Transmission of Escherichia coli O157:H7 from Contaminated Manure and Irrigation Water to Lettuce Plant Tissue and Its Subsequent Internalization

Bacteria.

E. coli O157:H7 (ATCC 43895) was transformed using the pGFP plasmid (Clontech, Palo Alto, Calif.), encoding GFP. The GFP reporter system was selected for its utility in visualizing bacteria in biological systems and because cells can be studied nondestructively, without further processing or substrate addition (4). GFP-expressing E. coli O157:H7 (E. coli O157:H7/pGFP) was cultured at 37°C for 24 h in tryptic soy broth (Difco, Cockeysville, Md.) supplemented with 100 μg of ampicillin (Sigma, St. Louis, Mo.) ml⁻¹. The cells were harvested by centrifugation (3,500 × g; 10 min.) and resuspended in sterile distilled water (SDW). Inocula were prepared by serial dilution in SDW to achieve the desired cell concentrations. All experiments were conducted using E. coli O157:H7/pGFP.

Preparation of planting mixture.

Fresh cow manure (475 g) collected at the Rutgers University dairy barn was inoculated with a suspension of E. coli O157:H7/pGFP and vigorously mixed by hand. Manure collected from the farm for inclusion in research experiments is routinely screened for the presence of E. coli O157:H7 and is consistently negative. The manure was collected immediately following evacuation from the animal and was used in experiments within 48 h. The inoculated manure was then mixed with 4.5 kg of soil (sandy loam; pH 7.13) to give 5 kg of planting mixtures with final E. coli O157:H7/pGFP concentrations of approximately 10⁸, 10⁶, and 10⁴ CFU g⁻¹. The planting mixtures were dispensed into vegetable flats, and seeds of green ice lettuce (lot no. 52977; W. Atlee Burpee & Co., Warminster, Pa.) were planted. The flats were kept at 20°C, illuminated for 14 h using Agro-Lite lights (Philips Lighting Company, Somerset, N.J.), and watered daily.

Sampling procedures and detection by culture methods.

On days 3, 6, and 9 postplanting, seedlings were collected from each flat. The seedlings were cut from the root systems approximately 1 cm above the soil surface to minimize surface contamination of the edible portion of the plant through contact with the planting mixture. The seedlings were surface disinfected by being dipped in 80% ethanol for 5 s followed by immersion in 0.1% (wt/vol) HgCl₂ for either 5 or 10 min. The seedlings were washed twice in sterile water and allowed to air dry at room temperature in a laminar flow hood. Of the 16 seedlings treated for 5 min, 8 were placed directly on tryptic soy agar (TSA) plates supplemented with 100 μg of ampicillin (Amp) ml⁻¹. The remaining eight seedlings were sliced longitudinally to the base of the cotyledons, and the inner surfaces were placed on TSA-Amp plates. After incubation at 37°C for 1 h, the seedlings or sections of seedlings were removed and the plates were further incubated at 37°C overnight. The 16 seedlings immersed in HgCl₂ for 10 min were examined as described above. The plates were illuminated with UV light, and GFP-expressing colonies were enumerated. E. coli O157:H7/pGFP was recovered from the surfaces of sanitized seedlings grown in planting mixtures containing the highest levels of the target pathogen (Table 1). Based on culture, 10-min exposure of exterior surfaces of seedlings to HgCl₂ eliminated most culturable bacteria, suggesting that the target pathogen was located within the seedling tissue and therefore was protected from the action of the sanitizing agent. Under the experimental conditions outlined in the present study, E. coli O157:H7 maintained the plasmid encoding GFP.

Fluorescence microscopy and laser scanning confocal microscopy (LSCM).

Sections of seedlings were further examined by fluorescence microscopy on days 3, 6, and 9 postplanting. Samples were stained with propidium iodide (10 μg ml⁻¹; Molecular Probes, Eugene, Oreg.) for 30 min, washed twice in phosphate-buffered saline (Sigma), and then mounted on glass microscope slides and examined with an Olympus BH-2 epifluorescence microscope equipped with a 100× oil objective. Images were captured with a charge-coupled device camera (Photometrics, Tucson, Ariz.) and formatted using Adobe Photoshop. Cells of E. coli O157:H7/pGFP were visualized on the cotyledons and hypocotyl of the lettuce seedlings, regardless of the level of soil contamination or day of sampling (Fig. 1). The surfaces of the seedlings likely became contaminated as the seedlings grew and broke through the soil surface.

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FIG. 1.

Photomicrograph showing colonization of the surface of a 3-day-old lettuce seedling grown in soil containing 10⁶ CFU of E. coli O157:H7/pGFP g⁻¹. Cells appear as aggregates and attach preferentially to junction zones of lettuce cells. The arrow indicates foci of E. coli O157:H7 cells.

