Inactivation kinetics and virulence potential of Salmonella Typhimurium and Listeria monocytogenes treated by combined high pressure and nisin

Combined impact of high-pressure processing and slightly acidic electrolysed water on Listeria monocytogenes proteomes

Slightly acidic electrolysed water (SAEW) and high-pressure processing (HPP) are well-established non-thermal preservation technologies. This study investigated the deactivation mechanisms of Listeria monocytogenes by label-free quantitative proteomics analysis. Samples were treated through HPP (300 MPa for 3 min), SAEW (20 ppm available chlorine concentration), and their combinations. The KEGG pathway analysis found SAEW + HPP induced differentially expressed proteins (DEPs) associated to biofunctions of ribosomes, secondary metabolite biosynthesis, microbial metabolism in diverse environments, carbon metabolism, and biosynthesis of amino acid and aminoacyl-transfer RNA. The results showed these non-thermal treatments were able to induce the shifting of ribosome biogenesis to initiate translation in L. monocytogenes. During protein translation, the initiation stage was upregulated. However, subsequent elongation, termination, and recycling of used ribosomes were retarded. Comparing various treatments, the combination of hurdles showed greater deactivation of L. monocytogenes than any single one. The approaches developed in this study provided crucial information for minimally processing in the food industries on the application of foodborne listeriosis prevention.

Sensitivity of Planktonic Cells of Staphylococcus aureus to Elevated Hydrostatic Pressure as Affected by Mild Heat, Carvacrol, Nisin, and Caprylic Acid

Current study investigated effects of elevated hydrostatic pressure exposure in the presence of mild heat and natural antimicrobials against Staphylococcus aureus. Hydrostatic pressure of 350 to 550 MPa with nisin (5000 IU/mL), carvacrol, or caprylic acid (0.5% v/v) were applied for the reduction in four-strain mixture of S. aureus in HEPES buffer at 4 and 40 °C for up to 7 min. Results were statistically analyzed by ANOVA and D-values were additionally calculated using best-fitted linear model. Prior to exposure to treatments at 4 °C, counts of the pathogen were 7.95 ± 0.4 log CFU/mL and were reduced (p < 0.05) to 6.44 ± 0.3 log CFU/mL after 7 min of treatment at 450 MPa. D-value associated with this treatment was 5.34 min (R² = 0.72). At 40 °C, counts were 8.21 ± 0.7 and 5.77 ± 0.3 log CFU/mL before and after the 7-min treatments, respectively. D-value associated with 40 °C treatment was 3.30 min (R² = 0.62). Application of the antimicrobials provided additional pathogen reduction augmentation for treatments < 5 min. The results of the current study could be incorporated for meeting regulatory requirements such as Food Code, HACCP, and Preventive Control for Human Food of Food Safety Modernization Act for assuring microbiological safety of products against this prevalent pathogen of public health concern.

Augmenting the Pressure-Based Pasteurization of Listeria monocytogenes by Synergism with Nisin and Mild Heat

The current study investigated Listeria monocytogenes inactivation using mild heat with elevated hydrostatic pressure and nisin under buffered condition. A four-strain pathogen mixture was exposed to 0 (control) and up to 9 min of (1) 4 °C elevated pressure; (2) 4 °C elevated pressure and nisin; (3) 4 °C nisin; (4) heat at 40 °C; (5) 40 °C elevated pressure; (6) 40 °C elevated pressure and nisin; and (7) 40 °C nisin. Elevated hydrostatic pressure at 400 MPa (Hub880 Explorer, Pressure BioScience Inc., Easton, MA, USA) and nisin concentration of 5000 IU/mL were used in the trials. Analyses of variance were conducted, followed by Dunnett's- and Tukey-adjusted means separations. Under conditions of these experiments, nisin augmented (p < 0.05) decontamination efficacy of 40 °C heat and elevated hydrostatic pressure treatments, particularly at treatment interval of 3 min. This synergism with nisin faded away (p ≥ 0.05) as the treatment time for thermal, high-pressure, and thermal-assisted pressure processing increased. The results of our study, thus, exhibit that practitioners and stakeholders of pressure-based technologies could benefit from synergism of mild heat and nisin for short-term, high-pressure pasteurization treatments to achieve microbial safety and economic feasibility comparable to traditional heat-treated products.

