Quorum sensing in thermophiles: prevalence of autoinducer-2 system

Improved protocol for metabolite extraction and identification of respiratory quinones in extremophilic Archaea grown on mineral materials

We investigated the metabolome of the iron- and sulfur-oxidizing, extremely thermoacidophilic archaeon Metallosphaera sedula grown on mineral pyrite (FeS2). The extraction of organic materials from these microorganisms is a major challenge because of the tight contact and interaction between cells and mineral materials. Therefore, we applied an improved protocol to break the microbial cells and separate their organic constituents from the mineral surface, to extract lipophilic compounds through liquid-liquid extraction, and performed metabolomics analyses using MALDI-TOF MS and UHPLC-UHR-Q/TOF. Using this approach, we identified several molecules involved in central carbon metabolism and in the modified Entner-Doudoroff pathway found in Archaea, sulfur metabolism-related compounds, and molecules involved in the adaptation of M. sedula to extreme environments, such as metal tolerance and acid resistance. Furthermore, we identified molecules involved in microbial interactions, i.e., cell surface interactions through biofilm formation and cell-cell interactions through quorum sensing, which relies on messenger molecules for microbial communication. Moreover, we successfully extracted and identified different saturated thiophene-bearing quinones using software for advanced compound identification (MetaboScape). These quinones are respiratory chain electron carriers in M. sedula, with biomarker potential for life detection in extreme environmental conditions.

The role of single and mixed biofilms in Clostridioides difficile infection and strategies for prevention and inhibition

Clostridioides difficile infection (CDI) is a serious disease with a high recurrence rate. The single and mixed biofilms formed by C. difficile in the gut contribute to the formation of recurrent CDI (rCDI). In parallel, other gut microbes influence the formation and development of C. difficile biofilms, also known as symbiotic biofilms. Interactions between members within the symbiotic biofilm are associated with the worsening or alleviation of CDI. These interactions include effects on C. difficile adhesion and chemotaxis, modulation of LuxS/AI-2 quorum sensing (QS) system activity, promotion of cross-feeding by microbial metabolites, and regulation of intestinal bile acid and pyruvate levels. In the process of C. difficile biofilms control, inhibition of C. difficile initial biofilm formation and killing of C. difficile vegetative cells and spores are the main targets of action. The role of symbiotic biofilms in CDI suggested that targeting interventions of C. difficile-promoting gut microbes could indirectly inhibit the formation of C. difficile mixed biofilms and improved the ultimate therapeutic effect. In summary, this review outlines the mechanisms of C. difficile biofilm formation and summarises the treatment strategies for such single and mixed biofilms, aiming to provide new ideas for the prevention and treatment of CDI.

The intricate symbiotic relationship between lactic acid bacterial starters in the milk fermentation ecosystem

Fermentation is one of the most effective methods of food preservation. Since ancient times, food has been fermented using lactic acid bacteria (LAB). Fermented milk is a very intricate fermentation ecosystem, and the microbial metabolism of fermented milk largely determines its metabolic properties. The two most frequently used dairy starter strains are Streptococcus thermophilus (S. thermophilus) and Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). To enhance both the culture growth rate and the flavor and quality of the fermented milk, it has long been customary to combine S. thermophilus and L. bulgaricus in milk fermentation due to their mutually beneficial and symbiotic relationship. On the one hand, the symbiotic relationship is reflected by the nutrient co-dependence of the two microbes at the metabolic level. On the other hand, more complex interaction mechanisms, such as quorum sensing between cells, are involved. This review summarizes the application of LAB in fermented dairy products and discusses the symbiotic mechanisms and interactions of milk LAB starter strains from the perspective of nutrient supply and intra- and interspecific quorum sensing. This review provides updated information and knowledge on microbial interactions in a fermented milk ecosystem.