Based on fluorescence microscopy, seedlings found to contain surface-associated E. coli O157:H7/pGFP were further examined using LSCM to determine if the target pathogen was located below the tissue surface. Slides were examined using a Zeiss Axioplan 410 microscope equipped with an Ar-Kr laser source and a 100× oil objective. E. coli O157:H7/pGFP was excited using the 488-nm laser line. Propidium iodide-stained tissue was excited with the 568-nm laser line. Emissions were detected using a 515- to 540-nm band-pass filter for E. coli O157:H7/pGFP and a 590-nm long-pass filter for propidium iodide-stained lettuce tissue. Confocal images were captured and merged using the Zeiss LSM software. In some instances, target bacteria were not visualized on the surface of lettuce tissue but were found in high numbers at subsurface locations (Fig. 2). The target pathogen was visualized at depths of up to 45 μm below the tissue surface, suggesting migration to an internal location (Fig. 3). These results confirm the culture results of the present study (Table 1) and previous studies (13) indicating E. coli O157:H7 can localize within lettuce tissue.

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FIG. 2.

Representative LSCM optical thin section of a lettuce seedling contaminated with E. coli O157:H7/pGFP. E. coli O157:H7/pGFP cells can be found in the subsurface tissue of the seedling. E. coli O157:H7/pGFP cells appear green (arrows), while lettuce tissue appears red. Each successive image progresses 1 μm deeper into the lettuce seedling.

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FIG. 3.

LSCM photomicrograph of lettuce leaf showing cells of E. coli O157:H7/pGFP at an internal location 45 μm from the outer leaf surface. E. coli O157:H7/pGFP cells (arrow) were not randomly dispersed but rather formed a band of aggregates restricted to the intercellular space.

Effect of irrigation with contaminated water and manure slurry.

To determine whether direct surface contact with the edible portion of the plant is required for internal contamination, 25 green ice lettuce plants were grown in 15-cm-diameter plastic pots containing Pro-Mix BX (Premier Horticulture Inc., Red Hill, Pa.). The plants were fertilized weekly with Peter’s General Purpose 20-20-20 fertilizer (Grace Sierra Horticultural Products, Milpitas, Calif.) in the Rutgers University greenhouse. Mature plants (approximately 50 days old) were moved to our laboratory and bundled with twine to prevent the edible portion of the plant from touching the soil. E. coli O157:H7/pGFP was processed as described above and resuspended in SDW. The soil in each of 15 pots was irrigated with 200 ml of water containing 7.5 × 10⁷ CFU of E. coli O157:H7/pGFP ml⁻¹. The inoculum was applied carefully to prevent splashing of the inoculum onto the edible portion of the lettuce plant. Five plants were harvested on days 1, 3, and 5 postinoculation and processed as follows. The plants were cut 2 cm above the soil surface with a sterile scalpel; the entire edible portion of the plant was combined with 200 ml of SDW in a sterile polyethylene bag and homogenized for 2 min in a stomacher (Dynatech Laboratories, Alexandria, Va.). The liquid phase was removed, centrifuged (3,500 × g; 10 min.), resuspended in 1 ml of SDW, and plated onto the surface of a TSA-Amp plate. The plates were incubated at 37°C overnight, and GFP-expressing colonies were visualized under UV light.

Contamination of the edible portion of the lettuce plant through exposure of soil, and consequently the plant root system, to manure runoff was also examined. Manure slurry was prepared by the method of Calicioglu et al. (5) and inoculated to achieve a concentration of 1.25 × 10⁸ CFU of E. coli O157:H7/pGFP ml⁻¹. Inoculated slurry (200 ml) was applied to the soil of the 10 remaining lettuce plants. On days 1 and 3 postinoculation, five plants were processed as described above, and the presence of E. coli O157:H7/pGFP colonies was determined. The results indicate that E. coli O157:H7 is capable of entering the roots of mature lettuce plants and can be transported upward to locations within the edible portions of the plant (Table 2). Direct contact between the leaves and a contamination source is not required for the organism to become integrated into edible lettuce tissue.

Application of E. coli O157:H7-contaminated manure to the production field or irrigation with E. coli O157:H7-contaminated water may result in contamination of the crop in the field. Studies have indicated that E. coli can survive for extended periods in manure and water (7, 11). We have demonstrated that lettuce grown in soil containing contaminated manure or irrigated with contaminated water results in contamination of the edible portion of the lettuce plant. Moreover, the results suggest that edible portions of a plant can become contaminated without direct exposure to a pathogen but rather through transport of the pathogen into the plant by the root system. We recognize that the levels of E. coli O157:H7 used in this study are far greater than what may be found on an agricultural field; however, numbers of bacteria were used that could be readily detected by the assays used in the present study. Under natural conditions, even a low level of contamination could present a significant human health risk, since the infective dose of E. coli O157:H7 is less than 1,000 cells (1). Research suggests that surface sanitizing of lettuce is not an effect method to eliminate all E. coli O157:H7 cells (3, 14). The inaccessibility of a large number of organisms, as a consequence of their subsurface location, is perhaps the reason for the lack of effectiveness of surface-sanitizing treatments. The impacts of on-farm practices which may result in E. coli O157:H7 becoming associated with lettuce, or for that matter other crops, have not been sufficiently explored.