Sub-lethal injury in foodborne microorganisms: An experimental comparison between ultrasound and traditional inactivation techniques

After applying chemical or physical treatments, various effects could occur on foodborne microorganisms, as the microbiota could survive, or be inactivated; however, surviving cells could experience a sub-lethal injury, which can strongly affect the microbiological dynamics during storage. In this research, the effects of ultrasound (US, power at 60–80 % for 4–8 min), heat-treatments (up to 65 °C for 8 min), thymol (up to 900 mg/L), trans-2-hexenal (50–150 mg/L), and sodium benzoate (up to 450 mg/L) were tested towards Saccharomyces cerevisiae, Pseudomonas fluorescens, and Levilactobacillus brevis assessing both cell inactivation and the existence of a sub-lethal injury on surviving cells, as well as the release of cell components for injured microorganisms.

Ultrasound treatments showed microorganism-dependent effects: S. cerevisiae exhibited 100 % sub-lethal injury only under 60 % power for 8 min, while 80 % power led to full inactivation. P. fluorescens showed progressive inactivation with increasing ultrasound intensity, reaching 100 % injury at 80 % power for 8 min. Lactobacillus brevis displayed the highest resistance, with no significant injury recovered. Among antimicrobial compounds, trans-2-hexenal showed the strongest activity, inducing over 80 % sub-lethal injury in S. cerevisiae and complete injury in P. fluorescens after 2 days of exposure. Prolonged exposure (6 days) to trans-2-hexenal and thymol caused complete inactivation of L. brevis.

Analysis of cellular component release indicated that only antimicrobial treatments led to significant membrane damage in bacteria.