Bacillus cereus non-haemolytic enterotoxin activates the NLRP3 inflammasome

A rare waterborne outbreak of Bacillus paranthracis in Shandong province, China, 2020: epidemiologic survey, genomic insights, and virulence characteristics

Bacillus paranthracis, a Gram-positive conditional pathogen of Bacillus cereus group species, is capable of causing foodborne and waterborne illnesses, leading to intestinal diseases in humans characterized by diarrhoea and vomiting. However, documented cases of B. paranthracis infection outbreaks are rare in the world, and the genomic background of outbreak strains is seldom characterized. This study retrospectively analyzed strains obtained from an outbreak in schools, as well as from water systems in peri-urban areas, China, in 2020. In total, 28 B. cereus group isolates were retrieved, comprising 6 from stool samples and 22 from water samples. Epidemiological and phylogenetic investigations indicated that the B. paranthracis isolate from drinking water as the causative agent of the outbreak. The genomic comparison revealed a high degree of consistency among 8 outbreak-related strains in terms of antimicrobial resistance gene profiles, virulence gene profiles, genomic content, and multilocus sequence typing (MLST). The strains related to the outbreak show highly similar genomic ring diagrams and close phylogenetic relationships. Additionally, this study shed light on the pathogenic potential and complexity of B. cereus group through its diversity in virulence genes and mice infection model. The findings highlight the usefulness of B. paranthracis genomes in understanding genetic diversity within specific environments and in tracing the source of pathogens during outbreak situations, thereby enabling targeted infection control interventions.

Bacillus cereus CwpFM induces colonic tissue damage and inflammatory responses through oxidative stress and the NLRP3/NF-κB pathway

Bacillus cereus (B. cereus) from cow milk poses a threat to public health, causing food poisoning and gastrointestinal disorders in humans. We identified CwpFM, an enterotoxin from B. cereus, caused oxidative stress and inflammatory responses in mouse colon and colonic epithelial cells. Colon proteomics revealed that CwpFM elevated proteins associated with inflammation and oxidative stress. Notably, CwpFM induced activation of the NLRP3/NF-κB signaling, but suppressed antioxidant NFE2L2/HO-1 expression in the intestine and epithelial cells. Consistently, CwpFM exposure led to cytotoxicity and ROS accumulation in Caco-2 cells in a dose-dependent manner. Further, NAC (ROS inhibitor) treatment abolished NLRP3/NF-κB activation due to CwpFM. Moreover, overexpression of Nfe2l2 or activation of NFE2L2 by NK-252 reduced ROS production and inhibited activation of the NLRP3/NF-κB pathway. Inhibition of NF-κB by ADPC and/or suppression of NLRP3 by MCC950 attenuated CwpFM-induced inflammatory responses in Caco-2 cells. Collectively, CwpFM induced oxidative stress and NLRP3/NF-κB activation by inhibiting the NFE2L2/HO-1 signaling and ROS accumulation, leading to the development of intestinal inflammation. Our data elucidate the role of oxidative stress and innate immunity in CwpFM enterotoxicity and contribute to developing diagnostic and therapeutic products for B. cereus-related food safety issues.

Revealing the Mechanism of NLRP3 Inflammatory Pathway Activation through K+ Efflux Induced by PLO via Signal Point Mutations

Trueperella pyogenes is an important opportunistic pathogenic bacterium widely distributed in the environment. Pyolysin (PLO) is a primary virulence factor of T. pyogenes and capable of lysing many different cells. PLO is a member of the cholesterol-dependent cytolysin (CDC) family of which the primary structure only presents a low level of homology with other members from 31% to 45%. By deeply studying PLO, we can understand the overall pathogenic mechanism of CDC family proteins. This study established a mouse muscle tissue model infected with recombinant PLO (rPLO) and its single-point mutations, rPLO N139K and rPLO F240A, and explored its mechanism of causing inflammatory damage. The inflammatory injury abilities of rPLO N139K and rPLO F240A are significantly reduced compared to rPLO. This study elaborated on the inflammatory mechanism of PLO by examining its unit point mutations in detail. Our data also provide a theoretical basis and practical significance for future research on toxins and bacteria.