Using next generation antimicrobials to target the mechanisms of infection

The remarkable impact of antibiotics on human health is being eroded at an alarming rate by the emergence of multidrug resistant pathogens. There is a recognised consensus that new strategies to tackle infection are urgently needed to limit the devasting impact of antibiotic resistance on our global healthcare infrastructure. Next generation antimicrobials (NGAs) are compounds that target bacterial virulence factors to disrupt pathogenic potential without impacting bacterial viability. By disabling the key virulence factors required to establish and maintain infection, NGAs make pathogens more vulnerable to clearance by the immune system and can potentially render them more susceptible to traditional antibiotics. In this review, we discuss the developing field of NGAs and how advancements in this area could offer a viable standalone alternative to traditional antibiotics or an effective means to prolong antibiotic efficacy when used in combination.

Quorum Sensing in Halorubrum saccharovorum Facilitates Cross-Domain Signaling between Archaea and Bacteria

Quorum Sensing (QS) is a well-studied intercellular communication mechanism in bacteria, regulating collective behaviors such as biofilm formation, virulence, and antibiotic resistance. However, cell-cell signaling in haloarchaea remains largely unexplored. The coexistence of bacteria and archaea in various environments, coupled with the known cell-cell signaling mechanisms in both prokaryotic and eukaryotic microorganisms and the presence of cell-cell signaling mechanisms in both prokaryotic and eukaryotic microorganisms, suggests a possibility for haloarchaea to possess analogous cell-cell signaling or QS systems. Recently, N-acylhomoserine lactone (AHL)-like compounds were identified in haloarchaea; yet, their precise role-for example, persister cell formation-remains ambiguous. This study investigated the capacity of crude supernatant extract from the haloarchaeon Halorubrum saccharovorum CSM52 to stimulate bacterial AHL-dependent QS phenotypes using bioreporter strains. Our findings reveal that these crude extracts induced several AHL-dependent bioreporters and modulated pyocyanin and pyoverdine production in Pseudomonas aeruginosa. Importantly, our study suggests cross-domain communication between archaea and bacterial pathogens, providing evidence for archaea potentially influencing bacterial virulence. Using Thin Layer Chromatography overlay assays, lactonolysis, and colorimetric quantification, the bioactive compound was inferred to be a chemically modified AHL-like compound or a diketopiperazine-like molecule, potentially involved in biofilm formation in H. saccharovorum CSM52. This study offers new insights into putative QS mechanisms in haloarchaea and their potential role in interspecies communication and coordination, thereby enriching our understanding of microbial interactions in diverse environments.

Virtual Screening and In Vitro Experimental Verification of LuxS Inhibitors for Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli O157:H7 is an important foodborne pathogen that forms biofilms. In this study, three quorum-sensing (QS) inhibitors (M414-3326, 3254-3286, and L413-0180) were obtained through virtual screening, and their in vitro antibiofilm activities were validated. Briefly, the three-dimensional structure model of LuxS was constructed and characterized using the SWISS-MODEL. High-affinity inhibitors were screened from the ChemDiv database (1,535,478 compounds) using LuxS as a ligand. Five compounds (L449-1159, L368-0079, M414-3326, 3254-3286, and L413-0180) with a good inhibitory effect (50% inhibitory concentration <10 μM) on type II QS signal molecule autoinducer-2 (AI-2) were obtained using a AI-2 bioluminescence assay. The absorption, distribution, metabolism, excretion, and toxicity (ADMET) properties predicated that the five compounds had high intestinal absorption levels (high) and plasma protein binding (absorbent strong) and did not inhibit the metabolism of CYP2D6 metabolic enzymes. In addition, molecular dynamics simulation showed that compounds L449-1159 and L368-0079 could not stably bind with LuxS. Thus, these compounds were excluded. Furthermore, surface plasmon resonance results showed that the three compounds could specifically bind to LuxS. IN addition, the three compounds could effectively inhibit the biofilm formation without affecting the growth and metabolism of the bacteria. Finally, the reverse transcription-quantitative PCR results showed that the three compounds downregulated the expression of the LuxS gene. Overall, these results revealed that the three compounds obtained through virtual screening could inhibit biofilm formation of E. coli O157:H7 and are potential LuxS inhibitors that can be used to treat E. coli O157:H7 infections. IMPORTANCE E. coli O157:H7 is a foodborne pathogen of public health importance. Quorum sensing (QS) is a form of bacterial communication that can regulate various group behaviors, including biofilm formation. Here, we identified three QS AI-2 inhibitors (M414-3326, 3254-3286, and L413-0180) that can stably and specifically bind to LuxS protein. The three QS AI-2 inhibitors inhibited biofilm formation without affecting the growth and metabolic activity of E. coli O157:H7. The three QS AI-2 inhibitors are promising agents for treating E. coli O157:H7 infections. Further studies to identify the mechanism of the three QS AI-2 inhibitors are needed to develop new drugs to overcome antibiotic resistance.

