Evidence That VirS Is a Receptor for the Signaling Peptide of the Clostridium perfringens Agr-like Quorum Sensing System

The presence of differentiated C2C12 muscle cells enhances toxin production and growth by Clostridium perfringens type A strain ATCC3624

Clostridium perfringens type A causes gas gangrene, which involves muscle infection. Both alpha toxin (PLC), encoded by the plc gene, and perfringolysin O (PFO), encoded by the pfoA gene, are important when type A strains cause gas gangrene in a mouse model. This study used the differentiated C2C12 muscle cell line to test the hypothesis that one or both of those toxins contributes to gas gangrene pathogenesis by releasing growth nutrients from muscle cells. RT-qPCR analyses showed that the presence of differentiated C2C12 cells induces C. perfringens type A strain ATCC3624 to upregulate plc and pfoA expression, as well as increase expression of several regulatory genes, including virS/R, agrB/D, and eutV/W. The VirS/R two component regulatory system (TCRS) and its coupled Agr-like quorum sensing system, along with the EutV/W TCRS (which regulates expression of genes involved in ethanolamine [EA] utilization), were shown to mediate the C2C12 cell-induced increase in plc and pfoA expression. EA was demonstrated to increase toxin gene expression. ATCC3624 growth increased in the presence of differentiated C2C12 muscle cells and this effect was shown to involve both PFO and PLC. Those membrane-active toxins were each cytotoxic for differentiated C2C12 cells, suggesting they support ATCC3624 growth by releasing nutrients from differentiated C2C12 cells. These findings support a model where, during gas gangrene, increased production of PFO and PLC in the presence of muscle cells causes more damage to those host cells, which release nutrients like EA that are then used to support C. perfringens growth in muscle.

Anemoside B4 attenuates necrotic enteritis of laying hens induced by Clostridium perfringens via inhibiting NF-κB and PI3K/Akt/mTOR signalling pathways

Poultry necrotic enteritis is an important enteric disease which might be controlled by antibiotics. However, with the excessive use of antibiotics, the phenomenon of drug resistance of Clostridium perfringens is becoming increasingly prominent. Anemoside B4 exhibits important anti-inflammatory, antioxidant and immunomodulatory effects. This study was performed to estimate the effect of Anemoside B4 on chicken necrotic enteritis induced by C. perfringens in vivo and in vitro. In the in vivo experiment we investigated the efficacy of Anemoside B4 on the growth curve, biofilm formation, haemolytic activity, virulence-related gene expression and NF-κB and PI3K/AKT/mTOR activation in Caco-2 cells induced by C. perfringens. The results showed that 12.5-50 μg/mL Anemoside B4 had no antibacterial activity but could inhibit biofilm formation, attenuate haemolytic activity and virulence-related gene expression of C. perfringens and weaken NF-κB and PI3K/Akt/mTOR activation triggered by C. perfringens in Caco-2 cells. In the in vivo experiment, 60 17-day-old healthy White Leghorns were randomly divided into six groups. The growing laying hens of the control group were fed a basic diet, and those of the five challenged groups were fed a basic diet (infection group), added 0.43 g/kg Anemoside B4 (0.43 g/kg Ane group), 0.86 g/kg Anemoside B4 (0.86 g/kg Ane group), 1.72 g/kg Anemoside B4 (1.72 g/kg Ane group) and 40 mg/kg lincomycin (lincomycin group), respectively. All challenged laying hens were infected with 1 × 109 CFU C. perfringens from day 17-20. Blood and intestinal samples were obtained, and the data demonstrated that Anemoside B4 improved the blood biochemical parameters, attenuated jejunum tissue injury, increased the spleen, thymus, bursa of fabricius index, and decreased lesion scores of the jejunum and the ileum. In the jejunum, Anemoside B4 and lincomycin downregulated the expression of IL-1β, IL-6, IL-10, TNF-α and IFN-γ at mRNA levels. Moreover, Anemoside B4 significantly enhanced both mRNA and protein levels of tight junctions ZO-1, Claudin-1 and MUC-2 in the jejunum. Anemoside B4 weakened p-P65, p-PI3K, p-Akt and p-mTOR protein expression in the jejunum infected by C. perfringens. Diets supplemented with Anemoside B4 alleviated C. perfringens-induced necrotic enteritis in laying hens by inhibiting NF-κB and PI3K/Akt/mTOR signalling pathways and improving intestinal barrier functions.

Cooperation of quorum sensing and central carbon metabolism in the pathogenesis of Gram-positive bacteria

Quorum sensing (QS), an integral component of bacterial communication, is essential in coordinating the collective response of diverse bacterial pathogens. Central carbon metabolism (CCM), serving as the primary metabolic hub for substances such as sugars, lipids, and amino acids, plays a crucial role in the life cycle of bacteria. Pathogenic bacteria often utilize CCM to regulate population metabolism and enhance the synthesis of specific cellular structures, thereby facilitating in adaptation to the host microecological environment and expediting infection. Research has demonstrated that QS can both directly or indirectly affect the CCM of numerous pathogenic bacteria, thus altering their virulence and pathogenicity. This article reviews the interplay between QS and CCM in Gram-positive pathogenic bacteria, details the molecular mechanisms by which QS modulates CCM, and lays the groundwork for investigating bacterial pathogenicity and developing innovative infection treatment drugs.

