Bacteria of tomatoes managed with well water and pond water: Impact of agricultural water sources on carposphere microbiota
Abstract
We know that contamination of crops by human pathogens can occur in agricultural settings but we still do not understand precisely which environmental sources represent the highest risks. Human pathogens maybe introduced by wind, worker hygiene, plant mediated factors, insects, water sources, or any combination of these factors. To safeguard against risks to consumers from agricultural waters, FSMA regulations for U.S. crop production require the use of water with an average of less than 126 CFU per 100 ml for applications that come in direct contact with a crop. Due to availability/scarcity however, water from other sources such as agricultural ponds is commonly used. To better understand risks that may be associated with the use of “surface”(often higher microbial load) water sources, we described the bacterial microbiota associated with an agricultu ral pond, an agricultural well and the corresponding microbiota of tomato carpospheres managed with each water source (also described as phyllosphere). 16S rRNA gene amplicons were used for bacterial profiling of waters and tomato surfaces at four time poi nts over a growing season. Microbial profiles differentiated surface and ground water samples throughout the season, however no significant influence on tomato fruit surfaces could be correlated to either water source. These results suggest that in certain cases, environmental pressures such as wind, dust or other airborne factors may have a more significant impact on the surface microbiology of field crops than irrigation or management water sources do.
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Introduction
Identifying the source of food borne outbreaks is one of the most important objectives for food safety research. Contamination of fresh produce can occur at any point in the farm to fork continuum but there is evidence in certain cases – that human pathogens become associated with agricultural commodities while in the agricultural environment(7).
Contamination in the field could result from wind pressures, water sources, soils, manures, plant microbiology, insects, worker hygiene or any combination of these factors. To better understand the influence that agricultural waters may have on themicrobiology of a crop, we designed the study presented here. The Food Safety Modernization Act (FSMA) for produce safety, requires the use of water with a geometric mean of 126 or less CFU of generic E.coliper 100 mL of water with a statistical threshold of 410 CFU or less of generic E.coli in 100 mL of water for applications that come in direct contact with a crop:http://www.fda.gov/Food/GuidanceRegulation/FSMA/ucm334114.htm. Given the fact that large volumes of water are needed for pesticide applications and irrigation, water demand cannot always be met with water that meets the recently mandated quality specifications. Growers routinely use water from farm ponds when other sources are no longer available. There is concern about the microbiological risks that may be introduced to a crop from the complex consortia of microbiology that is supported in waters exposed to wildlife excretions, anthropogenic pollutants, and sewage runoff – all of which have the potential to support a robust array of enteric pathogens. Groundwater sources (wells) are believed to support fewer enteric pathogens due to natural filtering mechanisms of soils and protection from wildlife pressures(11).There is of course, still ample opportunity for underground water sources to become contaminated due to depth and quality of wells, proximity to urban areas, animal husbandry, industrial pressures, seawater intrusion into aquifers and many other events. It makes sense to assume that cleaner water will result in a safer crop but few, if any studies have provided data that support this assumption. There is currently a data gap surrounding our understanding of food safety risks that may be associated with agricultural use of surface or reclaimed water sources. Studies conducted in California examined crops spray-irrigated with two types of treated wastewater compared to well water. The treated waters showed higher fecal coliform counts than the well water but no differences could be correlated to the quality of the resulting crop(14).
Drought pressures in agriculture pose significant concerns and highlight the need for the improved understanding of risks that may be associated with all types of available water sources. Water shortages will preclude long term access to low microbial count waters for certain agricultural areas and this situation will only become more dire in the years to come, so the improved understanding of food safety risks that correlate with use of surface or reclaimed waters is valuable to sustainable agriculture and public health. To characterize the risks that may be associated with the use of a “surface” water source (agricultural pond), compared to a ground water source (water from an agricultural well), we described the bacterial microbiota of the pond and the well and the corresponding microbiota of tomato surfaces managed with each water type.
Conclusion
Despite the aforementioned caveats, we demonstrated with confidence that the primary drivers of the bacterial microbiology observed in the tomato carposphere (phyllosphere) was not significantly influenced by water sou rces as diverse as well water and agricultural pond water. As we continue to evolve guidance recommendations and regulations for crop safety, it makes sense to continue monitoring microbial loads in agricultural waters but also perhaps to include addition al surveillance of the microbial loads associated with air pressures that come in contact with crops, as these may represent the primary drivers of carposphere surface microbiota.
