Cross-adaptation is one of the most important phenotypes in foodborne pathogens and poses a potential risk to food safety and human health. In this work, we found that pretreatment with acetic acid, citric acid, and lactic acid could induce subsequent heat tolerance development in S. aureus. ABSTRACT Staphylococcus aureus is a typical enterotoxin-producing bacterium that causes food poisoning. In the food industry, pasteurization is the most widely used technique for food decontamination. However, pre-exposure to an acidic environment might make bacteria more resistant to heat treatment, which could compromise the bactericidal effect of heat treatment and endanger food safety. In this work, the organic acid-induced cross-adaptation of S. aureus isolates to heat and the associated mechanisms were investigated. Cross-adaptation area analysis indicated that pre-exposure to organic acids induced cross-adaptation of S. aureus to heat in a strain-dependent manner. Compared with other strains, S. aureus strain J15 showed extremely high heat resistance after being stressed by acetic acid, citric acid, and lactic acid. S. aureus strains J19, J9, and J17 were found to be unable to develop cross-adaptation to heat with pre-exposure to acetic acid, citric acid, and lactic acid, respectively. Analysis of the phenotypic characteristics of the cell membrane demonstrated that the acid-heat-cross-adapted strain J15 retained cell membrane integrity and functions through enhanced Na+K+-ATPase and FoF1-ATPase activities. Cell membrane fatty acid analysis revealed that the ratio of anteiso to iso branched-chain fatty acids in the acid-heat-cross-adapted strain J15 decreased and the content of straight-chain fatty acids exhibited a 2.9 to 4.4% increase, contributing to the reduction in membrane fluidity. At the molecular level, fabH was overexpressed with preconditioning by organic acid, and its expression was further enhanced with subsequent heat exposure. Organic acids activated the Gr ✉ Corresponding author. 14 3 2023 11 2 e03832-22 e03832-22 14 4 2023 Copyright © 2023 Liao et al. This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license . ABSTRACT Staphylococcus aureus is a typical enterotoxin-producing bacterium that causes food poisoning. In the food industry, pasteurization is the most widely used technique for food decontamination. However, pre-exposure to an acidic environment might make bacteria more resistant to heat treatment, which could compromise the bactericidal effect of heat treatment and endanger food safety. In this work, the organic acid-induced cross-adaptation of S. Organic acids activated the GroESL system, which participated in the heat shock response of S. aureus to the subsequent heat stress. IMPORTANCE Cross-adaptation is one of the most important phenotypes in foodborne pathogens and poses a potential risk to food safety and human health. In this work, we found that pretreatment with acetic acid, citric acid, and lactic acid could induce subsequent heat tolerance development in S. aureus . Various S. aureus strains exhibited different acid-heat cross-adaptation areas. The acid-induced cross-adaptation to heat might be attributable to membrane integrity maintenance, stabilization of the charge equilibrium to achieve a normal internal pH, and membrane fluidity reduction achieved by decreasing the ratios of anteiso to iso fatty acids. The fabH gene, which is involved in fatty acid biosynthesis, and groES/groEL, which are related to heat shock response, contributed to the development of the acid-heat cross-adaptation phenomenon in S. aureus . The investigations of the stress cross-adaptation phenomenon in foodborne pathogens could help optimize food processing to better control S. aureus . KEYWORDS: acid-heat cross-adaptation, cell membrane, food safety, Staphylococcus aureus , stress response status released display-pdf yes is-olf no is-manuscript no is-preprint no is-journal-matter no is-scanned no is-retracted no Received 2022 Sep 21; Accepted 2023 Feb 14; Collection date 2023 Mar-Apr. INTRODUCTION Staphylococcus aureus is a common Gram-positive bacterium. It can survive in a pH range of 4.2 to 9.3 and a temperature range of 7°C to 48.5°C ( 1 ). Meat, poultry, eggs, milk and dairy The ingestion of food contaminated by staphylococcal enterotoxins results in food poisoning, with symptoms such as nausea, stomach cramps, diarrhea, and vomiting ( 3 ). Food contamination caused by S. aureus is a significant concern in public health worldwide ( 4 ). According to the European Food Safety Authority (EFSA), staphylococcal enterotoxins were listed as one of the top four bacterial toxins causing foodborne outbreaks (27.8% of the total) in the European Union in 2020 ( 5 ). In the United States, it is estimated that S. The widely used preservatives in the food industry include lactic acid, citric acid, sorbic acid, and benzoic acid ( 9 ). In the process of slaughtering, organic acids are usually sprayed on the surface of carcasses to prevent microbial contamination ( 10 , 11 ). With the occurrence of glycolysis in the meat matrix, pyruvate, lactic acid, and other acidic substances are produced, forming a weakly acidic environment ( 12 ). Under external stress, pathogenic bacteria can initiate a defensive response and develop stress resistance ( 13 , 14 ). A growing number of studies have found that in addition to a single stress response, initial exposure to one specific stress can confer resistance to subsequent similar or heterogeneous stress to bacteria ( 15 , 16 ). Under sublethal stress, bacteria activate the stress response to help repair damage and maintain cellular homeostasis, thus enhancing the tolerance of the bacteria to other adversities, known as cross-adaptation ( 15 , 17 ). It has been reported that traditional food processing and storage methods can induce cross-adaptation of S. aureus . Cebrián et al.

Weak organic acids are widely used as preservatives and disinfectants in the food industry. Despite their widespread use, the antimicrobial mode of action of organic acids is still not fully understood. This study investigated the effect of acetic acid on the cell membranes and cellular energy generation of four Salmonella strains. Using a nucleic acid/protein assay, it was established that acetic acid did not cause leakage of intracellular components from the strains. A scanning electron microscopy study further confirmed that membrane disruption was not the antimicrobial mode of action of acetic acid. Some elongated Salmonella cells observed in the micrographs indicated a possibility that acetic acid may inhibit DNA synthesis in the bacterial cells. Using an ATP assay, it was found that at a neutral pH, acetic acid caused cellular energy depletion with an ADP/ATP ratio in the range between 0.48 and 2.63 ( p <0.05) that was apparent for the four Salmonella strains. We suggest that this effect was probably due solely to the action of undissociated acid molecules. The antimicrobial effect of acetic acid was better under acidic conditions (ADP/ATP ratio of 5.56±1.27; p <0.05), where the role of both pH and undissociated acid molecules can act together. We concluded that the inhibitory effect of acetic acid is not solely attributable to acidic pH but also to undissociated acid molecules. This finding has implication for the use of acetic acid as an antimicrobial against Salmonella on food products, such as chicken meat, which can buffer its pH.

Bioactive peptides (BAPs), which are protein fragments defined by their distinct structural features and amino acid sequences, impart health benefits to foods enriched with them. These benefits include antioxidant, antihypertensive, antidiabetic, and immunomodulatory properties. BAPs have emerged as potential substitutes for synthetic molecules in various applications. Nevertheless, there is inadequate of information on the production of BAPs via novel fermented plant-based foods (FPFs), and many BAPs resources in FPFs have not been effectively explored. This review extensively discussed recent trends in FPFs, ranging from vinegar, cheese liquor, fermented rice, etc., from 2019 to 2024. We further discuss the beneficial health impacts of BAPs derived from FPFs, factors affecting the bioavailability of bioactive peptides, trends, and future perspectives in bioactive compounds from fermentation foods. This review will add to the existing body of knowledge and improve our understanding of the potential of BAPs derived from fermented plant-based foods.