The AI-2/luxS Quorum Sensing System Affects the Growth Characteristics, Biofilm Formation, and Virulence of Haemophilus parasuis

Transcriptome analysis reveals a new virulence-associated trimeric autotransporter responsible for Glaesserella parasuis autoagglutination

Capsular polysaccharide is an important virulence factor of Glaesserella parasuis. An acapsular mutant displays multiple phenotype variations, while the underlying mechanism for these variations is unknown. In this study, we created an acapsular mutant by deleting the wza gene in the capsule locus. We then used transcriptome analysis to compare the gene expression profiles of the wza deletion mutant with those of the parental strain to understand the possible reasons for the phenotypic differences. The mutant Δwza, which has a deleted wza gene, secreted less polysaccharide and lost its capsule structure. The Δwza exhibited increased autoagglutination, biofilm formation and adherence to eukaryotic cells, while the complementary strain C-Δwza partially restored the phenotype. Transcriptome analysis revealed several differentially expressed genes (DEGs) in Δwza, including up-regulated outer membrane proteins and proteins involved in peptidoglycan biosynthesis, suggesting that wza deletion affects the cell wall homeostasis of G. parasuis. Transcriptome analysis revealed the contribution of non-coding RNAs in the regulation of DEGs. Moreover, a new virulence-associated trimeric autotransporter, VtaA31 is upregulated in Δwza. It is responsible for enhanced autoagglutination but not for enhanced biofilm formation and adherence to eukaryotic cells in Δwza. In conclusion, these data indicate that wza affects the expression of multiple genes, especially those related to cell wall synthesis. Furthermore, they provide evidence that vtaA31 is involved in the autoagglutination of G. parasuis.

The absence of luxS reduces the invasion of Avibacterium paragallinarum but is not essential for virulence

The contagious respiratory pathogen, Avibacterium paragallinarum, contributes to infectious coryza in poultry. However, commercial vaccines have not shown perfect protection against infectious coryza. To search for an alternative approach, this research aimed to investigate whether the quorum-sensing system of pathogens plays a crucial role in their survival and pathogenicity. The LuxS/AI-2 quorum-sensing system in many Gram-negative and Gram-positive bacteria senses environmental changes to regulate physiological traits and virulent properties, and the role of the luxS gene in Av. paragallinarum remains unclear. To investigate the effect of the luxS gene in the quorum-sensing system of Av. paragallinarum, we constructed a luxS mutant. Bioluminescence analysis indicated that the luxS gene plays a vital role in the LuxS/AI-2 quorum-sensing system. The analysis of the LuxS/AI-2 system-related genes showed the level of pfs mRNA to be significantly increased in the mutant strain; however, lsrR, lsrK, and lsrB mRNA levels were not significantly different compared with the wild type. The ability of the luxS mutant strain to invade HD11 and DF-1 cells was significantly decreased compared with the wild-type strain. In addition, all chickens challenged with various doses of the luxS mutant strain developed infections and symptoms, and those challenged with the lowest dose exhibited only minor differences compared to chickens challenged with the wild-type strain. Thus, the deletion of the luxS gene reduces the invasion, but the luxS gene does not play an essential role in the pathogenesis of A. paragallinarum.

Ribosomal protein L32 contributes to the growth, antibiotic resistance and virulence of Glaesserella parasuis

Glaesserella parasuis is the pathogen that causes Glässer's disease in pigs, which is characterized by fibrinous polyserositis, arthritis and meningitis. Research on ribosomal protein L32 in microorganisms has mainly focused on regulating gene transcription and translation, but its effect on bacterial virulence is unclear. The role of L32 gene in G. parasuis is not clear, and in order to study the function of L32 gene, a suicide plasmid-mediated natural transformation method was used to construct a L32 gene deletion mutant. We found that although L32 was shown to be non-essential for cell proliferation, the growth curve of ΔL32 is clearly different compared with that of ZJ1208. ΔL32 produced more outer membrane vesicles (OMVs) with a variety of irregular shapes, but produced similar biofilm to the parental strain. ΔL32 is more sensitive to osmotic pressure, oxidation pressure and heat shock stress. Meanwhile, ΔL32 is significantly more susceptible to antimicrobials such as spectinomycin, apramycin, sulfafurazole, but not to other antibiotics used in this study. In the mouse challenge experiment, the mortality of mice infected with the mutant strain decreased by 40% compared to those infected with the wild-type strain, indicating that L32 is a virulence-associated factor which contributes to bacterial fitness in host environments. The above results show that L32 is important for the growth, stress resistance and virulence of G. parasuis, and this study also confirms for the first time that L32 plays an important role in antibiotic resistance against aminoglycosides and sulfonamides.

The role of cytolethal distending toxin in Glaesserella parasuis JS0135 strain infection: Cytotoxicity, phagocytic resistance and pathogenicity

Glaesserella parasuis is an important porcine pathogen that commonly colonizes the upper respiratory tract of pigs and is prone to causing Glässer's disease under complex conditions. As yet, the disease has led to serious economic losses to the swine industry worldwide. Studies so far have found that several virulence factors are associated with the pathogenicity of G. parasuis, but the pathogenic mechanism is still not fully understood. Cytolethal distending toxin (CDT), a potential virulence factor in G. parasuis, is involved in cytotoxicity, serum resistance, adherence to and invasion of host cells in vitro. Here, to further investigate the pathogenic role of CDT during G. parasuis infection in vitro and in vivo, a double cdt1 and cdt2 deletion mutant (Δcdt1Δcdt2) without selectable marker was first generated in G. parasuis JS0135 strain by continuous natural transformations and replica plating. Morphological observation and lactate dehydrogenase assay showed that the Δcdt1Δcdt2 mutant was defective in cytotoxicity. Additionally, the Δcdt1Δcdt2 mutant was more susceptible to phagocytosis caused by 3D4/2 macrophages compared to the wild-type JS0135 strain. Moreover, by focusing on clinical signs, necropsy, bacterial recovery and pathological observation, we found that the deletion of cdt1 and cdt2 genes led to a significant attenuation of virulence in G. parasuis. Taken together, these findings suggest that as an important virulence factor, CDT can significantly affect the pathogenicity of G. parasuis.

Quorum Sensing Inhibitors: An Alternative Strategy to Win the Battle against Multidrug-Resistant (MDR) Bacteria

Antibiotic resistance is a major problem and a major global health concern. In total, there are 16 million deaths yearly from infectious diseases, and at least 65% of infectious diseases are caused by microbial communities that proliferate through the formation of biofilms. Antibiotic overuse has resulted in the evolution of multidrug-resistant (MDR) microbial strains. As a result, there is now much more interest in non-antibiotic therapies for bacterial infections. Among these revolutionary, non-traditional medications is quorum sensing inhibitors (QSIs). Bacterial cell-to-cell communication is known as quorum sensing (QS), and it is mediated by tiny diffusible signaling molecules known as autoinducers (AIs). QS is dependent on the density of the bacterial population. QS is used by Gram-negative and Gram-positive bacteria to control a wide range of processes; in both scenarios, QS entails the synthesis, identification, and reaction to signaling chemicals, also known as auto-inducers. Since the usual processes regulated by QS are the expression of virulence factors and the creation of biofilms, QS is being investigated as an alternative solution to antibiotic resistance. Consequently, the use of QS-inhibiting agents, such as QSIs and quorum quenching (QQ) enzymes, to interfere with QS seems like a good strategy to prevent bacterial infections. This review sheds light on QS inhibition strategy and mechanisms and discusses how using this approach can aid in winning the battle against resistant bacteria.

Inhibitory effect of Lonicera japonica flos on Streptococcus mutans biofilm and mechanism exploration through metabolomic and transcriptomic analyses

Introduction

Streptococcus mutans was the primary pathogenic organism responsible for dental caries. Lonicera japonica flos (LJF) is a traditional herb in Asia and Europe and consumed as a tea beverage for thousands of years.

