When the E. coli hits the fan! Evaluating the risks of dust-associated produce cross-contamination

Dust represents an understudied vehicle for microbial dispersal and produce contamination by pathogens. Dust deposition onto crops during cultivation is inevitable as plant surfaces serve as a major aerosol sink and dust can serve as a vehicle for bacteria. Wind-driven distribution of dust in agricultural environments can also impact food safety when the sources of dust include particles from natural and human-related reservoirs of pathogens. While the populations of enteric pathogens in water is frequently determined and the microbiological quality of soils are monitored, the evaluation of dust and soil-borne particulates is rarely conducted. This study proposes the following: 1). To evaluate the role of dust in transferring foodborne pathogens to produce surfaces grown in eastern and western regions of the US, 2). To determine the role of humidity in the deposition of dust on produce and the survival of pathogens in dust, and 3). To test dust particulates from animal operations in both regions for the presence of biomarkers indicative of fecal contamination and potentially the presence of pathogens. This study will enhance our understanding of pathogen transport from feces into and through produce fields and will quantify the risk associated with contamination from dust under varying environmental/atmospheric conditions.

Dust, broadly defined as fine particulate matter resulting from wind erosion on land surfaces and suspended in the air, is an inseparable component of the atmosphere. Dust represents an understudied vehicle for microbial dispersal in agricultural environments and produce contamination by microorganisms pathogenic to humans. Dust not only affects biological processes in plants, such as stomatal gas exchange, but also the plant surface microbiome. Dust deposition onto crops during field cultivation is inevitable as plant surfaces serve as a major aerosol sink. Studies have indicated that dust can serve as a vehicle for bacteria. Wind-driven distribution of dust in agricultural environments could also impact food safety when the sources of dust include particles from natural (soil, decaying vegetation, feral/wild animal droppings) and human-related (manure-amended soils, silage, municipal sewage-based biosolids, composting, and animal production facilities) reservoirs of human pathogens. While the populations of enteric pathogens in water is frequently determined through periodic testing as recommended by the Food Safety Modernization Act (FSMA) and the microbiological quality of soils are monitored, the evaluation of dust and soil borne particulates is rarely carried out. This study proposes the following: 1). To evaluate the role of dust in transferring foodborne pathogens to the surfaces of produce commodities specific to the eastern and western agricultural regions of the United States, 2). To determine the role of humidity in the deposition of dust on produce and the survival of foodborne pathogens in dust particulates, and 3). To test dust particulates from animal operations in Georgia and Arizona for the presence of biomarkers indicative of fecal contamination and the presence of enteric pathogens. This project will enhance our understanding of pathogen transport from feces into and through produce fields and will quantify the risk associated with contamination from dust under varying environmental and atmospheric conditions.

1. Evaluate the role of dust in transferring foodborne pathogens to the surfaces of produce commodities specific to the eastern and western agricultural regions of the United States. 

2. Understand the role of humidity in the deposition of dust on produce and the survival of foodborne pathogens in dust particulates. 

3. Test dust particulates from animal operations for the presence of biomarkers indicative of fecal contamination and the presence of enteric pathogens.

During this study, the survival and cross-contamination of produce by several foodborne bacterial pathogens were studied on dust from organic soils from two vastly different produce growing regions within the United States – Arizona and Georgia. These regions differ geographically, by growing season (winter in Arizona, summer in Georgia), by soil consistency and characteristics, and by average atmospheric conditions, particularly regarding relative humidity levels. Arizona is a dry / arid region, whereas Georgia is lush and quite humid. The types of produce grown in each region also vary greatly. 

The survival and transfer studies conducted with dust in a range of particle sizes on produce grown in Georgia (tomatoes, bell peppers, apples, and peaches) and in Arizona (spinach, romaine lettuce outer leaves, and romaine lettuce inner leaves) provided several insights regarding the potential for contamination of produce by dust. For instance, it was determined that the dust particle size may play a role in certain situations in how the dust is transferred to the surfaces of the produce and in whether it is trapped on roduce surfaces. The produce type is also a significant determinant on the amount of dust transfer that occurs. For example, produce with irregular surfaces such as some leafy greens may capture dust more readily under certain conditions and produce with surface structures such as the fine hairs (trichomes) on peaches are quite good at trapping dust particles (particularly larger dust particles). There were also differences identified between different species of bacteria and even between different strains of the same species. For instance, under certain conditions, Salmonella Newport survived less well in inoculated dust than Salmonella Typhimurium, but was still able to contaminate produce at levels comparable to S. Typhimurium. E. coli not only survived relatively poorly in inoculated dust, but also was transferred to the surfaces of fresh produce at proportionally lower concentrations than the two Salmonella strains. The relative humidity can also play a role in the survival and transfer of bacterial pathogens in dust to the surfaces of fresh produce. 

For the field sampling, numerous samples were positive for the fecal genetic markers (particularly for LA35 poultry, Rum2Bac ruminant, and GFD avian markers); however, the vast majority of these were below the limit of quantification (LOQ) for the qPCR assays. The poultry marker was detected in most samples, even when poultry operations were not nearby. This could be due to the use of chicken composts / fertilizers being used in nearby fields. Total coliforms (an indicator of fecal contamination) were detected in only a few samples and neither Salmonella nor E. coli were detected in any of the samples in either growing region. Heterotrophic plate count (HPC) bacteria were detected in nearly all samples and were found to have some correlations to other measurements of fecal contamination such as the presence of fecal bile salts and endotoxins (from the cell walls of Gram-negative bacteria). Thus, HPC bacteria could possibly be used in the future as an indicator for the potential for fecal contamination of dust from animal feeding operations. 

Although we could not determine an effect of proximity to animal feeding operations on the risk of contamination of dust by microbial pathogens, it is likely that a larger project with greater numbers of samples and locations is needed to conduct a quantitative microbial risk assessment that will determine the distance from such feeding operations that might be deemed safe for the production of fresh produce.