Deciphering agr quorum sensing in Staphylococcus aureus: insights and therapeutic prospects

The emergence of superbugs like methicillin-resistant Staphylococcus aureus exposed the limitations of treating microbial infections using antibiotics. At present, the discovery of novel and convincing therapeutic methods are being executed increasingly as possible substitutes to conventional antibiotic therapies. The quorum sensing helps Staphylococcus aureus become more viable through their signaling mechanisms. In recent years, targeting the prominent factors of quorum sensing has obtained remarkable attention as a futuristic approach to dealing with bacterial pathogenicity. The standard antibiotic therapy intends to inhibit the organism by targeting specific molecules and afford a chance for the evolution of antibiotic resistance. This prompts the development of novel therapeutic strategies like inhibiting quorum sensing that can limit bacterial virulence by decreasing the selective pressure, thereby restricting antibiotic resistance evolution. This review furnishes new insights into the accessory gene regulator quorum sensing in Staphylococcus aureus and its inhibition by targeting the genes that regulate the operon. Further, this review comprehensively explores the inhibitors reported up to date and their specific targets and discusses their potentially ineffective alternative therapy against methicillin-resistant Staphylococcus aureus.

Effect of in-water administration of quorum system inhibitors in broilers' productive performance and intestinal microbiome in a mild necrotic enteritis challenge

The poultry industry has been facing the impact of necrotic enteritis (NE), a disease caused by the bacterium Clostridium perfringens producing the haemolytic toxin NetB. NE severity may vary from mild clinical to prominent enteric signs causing reduced growth rates and affecting feed conversion ratio. NetB production is controlled by the Agr-like quorum-sensing (QS) system, which coordinates virulence gene expression in response to bacterial cell density. In this study, the peptide-containing cell-free spent media (CFSM) from Enterococcus faecium was tested in NE challenged broilers in two battery cage and one floor pen studies. Results showed a significant reduction of NE mortality. Metagenomic sequencing of the jejunum microbiome revealed no impact of the CFSM on the microbial community, and growth of C. perfringens was unaffected by CFSM in vitro. The expression of QS-controlled virulence genes netB, plc and pfoA was found to be significantly repressed by CFSM during the mid-logarithmic stage of C. perfringens growth and this corresponded with a significant decrease in haemolytic activity. Purified fractions of CFSM containing bioactive peptides were found to cause reduced haemolysis. These results showed that bioactive peptides reduce NE mortality in broilers by interfering with the QS system of C. perfringens and reducing bacterial virulence. Furthermore, the microbiome of C. perfringens-challenged broilers is not affected by quorum sensing inhibitor containing CFSM.

Baicalin-aluminum alleviates necrotic enteritis in broiler chickens by inhibiting virulence factors expression of Clostridium perfringens

Clostridium perfringens type A is the main cause of necrotic enteritis (NE) in chickens. Since the use of antibiotics in feed is withdrawn, it is imperative to find out suitable alternatives to control NE. Baicalin-aluminum complex is synthesized from baicalin, a flavonoid isolated from Scutellaria baicalensis Georgi. The present study investigated the effects of baicalin-aluminum on the virulence-associated traits and virulence genes expression of C. perfringens CVCC2030, it also evaluated the in vivo therapeutic effect on NE. The results showed that baicalin-aluminum inhibited bacterial hemolytic activity, diminished biofilm formation, attenuated cytotoxicity to Caco-2 cells, downregulated the expression of genes encoding for clostridial toxins and extracellular enzymes such as alpha toxin (CPA), perfringolysin O (PFO), collagenase (ColA), and sialidases (NanI, NanJ). Additionally, baicalin-aluminum was found to negatively regulate the expression of genes involved in quorum sensing (QS) communication, including genes of Agr QS system (agrB, agrD) and genes of VirS/R two-component regulatory system (virS, virR). In vivo experiments, baicalin-aluminum lightened the intestinal lesions and histological damage, it inhibited pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) expression in the jejunal and ileal tissues. Besides, baicalin-aluminum alleviated the upregulation of C. perfringens and Escherichia coli and raised the relative abundance of Lactobacillus in the ileal digesta. This study suggests that baicalin-aluminum may be a potential candidate against C. perfringens infection by inhibiting the virulence-associated traits and virulence genes expression.

