Citrulline Accumulation Mechanism of Pediococcus acidilactici and Weissella confusa in Soy Sauce and the Effects of Phenolic Compound on Citrulline Accumulation

Optimization of citrulline production from a Bacillus subtilis BH-01 isolated from raw buffalo milk

The main purpose of this study was to optimize the L-citrulline production process using Plackett-Burman and Box-Behnken designs. L-citrulline-producing bacterium BH-01 was isolated from raw buffalo milk. The isolate was tested for probiotic activities such as tolerance to simulated gastric and intestinal juices, antagonistic activity against six antibiotic-resistant bacteria, and temperature tolerance. L-citrulline production and arginine deiminase (ADI) activity were optimized using statistical designs. The bacterial isolate was molecularly identified as Bacillus subtilis strain AUMC B-498 (accession number PP574248.1). The strain exhibited resistance at pH 2.0 and bile salt 0.5% for a two-hour exposure period. It could inhibit the growth of Escherichia coli, Klebsiella pneumonia, Serratia sp., Staphylococcus aureus, methicillin-resistant Staphylococcus aureus (MRSA), and Streptococcus pneumoniae. From the results of statistical optimization, the Plackett-Burman design identified temperature, L-arginine, incubation period, and peptone as the most effective factors among the eight selected variables. Based on these, the Box-Behnken design was used to optimize the factors required to maximize citrulline production. The maximum L-citrulline was 632.5 µg/L, and ADI activity was 1.42 U/mL. Therefore, BH-01 isolated from Buffalo milk might be a promising candidate in food, biotechnological, and pharmaceutical applications due to its dual functionality for citrulline production and probiotic characteristics.

Intracellular and Extracellular Metabolic Response of the Lactic Acid Bacterium Weissella confusa Under Salt Stress

BACKGROUND

Weissella confusa is a member of the lactic acid bacterium group commonly found in many salt-fermented foods. Strains of W. confusa isolated from high-salinity environments have been shown to tolerate salt stress to some extent. However, the specific responses and mechanisms of W. confusa under salt stress are not fully understood.

METHODS

To study the effect of NaCl stress on W. confusa, growth performance and metabolite profiles of the strains were compared between a NaCl-free group and a 35% NaCl-treated group. Growth performance was assessed by measuring viable cell counts and examining the cells using scanning electron microscopy (SEM). Intracellular and extracellular metabolites were analyzed by non-targeted metabolomics based on liquid chromatography-mass spectrometry (LC-MS).

RESULTS

It was found that the viable cell count of W. confusa decreased with increasing salinity, and cells could survive even in saturated saline (35%) medium for 24 h. When exposed to 35% NaCl, W. confusa cells exhibited surface pores and protein leakage. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, 42 different metabolites were identified in the cells and 18 different metabolites in the culture medium. These different metabolites were mainly involved in amino acid metabolism, carbohydrate metabolism, and nucleotide metabolism. In addition, salt-exposed cells exhibited higher levels of intracellular ectoine and lactose, whose precursors, such as aspartate, L-2,4-diaminobutanoate, and galactinol, were reduced in the culture medium.

CONCLUSIONS

This study provides insight into the metabolic responses of W. confusa under salt stress, revealing its ability to maintain viability and alter metabolism in response to high NaCl concentrations. Key metabolites such as ectoine and lactose, as well as changes in amino acid and nucleotide metabolism, may contribute to its tolerance to salt. These findings may improve our understanding of the bacterium's survival mechanisms and have potential applications in food fermentation and biotechnology.

Microbiota and Mycobiota of Soy Sauce-Supplied Lactic Acid Bacteria Treated with High Pressure

Background: Ethyl carbamate (EC), a byproduct that naturally forms in fermented foods, can cause tumors and cell death and is classified as a probable human carcinogen (Group 2A). EC is naturally formed through the alcoholysis reaction between ethanol and carbamyl compounds. The major precursors and dominantly emerging stages of EC differ with disparate food types, including soy sauce. This work aimed to clarify the formation of EC and its influence factors throughout the soy sauce production process with or without high-pressure process (HPP) treatment. Methods: Tetragenococcus halophilus, Pediococcus acidilactici, Zygosaccharomyces rouxii, and Candida versatilis were added to soy sauce. The levels of citrulline and EC were measured, and a 16S and ITS assay investigated the microbiota. Results: L-citrulline production was found in each group after fermentation for one month. In addition, L-citrulline levels were generated the most in group D (500 MPa treated raw soy sauce with 12% saltwater and mixed fermentation bacteria, including T. halophilus,P. acidilactici,Z. rouxii, and C. versatilis) and group E (soy sauce fermentation with 12% saltwater without HPP treatment) compared to group F (soy sauce fermentation with 18% saltwater without HPP treatment). Conclusions: These results indicated that salt concentration and mixed fermentation bacteria (T. halophilus,P. acidilactici,Z. rouxii,C. versatilis) might not be major factors for L-citrulline production.