Plant species-dependent transmission of Escherichia coli O157:H7 from the spermosphere to cotyledons and first leaves

Abstract

The colonization of six edible plant species: alfalfa, broccoli, coriander, lettuce, parsley and rocket, by the human pathogen Shigatoxigenic Escherichia coli was investigated following two modes of artificial inoculation of seeds, by soaking or watering. The frequency and extent of colonization of cotyledons depended on the mode of inoculation, with three, rapidly germinating species being successfully colonized after overnight soaking, but slower germinating species requiring prolonged exposure to bacteria by watering of the surrounding growth media. Separate analysis of the cotyledons and leaves from individual plants highlighted that successful colonization of the true leaves was also species dependent. For three species, failure of transfer, or lack of nutrients or suitable microhabitat on the leaf surface resulted in infrequent bacterial colonization. Colonization of leaves was lower and generally in proportion to that in cotyledons, if present. The potential risks associated with consumption of leafy produce are discussed.

FIGURE 1

Plants and seeds. (A) Plants of alfalfa, broccoli, coriander, lettuce parsley and rocket within plastic collars recorded immediately prior to harvest, indicating the approximate point of division (magenta line) between cotyledon (c) and leaf (l) samples. The diameter of each compost‐containing cavity is 5 cm to provide an estimate of scale. (B) Seeds of each species (scale bar = 1 cm)

FIGURE 2

Boxplot of median estimates of bacterial numbers recovered from (A) cotyledon, or (B) leaf expressed as log₁₀ MPN g⁻¹ FW with 25 and 75% quartile ranges and 95% confidence intervals for soaked and watered plant species. The number of plants analysed is shown below the x‐axis.

FIGURE 3

Relationship between leaf and cotyledon colonization (log₁₀ MPN g⁻¹ FW) for broccoli (circle), rocket (triangle) and parsley (square)