Inactivation of Listeria monocytogenes, Staphylococcus aureus, and Salmonella enterica under High Hydrostatic Pressure: A Quantitative Analysis of Existing Literature Data

High hydrostatic pressure processing (HPP) is a mild preservation technique, and its use for processing foods has been widely documented in the literature. However, very few quantitative synthesis studies have been conducted to gather and analyze bacterial inactivation data to identify the mechanisms of HPP-induced bacterial inactivation. The purpose of this study was to conduct a quantitative analysis of three-decimal reduction times (t_3δ) from a large set of existing studies to determine the main influencing factors of HPP-induced inactivation of three foodborne pathogens (Listeria monocytogenes, Staphylococcus aureus, and Salmonella enterica) in various foods. Inactivation kinetics data sets from 1995 to 2017 were selected, and t_3δ values were first estimated by using the nonlinear Weibull model. Bayesian inference was then used within a metaregression analysis to build and test several models and submodels. The best model (lowest error and most parsimonious) was a hierarchical mixed-effects model including pressure intensity, temperature, study, pH, species, and strain as explicative variables and significant factors. Values for t_3δ and Z_P associated with inactivation under HPP were estimated for each bacterial pathogen, with their associated variability. Interstudy variability explained most of the variability in t_3δ values. Strain variability was also important and exceeded interstudy variability for S. aureus, which prevented the development of an overall model for this pathogen. Meta-analysis is not often used in food microbiology but was a valuable quantitative tool for modeling inactivation of L. monocytogenes and Salmonella in response to HPP treatment. Results of this study could be useful for refining quantitative assessment of the effects of HPP on vegetative foodborne pathogens or for more precisely designing costly and labor-intensive experiments with foodborne pathogens.

Insights into the Mode of Action of the Sactibiotic Thuricin CD

Thuricin CD is a two-component bacteriocin, consisting of the peptides Trnα and Trnβ, and belongs to the newly designated sactibiotic subclass of bacteriocins. While it is clear from studies conducted thus far that it is a narrow-spectrum bacteriocin, requiring the synergistic activity of the two peptides, the precise mechanism of action of thuricin CD has not been elucidated. This study used a combination of flow cytometry and traditional culture-dependent assays to ascertain the effects of the thuricin CD peptides on the morphology, physiology and viability of sensitive Bacillus firmus DPC6349 cells. We show that both Trnα and Trnβ are membrane-acting and cause a collapse of the membrane potential, which could not be reversed even under membrane-repolarizing conditions. Furthermore, the depolarizing action of thuricin CD is accompanied by reductions in cell size and granularity, producing a pattern of physiological alterations in DPC6349 cells similar to those triggered by the pore-forming single-component bacteriocin Nisin A, and two-component lacticin 3147. Taken together, these results lead us to postulate that the lytic activity of thuricin CD involves the insertion of thuricin CD peptides into the membrane of target cells leading to permeabilization due to pore formation and consequent flux of ions across the membrane, resulting in membrane depolarization and eventual cell death.

Gene expression profiling of a nisin-sensitive Listeria monocytogenes Scott A ctsR deletion mutant

Listeria monocytogenes is a food-borne pathogen of significant threat to public health. Nisin is the only bacteriocin that can be used as a food preservative. Due to its antimicrobial activity, it can be used to control L. monocytogenes in food; however, the antimicrobial mechanism of nisin activity against L. monocytogenes is not fully understood. The CtsR (class III stress gene repressor) protein negatively regulates the expression of class III heat shock genes. A spontaneous pressure-tolerant ctsR deletion mutant that showed increased sensitivity to nisin has been identified. Microarray technology was used to monitor the gene expression profiles of the ctsR mutant under treatments with nisin. Compared to the nisin-treated wild type, 113 genes were up-regulated (>2-fold increase) in the ctsR deletion mutant whereas four genes were down-regulated (<-2-fold decrease). The up-regulated genes included genes that encode for ribosomal proteins, membrane proteins, cold-shock domain proteins, translation initiation and elongation factors, cell division, an ATP-dependent ClpC protease, a putative accessory gene regulator protein D, transport and binding proteins, a beta-glucoside-specific phosphotransferase system IIABC component, as well as hypothetical proteins. The down-regulated genes consisted of genes that encode for virulence, a transcriptional regulator, a stress protein, and a hypothetical protein. The gene expression changes determined by microarray assays were confirmed by quantitative real-time PCR analyses. Moreover, an in-frame deletion mutant for one of the induced genes (LMOf2365_1877) was constructed in the wild-type L. monocytogenes F2365 background. ΔLMOf2365_1877 had increased nisin sensitivity compared to the wild-type strain. This study enhances our understanding of how nisin interacts with the ctsR gene product in L. monocytogenes and may contribute to the understanding of the antibacterial mechanisms of nisin.