Fate of Shiga Toxin-producing Escherichia coli during Storage at Different Temperatures of Gamma Irradiated Spinach (Tetragonia expansa)

Introduction: In recent years, vegetables, including spinach, have been associated with many foodborne outbreaks. Among the pathogenic microorganisms that can be transmitted by these foods, Shiga toxin-producing Escherichia coli deserve attention. As the use of sanitizers does not significantly reduce the population of microorganisms present in vegetables, the application of more efficient techniques, such as irradiation, is necessary. 

Purpose: To evaluate the behavior of Shiga toxin-producing E. coli in spinach after exposition to gamma radiation, during storage at different temperatures.

Methods: Samples of spinach (Tetragonia expansa) were inoculated with a cocktail of three strains of Shiga toxin-producing E. coli O157:H7 (10⁵ – 10⁶ CFU/g) and exposed to 0.85 kGy. The samples were stored at 4°C and at abuse temperature (10°C) for up to 12 days. Population was enumerated on days 0, 3, 6, 9 and 12, using tryptone soya agar (TSA) with an overlay of sorbitol MacConkey agar plus cefixime and potassium tellurite. Plates were incubated at 37°C/24 h. 

Results: The applied dose (0.85 kGy) reduced the population of Shiga toxin-producing E. coli in approximately 6 log on day zero. Populations of E. coli remained below 1 log when samples were stored at 4°C and at 10°C for up to 12 days. 

Significance: The results showed that gamma radiation combined with storage at cold temperatures can reduce the risk posed by Shiga toxin-producing E. coli to consumers when it is contaminating minimally processed vegetables.