The Role of Quorum Sensing Molecules in Bacterial-Plant Interactions

Quorum sensing (QS) is a system of communication of bacterial cells by means of chemical signals called autoinducers, which modulate the behavior of entire populations of Gram-negative and Gram-positive bacteria. Three classes of signaling molecules have been recognized, Al-1, Al-2, Al-3, whose functions are slightly different. However, the phenomenon of quorum sensing is not only concerned with the interactions between bacteria, but the whole spectrum of interspecies interactions. A growing number of research results confirm the important role of QS molecules in the growth stimulation and defense responses in plants. Although many of the details concerning the signaling metabolites of the rhizosphere microflora and plant host are still unknown, Al-1 compounds should be considered as important components of bacterial-plant interactions, leading to the stimulation of plant growth and the biological control of phytopathogens. The use of class 1 autoinducers in plants to induce beneficial activity may be a practical solution to improve plant productivity under field conditions. In addition, researchers are also interested in tools that offer the possibility of regulating the activity of autoinducers by means of degrading enzymes or specific inhibitors (QSI). Current knowledge of QS and QSI provides an excellent foundation for the application of research to biopreparations in agriculture, containing a consortia of AHL-producing bacteria and QS inhibitors and limiting the growth of phytopathogenic organisms.

Regulatory Mechanisms between Quorum Sensing and Virulence in Salmonella

Salmonella is a foodborne pathogen that causes enterogastritis among humans, livestock and poultry, and it not only causes huge economic losses for the feed industry but also endangers public health around the world. However, the prevention and treatment of Salmonella infection has remained poorly developed because of its antibiotic resistance. Bacterial quorum sensing (QS) system is an intercellular cell-cell communication mechanism involving multiple cellular processes, especially bacterial virulence, such as biofilm formation, motility, adherence, and invasion. Therefore, blocking the QS system may be a new strategy for Salmonella infection independent of antibiotic treatment. Here, we have reviewed the central role of the QS system in virulence regulation of Salmonella and summarized the most recent advances about quorum quenching (QQ) in virulence attenuation during Salmonella infection. Unraveling the complex relationship between QS and bacterial virulence may provide new insight into the therapy of pathogen infection.

LuxS in Lactobacillus plantarum SS-128 Improves the Texture of Refrigerated Litopenaeus vannamei: Mechanism Exploration Using a Proteomics Approach

This study illustrated the texture changes of Shewanella baltica-inoculated Litopenaeus vannamei during refrigerated storage with the exogenous addition of Lactobacillus plantarum SS-128. The group inoculated with SS-128 had an improved texture compared with that inoculated with the luxS-mutant group (ΔluxS). Proteomics were conducted to analyze the protein alterations in L. vannamei and supernatant, respectively. During storage, many texture-related proteins, including myosin heavy chain and beta-actin, were maintained due to luxS. Some endogenous enzymes related to the energy metabolism and hydrolysis of L. vannamei were downregulated. The luxS-induced interaction with S. baltica showed significant changes in the expression of some critical enzymes and pathways. The ATP-dependent zinc metalloprotease FtsH and protease subunit HslV were downregulated, and the oxidative phosphorylation and glycosaminoglycan degradation pathways in S. baltica were inhibited, resulting in the slow deterioration of L. vannamei. By exploring the mechanism underlying SS-128-led manipulation of the metabolism of spoilage bacteria, we clarified the texture maintenance mechanism of luxS in SS-128, providing theoretical evidence for SS-128 application in food preservation.

