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United States Department of Agriculture

Agricultural Research Service

Research Project: MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
2007 Annual Report


1a.Objectives (from AD-416)
1. Determine the prevalence, diversity, and quantity of bacteria associated with organic as compared to conventional fresh produce in Maryland and Virginia. 2. Determine colonization and survival rates of bacterial pathogens associated with selected organic and conventional fresh produce. 3. Investigate the mechanism(s) of introduction and transference of E. coli O157:H7 to lettuce and leafy greens during growing, harvest, postharvest handlings and processing operations. 4. Determine the persistence and survival of Escherichia coli O157:H7 on fresh and fresh-cut leafy green produce. 5. Compare the extent of enteric pathogen transmission by a chewing insect, Colorado Potato Beetle, in organically- and conventionally-grown produce.


1b.Approach (from AD-416)
A farm-based investigation will be conducted to assess the prevalence, diversity, and quantity of epi- and endophytic saprophytes and gastroenteric bacteria associated with organic as compared to conventional fresh produce. Bacterial analyses will include use of microbial cultural and molecular methods to characterize the enteric and saprophytic microflora on fresh market produce types most commonly reported in foodborne illness outbreaks: tomatoes, salad greens: lettuce, arugula, mesculin, spinach, beets; herbs: basil, cilantro, parsley; strawberries; cucurbits: cucumbers and melons. In addition, practices at each farm will be identified and characterized to associate analytical results with microbial quality of farm inputs and operations. Bacterial analysis will include total aerobic heterotrophic bacteria, enteric bacteria (coliforms), Aeromonas spp., Bacillus cereus, C. perfringens, E. coli, Enterococcus spp., L. monocytogenes, and Salmonella. Microbial community analysis of epi- and endophytic bacteria by DGGE will be used for tomato fruit and salad greens from organic and conventional farms. Growth chamber and field plot experiments will be conducted to establish the pathogen concentrations required to establish populations that survive on different plants (e.g., tomatoes, greena, basil, scallions, carrots, and strawberries). Survival of pathogens (Aeromonas, EHEC, L. monocytogenes, and Salmonella) on/in plants grown in organic and conventional soils with composted or aged manure (spike with pathogens) and irrigated with contaminated river water or sprayed with compost tea will be determined by enrichment and enumeration of rhizosphere, cortical root tissue, and/or on the edible portion of plants. Organic practices that can prevent pathogen growth in compost tea and internal plant tissues will be evaluated in growth chamber and field studies with tomato. Finally, the potential for a chewing insect, Colorado Potato Beetle, to transfer bacterial pathogens to the surface and interior tissue of tomato plants and initiate systemic colonization will be investigated for organic and conventional production methods.


3.Progress Report
A farm-based investigation was conducted to assess the prevalence, diversity, and quantity of epi- and endophytic saprophytes and gastroenteric bacteria associated with organic as compared to conventional fresh produce in the mid-Atlantic region. Bacterial analyses included use of microbial cultural and molecular methods to characterize the enteric and saprophytic microflora on fresh market produce types most commonly reported in foodborne illness outbreaks: tomatoes, leafy greens: lettuce, spinach, radicchio, beets. In addition, practices at each farm were systematically recorded to relate analytical results with microbial quality of farm inputs and operations. Bacterial analysis included total aerobic heterotrophic bacteria and fungi, enteric bacteria (coliforms), E. coli, L. monocytogenes, and Salmonella. To date, from the 200 samples each of commodity, soil, and water, analyzed, no E. coli O157:H7 has been obtained, although E. coli has occasionally been recovered from soil and rarely from commodity. No salmonella was recovered from any source. Listeria monocytogenes was recovered from only one commodity and soil sample, which the farm used only for personal consumption; the corresponding well water sample from the farm was negative for all three pathogens from two separate test events. None of the farms were enrolled in a GAPS audit program, most were unaware of the program, yet in general their practices were compliant with the main GAPS recommendations. All leafy green growers supplying fresh market product were irrigating with potable water. A growth chamber experiment was initiated to examine the internalization and translocation of Cryptosporidium parvum in alfalfa, rye, and spinach. Additional growth chamber experiments to examine internalization and translocation of E. coli O157:H7 and Salmonella in spinach were initiated. Growth chamber studies to examine endo- and epiphytic survival of Salmonella on organic and conventional tomatoes have been initiated. Options for additional field plots for experiments to be initiated in FY08 to evaluate pathogen survival and growth interventions on different plants (e.g., tomatoes, spinach, basil, scallions, carrots, and strawberries) are under consideration with collaborators in other federal agencies. Additional plans were established and extramural support obtained to evaluate the generation and transport of airborne particulates containing viable E. coli O157:H7 from animal, pasture, and sewage treatment operations adjacent to leafy green fields in the California central coast valley.


4.Accomplishments
Compost tea is an unheated on-farm infusion of compost used as a spray or soil drench to promote plant growth and control foliar and root diseases. This report describes the effects of two production processes, aerated and nonaerated, on growth and survival of hetertrophic and fecal coliform bacteria, E. coli and Salmonella. Seven commercially available nutrients used for compost tea production were tested individually and in combination for their effects on the growth of E. coli O157:H7 and Salmonella Enteritidis when present at very low to moderate initial concentrations (10 to 1,000 colonies per g). Pathogen and fecal coliform populations were undetectable by 8.5 days in nonaerated compost tea without nutrient supplements. E. coli O157:H7 decreased below detection levels in aerated compost tea at 36 h without supplements. In contrast, the addition of commercially formulated mixtures or combinations of nutrient supplements resulted in growth of E. coli O157: H7, Salmonella, and fecal coliforms up to 10,000 colonies per g in both aerated and nonaerated compost tea. Addition of nutrient supplements during compost tea production should be avoided to reduce risk of contamination of fresh produce by pathogens in this organic spray.

This research falls within component 1.1 (Pathogens, Toxins and Chemical Contaminants, Preharvest) of NP 108. The project primarily addresses problem statement 1.1.2 (Epidemiology) in that it will provide an assessment of the potential for contamination of organic as compared to conventionally grown fresh market produce. It also addresses Problem Statement 1.1.4 (Interventions) in that it will provide information which may useful in minimizing the potential for contamination of fresh market produce.


5.Significant Activities that Support Special Target Populations
Co-wrote proposal for research on pathogen detection, survival, and reduction by composting with 1890 college (Univ. Md. Eastern Shore); proposal selected for funding by CSREES.


6.Technology Transfer

Number of new CRADAs and MTAs1
Number of active CRADAs and MTAs1
Number of non-peer reviewed presentations and proceedings8

Review Publications
Ingram, D.T., Millner, P.D. 2006. Factors affecting compost tea as a potential source of Escherichia coli and Salmonella on fresh produce. Journal of Food Protection. 70(4):828-834.

Last Modified: 9/10/2014
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