Images obtained by fluorescence microscopy technique for monitoring diffusion of PI molecules into pressure-treated Listeria monocytogenes cells

The effect of environmental stresses on microorganisms is well studied and cellular response to stresses such as heat, cold, acids, and salts was extensively discussed. Although high pressure processing (HPP) as a preservation method is becoming more popular in the food industry, the characteristics of the cellular damage caused by high pressure are unclear, and the microbial response to this stress is not well explored yet. We exposed the pathogen Listeria monocytogenes to HPP (400 MPa, 8 min, 8°C). Using a common staining technique with propidium iodide (PI) combined with high-frequency fluorescence microscopy, we monitored the diffusion rate of PI molecules into hundreds of bacterial cells at 0, 1, 2, 3, and 4 days after pressurization. We found that the diffusion rate of PI into the cells was reduced over four consecutive days after the exposure to high pressure, indicating a temporal repair of pores in the membrane caused by HPP.