Methods

The inhibitory effect and mechanism of LJF on biofilm formation by S. mutans was investigated. The active extracts of LJF were validated for their inhibitory activity by examining changes in surface properties such as adherence, hydrophobicity, auto-aggregation abilities, and exopolysaccharides (EPS) production, including water-soluble glucan and water-insoluble glucan.

Results and discussion

LJF primarily inhibited biofilm formation through the reduction of EPS production, resulting in alterations in cell surface characteristics and growth retardation in biofilm formation cycles. Integrated transcriptomic and untargeted metabolomics analyses revealed that EPS production was modulated through two-component systems (TCS), quorum sensing (QS), and phosphotransferase system (PTS) pathways under LJF stress conditions. The sensing histidine kinase VicK was identified as an important target protein, as LJF caused its dysregulated expression and blocked the sensing of autoinducer II (AI-2). This led to the inhibition of response regulator transcriptional factors, down-regulated glycosyltransferase (Gtf) activity, and decreased production of water-insoluble glucans (WIG) and water-soluble glucans (WSG). This is the first exploration of the inhibitory effect and mechanism of LJF on S. mutans, providing a theoretical basis for the application of LJF in functional food, oral health care, and related areas.

AI-2 quorum sensing controlled delivery of cytolysin-A by tryptophan auxotrophic low-endotoxic Salmonella and its anticancer effects in CT26 mice with colon cancer

INTRODUCTION

The limitations of conventional cancer therapies necessitate target-oriented, highly invasive, and safe treatment approaches. Hence, the intrinsic anti-tumor activity of Salmonella can offer better options to combat cancers.

OBJECTIVES

This study aims to utilize attenuated Salmonella and deliver cytolytic protein cytolysin A (ClyA) under quorum sensing (QS) signaling for precise localized expression in tumors but not in healthy organs.

METHODS

The therapeutic delivery strain was imposed with tryptophan auxotroph for selective colonization in tumors by trpA and trpE deletion, and lipid-A and O-antigen were altered by pagL and rfaL deletions using lambda red recombination method. The strain was transformed with the designed QS-controlled ClyA expression vector which was validated by western blot. The in vivo passaged therapeutic strain was used for treatment four times at a weekly interval, with a dose of 5 × 106 CFU/mouse for cancer therapy.

RESULTS

The attenuated strain induced minimal endotoxicity-related cytokines TNF-α, IL-1β, and IFN-γ and exhibited adequate colonization despite earlier exposure in mice. The QS-controlled ClyA expression was confirmed by western blot from bacterial cultures grown at different cell densities. The results demonstrated that the in vivo passaged strain preferentially colonized the tumor after vacating the spleen, liver, and lung, leaving no outward histological scars. The anti-cancer effect of the designed construct was evaluated in the murine CT26 colon cancer model. The expression of ClyA increased tumoricidal activity by 67 % compared to vector control.

CONCLUSION

Hence, the anti-tumor effect of the engineered Salmonella strain was improved by ClyA expression via QS activation after achieving the threshold bacterial cell density. Further, immunohistochemical staining of the tumor and other organs corroborated the QS-controlled tumor-specific expression of ClyA. Overall, the results imply that the developed anti-cancer Salmonella has low endotoxicity and QS-controlled expression of ClyA as beneficial safety elements and supports recurrent Salmonella inoculation by O-antigen deficiency.

Communication leads to bacterial heterogeneous adaptation to changing conditions in partial nitrification reactor

Recently, it is reported that bacterial communication coordinates the whole consortia to jointly resist the adverse environments. Here, we found the bacterial communication inevitably distinguished bacterial adaptation among different species in partial nitrification reactor under decreasing temperatures. We operated a partial nitrification reactor under temperature gradient from 30 °C to 5 °C and found the promotion of bacterial communication on adaptation of ammonia-oxidizing bacteria (AOB) was greater than that of nitrite-oxidizing bacteria (NOB). Signal pathways with single-component sensing protein in AOB can regulate more genes involved in bacterial adaptation than that with two-component sensing protein in NOB. The negative effects of bacterial communication, which were seriously ignored, have been highlighted, and Clp regulator downstream diffusible signal factor (DSF) based signal pathways worked as transcription activators and inhibitors of adaptation genes in AOB and NOB respectively. Bacterial communication can induce differential adaptation through influencing bacterial interactions. AOB inclined to cooperate with DSF synthesis bacteria as temperature declined, however, cooperation between NOB and DSF synthesis bacteria inclined to get weakening. According to the regulatory effects of signal pathways, bacterial survival strategies for self-protection were revealed. This study hints a potential way to govern niche differentiation in the microbiota by bacterial communication, contributing to forming an efficient artificial ecosystem.

AI-2 quorum sensing signal disrupts coral symbiotic homeostasis and induces host bleaching

Symbiotic microorganisms play critical ecophysiological roles that facilitate the maintenance of coral health. Currently, information on the gene and protein pathways contributing to bleaching responses is lacking, including the role of autoinducers. Although the autoinducer AI-1 is well understood, information on AI-2 is insufficient. Here, we observed a 3.7-4.0 times higher abundance of the AI-2 synthesis gene luxS in bleached individuals relative to their healthy counterparts among reef-building coral samples from the natural environment. Laboratory tests further revealed that AI-2 contributed significantly to an increase in coral bleaching, altered the ratio of potential probiotic and pathogenic bacteria, and suppressed the antiviral activity of specific pathogenic bacteria while enhancing their functional potential, such as energy metabolism, chemotaxis, biofilm formation and virulence release. Structural equation modeling indicated that AI-2 influences the microbial composition, network structure, and pathogenic features, which collectively contribute to the coral bleaching status. Collectively, our results offer novel potential strategies for coral conservation based on a signal manipulation approach.

Baicalin Weakens the Virulence of Porcine Extraintestinal Pathogenic Escherichia coli by Inhibiting the LuxS/AI-2 Quorum-Sensing System

Porcine extraintestinal pathogenic Escherichia coli (ExPEC) is a pathogenic bacterium that causes huge economic losses to the pig farming industry and considerably threatens human health. The quorum sensing (QS) system plays a crucial role in the survival and pathogenesis of pathogenic bacteria. Hence, it is a viable approach to prevent ExPEC infection by compromising the QS system, particularly the LuxS/AI-2 system. In this study, we investigated the effects of baicalin on the LuxS/AI-2 system of ExPEC. Baicalin at concentrations of 25, 50, and 100 μg/mL significantly diminished the survival ability of ExPEC in hostile environments and could inhibit the biofilm formation and autoagglutination ability in ExPEC. Moreover, baicalin dose-dependently decreased the production of AI-2 and down-regulated the expression level of luxS in PCN033. These results suggest that baicalin can weaken the virulence of PCN033 by inhibiting the LuxS/AI-2 system. After the gene luxS was deleted, AI-2 production in PCN033 was almost completely eliminated, similar to the effect of baicalin on the production of AI-2 in PCN033. This indicates that baicalin reduced the production of AI-2 by inhibiting the expression level of luxS in ExPEC. In addition, the animal experiment further showed the potential of baicalin as a LuxS/AI-2 system inhibitor to prevent ExPEC infection. This study highlights the potential of baicalin as a natural quorum-sensing inhibitor for therapeutic applications in preventing ExPEC infection by targeting the LuxS/AI-2 system.

