Summary
Contaminated produce continues to be a leading cause of foodborne illness. Yet farmers still lack the ability to predict when their crops are at-risk and effective strategies to manage those risks. While evidence is accumulating regarding the efficacy of many practices, results are often not made available to growers in a useable way. We plan to synthesize existing literature to develop data-driven, pre-harvest tools to help growers predict and mitigate risks associated with foodborne pathogens. This tool will be based on existing literature and will be customizable to each farmers’ unique management practices. In addition to creating a predictive tool, we seek to explore novel methods for suppressing pathogens, when they do occur on farms. Specifically, we will study how soil amendments and farm management affect the ability of feces-feeding soil bacteria to suppress pathogens (E. coli and Listeria). Farmers, industry, and conservation organizations have expressed strong interest in making informed decisions about on-farm practices to improve produce safety without comprising environmental health. By combining literature syntheses with lab and field experiments, this project will provide growers with both new strategies for mitigating pathogen prevalence and an effective tool to assist growers in navigating decisions regarding the food safety/conservation “stale-mate.”
Technical Abstract
Despite ongoing efforts to prevent fecal contamination, fresh produce is a leading cause of foodborne illness (Painter et al. 2013). While widely accepted GAPs (LGMA, 2013) aim to reduce the transmission of foodborne pathogens to produce, growers continue to lack the ability to predict when their crops are at highest food-safety risk as well as effective strategies to mitigate those risks. Moreover, existing practices can be costly and ineffective. For example, our prior research found that, despite widespread implementation (Karp et al. 2015a; Baur et al. 2016), removing non-crop vegetation did not reduce pathogen prevalence on California farms (Karp et al. 2015). Similarly, many growers are replacing compost with synthetic fertilizers or heat-treated manures (Baur et al. 2016), but we also discovered strong relationships between soil organic matter, microbial diversity, and natural pathogen suppression. More generally, while evidence is accumulating regarding the efficacy of many food-safety practices, conclusions are often hidden behind academic paywalls and communicated in forms inaccessible to growers. Our goals are two-fold. First, we seek to synthesize existing literature to develop data-driven, pre-harvest, decision-support tools to help growers predict and mitigate risks associated with foodborne pathogens. Second, we seek to explore novel methods for suppressing foodborne pathogens. Specifically, we will study how soil amendments and management history affect the community dynamics of feces-feeding soil bacteria in order to harness their activity to suppress pathogens. To do so, we will formally review studies that assess how farming practices and surrounding landscapes effect pathogen occurrence or suppression. Rather than committing to a traditional meta-analysis (a sufficient number of studies may not exist for this approach), we will implement a systematic framework for reviewing the efficacy of on-farm practices in which studies are collected, summarized, and then scored by a team of experts (CE, 2018; Dicks et al., 2010). Our experts will include an academic and grower advisory panel so that we can holistically compile information about the cost, feasibility, and efficacy of each practice. Working with our grower partners, we will then synthesize our findings into a web-based, decision-support tool that allows farmers to easily explore the evidence underlying current food-safety practices. To identify novel food-safety practices (Objective 2), we will utilize a 23-year soil experiment to manipulate microbial communities from soils with three distinct management histories: conventional, organic, and mixed (conventional with cover crops). In the lab, we will amend soils of each type with composted manure. We will then monitor changes in microbial communities and pathogen suppression over time, identifying the most effective pathogen suppression bacteria. Finally, we will leverage existing data to explore how surrounding landscape composition affects soil-based pathogen-suppression. Growers, industry, and conservation organizations have expressed strong interest in making more informed decisions about on-farm practices to improve produce safety without comprising environmental health. By combining literature syntheses and existing data with lab and field experiments, this project will provide growers with both new strategies for mitigating pathogen prevalence and an effective tool to assist growers in navigating decisions regarding the food safety/conservation “stale-mate.”
Research Objectives
1. To develop a grower-motivated decision-support tool that synthesizes evidence for the efficacy, feasibility, and costs of food-safety practices.
2. To understand the community dynamics of feces-feeding bacteria in order to harness their activity to bolster food safety.
Findings & Recommendations
Because they may introduce foodborne pathogens onto farms, growers are often reluctant to apply animal-based soil amendments to their farm fields. Indeed, surveys of growers suggest relatively low adoption of composting in California [6]. However, we found little evidence that properly treated organic soil amendments, such as compost, compromise food safety within the evidence synthesis presented here. Correspondingly, in our field study, we found that organic soils (amended with compost and cover crops) were not more conducive to pathogen survival. Instead, organic soils had more soil nutrients and more diverse soil microbial communities, both of which may have contributed to the comparatively higher rates of pathogen suppression observed early in the growing season. Even more promising, our results from lettuce farms across the Central Coast indicates that soil management affects different pathogens in remarkably similar ways (i.e., suppressive capacity of Listeria, Salmonella, and E. coli was strongly correlated across farm fields). This suggests that actions taken to reduce the hospitability of soils for one pathogen may be effective at suppressing another. Looking forward, it thus appears that tradeoffs between promoting soil health and food safety may not be as severe as previously considered. Organic soil amendments, including composts and cover crops, are known to improve soil health and, from our analyses, may promote bacterial communities that are particularly effective at suppressing pathogen survival. All in all, our work thus suggests that abandoning animal-based composts should be reconsidered given their benefits for soil health and potential pathogen suppression. That said, our evidence synthesis also suggests that proper composting is critical. Indeed, multiple studies demonstrated that untreated animal-based soil amendments (such as raw manure) introduce and promote foodborne pathogens on produce farms. More generally, our work highlights the utility of evidence synthesis for on-farm food-safety management, both in identifying promising practices and ineffective ones. For example, we found multiple studies reporting on the benefits of treatment wetlands for improving microbial water quality. The evidence synthesis also pointed towards practices to avoid, namely applying raw manures close to harvest and cultivating produce near livestock. On the other hand, the evidence synthesis also suggests that some current practices may be misguided. Very little evidence suggested that removing non-crop vegetation improved food safety, for example. Finally, a clear benefit of evidence synthesis is its ability to identify key research gaps. Many practices that may have a significant impact on food safety are severely understudied. Only a few papers published to date evaluate how foodborne pathogen persistence or prevalence change on North American produce with soil tillage, cover cropping, biosolid application, and livestock integration, among many other factors. Looking forward, we suggest that systematic evidence syntheses can serve as powerful tools for setting future research directions.