An RPA-Assisted CRISPR/Cas12a Assay Combining Fluorescence and Lateral Flow Strips for the Rapid Detection of Enterotoxigenic Bacillus cereus

Bacillus cereus (B. cereus) is a foodborne pathogen that can produce tripartite enterotoxins, which can cause a variety of diseases after infection. It is critical to rapidly and accurately detect strains with enteropathogenic potential to safeguard human health. In this study, a dual-signal visualized detection platform with fluorescence assay and paper-based lateral flow assay (LFA) based on recombinase polymerase amplification (RPA), CRISPR/Cas12a system, and self-developed CRISPR nucleic acid test strips was constructed for enterotoxigenic B. cereus. The genes that encode two tripartite enterotoxins─nheA, nheB, and nheC for nonhemolytic enterotoxin and hblA, hblC, and hblD for hemolysin BL─were utilized as detection targets. The platform was capable of detecting six enterotoxin genes at the same genomic DNA level. The limits of detection for each gene were 10-3 ng/μL in fluorescence assay and 10-4 ng/μL in LFA. Furthermore, 101-102 CFU/mL of B. cereus in pure culture was detected. Additionally, a smartphone miniprogram could assist in evaluating the results in LFA. The platform demonstrated good utility by detecting B. cereus in food samples, including milk and rice. The results indicate that our RPA-CRISPR/Cas12a dual-signal visualized detection platform can quickly and easily detect B. cereus with three-component enterotoxin-producing potentials. The whole analytic process took less than 60 min without complex operation or expensive equipment.

Haemolysins are essential to the pathogenicity of deep-sea Vibrio fluvialis

Vibrio fluvialis is an emerging foodborne pathogen that produces VFH (Vibrio fluvialis hemolysin) and δVFH (delta-Vibrio fluvialis hemolysin). The function of δVFH is unclear. Currently, no pathogenic V. fluvialis from deep sea has been reported. In this work, a deep-sea V. fluvialis isolate (V13) was examined for pathogenicity. V13 was most closely related to V. fluvialis ATCC 33809, a human isolate, but possessed 262 unique genes. V13 caused lethal infection in fish and induced pyroptosis involving activation of the NLRP3 inflammasome, caspase 1 (Casp1), and gasdermin D (GSDMD). V13 defective in VFH or VFH plus δVFH exhibited significantly weakened cytotoxicity. Recombinant δVFH induced NLRP3-Casp1-GSDMD-mediated pyroptosis in a manner that depended on K+ efflux and intracellular Ca2+ accumulation. δVFH bound several plasma membrane lipids, and these bindings were crucial for δVFH cytotoxicity. Together these results provided new insights into the function of δVFH and the virulence mechanism of V. fluvialis.

New Approach Methods to Assess the Enteropathogenic Potential of Strains of the Bacillus cereus Group, including Bacillus thuringiensis

Bacillus cereus (Bc) is a wide group of Gram-positive and spore-forming bacteria, known to be the etiological agents of various human infections, primarily food poisoning. The Bc group includes enteropathogenic strains able to germinate in the digestive tract and to produce enterotoxins such as Nhe, Hbl, and CytK. One species of the group, Bacillus thuringiensis (Bt), has the unique feature of producing insecticidal crystals during sporulation, making it an important alternative to chemical pesticides to protect crops from insect pest larvae. Nevertheless, several studies have suggested a link between the ingestion of pesticide strains and human cases of food poisoning, calling their safety into question. Consequently, reliable tools for virulence assessment are worth developing to aid decision making in pesticide regulation. Here, we propose complementary approaches based on two biological models, the human intestinal Caco-2 cell line and the insect Drosophila melanogaster, to assess and rank the enteric virulence potency of Bt strains in comparison with other Bc group members. Using a dataset of 48 Bacillus spp. strains, we showed that some Bc group strains, including Bt, were able to induce cytotoxicity in Caco-2 cells with concomitant release of IL-8 cytokine, a landmark of pro-inflammatory response. In the D. melanogaster model, we were able to sort a panel of 39 strains into four different classes of virulence, ranging from no virulence to strong virulence. Importantly, for the most virulent strains, mortality was associated with a loss of intestinal barrier integrity. Interestingly, although strains can share a common toxinotype, they display different degrees of virulence, suggesting the existence of specific mechanisms of virulence expression in vivo in the intestine.

