By examining how researchers, experts, and officials frame and design their projects and set up responses to problems, anthropology can open up new spaces for insight into the complexities of environmental politics, and the idea of islands by design helps me to do that in the purposefully particular place that is The Bahamas today.Bagged greens in the market are often labeled “pre-washed,” “triple-washed,” or “ready-to-eat,” and look shiny and clean. But are they really “clean” of harmful microbes? We cannot be so sure. Food safety has been threatened by contamination with human pathogens including bacteria, viruses, and parasites. Between 2000 and 2008, norovirus and Salmonella spp. contributed to 58 and 11% of forborne illnesses, respectively in the United States . In those same years, non-typhoidal Salmonella alone was ranked as the topmost bacterial pathogen contributing to hospitalizations and deaths . In 2007, 235 outbreaks were associated with a single food commodity; out of which 17% was associated with poultry, 16% with beef, and 14% with leafy vegetables that also accounted for the most episodes of illnesses . Apart from the direct effects on human health, enormous economic losses are incurred by contaminated food products recalls. The 8-day recall of spinach in 2006 cost $350 million to the US economy .
It should be realized that this is not the loss of one individual, but several growers, workers,grow strawberry in containers and distributors. This is a common scenario for any multistate food borne outbreak. Additionally, the skepticism of the general public toward consumption of a particular food product can lead to deficiencies of an important food source from the diet.Economic analysis shows that money spent on prevention of food borne outbreak by producers is much less than the cost incurred after the outbreak . Contamination of plants can occur at any step of food chain while the food travels from farm to table. Contaminated irrigation water, farm workers with limited means of proper sanitation, and fecal contamination in the farm by animals can expose plants to human pathogens before harvest of the edible parts . After harvest, contamination can occur during unclean modes of transportation, processing, and bagging . Mechanical damage during transport can dramatically increase the population of human pathogens surviving on the surface of edible plants . Control measures to decrease pathogen load on plant surfaces have been defined by the Food Safety Modernization Act and Hazard Analysis and Critical Control Point system . Using chlorine for post-harvest crop handling has been approved by US Department of Agriculture under the National Organic Program. However, some studies indicated that internalized human pathogens escape sanitization . Thus, understanding the biology of human pathogen-plant interactions is now crucial to prevent pathogen colonization of and survival in/on plants, and to incorporate additional, complementing measures to control food borne outbreaks.
We reasoned that as plants are recognized vectors for human pathogens, enhancing the plant immune system against them creates a unique opportunity to disrupt the pathogen cycle. In this cross-kingdom interaction, the physiology of both partners contribute to the outcome of the interactions . Bacterial factors important for interaction with plants have been discussed in recent, comprehensive reviews . Plant factors contributing to bacterial contamination is much less studied and discussed. In this review, we highlight current knowledge on plants as vectors for human pathogens, the molecular mechanisms of plant responses to human bacterial pathogens, and discuss common themes of plant defenses induced by phytopathogens and human pathogens. We have focused on human bacterial pathogens that are not recognized plant pathogens such as Salmonella enterica and Escherichia coli , but yet are major threats to food safety and human health.The leaf environment has long been considered to be a hostile environment for bacteria. The leaf surface is exposed to rapidly fluctuating temperature and relative humidity, UV radiation, fluctuating availability of moisture in the form of rain or dew, lack of nutrients, and hydrophobicity . Such extreme fluctuations, for example within a single day, are certainly not experienced by pathogens in animal and human gut. Thus, it is tempting to speculate that animal pathogens may not even be able to survive and grow in an environment as dynamic as the leaf surface. However, the high incidence of human pathogens such as S. enterica and E. coli O157:H7 on fresh produce, sprouts, vegetables, leading to foodborne illness outbreaks indicate a certain level of human pathogen fitness in/on the leaf.
The plant surface presents a barrier to bacterial invaders by the presence of wax, cuticle, cell wall, trichomes, and stomata. All except stomata, present a passive defense system to prevent internalization of bacteria. Nonetheless, several bacteria are able to survive on and penetrate within the plant interior. The surface of just one leaf is a very large habitat for any bacteria. The architecture of the leaf by itself is not uniform and provides areas of different environmental conditions. There are bulges and troughs formed by veins, leaf hair or trichomes, stomata, and hydathodes that form microsites for bacterial survival with increased water and nutrient availability, as well as temperature and UV radiation protection . Indeed, distinct microcolonies or aggregates of S. enterica were found on cilantro leaf surfaces in the vein region In addition, preference to the abaxial side of lettuce leaf by S. enterica may be is an important strategy for UV avoidance . Conversion of cells to viable but non-culturable state in E. coli O157:H7 on lettuce leaves may also be a strategy to escape harsh environmental conditions . Hence, localization to favorable microsites,hydroponic nft channel avoidance of harsh environments, and survival by aggregation or conversion to non-culturable state may allow these human pathogens to survive and at times multiply to great extent on the leaf surface. As stomata are abundant natural pores in the plant epidermis which serve as entrance points for bacteria to colonize the leaf interior , several studies addressed the question as to whether human bacterial pathogens could internalize leaves through stomata. Populations of E. coli O157:H7 and S. enterica SL1344 in the Arabidopsis leaf apoplast can be as large as four logs per cm2 of leaf after surface-inoculation under 60% relative humidity suggesting that these bacteria can and access the apoplast of intact leaves. Several microscopy studies indicated association of pathogens on or near guard cells. For instance, S. enterica serovar Typhimurium SL1344 was shown to internalize arugula and iceberg lettuce through stomata and bacterial cells were located in the sub-stomatal space . However, no internalization of SL1344 was observed into parsley where most cells were found on the leaf surface even though stomata were partially open . Cells of S. enterica serovar Typhimurium MAE110 , enteroaggregative E. coli , and E. coli O157:H7 were found to be associated with stomata in tomato, arugula leaves, and baby spinach leaves, respectively. In the stem E. coli O157:H7 and Salmonella serovar Typhimurium were found to be associated with the hypocotyl and the stem tissues including epidermis, cortex, vascular bundles, and pith when seedlings were germinated from contaminated seeds . The plant rhizosphere is also a complex habitat for microorganisms with different life styles including plant beneficial symbionts and human pathogens. Nutritionally rich root exudate has been documented to attract S. enterica to lettuce roots .
