Autoinducer-2-like activity in lactic acid bacteria isolated from minced beef packaged under modified atmospheres

Quorum sensing signal autoinducer-2 promotes hydrogen peroxide degradation in water by Gram-positive bacteria

Hydrogen peroxide is widely used to remedy bacterial and parasitic infections, but its excessive use will cause severe damage to aquatic animals. Moreover, there is no safe, efficient and low-cost method to degrade residual hydrogen peroxide in water. Here we developed a hydrogen peroxide removal mechanism by which autoinducer-2 (AI-2), a quorum sensing signal molecule that can promote the hydrogen peroxide degradation by Gram-positive bacteria. Here, we investigated the promotion effect of AI-2 on hydrogen peroxide degradation by Deinococcus sp. Y35 and the response of the antioxidant system. We further sought to understand the key mechanism underlying the promotion effect of AI-2 on hydrogen peroxide degradation is that, AI-2 contributed to the resistance of strain Y35 to oxidative stress induced by hydrogen peroxide, and altered membrane permeability of strain Y35 that allowed more hydrogen peroxide to enter bacterial cells and be degraded. Additionally, AI-2 can also encourage multiple Gram-positive bacteria to degrade hydrogen peroxide. Accordingly, our study serves as a reference for the regulation mechanism of the signal molecule AI-2 and provides the development of new strategies for hydrogen peroxide degradation.

Quorum sensing in human gut and food microbiomes: Significance and potential for therapeutic targeting

Human gut and food microbiomes interact during digestion. The outcome of these interactions influences the taxonomical composition and functional capacity of the resident human gut microbiome, with potential consequential impacts on health and disease. Microbe-microbe interactions between the resident and introduced microbiomes, which likely influence host colonisation, are orchestrated by environmental conditions, elements of the food matrix, host-associated factors as well as social cues from other microorganisms. Quorum sensing is one example of a social cue that allows bacterial communities to regulate genetic expression based on their respective population density and has emerged as an attractive target for therapeutic intervention. By interfering with bacterial quorum sensing, for instance, enzymatic degradation of signalling molecules (quorum quenching) or the application of quorum sensing inhibitory compounds, it may be possible to modulate the microbial composition of communities of interest without incurring negative effects associated with traditional antimicrobial approaches. In this review, we summarise and critically discuss the literature relating to quorum sensing from the perspective of the interactions between the food and human gut microbiome, providing a general overview of the current understanding of the prevalence and influence of quorum sensing in this context, and assessing the potential for therapeutic targeting of quorum sensing mechanisms.

Comprehensive molecular, probiotic, and quorum-sensing characterization of anti-listerial lactic acid bacteria, and application as bioprotective in a food (milk) model

Listeria monocytogenes is a major foodborne pathogen that adversely affects the food industry. In this study, 6 anti-listerial lactic acid bacteria (LAB) isolates were screened. These anti-listerial LAB isolates were identified via 16S rRNA gene sequencing and analyzed via repetitive extragenic palindromic-PCR. Probiotic assessment of these isolates, comprising an evaluation of the antibiotic susceptibility, tolerance to lysozyme, simulated gastric and intestinal juices, and gut conditions (low pH, bile salts, and 0.4% phenol), was carried out. Most of the isolates were resistant to streptomycin, vancomycin, gentamycin, kanamycin, and ciprofloxacin. All of the isolates were negative for virulence genes, including agg, ccf, cylA, cylB, cylLL, cylLS, cylM, esp, and gelE, and hemolytic activity. Furthermore, autoinducer-2 (a quorum-sensing molecule) was detected and quantified via HPLC with fluorescence detection after derivatization with 2,3-diaminonaphthalene. Metabolites profiles of the Lactobacillus sakei D.7 and Lactobacillus plantarum I.60 were observed and presented various organic acids linked with antibacterial activity. Moreover, freeze-dried cell-free supernatants from Lb. sakei (55 mg/mL) and Lb. plantarum (40 mg/mL) showed different minimum effective concentration (MEC) against L. monocytogenes in the food model (whole milk). In summary, these anti-listerial LAB isolates do not pose a risk to consumer health, are eco-friendly, and may be promising candidates for future use as bioprotective cultures and new probiotics to control contamination by L. monocytogenes in the food and dairy industries.