Gene-trait matching analysis reveals putative genes involved in Bifidobacterium spp. biofilm formation

Biofilm formation by bacteria represents an adaptation strategy to the environment, and some special genes may lead to a strong biofilm phenotype. In this study, we attempted to find functional genes associated with bifidobacterial biofilm formation. Firstly, we evaluated the biofilm formation ability of bifidobacterial strains from six species, which showed that Bifidobacterium longum, Bifidobacterium breve, Bifidobacterium animalis, Bifidobacterium adolescentis, and Bifidobacterium pseudocatenulatum had biofilm-forming and non-biofilm-forming strains, while all Bifidobacterium bifidum strains could form strong biofilms. Then 48 strains were selected for genome sequencing and comparative analysis. The gene-trait matching analysis revealed that B. bifidum biofilm formation phenotype may associate with their unique genes, involving in stress response, quorum sensing, two components, and peptide synthesis. B. pseudocatenulatum biofilm formation was positively correlated with the eps cluster (rfbX). While no genotype related to the biofilm phenotype was found in B. longum using this analysis, but all contain autoinducer-2 (AI-2) receptor genes. Moreover, luxS, rbsB, rfbX were selected for real-time qPCR analysis, suggesting that their expression are important to biofilm formation. These results indicated that strains carrying certain genes tend to form stronger biofilms than those formed by strains without these genes.

Structure and Signal Regulation Mechanism of Interspecies and Interkingdom Quorum Sensing System Receptors

Quorum sensing (QS) is a signaling mechanism for cell-to-cell communication between bacteria, fungi, and even eukaryotic hosts such as plant and animal cells. Bacteria in real life do not exist as isolated organisms but are found in complex, dynamic, and microecological environments. The study of interspecies QS and interkingdom QS is a valuable approach for exploring bacteria-bacteria interactions and bacteria-host interaction mechanisms and has received considerable attention from researchers. The correct combination of QS signals and receptors is key to initiating the QS process. Compared with intraspecies QS, the signal regulation mechanism of interspecies QS and interkingdom QS is often more complicated, and the distribution of receptors is relatively wide. The present review focuses on the latest progress with respect to the distribution, structure, and signal transduction of interspecies and interkingdom QS receptors and provides a guide for the investigation of new QS receptors in the future.

Evolution of two-component quorum sensing systems

Quorum sensing (QS) is a cell-to-cell communication system that enables bacteria to coordinate their gene expression depending on their population density, via the detection of small molecules called autoinducers. In this way bacteria can act collectively to initiate processes like bioluminescence, virulence and biofilm formation. Autoinducers are detected by receptors, some of which are part of two-component signal transduction systems (TCS), which comprise of a (usually membrane-bound) sensor histidine kinase (HK) and a cognate response regulator (RR). Different QS systems are used by different bacterial taxa, and their relative evolutionary relationships have not been extensively studied. To address this, we used the Kyoto Encyclopedia of Genes and Genomes (KEGG) database to identify all the QS HKs and RRs that are part of TCSs and examined their conservation across microbial taxa. We compared the combinations of the highly conserved domains in the different families of receptors and response regulators using the Simple Modular Architecture Research Tool (SMART) and KEGG databases, and we also carried out phylogenetic analyses for each family, and all families together. The distribution of the different QS systems across taxa, indicates flexibility in HK–RR pairing and highlights the need for further study of the most abundant systems. For both the QS receptors and the response regulators, our results indicate close evolutionary relationships between certain families, highlighting a common evolutionary history which can inform future applications, such as the design of novel inhibitors for pathogenic QS systems.