Impact of LuxS on virulence and pathogenicity in Klebsiella pneumoniae exhibiting varied mucoid phenotypes

How the LuxS/AI-2 quorum sensing (QS) system influences the pathogenicity of K. pneumoniae is complicated by the heterogeneity of the bacterial mucoid phenotypes. This study aims to explore the LuxS-mediated regulation of the pathogenicity of K. pneumoniae with diverse mucoid phenotypes, including hypermucoid, regular-mucoid, and nonmucoid. The wild-type, luxS knockout, and complemented strains of three K. pneumoniae clinical isolates with distinct mucoid phenotypes were constructed. The results revealed the downregulation of virulence genes of regular-mucoid, and nonmucoid but not hypermucoid strains. The deletion of luxS reduced the pathogenicity of the regular-mucoid, and nonmucoid strains in mice; while in hypermucoid strain, luxS knockout reduced virulence in late growth but enhanced virulence in the early growth phase. Furthermore, the absence of luxS led the regular-mucoid and nonmucoid strains to be more sensitive to the host cell defense, and less biofilm-productive than the wild-type at both the low and high-density growth state. Nevertheless, luxS knockout enhanced the resistances to adhesion and phagocytosis by macrophage as well as serum-killing, of hypermucoid K. pneumoniae at its early low-density growth state, while it was opposite to those in its late high-density growth phase. Collectively, our results suggested that LuxS plays a crucial role in the pathogenicity of K. pneumoniae, and it is highly relevant to the mucoid phenotypes and growth phases of the strains. LuxS probably depresses the capsule in the early low-density phase and promotes the capsule, biofilm, and pathogenicity during the late high-density phase, but inhibits lipopolysaccharide throughout the growth phase, in K. pneumoniae.IMPORTANCECharacterizing the regulation of physiological functions by the LuxS/AI-2 quorum sensing (QS) system in Klebsiella pneumoniae strains will improve our understanding of this important pathogen. The genetic heterogeneity of K. pneumoniae isolates complicates our understanding of its pathogenicity, and the association of LuxS with bacterial pathogenicity has remained poorly addressed in K. pneumoniae. Our results demonstrated strain and growth phase-dependent variation in the contributions of LuxS to the virulence and pathogenicity of K. pneumoniae. Our findings provide new insights into the important contribution of the LuxS/AI-2 QS system to the networks that regulate the pathogenicity of K. pneumoniae. Our study will facilitate our understanding of the regulatory mechanisms of LuxS/AI-2 QS on the pathogenicity of K. pneumoniae under the background of their genetic heterogeneity and help develop new strategies for diminished bacterial virulence within the clinical K. pneumoniae population.

Characterization of AI-2/LuxS quorum sensing system in biofilm formation, pathogenesis of Streptococcus equi subsp. zooepidemicus

Streptococcus equi subsp. zooepidemicus (SEZ) is an opportunistic pathogen of both humans and animals. Quorum sensing (QS) plays an important role in the regulation of bacterial group behaviors. The aim of this study was to characterize the LuxS in SEZ and evaluate its impact on biofilm formation, pathogenesis and gene expression. The wild-type SEZ and its LuxS mutant (ΔluxS) were examined for growth, biofilm formation, virulence factors, and transcriptomic profiles. Our results showed that LuxS deficiency did not affect SEZ hemolytic activity, adhesion or capsule production. For biofilm assay demonstrated that mutation in the luxS gene significantly enhances biofilm formation, produced a denser biofilm and attached to a glass surface. RAW264.7 cell infection indicated that ΔluxS promoted macrophage apoptosis and pro-inflammatory responses. In mice infection, there was no significant difference in mortality between SEZ and ΔluxS. However, the bacterial load in the spleen of mice infected with ΔluxS was significantly higher than in those infected with SEZ. And the pathological analysis further indicated that spleen damage was more severe in the ΔluxS group. Moreover, transcriptomics analysis revealed significant alterations in carbon metabolism, RNA binding and stress response genes in ΔluxS. In summary, this study provides the first evidence of AI-2/LuxS QS system in SEZ and reveals its regulatory effects on biofilm formation, pathogenicity and gene expression.

Pathogenicity and virulence of Borrelia burgdorferi

Infection with Borrelia burgdorferi often triggers pathophysiologic perturbations that are further augmented by the inflammatory responses of the host, resulting in the severe clinical conditions of Lyme disease. While our apprehension of the spatial and temporal integration of the virulence determinants during the enzootic cycle of B. burgdorferi is constantly being improved, there is still much to be discovered. Many of the novel virulence strategies discussed in this review are undetermined. Lyme disease spirochaetes must surmount numerous molecular and mechanical obstacles in order to establish a disseminated infection in a vertebrate host. These barriers include borrelial relocation from the midgut of the feeding tick to its body cavity and further to the salivary glands, deposition to the skin, haematogenous dissemination, extravasation from blood circulation system, evasion of the host immune responses, localization to protective niches, and establishment of local as well as distal infection in multiple tissues and organs. Here, the various well-defined but also possible novel strategies and virulence mechanisms used by B. burgdorferi to evade obstacles laid out by the tick vector and usually the mammalian host during colonization and infection are reviewed.

Antimicrobial resistance expansion in pathogens: a review of current mitigation strategies and advances towards innovative therapy

The escalating problem of antimicrobial resistance (AMR) proliferation in clinically important pathogens has become one of the biggest threats to human health and the global economy. Previous studies have estimated AMR-associated deaths and disability-adjusted life-years (DALYs) in many countries with a view to presenting a clearer picture of the global burden of AMR-related diseases. Recently, several novel strategies have been advanced to combat resistance spread. These include efflux activity inhibition, closing of mutant selection window (MSW), biofilm disruption, lytic bacteriophage particles, nanoantibiotics, engineered antimicrobial peptides, and the CRISPR-Cas9 gene-editing technique. The single or integrated deployment of these strategies has shown potentialities towards mitigating resistance and contributing to valuable therapeutic outcomes. Correspondingly, the new paradigm of personalized medicine demands innovative interventions such as improved and accurate point-of-care diagnosis and treatment to curtail AMR. The CRISPR-Cas system is a novel and highly promising nucleic acid detection and manipulating technology with the potential for application in the control of AMR. This review thus considers the specifics of some of the AMR-mitigating strategies, while noting their drawbacks, and discusses the advances in the CRISPR-based technology as an important point-of-care tool for tracking and curbing AMR in our fight against a looming 'post-antibiotic' era.

Design of β-Keto Esters with Antibacterial Activity: Synthesis, In Vitro Evaluation, and Theoretical Assessment of Their Reactivity and Quorum-Sensing Inhibition Capacity

This work proposes the design of β-keto esters as antibacterial compounds. The design was based on the structure of the autoinducer of bacterial quorum sensing, N-(3-oxo-hexanoyl)-l-homoserine lactone (3-oxo-C6-HSL). Eight β-keto ester analogues were synthesised with good yields and were spectroscopically characterised, showing that the compounds were only present in their β-keto ester tautomer form. We carried out a computational analysis of the reactivity and ADME (absorption, distribution, metabolism, and excretion) properties of the compounds as well as molecular docking and molecular dynamics calculations with the LasR and LuxS quorum-sensing (QS) proteins, which are involved in bacterial resistance to antibiotics. The results show that all the compounds exhibit reliable ADME properties and that only compound 7 can present electrophile toxicity. The theoretical reactivity study shows that compounds 6 and 8 present a differential local reactivity regarding the rest of the series. Compound 8 presents the most promising potential in terms of its ability to interact with the LasR and LuxS QS proteins efficiently according to its molecular docking and molecular dynamics calculations. An initial in vitro antimicrobial screening was performed against the human pathogenic bacteria Pseudomonas aeruginosa and Staphylococcus aureus as well as the phytopathogenic bacteria Pseudomonas syringae and Agrobacterium tumefaciens. Compounds 6 and 8 exhibit the most promising results in the in vitro antimicrobial screening against the panel of bacteria studied.

Effects of luxS gene on growth characteristics, biofilm formation, and antimicrobial resistance of multi-antimicrobial-resistant Vibrio parahaemolyticus Vp2015094 isolated from shellfish

AIMS

Vibrio parahaemolyticus is an important foodborne pathogen worldwide, which can cause gastroenteritis. This study aimed to investigate the effect of quorum sensing system LuxS/AI-2-related gene luxS on the biological characteristics and antimicrobial resistance of V. parahaemolyticus Vp2015094 from shellfish, which carried a multi-antimicrobial-resistant plasmid.