Ιn Vitro Screening of γ-Aminobutyric Acid and Autoinducer-2 Signalling in Lactic Acid Bacteria Exhibiting Probiotic Potential Isolated from Natural Black Conservolea Olives

In the present study, 33 strains of lactic acid bacteria (LAB) previously isolated from natural black Conservolea olives were assessed for their probiotic potential in vitro, as well as for their autoinducer-2 (AI-2) activity under standard growth conditions and the production of γ-aminobutyric acid (GABA). The probiotic tests included the in vitro resistance to low pH and resistance to bile salts, the evaluation of bile salt hydrolase activity, as well as safety tests regarding their possible haemolytic activity and their antimicrobial activity against pathogens. The results indicated that 17 strains were able to survive in low pH and in the presence of bile, with 15 of them also exhibiting partial bile salt hydrolase activity. None of the strains exhibited haemolytic activity or inhibited the growth of any of the examined pathogens. Moreover, the strains displayed generally low AI-2 activity under the growth conditions tested, regardless of the species. Interestingly, in contrast to what has been found in most foods, none of the isolates were found to produce GABA after 48 h of growth. The results from the AI-2 activity and extracellular GABA detection were considered as unexpected for LAB with probiotic attributes.

Proteomic and genetics insights on the response of the bacteriocinogenic Lactobacillus sakei CRL1862 during biofilm formation on stainless steel surface at 10°C

Some lactic acid bacteria have the ability to form biofilms on food-industry surfaces and this property could be used to control food pathogens colonization. Lactobacillus sakei CR1862 was selected considering its bacteriocinogenic nature and ability to adhere to abiotic surfaces at low temperatures. In this study, the proteome of L. sakei CRL1862 grown either under biofilm on stainless steel surface and planktonic modes of growth at 10°C, was investigated. Using two-dimensional gel electrophoresis, 29 out of 43 statistically significant spots were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Ten proteins resulted up-regulated whereas 16 were down-regulated during biofilm formation. Differentially expressed proteins were found to belong to carbohydrate, nucleotide, aminoacid and lipid metabolisms as well as translation, peptide hydrolysis, cell envelope/cell wall biosynthesis, adaption to atypical conditions and protein secretion. Some proteins related to carbohydrate and nucleotide metabolisms, translation and peptide degradation were overexpressed whereas those associated to stress conditions were synthesized in lower amounts. It seems that conditions for biofilm development would not imply a stressful environment for L. sakei CRL1862 cells, directing its growth strategy towards glycolytic flux regulation and reinforcing protein synthesis. In addition, L. sakei CRL1862 showed to harbor nine out of ten assayed genes involved in biofilm formation and protein anchoring. By applying qRT-PCR analysis, four of these genes showed to be up regulated, srtA2 being the most remarkable. The results of this study contribute to the knowledge of the physiology of L. sakei CRL1862 growing in biofilm on a characteristic food contact surface. The use of this strain as green biocide preventing L. monocytogenes post-processing contamination on industrial surfaces may be considered.

Autoinducer-2 properties of kimchi are associated with lactic acid bacteria involved in its fermentation