Evaluation of the differences between biofilm and planktonic Brucella abortus via metabolomics and proteomics

This study analyzed the difference between biofilm and planktonic Brucella abortus using metabolomics and proteomics. Brucella abortus was cultured in different media to induce Brucella abortus biofilm formation and planktonic cells, followed by metabolomics and proteomics analyses for these two samples. Significant differential metabolites were identified, followed by KEGG pathway analysis. Differentially expressed proteins were identified, followed by subcellular localization, GO annotation, and KEGG pathway enrichment. Additionally, a correlation analysis of metabolomics and proteomics was performed. Metabolomics analysis showed 7682 positive and 4433 negative metabolites, including 188 positive and 117 negative significant differential metabolites. These differential metabolites were enriched in fatty acid/unsaturated fatty acid biosynthesis and linoleic acid metabolism. Proteomics analysis revealed 1759 proteins, including 486 differentially expressed proteins, which were enriched in various metabolic and degradation-related pathways. Subcellular localization showed that 74.3% of the differential proteins were cytoplasmic proteins. Correlation analysis showed that 1-palmitoyl-2-oleoyl-phosphatidylglycerol had the most significant correlations with proteins, followed by cytosine. Both metabolites correlated with the protein Q57EI7 (RbsB-1, ribose ABC transporter). One common pathway, fatty acid biosynthesis, was identified by both proteomics and metabolomics analyses that involved the metabolites, oleic acid, and protein Q57DK3 (biotin carboxylase). There were metabolomic and proteomic differences between Brucella abortus biofilm and planktonic cells, and these results provide novel insights into the biofilm-forming process of Brucella abortus.

Transcriptome Analysis Reveals the Genes Involved in Bifidobacterium Longum FGSZY16M3 Biofilm Formation

Biofilm formation has evolved as an adaptive strategy for bacteria to cope with harsh environmental conditions. Currently, little is known about the molecular mechanisms of biofilm formation in bifidobacteria. A time series transcriptome sequencing analysis of both biofilm and planktonic cells of Bifidobacterium longum FGSZY16M3 was performed to identify candidate genes involved in biofilm formation. Protein–protein interaction network analysis of 1296 differentially expressed genes during biofilm formation yielded 15 clusters of highly interconnected nodes, indicating that genes related to the SOS response (dnaK, groS, guaB, ruvA, recA, radA, recN, recF, pstA, and sufD) associated with the early stage of biofilm formation. Genes involved in extracellular polymeric substances were upregulated (epsH, epsK, efp, frr, pheT, rfbA, rfbJ, rfbP, rpmF, secY and yidC) in the stage of biofilm maturation. To further investigate the genes related to biofilm formation, weighted gene co-expression network analysis (WGCNA) was performed with 2032 transcript genes, leading to the identification of nine WGCNA modules and 133 genes associated with response to stress, regulation of gene expression, quorum sensing, and two-component system. These results indicate that biofilm formation in B. longum is a multifactorial process, involving stress response, structural development, and regulatory processes.

Databases for the study of biofilms: current status and potential applications

Biofilms play an important role in health, being associated with >80% of all microbial infections in the body and in the development of antibiotic resistance. Research in this field has continuously produced large volumes of data. Being able to handle all this information will be paramount for progress in this field. However, this places a heavy burden on the development of strategies to gather, organize and make this information available in a way that can be readily and effectively used by those requiring it. Lately, efforts towards this goal have been reported, particularly with the development of Quorumpeps, BiofOmics, BaAMPs, QSPpred, dPABBs, aBiofilm and the Biofilms Structural Database. This work reviews these databases and highlights their applicability and potential, while stressing some of the challenges for the coming years in database development and usage brought about by the use of big data and machine learning.