METHODS AND RESULTS

The critical gene luxS related to the synthesis of AI-2 in V. parahaemolyticus Vp2015094 was knocked out by homologous recombination with suicide plasmid. The effect of luxS on the biological characteristics of V. parahaemolyticus was determined by comparing the growth, AI-2 activity, motility, biofilm formation ability, and antibiotic resistance between the wildtype strain and the luxS deletion mutant. Compared with wildtype strain, the production of AI-2, the motility and biofilm formation ability, antimicrobial resistance, and conjugation frequency of luxS deletion mutant strain were decreased. The transcriptome sequencing showed that the transcriptional levels of many genes related to motility, biofilm formation, antimicrobial resistance, and conjugation were significantly downregulated after luxS deletion.

CONCLUSIONS

Quorum sensing system LuxS/AI-2-related gene luxS in V. parahaemolyticus Vp2015094 played an important role in growth characteristics, biofilm formation, antimicrobial resistance, and resistance genes' transfer.

Modeling Co-Infection by Streptococcus suis and Haemophilus parasuis Reveals Influences on Biofilm Formation and Host Response

Streptococcus suis (S. suis) and Haemophilus parasuis (H. parasuis) are two primary pathogens currently affecting the porcine industry. They often cause encephalitis and arthritis. They also frequently co-infect in clinical settings. In the current study, we identified significant correlations between S. suis and H. parasuis. The results from CI versus RIR suggested that S. suis and H. parasuis were competitive in general. Compared to mono-species biofilm, the biomass, bio-volume, and thickness of mixed-species biofilms were significantly higher, which was confirmed using crystal violet staining, confocal laser scanning microscopy, and scanning electron microscopy. Compared to mono-species biofilm, the viable bacteria in the mixed-species biofilms were significantly lower, which was confirmed using the enumeration of colony-forming units (CFU cm^-2). The susceptibility of antibiotics in the co-culture decreased in the planktonic state. In contrast, biofilm state bacteria are significantly more difficult to eradicate with antibiotics than in a planktonic state. Whether in planktonic or biofilm state, the expression of virulence genes of S. suis and H. parasuis in mixed culture was very different from that in single culture. Subsequently, by establishing a mixed infection model in mice, we found that the colonization of the two pathogens in organs increased after mixed infection, and altered the host's inflammatory response. In summary, our results indicate that S. suis and H. parasuis compete when co-cultured in vitro. Surprisingly, S. suis and H. parasuis synergistically increased colonization capacity after co-infection in vivo. This study elucidated the interaction between S. suis and H. parasuis during single infections and co-infections. Future studies on bacterial disease control and antibiotic treatment should consider the interaction of mixed species.

Curcumin and 10-undecenoic acid as natural quorum sensing inhibitors of LuxS/AI-2 of Bacillus subtilis and LasI/LasR of Pseudomonas aeruginosa

The quorum sensing (QS) system is related to cell-to-cell communication as a function of population density, which regulates several physiological functions including biofilm formation and virulence gene expression. QS inhibitors have emerged as a promising strategy to tackle virulence and biofilm development. Among a wide variety of phytochemicals, many of them have been described as QS inhibitors. Driven by their promising clues, this study aimed to identify active phytochemicals against LuxS/autoinducer-2 (AI-2) (as the universal QS system) from Bacillus subtilis and LasI/LasR (as a specific QS system) of Pseudomonas aeruginosa, through in silico analysis followed by in vitro validation. The optimized virtual screening protocols were applied to screen a phytochemical database containing 3479 drug-like compounds. The most promising phytochemicals were curcumin, pioglitazone hydrochloride, and 10-undecenoic acid. In vitro analysis corroborated the QS inhibitory activity of curcumin and 10-undecenoic acid, however, pioglitazone hydrochloride showed no relevant effect. Inhibitory effects on LuxS/AI-2 QS system triggered reduction of 33-77% by curcumin (at 1.25-5 µg/mL) and 36-64% by 10-undecenoic acid (at 12.5-50 µg/mL). Inhibition of LasI/LasR QS system was 21% by curcumin (at 200 µg/mL) and 10-54% by 10-undecenoic acid (at 15.625-250 µg/mL). In conclusion, in silico analysis allowed the identification of curcumin and, for the first time, 10-undecenoic acid (showing low cost, high availability, and low toxicity) as alternatives to counteract bacterial pathogenicity and virulence, avoiding the imposition of selective pressure usually related to classic industrial disinfection and antibiotics therapy.

Pan-genome wide association study of Glaesserella parasuis highlights genes associated with virulence and biofilm formation

Glaesserella parasuis is a gram-negative bacterium that causes fibrotic polyserositis and arthritis in pig, significantly affecting the pig industry. The pan-genome of G. parasuis is open. As the number of genes increases, the core and accessory genomes may show more pronounced differences. The genes associated with virulence and biofilm formation are also still unclear due to the diversity of G. parasuis. Therefore, we have applied a pan-genome-wide association study (Pan-GWAS) to 121 strains G. parasuis. Our analysis revealed that the core genome consists of 1,133 genes associated with the cytoskeleton, virulence, and basic biological processes. The accessory genome is highly variable and is a major cause of genetic diversity in G. parasuis. Furthermore, two biologically important traits (virulence, biofilm formation) of G. parasuis were studied via pan-GWAS to search for genes associated with the traits. A total of 142 genes were associated with strong virulence traits. By affecting metabolic pathways and capturing the host nutrients, these genes are involved in signal pathways and virulence factors, which are beneficial for bacterial survival and biofilm formation. This research lays the foundation for further studies on virulence and biofilm formation and provides potential new drug and vaccine targets against G. parasuis.

Bringing Antimicrobial Strategies to a New Level: The Quorum Sensing System as a Target to Control Streptococcus suis

Streptococcus suis (S. suis) is an important zoonotic pathogen. It mainly uses quorum sensing (QS) to adapt to complex and changeable environments. QS is a universal cell-to-cell communication system that has been widely studied for its physiological functions, including the regulation of bacterial adhesion, virulence, and biofilm formation. Quorum sensing inhibitors (QSIs) are highly effective at interfering with the QS system and bacteria have trouble developing resistance to them. We review the current research status of the S. suis LuxS/AI-2 QS system and QSIs. Studies showed that by inhibiting the formation of AI-2, targeting the LuxS protein, inhibiting the expression of luxs gene can control the LuxS/AI-2 QS system of S. suis. Other potential QSIs targets are summarized, which may be preventing and treating S. suis infections, including AI-2 production, transmission, LuxS protein, blockage of AI-2 binding to receptors, AI-2-mediated QS. Since antibiotics are becoming increasingly ineffective due to the emergence of resistant bacteria, including S. suis, it is thus critical to find new antibacterial drugs with different mechanisms of action. QSIs provide hope for the development of such drugs.

1,8-Cineole inhibits biofilm formation and bacterial pathogenicity by suppressing luxS gene expression in Escherichia coli

In recent years, with frequent reports of multi-drug resistant strains, bacteria antibiotic resistance has become an increasingly serious health problem worldwide. One of the most promising ways for combating bacterial infections and antibiotic resistance is development of quorum-sensing (QS) interfering drugs. In this study, the results show that 1,8-cineole inhibited the expression of QS as well as the virulence genes in Escherichia coli O101 (E. coli O101) with a 65% inhibition rate against luxS gene. Therefore, we hypothesized that 1,8-cineole may inhibit the biofilm formation and reduce the pathogenicity of E. coli O101 by inhibiting the expression of luxS gene. To confirm our hypotheses, a luxS gene deleted E. coli O101 was constructed. The results show that the biofilm formation, motility, structure and pathogenicity of E. coli O101 were significantly inhibited following deletion of the luxS gene. In addition, the transcript levels of QS and virulence genes of E. coli O101 were also significantly down-regulated. Interestingly, 1,8-cineole no longer had a significant inhibitory effect on the related phenotype and gene expression of E. coli O101 without luxS gene. In conclusion, the results show that 1,8-cineole can affect bacterial biofilm formation and pathogenicity by suppressing the expression of luxS gene in E. coli O101, which could provide a new perspective for dealing with the biofilm problem of pathogenic bacteria.

