In a recent study published in the journal Food Microbiology, researchers summarized the evidence on interactions of enteric bacterial pathogens with ready-to-eat (RTE) fruits and vegetables.
Fruit and vegetable consumption is associated with diverse health benefits and has substantially increased from 1960 to 2019, along with a parallel increase in foodborne illnesses. The agri-food industry has risks of introducing foodborne pathogens. Bacteria, fungi, viruses, parasites, and mycotoxins cause foodborne illnesses.
Review: From field to plate: How do bacterial enteric pathogens interact with ready-to-eat fruit and vegetables, causing disease outbreaks? Image Credit: Ezume Images / Shutterstock
Norovirus was the primary contaminant in fruits and vegetables during foodborne illness outbreaks in the European Union (EU) and the United States (US) during 2004-12, followed by bacterial pathogens. Specifically, three bacterial pathogens, Listeria monocytogenes, Escherichia coli, and Salmonella enterica, were responsible for 82% of hospitalizations and deaths due to foodborne illnesses in the US during 2009-15. The present study discussed how enteric bacterial pathogens interact with RTE vegetables and fruits, causing disease outbreaks.
Enteric bacterial pathogens
Food surveillance in the EU observed that approximately 1% of RTE fruits and vegetables were Salmonella-positive. Sprouted vegetables, tomatoes, cucumbers, melon/cantaloupe, and papaya have been reported as common vectors for Salmonella spp. Although E. coli is non-pathogenic and forms part of the commensal flora of mammals, some strains of E. coli cause urinary tract infections (UTIs), meningitis, diarrhea, and sepsis in humans.
Diarrheagenic E. coli is classified into seven pathotypes that differ by the presence of specific virulence factors and by somatic, flagellar, and capsular surface antigens. The Shiga toxin-producing E. coli (STEC) was the pathotype most associated with foodborne illness outbreaks in the US, accounting for about 92% of cases from 1998 to 2013. Although L. monocytogenes has caused fewer outbreaks than E. coli or Salmonella, listeriosis has the highest fatality rate of the three pathogens.
Contamination routes
Various contamination sources in the crop cycle that allow bacteria to establish and survive/multiply under favorable conditions have been identified. Soil is one of the primary sources of contamination, mainly if the sites were previously used for waste disposal or animal rearing. Moreover, L. monocytogenes is ubiquitous in the environment and frequently isolated from soil.
Extreme weather events, including dust storms and flooding, could cause foodborne illnesses. Besides, two mechanisms of seed contamination have been reported – 1) germinating seeds could attract enteric pathogens in soil, and 2) sowing of pre-contaminated seeds. Pathogens can spread to the edible portions once (contaminated) seeds germinate.
Irrigation water is another known source of contamination. Lake/river water may introduce enteric pathogens through contamination due to sewage, animal feces, or soil. Multiple studies have detected enteric pathogens in crops irrigated with contaminated water. Animals serve as a source of contamination through their fecal matter or as vectors of various pathogens.
Salmonella, L. monocytogenes, and E. coli have been detected in livestock. Contamination is also possible during post-harvest operations, such as preparation, packaging, and storage, if not controlled according to good manufacturing practices. Leaf damage post-harvest can alter the phyllosphere environment, providing sites for pathogen adhesion.
Bacterial interactions with plants
Enteric pathogens do not form part of the leaf phyllosphere. Plant surfaces are stressful for enteric pathogens as they are not rich in nutrients compared to their (warm-blooded) hosts. Besides, the microorganisms face fluctuations in wind, temperature, rainfall, and solar radiation. In general, bacterial colonization of leaves – attachment of bacteria, multiplication and formation of aggregates, and internalization through pores.
Bacteria attach to the leaf surface through flagella, fimbriae, and pili. Studies have highlighted the potential role of flagella in adhesion to fresh produce. Deleting the primary subunit of the flagellum has been shown to reduce the adhesive capability of E. coli clones. Bacteria-secreted cellulose acts as the constituent of the biofilm matrix and could be crucial during initial attachment to plants.
Studies have demonstrated the critical role of the cellulose synthase complex in Salmonella to attach to fresh produce; however, it may not be as crucial for E. coli as the deletion of the catalytic subunit failed to impair the attachment of STEC to spinach. In L. monocytogenes, cellulose binding may be essential for attachment to plant matrices, as deleting a putative cellulose binding protein reduced attachment to lettuce, cantaloupe, and baby spinach.
Survival of the bacteria (on the plant surface) after adhesion is a key determinant of their ability to cause foodborne illness. The biofilms provide an adaptive strategy to persist on plants and resist disinfectants. Numerous studies have described that bacterial pathogens can survive on leaves for several weeks to months. A few studies have reported the role of type 3 secretion system (T3SS) in Salmonella colonization of Arabidopsis.
Specifically, deficiency of a T3SS effector protein in Salmonella mutants reduced growth on leaves. Internalizing bacteria into plant tissue through surface pores helps them evade disinfection, which may be preceded by stomatal colonization. Salmonella, L. monocytogenes, and E. coli have been reported to colonize around stomatal pores. Although there is extensive research on genetic components mediating the internalization of Salmonella and E. coli, less is known about L. monocytogenes.
Plant responses to pathogen presence
Although plants have long been regarded as passive transmission vectors, increasing evidence suggests they may recognize enteric pathogens. Plants have an immune system for pathogen detection and restriction by recognizing surface molecules, such as pathogen-associated molecular patterns (PAMPs).
Interactions between PAMPs and plant cellular pattern recognition receptors activate a downstream signaling cascade, conferring resistance. This signaling cascade, called pathogen-triggered immunity (PTI), involves downstream processes such as reactive oxygen species (ROS) production, elevated pathogenesis-related gene expression, and activation of plant defense signaling pathways, among others.
Although research has focused on plant pathogens, enteric pathogens have been increasingly studied. Flagellin is a well-reported PAMP in both plant and enteric pathogens. E. coli and S. typhimurium have been shown to activate flagella-mediated PTI in Arabidopsis thaliana. Notably, immune responses to flagellin may be species-specific, as exposure to epitopes from E. coli induced ROS burst in tomato but not in Arabidopsis.
Further, A. thaliana is unresponsive to L. monocytogenes flagella. Stomatal closure occurs following pathogen recognition to prevent entry and pathogenesis. Notwithstanding, some plant pathogens can inhibit stomatal closure. Likewise, enteric pathogens have evolved mechanisms to overcome stomatal closure.
Concluding remarks
Taken together, increasing crop yields is necessary to ensure sufficient food for the increasing population, as the United Nations estimates the global population to exceed 10 billion by 2100. Reducing foodborne illnesses and product waste can help achieve food security. Moreover, the risk of crop contamination by enteric pathogens increases as urban populations extend into the countryside. As such, understanding the interactions between plants and microbes, aimed at reducing adhesion and colonization throughout supply, could help mitigate this excess risk.
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