Postantibiotic leukocyte enhancement-mediated reduction of intracellular bacteria by macrophages

INTRODUCTION

Potentiation of the bactericidal activities of leukocytes, including macrophages, upon antibacterial agent administration has been observed for several decades and is summarized as the postantibiotic leukocyte enhancement (PALE) theory. Antibiotics-induced bacterial sensitization to leukocytes is commonly recognized as the mechanism of PALE. However, the degree of sensitization drastically varies with antibiotic classes, and little is known about whether and how the potentiation of leukocytes contributes to PALE.

OBJECTIVES

In this study, we aim to develop a mechanistic understanding of PALE by investigating the immunoregulation of traditional antibiotics on macrophages.

METHODS

Interaction models between bacteria and macrophages were constructed to identify the effects of different antibiotics on the bactericidal activities of macrophages. Oxygen consumption rate, expression of oxidases, and antioxidants were then measured to evaluate the effects of fluoroquinolones (FQs) on the oxidative stress of macrophages. Furthermore, the modulation in endoplasmic reticulum stress and inflammation upon antibiotic treatment was detected to analyze the mechanisms. At last, the peritoneal infection model was utilized to verify the PALE in vivo.

RESULTS

Enrofloxacin significantly reduced the intracellular burden of diverse bacterial pathogens through promoting the accumulation of reactive oxygen species (ROS). The upregulated oxidative response accordingly reprograms the electron transport chain with decreased production of antioxidant enzymes to reduce internalized pathogens. Additionally, enrofloxacin modulated the expression and spatiotemporal localization of myeloperoxidase (MPO) to facilitate ROS accumulation to target invaded bacteria and downregulated inflammatory response to alleviate cellular injury.

CONCLUSION

Our findings demonstrate the crucial role of leukocytes in PALE, shedding light on the development of new host-directed antibacterial therapies and the design of rational dosage regimens.

Bacillus cereus cereolysin O induces pyroptosis in an undecapeptide-dependent manner

Bacillus cereus is a clinically significant foodborne pathogen that causes severe gastrointestinal and non-gastrointestinal disease. Cereolysin O (CLO) is a putative virulence factor of B. cereus, and its function remains to be investigated. In this study, we examined the biological activity of CLO from a deep sea B. cereus isolate. CLO was highly toxic to mammalian cells and triggered pyroptosis through NLRP3 inflammasome-mediated caspase 1 and gasdermin D activation. CLO-induced cell death involved ROS accumulation and K+ efflux, and was blocked by serum lipids. CLO bound specifically to cholesterol, and this binding was essential to CLO cytotoxicity. The structural integrity of the three tryptophan residues in the C-terminal undecapeptide was vital for CLO to interact with membrane lipids and cause membrane perforation. Taken together, these results provided new insights into the molecular mechanism of B. cereus CLO-mediated cytotoxicity.

A detailed molecular network map and model of the NLRP3 inflammasome

The NLRP3 inflammasome is a key regulator of inflammation that responds to a broad range of stimuli. The exact mechanism of activation has not been determined, but there is a consensus on cellular potassium efflux as a major common denominator. Once NLRP3 is activated, it forms high-order complexes together with NEK7 that trigger aggregation of ASC into specks. Typically, there is only one speck per cell, consistent with the proposal that specks form - or end up at - the centrosome. ASC polymerisation in turn triggers caspase-1 activation, leading to maturation and release of IL-1β and pyroptosis, i.e., highly inflammatory cell death. Several gain-of-function mutations in the NLRP3 inflammasome have been suggested to induce spontaneous activation of NLRP3 and hence contribute to development and disease severity in numerous autoinflammatory and autoimmune diseases. Consequently, the NLRP3 inflammasome is of significant clinical interest, and recent attention has drastically improved our insight in the range of involved triggers and mechanisms of signal transduction. However, despite recent progress in knowledge, a clear and comprehensive overview of how these mechanisms interplay to shape the system level function is missing from the literature. Here, we provide such an overview as a resource to researchers working in or entering the field, as well as a computational model that allows for evaluating and explaining the function of the NLRP3 inflammasome system from the current molecular knowledge. We present a detailed reconstruction of the molecular network surrounding the NLRP3 inflammasome, which account for each specific reaction and the known regulatory constraints on each event as well as the mechanisms of drug action and impact of genetics when known. Furthermore, an executable model from this network reconstruction is generated with the aim to be used to explain NLRP3 activation from priming and activation to the maturation and release of IL-1β and IL-18. Finally, we test this detailed mechanistic model against data on the effect of different modes of inhibition of NLRP3 assembly. While the exact mechanisms of NLRP3 activation remains elusive, the literature indicates that the different stimuli converge on a single activation mechanism that is additionally controlled by distinct (positive or negative) priming and licensing events through covalent modifications of the NLRP3 molecule. Taken together, we present a compilation of the literature knowledge on the molecular mechanisms on NLRP3 activation, a detailed mechanistic model of NLRP3 activation, and explore the convergence of diverse NLRP3 activation stimuli into a single input mechanism.