First report on the characterization of pathogenic Rahnella aquatilis KCL-5 from crucian carp: Revealed by genomic and proteomic analyses

Rahnella aquatilis is an important pathogen of several aquatic organisms and is found widely distributed in the freshwater, soil, fish and human clinical samples. Our previously published study reported a novel pathogenic R. aquatilis strain KCL-5 to crucian carp (Carassius auratus). To further investigate the characteristics and pathogenesis caused by R. aquatilis, we here report on the pathological changes, bacterial genomic and proteomic analyses of strain KCL-5. Significantly pathological changes in liver, intestine, spleen and gills were observed in infected fish. The genome consists of one circular chromosome 5,062,299 bp with 52.02% GC content and two plasmids (506,827 bp, 52.16%; 173,433 bp, 50.00%) and predicted 5,653 genes, 77 tRNAs and 22 rRNAs. Some virulence factors were characterized, including outer membrane protein, haemolysin, RTX toxin, chemotaxis and T3SS secretion system. Antimicrobial resistance genes such as EmrAB-TolC, MexABC-OpmB and RosAB efflux pump were found in strain KCL-5. KEGG analysis showed that mainly functional modules were ABC transporters, biosynthesis of amino acids, two-component system, quorum sensing, flagellum assembly and chemotaxis, in which most of them were identified by using 2-DE/MS analyses. To our knowledge, this was first report on the molecular characteristics of R. aquatilis by multi-omics approaches, which will provide insights into the pathogenic mechanism of R. aquatilis infection in fish.

Modelling and analysing biological oscillations in quorum sensing networks

Recent experiments have shown that the biological oscillation of quorum sensing (QS) system play a vital role not only in the process of bacterial synthesis but also in the treatment of cancer by releasing drugs. As known, these five substances TetR, CI, LacI, AiiA and AI are the core components of the QS system. However, the effects of AiiA and protein synthesis time delay on QS system are often ignored in the theoretical model, which is taken as a priority in the proposed research. Therefore, the authors developed a new mathematical model to explore the effects of AiiA and time delay on the dynamical behaviour of QS system theoretically and numerically. The results show that time delay can induce oscillation of QS system. Concretely, there exists a time delay threshold [inline-formula removed]. When time delay is less than [inline-formula removed], the system is stable. With the increasing of time delay and once it passes [inline-formula removed], oscillation behaviour occurs. Moreover, the length of time delay determines the amplitude and period of the QS oscillation. In addition, the value of [inline-formula removed] is sensitive to AiiA. These results may enhance the understanding of QS oscillations and provide new insights for bacterial release drugs to treat cancer.

Expression of Meiothermus ruber luxS in E. coli alters the antibiotic susceptibility and biofilm formation

Quorum sensing (QS) and signal molecules used for interspecies communication are well defined in mesophiles, but there is still a plethora of microorganisms in which existence and mechanisms of QS need to be explored, thermophiles being among them. In silico analysis has revealed the presence of autoinducer-2 (AI-2) class of QS signaling molecules in thermophiles, synthesized by LuxS (AI-2 synthase), though the functions of this system are not known. In this study, LuxS of Meiothermus ruber was used for understanding the mechanism and functions of AI-2 based QS among thermophilic bacteria. The luxS gene of M. ruber was expressed in luxS^- deletion mutant of Escherichia coli. Complementation of luxS resulted in significant AI-2 activity, enhanced biofilm formation, and antibiotic susceptibility. Transcriptome analysis showed significant differential expression of 204 genes between the luxS-complemented and luxS^- deletion mutant of E. coli. Majority of the genes regulated by luxS belonged to efflux pumps. This elucidation may contribute towards finding novel alternatives against incessant antibiotic resistance in bacteria.Key Points• Expression of luxS in luxS^-E. coli resulted in increase in biofilm index. • Reduction in the MIC of antibiotics was observed after complementation of luxS. • Downregulation of efflux pump genes was observed after complementation of luxS. • Transcriptome analysis showed that 204 genes were differentially regulated significantly.

