Identification and Validation of Garlic (Allium sativum) Metabolites as Quorum Sensing Inhibitors of Bacillus cereus Targeting the PlcR Receptor: An In Silico and In Vitro Study.

This study aimed to investigate the influence of garlic metabolites on the quorum sensing (QS) of Bacillus cereus, a foodborne pathogen that controls its main virulence factor through QS. The QS signal receptor PlcR of B. cereus was targeted by molecular docking with 82 garlic metabolites to identify the most potent QS inhibitors. Five metabolites, quercetin, kaempferol, luteolin, flavone, and rutin, were selected for further evaluation of their impacts on the growth, toxin production, and virulence of B. cereus in vitro. The expression levels of key QS genes were also measured to verify their anti-QS ability. The results revealed that quercetin reduced enterotoxin production by B. cereus but did not affect the QS process at the transcriptional level; flavone and rutin in garlic interfered with the QS of B. cereus by competing with the autoinducing peptide (AIP) PapR7 for the PlcR binding site, resulting in decreased enterotoxin secretion and hemolysis without altering the bacterial growth. Interestingly, luteolin and kaempferol in garlic acted as AIP analogs and bound to PlcR to stimulate the QS process and virulence. Furthermore, kaempferol, luteolin, flavone, and rutin had distinct or opposite interactions with PapR7 at the Gln237 or Tyr275 residues of PlcR, which determined the suppression or enhancement of the QS process. The findings suggested that flavone and rutin were effective compounds to inhibit the QS process in garlic and could be used as alternative methods to control B. cereus.

A potential flavor culture: Lactobacillus harbinensis M1 improves the organoleptic quality of fermented soymilk by high production of 2,3-butanedione and acetoin.

Lactic acid bacteria (LAB) are commonly used in soymilk fermentation to improve health-related functionality, but their contribution to sensory qualities is less valued. We characterized Lactobacillus harbinensis M1, Lactobacillus mucosae M2, Lactobacillus fermentum M4, Lactobacillus casei M8 and Lactobacillus rhamnosus C1 from naturally-fermented tofu whey, along with Streptococcus thermophilus ST3 from kefir XPL-1 fermented soymilk, to investigate their potential as starter cultures of fermented soymilk. They were characterized for antibiotic susceptibility, probiotic potential and their performance as starter cultures. All the LABs showed sensitivity to the tested antibiotics. L. casei M8 had strongest tolerance to synthetic gastrointestinal juice (<1.0 log CFU/mL loss), as well as antagonistic effects towards five food-borne pathogens. GC/MS analysis showed that L. harbinensis M1 produced significantly higher abundance (P < 0.05) of 2,3-butanedione (2.45 ppm) and acetoin (44.30 ppm), thus improving the overall sensory acceptability of fermented soymilk. The coding genes for the synthesis of 2,3-butanedione/acetoin (alsS, alsD, butA) were predicted from the whole-genome. A co-culture of L. harbinensis M1 and L. casei M8 produced a fermented soymilk product with both markedly improved flavor and good probiotic potential. It appears that L. harbinensis M1 has much potential for improving the organoleptic properties of fermented soymilk.

Lacticaseibacillus rhamnosus C1 effectively inhibits Penicillium roqueforti: Effects of antimycotic culture supernatant on toxin synthesis and corresponding gene expression.

Recently, consumers are increasingly concerned about the contamination of food by molds and the addition of chemical preservatives. As natural and beneficial bacteria, probiotics are a prospective alternative in food conservation because of their antimycotic activities, although the mechanism has not been explained fully at the level of metabolites. This study aimed at investigating the antifungal activities and their mechanisms of five potential probiotic strains (Lacticaseibacillus rhamnosus C1, Lacticaseibacillus casei M8, Lactobacillus amylolyticus L6, Schleiferilactobacillus harbinensis M1, and Limosilactobacillus fermentum M4) against Penicillium roqueforti, the common type of mold growth on the bread. Results showed that C1 emerged the strongest effectiveness at blocking mycelium growth, damaging the morphology of hyphae and microconidia, decreasing DNA content and interfering in the synthesis of the fungal toxins patulin, roquefortine C and PR-toxin, as well as downregulating the expression of key genes associated with the toxin biosynthesis pathways. Further metabonomic investigation revealed that protocatechuic acid with the minimum inhibitory concentration of 0.40 mg/mL, may be most likely responsible for positively correlated with the antimycotic effects of C1. Thus, C1 is expected to be both a potentially greatly efficient and environmental antimycotic for controlling P. roqueforti contamination in foods.