Bacteria use the cell density-dependent quorum signalling system to regulate particular gene expressions. In food microbiology, signalling is well known for its relation to (foodborne) pathogenicity, food spoilage, and biofilm formation. Quorum quenching and inhibition are thus being considered as a feasible approach in food preservation and safety. In the case of the luxS-mediated universal quorum sensing using autoinducer-2 (AI-2), however, it could be a different issue. Several studies have reported a luxS AI-2 synthase homologue in numerous bacteria, comprising both pathogens and beneficial strains. A recent study has shown the AI-2 signal to restore the balance of the major phyla of the gut microbiota in antibiotic-induced dysbiosis. We measured the AI-2 activity of the lactic fermented food, kimchi, and found different AI-2 signalling intensities. In order to trace the origin of the signal production, we obtained 229 lactic acid bacterial isolates from the kimchi samples, and detected the AI-2 properties of each isolate using a modified AI-2 bioluminescence assay. Our results showed isolates of dominant species of the genera Lactobacillus, Weissella and Leuconostoc which either produced or inhibited the AI-2 signal. No isolate of the dominant species Lactobacillus sakei (75 isolates) and Lactobacillus curvatus (28 isolates) showed AI-2 producing activity, while AI-2 inhibition could not be detected for any of the 31 Lactobacillus plantarum isolates. These results suggest the AI-2 activity of kimchi to result from the interaction of the associated microbial food cultures (MFCs) during fermentation. Thus far, only sparse information is available on AI-2 signalling interaction in fermented food, however, we suggest that fermented food may be a supplier of AI-2 signalling molecules via typical MFCs.

Assessment of the effect of a Salmonella enterica ser. Typhimurium culture supernatant on the single-cell lag time of foodborne pathogens

The objective of this study was the in vitro evaluation of the effect of a cell-free microbial supernatant, produced by a luxS-positive Salmonella enterica ser. Typhimurium strain, on the single-cell growth kinetic behavior of two strains of S. enterica (serotypes Enteritidis and Typhimurium) and a methicillin-resistant Staphylococcus aureus strain. The single-cell lag time (λ) of the pathogens was estimated in the absence and presence (20% v/v) of microbial supernatant based on optical density measurements. As demonstrated by the obtained results, the tested microbial supernatant had a strain-specific effect on the single-cell λ and its variability. Although the mean λ values were similar in the absence and presence of microbial supernatant in the case of Salmonella Enteritidis, a significant (P ≤ 0.05) reduction and increase in the mean value of this parameter in the presence of microbial supernatant were observed for Salmonella Typhimurium and St. aureus, respectively. With regard to the effect of the tested microbial supernatant on the single-cell variability of λ, similar λ distributions were obtained in its absence and presence for S. Enteritidis, while considerable differences were noted for the other two tested organisms; the coefficient of variation of λ in the absence and presence of microbial supernatant was 41.6 and 69.8% for S. Typhimurium, respectively, with the corresponding values for St. aureus being 74.0 and 56.9%. As demonstrated by the results of bioassays, the tested microbial supernatant exhibited autoinducer-2 activity, indicating a potential association of such quorum sensing compounds with the observed effects. Although preliminary in nature, the collected data provide a good basis for future research on the role of quorum sensing in the single-cell growth behavior of foodborne pathogens.

Quorum sensing in the context of food microbiology

Food spoilage may be defined as a process that renders a product undesirable or unacceptable for consumption and is the outcome of the biochemical activity of a microbial community that eventually dominates according to the prevailing ecological determinants. Although limited information are reported, this activity has been attributed to quorum sensing (QS). Consequently, the potential role of cell-to-cell communication in food spoilage and food safety should be more extensively elucidated. Such information would be helpful in designing approaches for manipulating these communication systems, thereby reducing or preventing, for instance, spoilage reactions or even controlling the expression of virulence factors. Due to the many reports in the literature on the fundamental features of QS, e.g., chemistry and definitions of QS compounds, in this minireview, we only allude to the types and chemistry of QS signaling molecules per se and to the (bioassay-based) methods of their detection and quantification, avoiding extensive documentation. Conversely, we attempt to provide insights into (i) the role of QS in food spoilage, (ii) the factors that may quench the activity of QS in foods and review the potential QS inhibitors that might "mislead" the bacterial coordination of spoilage activities and thus may be used as biopreservatives, and (iii) the future experimental approaches that need to be undertaken in order to explore the "gray" or "black" areas of QS, increase our understanding of how QS affects microbial behavior in foods, and assist in finding answers as to how we can exploit QS for the benefit of food preservation and food safety.