Bacillus cereus Sensu Lato Accelerate Cellular Bioenergetic Metabolism of Human Colorectal Adenocarcinoma Caco-2 Cell Line

How foodborne enterotoxigenic Bacillus cereus rewires energy metabolism during intestinal tract infection is still not understood. In this study, we used the Seahorse XFe technology to simultaneously analyze oxygen consumption and acidification rates to estimate bioenergetic changes in the intestinal Caco-2 cell line after exposure to the B. cereus sensu lato (s.l.) enterotoxin-producing pathotypes, American Type Culture Collection (ATCC) 14579 (836), NVH0391-98 (828), and NVH0075/95 (825). Infection of Caco-2 led to a more energetic phenotype due to increased flux through oxidative phosphorylation and glycolysis. Strain 836 caused the most pronounced effects toward the specific energy phenotype, followed by strains 828 and 825. However, the metabolic potential of Caco-2 cells was most strongly induced by the 828 strain. Furthermore, infected cells manifested an increased adenosine triphosphate (ATP) production rate. Strain 828 caused the highest glycolytic and mitochondrial ATP production rates, followed by the 836 and 825 B. cereus s.l. strains. The glycolytic stress assay showed that strains 828 and 826 slightly increased compensatory glycolysis, providing a better understanding of the pathogenicity of this versatile pathogen. The results of this study underline that extracellular flux measurement can be used to accurately estimate bioenergetic perturbations of Caco-2 cells as a consequence of infection. Our findings enhance our understanding of how intestinal cells adjust their metabolism during infection with B. cereus s.l.

Acid-tolerant Lactiplantibacillus plantarum ZDY2013 shows a colonization niche preference and interacts with enterotoxigenic Bacillus cereus in specific-pathogen-free mice

Probiotics have long been utilized as functional food and modulate gut microbial homeostasis, but their colonization niche is mostly unclear and transient, which restrains the development of microbiome-targeted strategies. Lactiplantibacillus (L.) plantarum ZDY2013 is an allochthonous species of the human gastrointestinal tract with acid-tolerant properties. It serves as an antagonistic agent against the food-borne pathogen Bacillus (B.) cereus and a potent regulator of the gut microbiota. However, there is a knowledge gap regarding the colonization dynamics of L. plantarum ZDY2013 in the host intestine and the colonization niche of its interaction with pathogens. Here, we designed a pair of specific primers targeting L. plantarum ZDY2013 based on its whole genome sequence. We evaluated their accuracy and sensitivity with other host-derived strains and confirmed their availability with artificially spiked fecal samples from different mouse models. Additionally, the content of L. plantarum ZDY2013 was quantified by qPCR in fecal samples from BALB/c mice, followed by the analysis of its colonization niche preference. Moreover, the interactions between L. plantarum ZDY2013 and enterotoxigenic B. cereus HN001 were also elucidated. The results revealed that the newly designed primers could identify L. plantarum ZDY2013 with high specificity and were resistant to the influence of the complex fecal matrix and gut microbes from different hosts. Interestingly, the content of mixed L. plantarum ZDY2013 and B. cereus HN001 when orally administered remained higher when compared with the single strain group in BALB/c mice upon discontinuation of intragastric administration. In addition, L. plantarum ZDY2013 was mainly enriched in the large intestine during the ingestion period and maintained the highest content in the stomach after discontinuing supplementation on day 7. Moreover, L. plantarum ZDY2013 colonization neither damaged the intestine nor ameliorated the damage triggered by B. cereus in BALB/c mice. Overall, our study constructed two efficient specific primers targeting L. plantarum ZDY2013 and provided the potential to explore the underlying mechanism of competition between L. plantarum ZDY2013 and pathogens in host species.

