Sub-lethal exposure of Lacticaseibacillus paracasei to atmospheric nonthermal plasma alters its membrane and response to low pH

Abstract

Lactic acid bacteria have an important role in food production, as probiotics and producers of lactic acid. Due to the generation of oxygen and nitrogen-based reactive species and UV radiation, non-thermal plasma (NTP) was found to be effective in microbial inactivation and food processing. In this study, we investigated the effect of atmospheric NTP on Lacticaseibacillus paracasei NRRL B-4564 survival, membrane alternations, and the response of treated cells to acidic stress. Cell suspensions in water were subjected to different treatment time intervals using a custom-made plasma needle. Argon was used as a feed gas, with a flow of 0.5 slm, while the distance between the needle tip and suspension surface was 1.5 cm. Immediately after the treatment, the viable cell number was estimated by the pour plate method, while cell membrane alternations were studied by analyzing zeta potential and membrane permeability (Crystal Violet assay). To ascertain if sub-lethal NTP stress could influence L. paracasei survival in an acidic environment, NTP-treated cells were challenged by pH 2.5 for 3h. The results demonstrate that the negative surface potential of the bacterial membrane (-29.83±1.49 mV for untreated) was gradually shifted towards neutrality (-7.59±0.54 mV after 180 s) with prolonged treatment time. Increasing cell exposure to NTP resulted in higher membrane permeability, which was correlated with viable cell reduction. The cells exposed to shorter treatment time (30 and 60 s) kept viability and showed better survival in low pH compared to untreated cells, suggesting the application of NTP in probiotic food processing.