The Role of Pea (Pisum sativum) Seeds in Transmission of Entero-Aggregative Escherichia coli to Growing Plants

Abstract

Crop plants can become contaminated with human pathogenic bacteria in agro-production systems. Some of the transmission routes of human pathogens to growing plants are well explored such as water, manure and soil, whereas others are less explored such as seeds. Fenugreek seeds contaminated with the entero-hemorrhagic Escherichia coli O104:H4 were suspected to be the principle vectors for transmission of the pathogen to sprouts at the food-borne disease outbreak in Hamburg and surrounding area in 2011. In this study we raised the questions of whether cells of the entero-aggregative E. coli O104:H4 strain 55989 is capable of colonizing developing plants from seeds and if it would be possible that, via plant internalization, these cells can reach the developing embryonic tissue of the next generation of seeds. To address these questions, we followed the fate of strain 55989 and of two other E. coli strains from artificially contaminated seeds to growing plants, and from developing flower tissue to mature seeds upon proximate introductions to the plant reproductive organs. Escherichia coli strains differing in origin, adherence properties to epithelial cells, and virulence profile were used in our experimentation to relate eventual differences in seed and plant colonization to typical E. coli properties. Experiments were conducted under realistic growth circumstances in greenhouse and open field settings. Entero-aggregative E. coli strain 55989 and the two other E. coli strains were able to colonize the root compartment of pea plants from inoculated seeds. In roots and rhizosphere soil, the strains could persist until the senescent stage of plant growth, when seeds had ripened. Colonization of the above-soil parts was only temporary at the start of plant growth for all three E. coli strains and, therefore, the conclusion was drawn that translocation of E. coli cells via the vascular tissue of the stems to developing pea seeds seems unlikely under circumstances realistic for agricultural practices. Proximate introductions of cells of E. coli strains to developing flowers also did not result in internal seed contamination, indicating that internal seed contamination with E. coli is an unlikely event. The fact that all three E. coli strains showed stronger preference for the root-soil zones of growing pea plants than for the above soil plant compartments, in spite of their differences in clinical behaviour and origin, indicate that E. coli in general will colonize root compartments of crop plants in production systems.

Transmission of Escherichia coli from Manure to Root Zones of Field-Grown Lettuce and Leek Plants

“…Upon inoculation, E. coli O157:H7 was isolated from surface-sterilized spinach plants [13]. However, internal plant colonization only sporadically occurred, as was demonstrated for E. coli O157:H7 in spinach and lettuce plants [14] and for a Shigatoxin-producing E. coli (STEC) strain, an extendedspectrum β-lactamase-producing E. coli (ESBL) strain, and an enteroaggregative E. coli strain in pea plants [15]. Furthermore, E. coli was found to persist on seeds and shown to grow out with seedling growth [15,16].…”

“…However, internal plant colonization only sporadically occurred, as was demonstrated for E. coli O157:H7 in spinach and lettuce plants [14] and for a Shigatoxin-producing E. coli (STEC) strain, an extendedspectrum β-lactamase-producing E. coli (ESBL) strain, and an enteroaggregative E. coli strain in pea plants [15]. Furthermore, E. coli was found to persist on seeds and shown to grow out with seedling growth [15,16]. On lettuce seeds, E. coli O157:H7 was demonstrated to survive for two years and still able to grow out with the young plants emerging from the seeds [16] and three different E. coli strains were able to colonize young plants from seeds [15].…”

“…Factors related to leaf axis (adaxial or abaxial sides), age and surface topography as well as local environmental circumstances, such as nutrient availability and temperature, were shown to play decisive roles in plant colonization by E. coli [18]. The root zone of plants is the habitat that is commonly favored by E. coli, as demonstrated for cabbage [20], lamb's lettuce [19], lettuce [5,6,21,22] pea [15], radish [5] and grass plants [23]. Escherichia coli can accumulate on/inside roots and in rhizosphere soil (together named the 'plant root zone') when plants come into contact with contaminated soil.…”

“…Furthermore, E. coli was found to persist on seeds and shown to grow out with seedling growth [15,16]. On lettuce seeds, E. coli O157:H7 was demonstrated to survive for two years and still able to grow out with the young plants emerging from the seeds [16] and three different E. coli strains were able to colonize young plants from seeds [15]. Cells of E. coli also were shown to adhere rapidly to leaf surfaces, and once adhered, they showed a remarkable resistance to removal by disinfecting agents [17,18].…”

“…In this study, it was shown that strains without stx genes survived longer in bulk soil than the ones carrying these genes. However, the roles that stx genes played in E. coli survival in the environment seems controversial, as no difference in pea root colonization was observed with three distinct E. coli strains, of which one contained stx genes [15].…”