Biophysical and Ecological Processes Impacting the Growth and Survival of E. Coli O157 On and In Vegetables

NON-TECHNICAL SUMMARY: Fresh fruits and vegetables are an increasing source of foodborne disease. Factors that impact the survival of pathogenic bacteria on and in fruits and vegetables are poorly understood. This project explores the interaction between the human pathogen E. coli O157 and other bacteria that can normally be found on plant surfaces, including those bacteria that cause diseases of plants. the purpose of this project is to identify ways to make E. coli O157 and other pathogens less likely to contaminate and survive on fruits and vegetables.

<P>
APPROACH: The long-term goal is to enhance human health through prevention of foodborne infections. Given the increase in identified foodborne diseases outbreaks associated with fruits and vegetables, there is a critical need to understand the ecology of bacterial foodborne pathogens on plant surfaces. It is essential to determine the roles plant physiology and epiphytic microbiology play in the contamination (and potential for decontamination) of fruits and vegetables with human foodborne pathogens such as E. coli O157. Our objective is to determine how interactions between human foodborne pathogens and fruits and vegetables decrease the microbiological safety of these products. Preliminary findings from our laboratory suggest that there is significant interaction between human pathogens and plants and their associated epiphytic flora during the pre-harvest stages of production. Further, the bacterial flora of plants may serve as a significant reservoir for antimicrobial resistance genes. Our team of microbiologists, a plant pathologist, and food scientist will achieve these goals through controlled laboratory experiments that are designed to assess the impact plant pathogens on human pathogen uptake, proliferation, exchange of antibiotic resistance genes, and protection from subsequent post-harvest disinfection. Once factors influencing the microbiological interactions on plant surfaces are identified and quantified, science-based intervention and education strategies for enhancing food safety of U.S. agricultural products can be developed, evaluated, and communicated to producers and processors. The end result is a safer food supply and fewer foodborne illnesses, extending from the local scale in community-based food products to our international trading partners.

<P>

PROGRESS: 2006/09 TO 2007/09 <br>
The ecology of the vegetable leaf surface is important to the survival and proliferation of enteric pathogens. Understanding the effects of changes in these environments created by physical damage and phytopathogen infection on foodborne pathogens can lead to development of approaches to minimize the hazard of contamination of fresh fruits and vegetables. Output Activities: In one series of experiments, healthy plants were compared to physically damage and bacterial phytopathogen- infected lettuce plants for their ability to harbor Escherichia coli O157 over 10 days. Leaves from lettuce plants cracked along the central vein, plants infected with Xanthomonas campestris pv. vitians, and healthy plants were inoculated with E. coli O157. Escherichia coli O157 populations were counted on these leaves and also on non-inoculated leaves on the same plants for 10 days. In a second series of experiments Escherichia coli O157 proliferation was determined on tomato leaves infected with Pseudomonas syringae and Xanthomonas campestris. Gas chromatography was used to determine the sugars and sugar alcohols released by tomato leaves damaged by the same biotropic plant pathogens. In terms of Extension Programming, An expert technical model was developed concerning the contamination and prevention of contamination of vegetables with pathogens. Subsequently, mental models to ascertain the fundamental beliefs and practices among a cohort of 12 large-scale vegetable farmers was conducted to determine where the model developed by experts deviated from that produced following producer elicitation.
<br> <br>
IMPACT: 2006/09 TO 2007/09<br>
In the first experiments the density of E. coli O157 decreased on lettuce leaves regardless of the treatment over time, the decrease was greatest on healthy plants. E. coli O157 populations on day 10 were significantly higher on physically-damaged leaves than on undamaged control leaves (P <0.05)), but not different from those on Xanthomonas-infected leaves. E. coli O157 populations on Xanthomonas-infected leaves were also not significantly different from those on control leaves on day 10. There was little dissemination of the enteric pathogen to non-inoculated leaves. Damage may impact the survivability of enteric pathogens by providing some needed nutrients and protection from environmental stress. Maintaining healthy plants might help limit the dissemination, persistence and proliferation of foodborne pathogens on fresh vegetables and thus improve the safety of fresh produce. The in vitro proliferation of E. coli O157, P. syringae, and X. campestris incubated in the presence of different combinations of exuded sugars was analyzed using a mixture model design. Escherichia coli O157 survived better on tomato plants damaged by Xanthomonas campestris than on healthy plants (P = 0.012). The most common sugars and sugar alcohols in the leaf exudate were glucose, fructose, inositol, and sucrose. The abundance of sucrose and inositol differed between the healthy and infected plants (P<0.05). Phytopathogen damaged increased nutrient availability and offered attachment sites for E. coli O157. Keeping plants free from disease could improve the overall safety of the product.