Defeating Antibiotic-Resistant Bacteria: Exploring Alternative Therapies for a Post-Antibiotic Era

Antibiotics are one of the greatest medical advances of the 20th century, however, they are quickly becoming useless due to antibiotic resistance that has been augmented by poor antibiotic stewardship and a void in novel antibiotic discovery. Few novel classes of antibiotics have been discovered since 1960, and the pipeline of antibiotics under development is limited. We therefore are heading for a post-antibiotic era in which common infections become untreatable and once again deadly. There is thus an emergent need for both novel classes of antibiotics and novel approaches to treatment, including the repurposing of existing drugs or preclinical compounds and expanded implementation of combination therapies. In this review, we highlight to utilize alternative drug targets/therapies such as combinational therapy, anti-regulator, anti-signal transduction, anti-virulence, anti-toxin, engineered bacteriophages, and microbiome, to defeat antibiotic-resistant bacteria.

Cooperation in Microbial Populations: Theory and Experimental Model Systems

Cooperative behavior, the costly provision of benefits to others, is common across all domains of life. This review article discusses cooperative behavior in the microbial world, mediated by the exchange of extracellular products called public goods. We focus on model species for which the production of a public good and the related growth disadvantage for the producing cells are well described. To unveil the biological and ecological factors promoting the emergence and stability of cooperative traits we take an interdisciplinary perspective and review insights gained from both mathematical models and well-controlled experimental model systems. Ecologically, we include crucial aspects of the microbial life cycle into our analysis and particularly consider population structures where ensembles of local communities (subpopulations) continuously emerge, grow, and disappear again. Biologically, we explicitly consider the synthesis and regulation of public good production. The discussion of the theoretical approaches includes general evolutionary concepts, population dynamics, and evolutionary game theory. As a specific but generic biological example, we consider populations of Pseudomonas putida and its regulation and use of pyoverdines, iron scavenging molecules, as public goods. The review closes with an overview on cooperation in spatially extended systems and also provides a critical assessment of the insights gained from the experimental and theoretical studies discussed. Current challenges and important new research opportunities are discussed, including the biochemical regulation of public goods, more realistic ecological scenarios resembling native environments, cell-to-cell signaling, and multispecies communities.

Regulatory Mechanisms of the LuxS/AI-2 System and Bacterial Resistance

The quorum-sensing (QS) system is an intercellular cell-cell communication mechanism that controls the expression of genes involved in a variety of cellular processes and that plays critical roles in the adaption and survival of bacteria in their environment. The LuxS/AI-2 QS system, which uses AI-2 (autoinducer-2) as a signal molecule, has been identified in both Gram-negative and Gram-positive bacteria. As one of the important global regulatory networks in bacteria, it responds to fluctuations in the numbers of bacteria and regulates the expression of a number of genes, thus affecting cell behavior. We summarize here the known relationships between the LuxS/AI-2 system and drug resistance, discuss the inhibition of LuxS/AI-2 system as an approach to prevent bacterial resistance, and present new strategies for the treatment of drug-resistant pathogens.

Cultivation technology development of Rhodothermus marinus DSM 16675

This work presents an evaluation of batch, fed-batch, and sequential batch cultivation techniques for production of R. marinus DSM 16675 and its exopolysaccharides (EPSs) and carotenoids in a bioreactor, using lysogeny broth (LB) and marine broth (MB), respectively, in both cases supplemented with 10 g/L maltose. Batch cultivation using LB supplemented with maltose (LB_malt) resulted in higher cell density (OD₆₂₀ = 6.6) than use of MB_malt (OD₆₂₀ = 1.7). Sequential batch cultivation increased the cell density threefold (OD₆₂₀ = 20) in LB_malt and eightfold (OD₆₂₀ = 14) in MB_malt. In both single and sequential batches, the production of carotenoids and EPSs using LB_malt was detected in the exponential phase and stationary phase, respectively, while in MB_malt formation of both products was detectable in both the exponential and stationary phases of the culture. Heteropolymeric EPSs were produced with an overall volumetric productivity (Q_E) of 0.67 (mg/L h) in MB_malt and the polymer contained xylose. In LB, Q_E was lower (0.1 mg/L h) and xylose could not be detected in the composition of the produced EPSs. In conclusion, this study showed the importance of a process design and medium source for production of R. marinus DSM 16675 and its metabolites.