Retrospective surveillance of viable Bacillus cereus group contaminations in commercial food and feed vitamin B2 products sold on the Belgian market using whole-genome sequencing

Bacillus cereus is a spore-forming bacterium that occurs as a contaminant in food and feed, occasionally resulting in food poisoning through the production of various toxins. In this study, we retrospectively characterized viable B. cereus sensu lato (s.l.) isolates originating from commercial vitamin B₂ feed and food additives collected between 2016 and 2022 by the Belgian Federal Agency for the Safety of the Food Chain from products sold on the Belgian market. In total, 75 collected product samples were cultured on a general medium and, in case of bacterial growth, two isolates per product sample were collected and characterized using whole-genome sequencing (WGS) and subsequently characterized in terms of sequence type (ST), virulence gene profile, antimicrobial resistance (AMR) gene profile, plasmid content, and phylogenomic relationships. Viable B. cereus was identified in 18 of the 75 (24%) tested products, resulting in 36 WGS datasets, which were classified into eleven different STs, with ST165 (n = 10) and ST32 (n = 8) being the most common. All isolates carried multiple genes encoding virulence factors, including cytotoxin K-2 (52.78%) and cereulide (22.22%). Most isolates were predicted to be resistant to beta-lactam antibiotics (100%) and fosfomycin (88.89%), and a subset was predicted to be resistant to streptothricin (30.56%). Phylogenomic analysis revealed that some isolates obtained from different products were closely related or even identical indicating a likely common origin, whereas for some products the two isolates obtained did not show any close relationship to each other or other isolates found in other products. This study reveals that potentially pathogenic and drug-resistant B. cereus s.l. can be present in food and feed vitamin B₂ additives that are commercially available, and that more research is warranted to assess whether their presence in these types of products poses a threat to consumers.

The Bacillus cereus toxin alveolysin disrupts the intestinal epithelial barrier by inducing microtubule disorganization through CFAP100

Bacillus cereus is a Gram-positive bacterium that mainly causes self-limiting emetic or diarrheal illness but can also cause skin infections and bacteremia. Symptoms of B. cereus ingestion depend on the production of various toxins that target the gastric and intestinal epithelia. From a screen of bacterial isolates from human stool samples that compromised intestinal barrier function in mice, we identified a strain of B. cereus that disrupted tight and adherens junctions in the intestinal epithelium. This activity was mediated by the pore-forming exotoxin alveolysin, which increased the production of the membrane-anchored protein CD59 and of cilia- and flagella-associated protein 100 (CFAP100) in intestinal epithelial cells. In vitro, CFAP100 interacted with microtubules and promoted microtubule polymerization. CFAP100 overexpression stabilized microtubules in intestinal epithelial cells, leading to disorganization of the microtubule network and perturbation of tight and adherens junctions. The disruption of cell junctions by alveolysin depended on the increase in CFAP100, which in turn depended on CD59 and the activation of PI3K-AKT signaling. These findings demonstrate that, in addition to forming membrane pores, B. cereus alveolysin can permeabilize the intestinal epithelium by disrupting epithelial cell junctions in a manner that is consistent with intestinal symptoms and may allow the bacteria to escape the intestine and cause systemic infections. Our results suggest the potential value of targeting alveolysin or CFAP100 to prevent B. cereus-associated intestinal diseases and systemic infections.

Bacillus cereus extracellular vesicles act as shuttles for biologically active multicomponent enterotoxins

BACKGROUND

Extracellular vesicles (EVs) from Gram-positive bacteria have gained considerable importance as a novel transport system of virulence factors in host-pathogen interactions. Bacillus cereus is a Gram-positive human pathogen, causing gastrointestinal toxemia as well as local and systemic infections. The pathogenicity of enteropathogenic B. cereus has been linked to a collection of virulence factors and exotoxins. Nevertheless, the exact mechanism of virulence factor secretion and delivery to target cells is poorly understood.

