Survival of E. coli on Microgreens during Growth

Introduction: Microgreens, as yet, have not been associated with any food borne illness outbreaks.  Microgreens are recognized as being nutritional powerhouses and are gaining in popularity and filling the niche previously occupied by sprouts. However, they are still mainly grown on a very small scale and they do share some characteristics of sprouts; primarily youth. The vulnerability of plants to pathogen colonization seems to be greatest during and immediately following germination.

Purpose: This study was undertaken to investigate the persistence of enteric bacteria on or within seedlings.

Methods: Mizuna seeds inoculated with a 3 strain cocktail of Escherichia coli MW416, MW423, and MW425, strains known to have superior attachment to plant tissue, were grown in peat based growth medium for up to 14 days. Plant trays were covered during germination and uncovered and exposed to light on different days to determine the effect of light and its consequent effects on the metabolic processes of the microgreens on the E. coli survival. Seedlings (2 to 6 g) from each treatment group were harvested by cutting stems above the soil surface each sampling day and plated.

Results: E coli populations declined over each of three 2-week test periods. While light initially had a modest effect on E. coli population reduction, as treatments were exposed to light for more than one day, the rate of decline stabilized. Surface sterilized seedlings showed up to 3 log cfu E. coli cells/ g of plant tissue indicating that at least some internalization occurred. Scanning electron microscopy was used to observe the location of E. coli cells within the plant tissue.

Significance: his study is an important step towards learning what factors are involved in the die off of enteric pathogens frequently seen in plants and to determine whether the decline is sufficient to result in safe product.