Reevaluation of whether a Functional Agr-like Quorum-Sensing System Is Necessary for Production of Wild-Type Levels of Epsilon-Toxin by Clostridium perfringens Type D Strains

Clostridium perfringens type B and D strains produce epsilon-toxin (ETX). Our 2011 mBio study (mBio 2:e00275-11, 2011, https://doi.org/10.1128/mBio.00275-11) reported that the Agr quorum-sensing (QS) system regulates ETX production by type D strain CN3718. However, subsequent studies have brought that conclusion into question. For example, we reported in 2012 (Infect Immun 80:3008–3017, 2012, https://doi.org/10.1128/IAI.00438-12) that the Agr-like QS system is not required for wild-type ETX production levels by two type B strains. Consequently, we reexamined whether the Agr-like QS system regulates ETX production in type D strains by using Targetron insertional mutagenesis to construct new agrB null mutants of two type D strains, CN3718 and CN2068. Western blotting showed that both agrB mutants still produce wild-type ETX levels. However, the newly constructed agrB mutants of both type D strains produced reduced amounts of alpha-toxin, and this effect was reversible by complementation, which confirms loss of functional AgrB production by these mutants since alpha-toxin production is known to be regulated by AgrB. Coupled with the previously published results for type B strains, these new findings indicate the Agr-like QS system is not usually necessary for C. perfringens to produce wild-type ETX levels.

Autoinducing peptide-based quorum signaling systems in Clostridioides difficile

The autoinducing peptide-based Agr system in Clostridioides difficile is involved in virulence factor expression, motility, and sporulation. This review highlights several of the recent discoveries regarding C. difficile Agr. Typical Agr systems rely on the combined activities of four proteins involved in peptide expression, peptide processing, peptide sensing, and transcriptional regulation. As emphasized in this review, at least two C. difficile Agr systems (Agr1 and Agr3) lack the set of proteins associated with this regulatory network. In line with this, recent finding indicate Agr1 can function in ways that may not depend on accumulation of extracellular peptide. Also, described are the similarities and differences in Agr systems within the pathogenic Clostridia.

Navarro M, Li J, Shrestha A, Uzal F, A. McClane B. Pathogenicity and virulence of Clostridium perfringens

Clostridium perfringens is an extremely versatile pathogen of humans and livestock, causing wound infections like gas gangrene (clostridial myonecrosis), enteritis/enterocolitis (including one of the most common human food-borne illnesses), and enterotoxemia (where toxins produced in the intestine are absorbed and damage distant organs such as the brain). The virulence of this Gram-positive, spore-forming, anaerobe is largely attributable to its copious toxin production; the diverse actions and roles in infection of these toxins are now becoming established. Most C. perfringens toxin genes are encoded on conjugative plasmids, including the pCW3-like and the recently discovered pCP13-like plasmid families. Production of C. perfringens toxins is highly regulated via processes involving two-component regulatory systems, quorum sensing and/or sporulation-related alternative sigma factors. Non-toxin factors, such as degradative enzymes like sialidases, are also now being implicated in the pathogenicity of this bacterium. These factors can promote toxin action in vitro and, perhaps in vivo, and also enhance C. perfringens intestinal colonization, e.g. NanI sialidase increases C. perfringens adherence to intestinal tissue and generates nutrients for its growth, at least in vitro. The possible virulence contributions of many other factors, such as adhesins, the capsule and biofilms, largely await future study.

Identifying the Basis for VirS/VirR Two-Component Regulatory System Control of Clostridium perfringens Beta-Toxin Production

Clostridium perfringens toxin production is often regulated by the Agr-like quorum sensing (QS) system signaling the VirS/VirR two-component regulatory system (TCRS), which consists of the VirS membrane sensor histidine kinase and the VirR response regulator. VirS/VirR is known to directly control expression of some genes by binding to a DNA binding motif consisting of two VirR boxes located within 500 bp of the target gene start codon. Alternatively, the VirS/VirR system can indirectly regulate production levels of other proteins by increasing expression of a small regulatory RNA, VR-RNA. Previous studies demonstrated that C. perfringens beta-toxin (CPB) production by C. perfringens type B and C strains is positively regulated by both the Agr-like QS and the VirS/VirR TCRS, but the mechanism has been unclear. The current study first inactivated the vrr gene encoding VR-RNA to show that VirS/VirR regulation of cpb expression does not involve VR-RNA. Subsequently, bioinformatic analyses identified a potential VirR binding motif, along with a predicted strong promoter, ∼1.4 kb upstream of the cpb open reading frame (ORF). Two insertion sequences were present between this VirR binding motif/promoter region and the cpb ORF. PCR screening of a collection of strains carrying cpb showed that the presence and sequence of this VirR binding motif/promoter is highly conserved among CPB-producing strains. Reverse transcription-PCR (RT-PCR) and a GusA reporter assay showed this VirR binding motif is important for regulating CPB production. These findings indicate that VirS/VirR directly regulates cpb expression via VirS binding to a VirR binding motif located unusually distant from the cpb start codon. IMPORTANCE Clostridium perfringens beta-toxin (CPB) is only produced by type B and C strains. Production of CPB is essential for the pathogenesis of type C-associated infections, which include hemorrhagic necrotizing enteritis and enterotoxemia in both humans and animals. In addition, CPB can synergize with other toxins during C. perfringens gastrointestinal diseases. CPB toxin production is cooperatively regulated by the Agr-like quorum sensing (QS) system and the VirS/VirR two-component regulatory system. This study now reports that the VirS/VirR regulatory cascade directly controls expression of the cpb gene via a process involving a VirR box binding motif located unusually far (∼1.4 kb) upstream of the cpb ORF. This study provides a better understanding of the regulatory mechanisms for CPB production by the VirS/VirR regulatory cascade.