In silico metatranscriptomic approach for tracking biofilm-related effectors in dairies and its importance for improving food safety

Sessile microorganisms are usually recalcitrant to antimicrobial treatments, and it is possible that finding biofilm-related effectors in metatranscriptomics datasets helps to understand mechanisms for bacterial persistence in diverse environments, by revealing protein-encoding genes that are expressed in situ. For this research, selected dairy-associated metatranscriptomics bioprojects were downloaded from the public databases JGI GOLD and NCBI (eight milk and 45 cheese samples), to screen for sequences encoding biofilm-related effectors. Based on the literature, the selected genetic determinants were related to adhesins, BAP, flagellum-related, intraspecific QS (AHL, HK, and RR), interspecific QS (LuxS), and QQ (AHL-acylases, AHL-lactonases). To search for the mRNA sequences encoding for those effector proteins, a custom database was built from UniprotKB, yielding 1,154,446 de-replicated sequences that were indexed in DIAMOND for alignment. The results revealed that in all the dairy-associated metatranscriptomic datasets obtained, there were reads assigned to genes involved with flagella, adhesion, and QS/QQ, but BAP-reads were found only for milk. Significant Pearson correlations (p < 0.05) were observed for transcripts encoding for flagella, RR, histidine kinases, adhesins, and LuxS, although no other significant correlations were found. In conclusion, the rationale used in this study was useful to demonstrate the presence of biofilm-associated effectors in metatranscriptomics datasets, pointing out to possible regulatory mechanisms in action in dairy-related biofilms, which could be targeted in the future to improve food safety.

Transcriptome analysis of heat resistance regulated by quorum sensing system in Glaesserella parasuis

The ability of bacteria to resist heat shock allows them to adapt to different environments. In addition, heat shock resistance is known for their virulence. Our previous study showed that the AI-2/luxS quorum sensing system affects the growth characteristics, biofilm formation, and virulence of Glaesserella parasuis. The resistance of quorum sensing system deficient G. parasuis to heat shock was obviously weaker than that of wild type strain. However, the regulatory mechanism of this phenotype remains unclear. To illustrate the regulatory mechanism by which the quorum sensing system provides resistance to heat shock, the transcriptomes of wild type (GPS2), ΔluxS, and luxS complemented (C-luxS) strains were analyzed. Four hundred forty-four differentially expressed genes were identified in quorum sensing system deficient G. parasuis, which participated in multiple regulatory pathways. Furthermore, we found that G. parasuis regulates the expression of rseA, rpoE, rseB, degS, clpP, and htrA genes to resist heat shock via the quorum sensing system. We further confirmed that rseA and rpoE genes exerted an opposite regulatory effect on heat shock resistance. In conclusion, the findings of this study provide a novel insight into how the quorum sensing system affects the transcriptome of G. parasuis and regulates its heat shock resistance property.

Differential Proteomic Analysis of Listeria monocytogenes during High-Pressure Processing

Simple Summary

High-pressure processing (HPP) is a prevailing non-thermal food preservation technology. The inactivation mechanisms of Listeria monocytogenes under sub-lethal to lethal damage by different levels of HPP treatments were conducted by label-free quantitative proteomic analysis. HPP might promote translation initiation due to upregulation of most ribosomal subunits and initiation factors. However, protein synthesis was arrested according to the shortage of proteins responsible for elongation, termination and recycling. The quantitative proteomics approaches provide fundamental information on L. monocytogenes under different HPP pressures, and provide theoretical support for HPP against Listeriosis illness and for promotion of safer ready-to-eat foods.

Abstract

High-pressure processing (HPP) is a prevailing non-thermal food preservation technology. The inactivation mechanisms of Listeria monocytogenes under HPP at 200 and 400 MPa for 3 min were investigated by label-free quantitative proteomic analysis and functional enrichment analysis in the Kyoto Encyclopedia of Genes and Genomes. HPP treatment at 400 MPa exhibited significant effects on proteins involved in translation, carbon, carbohydrate, lipid and energy metabolism, and peptidoglycan biosynthesis. HPP increased most ribosomal subunits and initiation factors, suggesting it might shift ribosomal biogenesis to translation initiation. However, protein synthesis was impaired by the shortage of proteins responsible for elongation, termination and recycling. HPP stimulated several ATP-dependent Clp proteases, and the global transcriptional regulator Spx, associating with activation of the stress-activated sigma factor Sigma B (σ^B) and the transcriptional activator positive regulatory factor A (PrfA) regulons. The quantitative proteomics approaches provide fundamental information on L. monocytogenes under different HPP pressures, and provide theoretical support for HPP against Listeriosis illness and for promotion of safer ready-to-eat foods.

The molecular diversity of transcriptional factor TfoX is a determinant in natural transformation in Glaesserella parasuis

Natural transformation is a mechanism by which a particular bacterial species takes up foreign DNA and integrates it into its genome. The swine pathogen Glaesserella parasuis (G. parasuis) is a naturally transformable bacterium. The regulation of competence, however, is not fully understood. In this study, the natural transformability of 99 strains was investigated. Only 44% of the strains were transformable under laboratory conditions. Through a high-resolution melting curve and phylogenetic analysis, we found that genetic differences in the core regulator of natural transformation, the tfoX gene, leads to two distinct natural transformation phenotypes. In the absence of the tfoX gene, the highly transformable strain SC1401 lost its natural transformability. In addition, when the SC1401 tfoX gene was replaced by the tfoX of SH0165, which has no natural transformability, competence was also lost. These results suggest that TfoX is a core regulator of natural transformation in G. parasuis, and that differences in tfoX can be used as a molecular indicator of natural transformability. Transcriptomic and proteomic analyses of the SC1401 wildtype strain, and a tfoX gene deletion strain showed that differential gene expression and protein synthesis is mainly centered on pathways related to glucose metabolism. The results suggest that tfoX may mediate natural transformation by regulating the metabolism of carbon sources. Our study provides evidence that tfoX plays an important role in the natural transformation of G. parasuis.

The role of bacterial signaling networks in antibiotics response and resistance regulation

Excessive use of antibiotics poses a threat to public health and the environment. In ecosystems, such as the marine environment, antibiotic contamination has led to an increase in bacterial resistance. Therefore, the study of bacterial response to antibiotics and the regulation of resistance formation have become an important research field. Traditionally, the processes related to antibiotic responses and resistance regulation have mainly included the activation of efflux pumps, mutation of antibiotic targets, production of biofilms, and production of inactivated or passivation enzymes. In recent years, studies have shown that bacterial signaling networks can affect antibiotic responses and resistance regulation. Signaling systems mostly alter resistance by regulating biofilms, efflux pumps, and mobile genetic elements. Here we provide an overview of how bacterial intraspecific and interspecific signaling networks affect the response to environmental antibiotics. In doing so, this review provides theoretical support for inhibiting bacterial antibiotic resistance and alleviating health and ecological problems caused by antibiotic contamination.

YqeH contributes to avian pathogenic Escherichia coli pathogenicity by regulating motility, biofilm formation, and virulence

Avian pathogenic Escherichia coli (APEC) is a pathotype of extraintestinal pathogenic E. coli and one of the most serious infectious diseases of poultry. It not only causes great economic losses to the poultry industry, but also poses a serious threat to public health worldwide. Here, we examined the role of YqeH, a transcriptional regulator located at E. coli type III secretion system 2 (ETT2), in APEC pathogenesis. To investigate the effects of YqeH on APEC phenotype and virulence, we constructed a yqeH deletion mutant (APEC40-ΔyqeH) and a complemented strain (APEC40-CΔyqeH) of APEC40. Compared with the wild type (WT), the motility and biofilm formation of APEC40-ΔyqeH were significantly reduced. The yqeH mutant was highly attenuated in a chick infection model compared with WT, and showed severe defects in its adherence to and invasion of chicken embryo fibroblast DF-1 cells. However, the mechanisms underlying these phenomena were unclear. Therefore, we analyzed the transcriptional effects of the yqeH deletion to clarify the regulatory mechanisms of YqeH, and the role of YqeH in APEC virulence. The deletion of yqeH downregulated the transcript levels of several flagellum-, biofilm-, and virulence-related genes. Our results demonstrate that YqeH is involved in APEC pathogenesis, and the reduced virulence of APEC40-ΔyqeH may be related to its reduced motility and biofilm formation.

