Abstract
The 3-Phenyllactic acid (PLA) produced by various lactic acid bacteria (LAB) possesses a broad spectrum of antimicrobial activity. In this study, the effect of PLA against Listeria innocua was studied with the aim to obtain additional information about its mechanism of action. The effect of pH on the antilisterial activity of PLA was investigated and a pH-dependent behavior, typical of weak acid, was detected. The antilisterial effect of PLA was firstly compared to that produced by lactic acid (LA) and than to that expressed by phenolic acids (gallic, caffeic, and ferulic acids) evaluating minimum inhibitory concentration (MIC), MBC, and survival kinetic parameters. PLA showed MIC values and death kinetic parameters significantly different from those exhibited by LA and by tested phenolic acids. In particular, the MIC value observed for PLA vs L. innocua resulted lower than that of the other preservative compounds studied herein, and consistent with the quantity generally produced by LAB. Moreover, the effect of PLA and phenolic acids on bacterial surface charge and loss of cellular content resulted different. The overall results highlighted strong differences in the antilisterial mechanism of action among PLA and other compounds such as LA and phenols. Specifically, it is possible to hypothesize that the antilisterial mechanism of action due to PLA is associated with the affinity to cell surface, which contributes to the cellular damage.
Highlights
In the last years, several bio-control strategies have been developed to improve the safety and to extend the shelf-life of various foods
minimum inhibitory concentration (MIC) was defined as the lowest concentration of Phenyllactic acid (PLA) at which bacteria failed to grow in liquid medium, but yet viable when 100 μL of the culture broth were plated on agar media
Cells suspended in MES buffer were added with lactic acid (LA) as follows: LA_MIC: LA was added to L. innocua suspension at the MIC concentration detected at pH 5.5; LA_PLA: LA was added at the same concentration of PLA used in the batch PLA_MIC to L. innocua suspension; the same experiment was carried out using GA, FA and CA at MIC and Minimum Bactericidal Concentration (MBC) concentration
Summary
Several bio-control strategies have been developed to improve the safety and to extend the shelf-life of various foods. The use of natural compounds, in particular antimicrobials produced by protective cultures, may represent an important tool in the food biopreservation, considering the consumer demand for safe and healthy foods, in terms of reduction in chemical additives and provision of beneficial effects (Reale et al, 2011; Succi et al, 2014) In this context, 3-phenyllactic acid [2-Hydroxy-3-Phenyl propionic acid (PLA)], an organic acid produced by several microorganisms used in the food industry, shows promising results. Ning et al (2017) supposed that PLA has a mode of action different to that of other organic acids, such as LA, and this fact is due to the PLA chemical structure, showing amphiphilic properties resulting from the hydrophobic group-benzene ring, and hydrophilic group-carboxy group For this reason, PLA could be able to interact with both lipids and proteins in the cell membrane, and to affect its permeability and integrity, with an antimicrobial mechanism similar to that highlighted in the case of other phenolic acids. Listeria innocua was used as a nonpathogenic surrogate for Listeria monocytogenes considering the high similarity between the two species (Tremonte et al, 2016b)
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