internalized bacteria. The study showed that washing with plain water was not effective at reducing the levels of the pathogen on either spinach or lettuce. The chemical treatment, a sodium hypochlorite solution, did not result in significant reductions of E. coli cells in spinach leaves, and an gave less than 90 percent reduction of E. coli in the romaine lettuce samples.
Ionizing radiation, in contrast, significantly reduced the pathogen population in both the spinach and the lettuce leaves. The level of kill was dependent on the dose applied, with reductions of 99.99 percent on romaine lettuce and 99.9 percent on spinach at the highest dose tested.
The researchers then conducted lab tests with biofilms to see how well different strains of Salmonella and E. coli, which were buried inside the biofilms, stood up to irradiation.
The biofilms that contained Salmonella tended to die more easily with irradiation, while those that were infected with E. coli were a bit more resistant, Niemira says.
"In the most resistant cases, we saw a difference of a few percent, but it was nothing at all compared to the resistance you might see if you were using a chemical treatment," he says.
The scientists now are conducting studies of biofilms on leafy green vegetables to better gauge how irradiation might work on plants in the field.
Niemira says it's still not clear if human pathogens can actually increase in population within plant tissues, or if they merely persist.
"This is an important question, because if the pathogens don't reproduce effectively within these protected spaces and stay below minimally infective population sizes, then the risk they pose to consumers is less," he says. "If they are able to reproduce inside, then they may increase to more dangerous levels."
Though some activist groups continue to speak against irradiation, consumer confidence in the application has grown steadily through the years as studies have shown its effectiveness in reducing pathogens that cause foodborne illnesses, says Christine Bruhn, Ph.D., who focuses on consumer issues in food safety and quality at the University of California at Davis.
"Sixty to 90 percent of consumers indicate that they would buy irradiated food when told of the benefits of the process and the endorsement of health authorities," Bruhn says.
She and Niemira have submitted a proposal to the USDA to further explore the applications of irradiation in leafy greens and to gauge consumer acceptance of this application.
Note for reporters' use only: For full information about the New Orleans meeting, including access to abstracts of more than 9,000 scientific papers and hundreds of non-technical summaries, visit http://www.eurekalert.org/acsmeet.php. The paper on this research, AGFD 136, will be presented at 9:00 a.m., Thursday, April 10, 2008, in the Marriott Convention Center, Blaine Kern E, during the symposium, "Intentional and Unintentional Contaminants of Food and Feed." Brendan A Niemira, Ph.D., is a microbiologist with the USDA's Agricultural Research Service in Wyndmoor, Pa. ALL PAPERS ARE EMBARGOED UNTIL DATE AND TIME OF PRESENTATION, UNLESS OTHERWISE NOTED AGFD 136Inactivation of microbial contaminants in fresh produce Program Selection: Division of Agricultural & Food ChemistryTopic Selection: Intentional and Unintentional Contaminants of Food and Feed: Potential Strategies to Prevent Contamination of Food
Abstract With the microbial safety of fresh produce of increasing concern, conventional sanitizing treatments need to be supplemented with effective new interventions to inactivate human pathogens. Our research group investigates physical and chemical treatments such as hot water pasteurization, gaseous chlorine dioxide, cold plasma and irradiation. Research in biological controls deals with the use of single or multiple isolates of antagonistic bacteria for inhibiting the outgrowth of bacterial human pathogens. Related research in microbial ecology determines how pathogen biofilm formation and interactions with native microflora may alter the efficacy of applied treatments and interventions. This presentation will summarize the advances made in these areas, as well as research results on the process of scaling up effective interventions from laboratory scale to pilot plant scale, including the critical process of evaluating the effects of the various interventions on sensory and nutritional quality attributes, yield, physiology, and shelf-life. ________________________________________Researcher Provided Non-Technical Summary Briefly explain in lay language what you have done, why it is significant and what are its implications (particularly to the general public)
Unless they are suppressed or killed, human pathogens can survive and grow on produce. Our research has shown that certain strains of good bacteria can prevent pathogens from growing. By suppressing the pathogens, the are less able to cause harm to consumers. Pathogens on the surfaces of fruits and vegetables frequently live in communities of bacteria called biofilms. Once they are in these tightly knit communities, they are much more resistant to chemical sanitizers such as chlorine. However, our research has shown that irradiation effectively kills these pathogens, even in these communities. We have also shown that pathogens that are inside a lettuce or spinach leaf are protected from chlorine washes, but they are not protected from irradiation. Our data suggests that irradiation effectively kills pathogens that are beyond the reach of conventional chemical sanitizers.
How new is this work and how does it differ from that of others who may be doing similar research?
The results on biocontrol organisms and the ability of irradiation to kill pathogens protected in biofims or inside a leaf are entirely new results, published within the last year. Although other labs have investigated chemical antimicrobials derived from carrots, the use of biocontrol organisms isolated from carrot microflora is a new direction of research. To our knowledge, the information on irradiation of biofilms is the first of its kind. Although previous studies of irradiation of lettuce have examined surface-inoculated leaves, this work is the first of its kind to examine leaves which were inoculated so as to draw the bacteria inside the inner spaces of the leaf.