Influence of Polysaccharides From Polygonatum kingianum on Short-Chain Fatty Acid Production and Quorum Sensing in Lactobacillus faecis

Polysaccharide is one of the main active ingredients of Polygonatum kingianum, which has been proven to regulate the balance of gut microbiota. For the first time, this study focused on the regulation of polysaccharides from Polygonatum kingianum (PS) on Lactobacillus faecis, a specific probiotic in the intestinal tract. PS effectively promoted the biomass, biofilm and acetic acid production in L. faecis 2-84, and enhanced quorum sensing (QS) signaling. The characteristics of gene sequence were analyzed using genomics approaches, and L. faecis 2-84 was found to encode 18 genes that are closely related to QS and 10 genes related to short-chain fatty acids (SCFAs). Additionally, transcriptome and proteome analysis demonstrated that PS could promote the QS system of L. faecis by enhancing the transcription of oppA gene and expression of oppD protein. PS also regulated the production and metabolism of SCFAs of L. faecis by upregulating the expression of ldh and metE gene and adh2 protein, and downregulating the expression of mvK gene. In conclusion, it was speculated that PS could affect intestinal SCFAs production by affecting the QS system and SCFAs production in L. faecis. The present study implied that PS might have a role in promoting the growth of intestinal probiotics, where the QS system and SCFAs might be two of the important mechanisms for the probiotic activity of PS.

Probiotics as Therapeutic Tools against Pathogenic Biofilms: Have We Found the Perfect Weapon?

Bacterial populations inhabiting a variety of natural and human-associated niches have the ability to grow in the form of biofilms. A large part of pathological chronic conditions, and essentially all the bacterial infections associated with implanted medical devices or prosthetics, are caused by microorganisms embedded in a matrix made of polysaccharides, proteins, and nucleic acids. Biofilm infections are generally characterized by a slow onset, mild symptoms, tendency to chronicity, and refractory response to antibiotic therapy. Even though the molecular mechanisms responsible for resistance to antimicrobial agents and host defenses have been deeply clarified, effective means to fight biofilms are still required. Lactic acid bacteria (LAB), used as probiotics, are emerging as powerful weapons to prevent adhesion, biofilm formation, and control overgrowth of pathogens. Hence, using probiotics or their metabolites to quench and interrupt bacterial communication and aggregation, and to interfere with biofilm formation and stability, might represent a new frontier in clinical microbiology and a valid alternative to antibiotic therapies. This review summarizes the current knowledge on the experimental and therapeutic applications of LAB to interfere with biofilm formation or disrupt the stability of pathogenic biofilms.

Deleting qseC downregulates virulence and promotes cross-protection in Pasteurella multocida

QseC, a histidine sensor kinase of the QseBC two-component system, acts as a global regulator of bacterial stress resistance, biofilm formation, and virulence. The function of QseC in some bacteria is well understood, but not in Pasteurella multocida. We found that deleting qseC in P. multocida serotype A:L3 significantly down-regulated bacterial virulence. The mutant had significantly reduced capsule production but increased resistance to oxidative stress and osmotic pressure. Deleting qseC led to a significant increase in qseB expression. Transcriptome sequencing analysis showed that 1245 genes were regulated by qseC, primarily those genes involved in capsule and LPS biosynthesis and export, biofilm formation, and iron uptake/utilization, as well as several immuno-protection related genes including ompA, ptfA, plpB, vacJ, and sodA. In addition to presenting strong immune protection against P. multocida serotypes A:L1 and A:L3 infection, live ΔqseC also exhibited protection against P. multocida serotype B:L2 and serotype F:L3 infection in a mouse model. The results indicate that QseC regulates capsular production and virulence in P. multocida. Furthermore, the qseC mutant can be used as an attenuated vaccine against P. multocida strains of multiple serotypes.

Inhibitory effects of Flavourzyme on biofilm formation, quorum sensing, and virulence genes of foodborne pathogens Salmonella Typhimurium and Escherichia coli

Salmonella enterica and Shiga toxin-producing (or verotoxin-producing) Escherichia coli are major foodborne pathogens, posing substantial food safety risks. Due to the negative effects of chemical treatment against foodborne pathogens, the application of enzyme-based techniques is currently receiving great attention. Here, we evaluated the inhibitory properties of Flavourzyme, a commercial peptidase, against these two foodborne pathogens. We noticed 4.0 and 5.5 log inhibition of biofilm formation by S. Typhimurium and E. coli, respectively, while treated with sub-minimum inhibitory concentrations of Flavourzyme for 24 h. For both bacteria, the enzyme exhibited quorum-quenching activity, preventing autoinducer-2 production completely by E. coli. In addition, Flavourzyme significantly suppressed the relative expression levels of biofilm-forming, quorum sensing, and virulence regulatory genes as measured by qRT-PCR. Based on our results, we suggest the use of Flavourzyme as a preventive agent against foodborne pathogens that possibly acts by inhibiting bacterial self-defense mechanisms following disruption of cellular proteins. This finding may shed light on how enzymes can be applied as a novel weapon to control foodborne illnesses to ensure food safety and public health.

Baicalin Protects Vascular Tight Junctions in Piglets During Glaesserella parasuis Infection

Glaesserella parasuis (G. parasuis) can cause Glässer's disease and severely affect swine industry worldwide. This study is an attempt to address the issue of the capability of G. parasuis to damage the vascular barrier and the effects of baicalin on vascular tight junctions (TJ) in order to investigate the interactions between the pathogen and the porcine vascular endothelium. Piglets were challenged with G. parasuis and treated with or without baicalin. The expressions of vascular TJ genes were examined using RT-PCR. The distribution patterns of TJ proteins were detected by immunofluorescence. The involved signaling pathways were determined by Western blot assays on related proteins. G. parasuis can downregulate TJ expression and disrupt the distribution of TJ proteins. Baicalin can alleviate the downregulation of vascular TJ mRNA, maintain the distribution, and prevent the abnormalities of TJ. These results provide ample evidence that baicalin has the capacity to protect vascular TJ damaged by G. parasuis through inhibiting PKC and MLCK/MLC pathway activation. As a result, baicalin is a promising candidate for application as a natural agent for the prevention and control of G. parasuis infection.

Biofilm characteristics and transcriptomic analysis of Haemophilus parasuis

Haemophilus parasuis (H. parasuis) is a conditional pathogen with the ability to form biofilms which can lead to ineffective drug treatment and severe chronic infections resulting in significant economic losses to the pig industry. Currently, knowledge of biofilm formation by H. parasuis is not well developed. The objective of this study was to investigate the three-dimensional morphology of biofilms and perform transcriptomic analysis on H. parasuis cells in biofilm versus planktonic forms. The results showed that proteins and DNA accounted for a large proportion of the H. parasuis biofilm extracellular matrix. Here, we have traced the entire biofilm formation process of H. parasuis from beginning to end for the first time. These biofilms grew rapidly in the first 48 h and became stable at 60 h. According to GO and KEGG analysis, the differentially expressed genes (DEG) artM, artQ, ssrS, pflA and HutX were implicated as being involved in bacterial colonisation and adhesion; these are the most likely genes to affect biofilm formation. Most functional gene enrichments were of those involved in metabolic pathways, biosynthesis of secondary metabolites, ATP-binding cassette (ABC) transporters, and starch and sucrose metabolism. Thus, in the present pilot study, the composition and characteristics of these biofilms were explored, and the genes related to biofilm formation were screened for. This research lays the foundation for further studies on mechanisms regulating biofilm formation, in order to find new drug targets and develop new therapeutic drugs against H. parasuis.

