Comparison of Ultraviolet Light against Log and Stationary Phase Listeria monocytogenes

Introduction:   The ubiquitous nature of Listeria monocytogenes and its ability to grow at refrigerated temperature has made L. monocytogenes a significant threat to the safety of ready-to-eat (RTE) meat products. Contamination by L. monocytogenes in RTE meat primarily occurs during slicing and packaging as a post process re-contamination. A non-thermal, post-process decontamination method was applied to study the efficacy of UV light against L. monocytogenesat different exposure times and intensities.

Purpose: To evaluate the efficacy of ultraviolet light against log and stationary phases of Listeria monocytogenes in growth media.

Methods:   Listeria monocytogenes serotype 4a was cultured in Brain Heart Infusion (BHI) broth at 37°C. Cells were harvested during their log and stationary phase and 1.5 ml of the cell suspension (3 mm in depth) was taken in a 3 cm petriplate, subjected to UV radiation at low (3 – 4 mW/cm²) and high (7-8 mW/cm²) intensity for 0, 10, 30,50,70,90 and 110 s. Cells were recovered on Modified Oxford agar (MOX) and data was analyzed using ANOVA to find significant differences between the means at P < 0.05.

Results:   Overall, populations of L. monocytogenes were significantly reduced (P < 0.05) after 10 and 30 s of exposure and further significant (P < 0.05) reductions were observed at 50, 70, 90, and 110 s when compared to the 10 and 30 s exposure times irrespective of the growth phase and UV light intensities. Irrespective of the UV light intensity and exposure times, significantly higher (P < 0.05) reductions were observed in the log phase cells as compared to the stationary phase cells.

Significance: Significant reduction in the log phase of L. monocytogenes populations as compared to the stationary phase are of importance for processors as it provides information that is helpful to design an effective UV light intervention system.