Oscillatory dynamics mechanism induced by protein synthesis time delay in quorum-sensing system

Recent experimental evidence reports that the oscillatory behavior of quorum sensing plays an extremely important role in the process of bacterial synthesis and release drug to fight cancer. As we know, the six substances AiiA, LuxI, internal AHL, external AHL, AHL substrate, and H_{2}O_{2} are the core parts of the quorum-sensing system. Here, the effects of several important factors, including time delay, variable H_{2}O_{2}, AHL synthesis rate induced by LuxI, and AHL degradation rate induced by AiiA on the oscillatory behavior of the quorum-sensing system are studied theoretically based on a part of mathematical model describing the interaction of the above six substances proposed by Prindle et al. [Nature 508, 387 (2014)10.1038/nature13238]. The results show that the time delay is a prerequisite for inducing oscillation of the quorum-sensing system. Furthermore, the length of time delay can determine the amplitude and period of oscillation. As a further matter, the change of H_{2}O_{2} concentration can induce the oscillatory behavior of the quorum-sensing system. In addition, under the regulation of H_{2}O_{2}, the period robustness of the quorum-sensing system is increased. Similarly, the quorum-sensing system exhibits periodic oscillation when AHL synthesis rate induced by LuxI less than a certain critical value, unless it displays a steady state. Additionally, a too-high or too-low level of AHL degradation rate induced by AiiA will fail to generate oscillation of the quorum-sensing system, only the intermediate level will cause oscillation. Finally, the two and three parameter regions in which the quorum-sensing system exhibits oscillation behavior are generated.

Interference With Quorum-Sensing Signal Biosynthesis as a Promising Therapeutic Strategy Against Multidrug-Resistant Pathogens

Faced with the global health threat of increasing resistance to antibiotics, researchers are exploring interventions that target bacterial virulence factors. Quorum sensing is a particularly attractive target because several bacterial virulence factors are controlled by this mechanism. Furthermore, attacking the quorum-sensing signaling network is less likely to select for resistant strains than using conventional antibiotics. Strategies that focus on the inhibition of quorum-sensing signal production are especially attractive because the enzymes involved are expressed in bacterial cells but are not present in their mammalian counterparts. We review here various approaches that are being taken to interfere with quorum-sensing signal production via the inhibition of autoinducer-2 synthesis, PQS synthesis, peptide autoinducer synthesis, and N-acyl-homoserine lactone synthesis. We expect these approaches will lead to the discovery of new quorum-sensing inhibitors that can help to stem the tide of antibiotic resistance.

Quorum sensing inhibitors as antipathogens: biotechnological applications

The mechanisms through which microbes communicate using signal molecules has inspired a great deal of research. Microbes use this exchange of information, known as quorum sensing (QS), to initiate and perpetuate infectious diseases in eukaryotic organisms, evading the eukaryotic defense system by multiplying and expressing their pathogenicity through QS regulation. The major issue to arise from such networks is increased bacterial resistance to antibiotics, resulting from QS-dependent mediation of the formation of biofilm, the induction of efflux pumps, and the production of antibiotics. QS inhibitors (QSIs) of diverse origins have been shown to act as potential antipathogens. In this review, we focus on the use of QSIs to counter diseases in humans as well as plants and animals of economic importance. We also discuss the challenges encountered in the potential applications of QSIs.