Although bacteria cannot directly penetrate through root cells, sites at the lateral root emergence and root cracks provide ports of entry for S. enterica and E. coli O157:H7 into root tissues , and in some instances between the epidermal cells . High colonization of S. enterica has been observed in the root-shoot transition area . Once internalized both bacterial pathogens have been found in the intercellular space of the root outer cortex of Medicago truncatula . Salmonella enterica was found in the parenchyma, endodermis, pericycle, and vascular system of lettuce roots and in the inner root cortex of barley . A detailed study on the localization of E. coli O157:H7 in live root tissue demonstrated that this bacterium can colonize the plant cell wall, apoplast, and cytoplasm . Intracellular localization of E. coli O157:H7 seems to be a rare event as most of the microscopy-based studies show bacterial cells in the intercellular space only. Bacterial translocation from roots to the phyllosphere may be by migration on the plant surface in a flagellum-dependent manner or presumably through the vasculature . The mechanism for internal movement of enteric bacterial cells from the root cortex to the root vasculature through the endodermis and casparian strips and movement from the roots to the phyllosphere through the vascular system is yet to be demonstrated. Several outbreaks of S. enterica have also been associated with fruits, especially tomatoes. Salmonella enterica is unlikely to survive on surface of intact fruits raising the question: what are the routes for human pathogenic bacteria penetration into fruits? It has been suggested that S. enterica can move from inoculated leaves , stems, and flowers to tomato fruits. However, the rate of internal contamination of fruits was low when leaves were surface-infected with S. enterica . The phloem has been suggested as the route of movement of bacteria to non-inoculated parts of the plant as bacterial cells were detected in this tissue by microscopy . Figure 1 depicts the observed phyllosphere and rhizosphere niches colonized by bacteria in/on intact plants and probable sources of contamination.Plants possess a complex innate immune system to ward off microbial invaders . Plants are able to mount a generalized step-one response that is triggered by modi- fied/degraded plant products or conserved pathogen molecules. These molecules are known as damage or pathogen associated molecular patterns . In many cases, conserved PAMPs are components of cell walls and surface structures such as flagellin, lipopolysaccharides, and chitin .PAMPs are recognized by a diverse set of plant extracellular receptors called pattern-recognition receptors that pass intracellular signals launching an army of defense molecules to stop the invasion of the pathogens. This branch of the immune system known as pathogen-triggered immunity is the first line of active defense against infection. Human pathogen on plants is an emerging field that only recently has caught the attention of plant biologists and phytopathologists. A few studies have been reported in the last 5–10 years, which focused on the most well studied PAMPs, flagellin and lipopolysaccharide , in the interaction of human pathogens with plants. Table 1 lists the plants, bacterial strains, and method details for such studies.Flagellin, the structural component of flagellum in bacteria, is involved in bacterial attachment and motility on the plant , is recognized by plant through the FLS2 receptor , and induces plant defenses . Similar to the well-studied PTI elicitor flg22 , the flg22 epitope of S. enterica serovar Typhimurium 14028 is also an effective PAMP and elicitor of downstream immune responses in Arabidopsis , tobacco, and tomato plants . Flagellum-deficient mutants of S. enterica serovar Typhimurium 14028 are better colonizers of wheat, alfalfa, and Arabidopsis roots as compared to the wild type bacterium further suggesting that the Salmonella flagellum induces plant defenses that may restrict bacterial colonization of several plant organs. However, the Salmonella flflg22 peptide is not the only PAMP for elicitation of plant immune response as flfls2 mutant of Arabidopsis still shows a low level of PTI activation in response to this PAMP . Purified flagellin or derived epitopes of E. coli O157:H7 has not been used to induce plant defenses. However, flagellum-deficient mutant of this strain does not activate the SA-dependent BGL2 gene promoter as much as the wild type strain and shows larger population in Arabidopsis than the wild type strain further suggesting that surface structures in the bacterial cell are perceived by plants. The differences in responses observed could be attributed to the presence of other microbial signatures eliciting plant defense. Variations in plant response to S. enterica flagellin could be owed to host-strain specificity as well. Although flagellin sequences from S. enterica strains and other bacteria are highly conserved, even a minor change of five amino acids in the flflg22 epitope leads to reduced activation of PTI in Arabidopsis, tobacco, and tomato plants .