Comparative transcriptomics analysis of Streptococcus mutans with disruption of LuxS/AI-2 quorum sensing and recovery of methyl cycle

OBJECTIVE

The LuxS/AI-2 quorum sensing (QS) system has critical roles in Streptococcus mutans cariogenicity. Whereas the molecular and cellular mechanisms of the LuxS/AI-2 QS system are not thoroughly understood. Given that LuxS has roles in QS and methyl cycle, its mutation can cause QS deficiency and methyl cycle disruption. The aim of this study was to investigate effects of the LuxS/AI-2 QS system on gene expression in Streptococcus mutans when methyl cycle was recovered with exogenous sahH gene.

METHODS

Our previous study introduced the exogenous sahH gene from Pseudomonas aeruginosa into an S. mutans luxS-null strain to restore the disrupted methyl cycle, and this produced the solely the LuxS/AI-2 QS system deficient strain. Here, we analyzed the transcriptomics of this strain to get insights into the molecular mechanisms of the LuxS/AI-2 QS system applying RNA-seq.

RESULTS

With recovery of methyl cycle, 84 genes didn't change in expression trends in S. mutans luxS-null strain. These genes mainly encode the ABC transporters, sugar transporter EII and enzymes of carbohydrate metabolism, and are rich in the Phosphotransferase system, Fructose and mannose metabolism, Amino sugar and nucleotide sugar metabolism, Galactose metabolism, Glycolysis/Gluconeogenesis, RNA degradation, Lysine biosynthesis, and Glycine, serine and threonine metabolism.

CONCLUSIONS

The LuxS/AI-2 QS system may mainly affect ABC transporters and carbohydrate transport, transformation and metabolism via EII subunits and enzymes to influence virulence-associated traits without effects of methyl cycle inStreptococcus mutans.

Research on the role of LuxS/AI-2 quorum sensing in biofilm of Leuconostoc citreum 37 based on complete genome sequencing

Leuconostoc citreum, a type of food-grade probiotic bacteria, plays an important role in food fermentation and intestinal probiotics. Biofilms help bacteria survive under adverse conditions, and LuxS/AI-2-dependent quorum sensing (QS) plays an important role in the regulation of their biofilm-forming activities. L. citreum 37 was a biofilm-forming strain isolated from dairy products. The aim of this study was to analyze genes involved in the LuxS/AI-2 system based on genome sequencing and biofilm formation of L. citreum 37. Genome assembly yielded two contigs (one chromosome and one plasmid), and the complete genome contained 1,946,279 base pairs (bps) with a G + C content of 38.91%. The genome sequence analysis showed that there were several pathways such as the two-component system, QS, and seven other signal pathways, and 26 genes (including luxS, pfs, and 24 other genes) may participate in QS related to biofilm formation. All these results showed that the LuxS/AI-2 system is complete in the genome of L. citreum 37. The quantitative polymerase chain reaction (qPCR) of pfs, luxS genes, and AI-2 production of L. citreum 37 in planktonic state and biofilm state showed that the expression of pfs and luxS genes was consistent with the production of AI-2 and was positively correlated with biofilm formation. After luxS of L. citreum 37 expressed in Escherichia coli BL21, AI-2 production was detected, suggesting that the luxS gene played an important role in AI-2 synthesis, Therefore, luxS may regulate the biofilm formation of L. citreum 37 by participating in AI-2 synthesis. It is projected that results of this study could help facilitate further understanding and application of L. citreum 37.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02747-2.

Probiotic Bacilli Inhibit Salmonella Biofilm Formation Without Killing Planktonic Cells

Salmonellosis is a foodborne infection caused by Salmonella. Domestic poultry species are one of the main reservoirs of Salmonella, which causes the foodborne infection salmonellosis, and are responsible for many cases of animal-to-human transmission. Keeping backyard chickens is now a growing trend, increasing the frequency of direct contact with the flock and, by consequence, the incidence of Salmonella infections. Bacillus subtilis KATMIRA1933 and Bacillus amyloliquefaciens B-1895 are probiotic bacilli that produce the bacteriocins subtilosin A and subtilin, respectively. The antimicrobial activity of the two strains was determined against the reference strain Micrococcus luteus ATCC 10420. The cell-free supernatant of B. subtilis KATMIRA1933 inhibited biofilm formation by Salmonella enterica subsp. enterica serovar Hadar, Salmonella enterica subsp. enterica serovar Enteritidis phage type 4, and Salmonella enterica subsp. enterica serovar Thompson by 51.1, 48.3, and 56.9%, respectively. The cell-free supernatant of B. amyloliquefaciens B-1895 inhibited the biofilm formation of these Salmonella strains by 30.4, 28.6, and 35.5%, respectively. These findings suggest that the bacillus strains may have the potential to be used as probiotics and antibiotic alternatives for the control of Salmonella in poultry. The number of planktonic cells was unaffected by treatment with the cell-free supernatant. A co-culture of the Salmonella strains with either bacilli showed no signs of growth inhibition, suggesting that it might have been quorum sensing that is affected by the two Bacillus strains.

Enhanced biofilm formation of Streptococcus gordonii with lipoprotein deficiency

Streptococcus gordonii is a commensal Gram-positive bacterium that acts as an opportunistic pathogen that can cause apical periodontitis, endocarditis, and pneumonia. Biofilm formation of bacteria is important for the initiation and progression of such diseases. Although lipoproteins play key roles in physiological functions, the role of lipoproteins of S. gordonii in its biofilm formation has not been clearly understood. In this study, we investigated the role of lipoproteins of S. gordonii in the bacterial biofilm formation using its lipoprotein-deficient strain (Δlgt). The S. gordonii Δlgt exhibited increased biofilm formation on the human dentin slices or on the polystyrene surfaces compared to the wild-type strain, while its growth rate did not differ from that of the wild-type. In addition, the S. gordonii Δlgt strain exhibited the enhanced LuxS mRNA expression and AI-2 production, which is known to be a positive regulator of biofilm formation, compared to the wild-type. Concordantly, the augmented biofilm formation of S. gordonii Δlgt was attenuated by an AI-2 inhibitor, D-ribose. In addition, lipoproteins from purified S. gordonii inhibited the biofilm formation of S. gordonii wild-type and Δlgt. Taken together, these results suggest that lipoprotein-deficient S. gordonii form biofilms more effectively than the wild-type strain, which might be related to the AI-2 quorum-sensing system.

The effects of baicalin on piglets challenged with Glaesserella parasuis

Glaesserella parasuis (G. parasuis) causes porcine vascular inflammation and damage. Baicalin is reported to have antioxidant and anti-inflammatory functions. However, whether baicalin protects piglets against G. parasuis challenge and the potential protective mechanism have not been investigated. Therefore, in this study, we comprehensively examined the protective efficacy of baicalin in piglets challenged with G. parasuis and the possible protective mechanism. Our results show that baicalin attenuated the release of the inflammation-related cytokines interleukin (IL) 1β, IL6, IL8, IL10, and tumour necrosis factor α (TNF-α) and reduced high mobility group box 1 (HMGB1) production and cell apoptosis in piglets infected with G. parasuis. Baicalin also inhibited the activation of the mitogen-activated protein kinase (MAPK) signalling pathway and protected piglets against G. parasuis challenge. Taken together, our data suggest that baicalin could protect piglets from G. parasuis by reducing HMGB1 release, attenuating cell apoptosis, and inhibiting MAPK signalling activation, thereby alleviating the inflammatory response induced by the bacteria. Our results suggest that baicalin has utility as a novel therapeutic drug to control G. parasuis infection.

Effects of Natural Products on Bacterial Communication and Network-Quorum Sensing

Quorum sensing (QS) has emerged as a research hotspot in microbiology and medicine. QS is a regulatory cell communication system used by bacterial flora to signal to the external environment. QS influences bacterial growth, proliferation, biofilm formation, virulence factor production, antibiotic synthesis, and environmental adaptation. Through the QS system, natural products can regulate the growth of harmful bacteria and enhance the growth of beneficial bacteria, thereby improving human health. Herein, we review advances in the discovery of natural products that regulate bacterial QS systems.

Whole Genome Sequencing and Comparative Genomic Analyses of Lysinibacillus pakistanensis LZH-9, a Halotolerant Strain with Excellent COD Removal Capability

Halotolerant microorganisms are promising in bio-treatment of hypersaline industrial wastewater. Four halotolerant bacteria strains were isolated from wastewater treatment plant, of which a strain LZH-9 could grow in the presence of up to 14% (w/v) NaCl, and it removed 81.9% chemical oxygen demand (COD) at 96 h after optimization. Whole genome sequencing of Lysinibacillus pakistanensis LZH-9 and comparative genomic analysis revealed metabolic versatility of different species of Lysinibacillus, and abundant genes involved in xenobiotics biodegradation, resistance to toxic compound, and salinity were found in all tested species of Lysinibacillus, in which Horizontal Gene Transfer (HGT) contributed to the acquisition of many important properties of Lysinibacillus spp. such as toxic compound resistance and osmotic stress resistance as revealed by phylogenetic analyses. Besides, genome wide positive selection analyses revealed seven genes that contained adaptive mutations in Lysinibacillus spp., most of which were multifunctional. Further expression assessment with Codon Adaption Index (CAI) also reflected the high metabolic rate of L. pakistanensis to digest potential carbon or nitrogen sources in organic contaminants, which was closely linked with efficient COD removal ability of strain LZH-9. The high COD removal efficiency and halotolerance as well as genomic evidences suggested that L. pakistanensis LZH-9 was promising in treating hypersaline industrial wastewater.

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.

LuxS/AI-2 Quorum Sensing System in Edwardsiella piscicida Promotes Biofilm Formation and Pathogenicity

LuxS/AI-2 is an important quorum sensing system which affects the growth, biofilm formation, virulence, and metabolism of bacteria. LuxS is encoded by the luxS gene, but how this gene is associated with a diverse array of physiological activities in Edwardsiella piscicida (E. piscicida) is not known. Here, we constructed an luxS gene mutant strain, the △luxS strain, to identify how LuxS/AI-2 affects pathogenicity. The results showed that LuxS was not found in the luxS gene mutant strain, and this gene deletion decreased E. piscicida growth compared to that of the wild-type strain. Meanwhile, the wild-type strain significantly increased penetration and motility in mucin compared to levels with the △luxS strain. The 50% lethal dose (LD₅₀) of the E. piscicida △luxS strain for zebrafish was significantly higher than that of the wild-type strain, which suggested that the luxS gene deletion could attenuate the strain's virulence. The AI-2 activities of EIB202 were 56-fold higher than those in the △luxS strain, suggesting that the luxS gene promotes AI-2 production. Transcriptome results demonstrated that between cells infected with the △luxS strain and those infected with the wild-type strain 46 genes were significantly differentially regulated, which included 34 upregulated genes and 12 downregulated genes. Among these genes, the largest number were closely related to cell immunity and signaling systems. In addition, the biofilm formation ability of EIB202 was significantly higher than that of the △luxS strain. The supernatant of EIB202 increased the biofilm formation ability of the △luxS strain, which suggested that the luxS gene and its product LuxS enhanced biofilm formation in E. piscicida All results indicate that the LuxS/AI-2 quorum sensing system in E. piscicida promotes its pathogenicity through increasing a diverse array of physiological activities.

Antivirulence properties and related mechanisms of spice essential oils: A comprehensive review

In recent decades, reduced antimicrobial effectiveness, increased bacterial infection, and newly emerged microbial resistance have become global public issues, leading to an urgent need to find effective strategies to counteract these problems. Strategies targeting bacterial virulence factors rather than bacterial survival have attracted increasing interest, since the modulation of virulence factors may prevent the development of drug resistance in bacteria. Spices are promising natural sources of antivirulence compounds owing to their wide availability, diverse antivirulence phytochemical constituents, and generally favorable safety profiles. Essential oils are the predominant and most important antivirulence components of spices. This review addresses the recent efforts of using spice essential oils to inhibit main bacterial virulence traits, including the quorum sensing system, biofilm formation, motility, and toxin production, with an intensive discussion of related mechanisms. We hope that this review can provide a better understanding of the antivirulence properties of spice essential oils, which have the potential to be used as antibiotic alternatives by targeting bacterial virulence.

Effect of cAMP Receptor Protein Gene on Growth Characteristics and Stress Resistance of Haemophilus parasuis Serovar 5

Haemophilus parasuis (HPS), a member of the family Pasteurellaceae, is a common bacteria in the upper respiratory tract of pigs but under certain circumstances can cause serious systemic disease (Glasser's disease) characterized by severe infection of the upper respiratory tract, fibrinous polyserositis, polyarthritis, and meningitis. cAMP receptor protein (CRP) is among the most important global regulators, playing a vital role in adapting to environmental changes during the process of bacterial infection. In order to investigate the function of the crp gene in the growth characteristics of H. parasuis serovar 5 (HPS5) and its ability to overcome adverse environmental stresses, a crp mutant strain (Δcrp) was constructed and verified. In this study, we found that the crp gene was involved in growth rate, biofilm formation, stress tolerance, serum resistance, and iron utilization. Compared with the wild type, both the growth rate of the crp mutant and its resistance to osmotic pressure decreased significantly. Similar phenomena were also found in biofilm formation and iron utilization. However, the resistance to heat shock and serum complement of the crp mutant were enhanced. This study aimed to reveal the function in growth characteristics and stress resistance of the crp gene in HPS5. Whether it relates to virulence requires additional in-depth research.

Challenges and Limitations of Anti-quorum Sensing Therapies

Quorum sensing (QS) is a mechanism allowing microorganisms to sense population density and synchronously control genes expression. It has been shown that QS supervises the activity of many processes important for microbial pathogenicity, e.g., sporulation, biofilm formation, and secretion of enzymes or membrane vesicles. This contributed to the concept of anti-QS therapy [also called quorum quenching (QQ)] and the opportunity of its application in fighting against various types of pathogens. In recent years, many published articles reported promising results indicating the possibility of reducing pathogenicity of tested microorganisms and their easier eradication when co-treated with antibiotics. The aim of the present article is to point to the opposite, negative side of the QQ therapy, with particular emphasis on three fundamental properties attributed to anti-QS substances: the selectivity, virulence reduction, and lack of resistance against QQ. This point of view may highlight new directions of research, which should be taken into account in the future before the widespread introduction of QQ therapies in the treatment of people.

Antibiofilm activity and modes of action of a novel β-sheet peptide against multidrug-resistant Salmonella enterica

S. enterica is an important foodborne pathogen worldwide. As some strains can form biofilms which may offer protection against antimicrobials, it is of interest to explore ways to prevent biofilm formation by S. enterica. In this study, we engineered a short β-sheet peptide WK2 (WKWKCTKSGCKWKW-NH₂) and examined its antimicrobial and anti-biofilm activities against various S. enterica strains, including the multidrug-resistant S. Typhimurium DT104. WK2 displayed bacteriostatic activity with a geometric mean (GM) minimum inhibitory concentration (MIC) of 4.17 μg/mL, and bactericidal activity, with a GM lethal concentration (LC) of 7.51 μg/mL. Crystal violet staining and fluorescence measurements demonstrated that WK2 inhibited S. Typhimurium DT104 biofilm formation at 0.5 μg/mL and killed the sessile cells in biofilms at 8 μg/mL. Real-time polymerase chain reaction (qPCR) and microscopic observation revealed that the anti-biofilm activity of WK2 likely arises through the formation of complexes with bacterial DNA, inhibition of surface organelle biosynthesis and interference with autoinducer-2 (AI-2)-mediated quorum sensing (QS). Therefore, WK2 is a promising antimicrobial agent for the prevention and control of biofilms produced by multidrug-resistant S. enterica.