RESULTS

Here, we investigate the production and characterization of enterotoxin-associated EVs from the enteropathogenic B. cereus strain NVH0075-95 by using a proteomics approach and studied their interaction with human host cells in vitro. For the first time, comprehensive analyses of B. cereus EV proteins revealed virulence-associated factors, such as sphingomyelinase, phospholipase C, and the three-component enterotoxin Nhe. The detection of Nhe subunits was confirmed by immunoblotting, showing that the low abundant subunit NheC was exclusively detected in EVs as compared to vesicle-free supernatant. Cholesterol-dependent fusion and predominantly dynamin-mediated endocytosis of B. cereus EVs with the plasma membrane of intestinal epithelial Caco2 cells represent entry routes for delivery of Nhe components to host cells, which was assessed by confocal microscopy and finally led to delayed cytotoxicity. Furthermore, we could show that B. cereus EVs elicit an inflammatory response in human monocytes and contribute to erythrocyte lysis via a cooperative interaction of enterotoxin Nhe and sphingomyelinase.

CONCLUSION

Our results provide insights into the interaction of EVs from B. cereus with human host cells and add a new layer of complexity to our understanding of multicomponent enterotoxin assembly, offering new opportunities to decipher molecular processes involved in disease development. Video Abstract.

Immunity against Moraxella catarrhalis requires guanylate-binding proteins and caspase-11-NLRP3 inflammasomes

Moraxella catarrhalis is an important human respiratory pathogen and a major causative agent of otitis media and chronic obstructive pulmonary disease. Toll-like receptors contribute to, but cannot fully account for, the complexity of the immune response seen in M. catarrhalis infection. Using primary mouse bone marrow-derived macrophages to examine the host response to M. catarrhalis infection, our global transcriptomic and targeted cytokine analyses revealed activation of immune signalling pathways by both membrane-bound and cytosolic pattern-recognition receptors. We show that M. catarrhalis and its outer membrane vesicles or lipooligosaccharide (LOS) can activate the cytosolic innate immune sensor caspase-4/11, gasdermin-D-dependent pyroptosis, and the NLRP3 inflammasome in human and mouse macrophages. This pathway is initiated by type I interferon signalling and guanylate-binding proteins (GBPs). We also show that inflammasomes and GBPs, particularly GBP2, are required for the host defence against M. catarrhalis in mice. Overall, our results reveal an essential role for the interferon-inflammasome axis in cytosolic recognition and immunity against M. catarrhalis, providing new molecular targets that may be used to mitigate pathological inflammation triggered by this pathogen.

The double sides of NLRP3 inflammasome activation in sepsis

Sepsis is defined as a life-threatening organ dysfunction induced by a dysregulated host immune response to infection. Immune response induced by sepsis is complex and dynamic. It is schematically described as an early dysregulated systemic inflammatory response leading to organ failures and early deaths, followed by the development of persistent immune alterations affecting both the innate and adaptive immune responses associated with increased risk of secondary infections, viral reactivations, and late mortality. In this review, we will focus on the role of NACHT, leucin-rich repeat and pyrin-containing protein 3 (NLRP3) inflammasome in the pathophysiology of sepsis. NLRP3 inflammasome is a multiproteic intracellular complex activated by infectious pathogens through a two-step process resulting in the release of the pro-inflammatory cytokines IL-1β and IL-18 and the formation of membrane pores by gasdermin D, inducing a pro-inflammatory form of cell death called pyroptosis. The role of NLRP3 inflammasome in the pathophysiology of sepsis can be ambivalent. Indeed, although it might protect against sepsis when moderately activated after initial infection, excessive NLRP3 inflammasome activation can induce dysregulated inflammation leading to multiple organ failure and death during the acute phase of the disease. Moreover, this activation might become exhausted and contribute to post-septic immunosuppression, driving impaired functions of innate and adaptive immune cells. Targeting the NLRP3 inflammasome could thus be an attractive option in sepsis either through IL-1β and IL-18 antagonists or through inhibition of NLRP3 inflammasome pathway downstream components. Available treatments and results of first clinical trials will be discussed.

The NLRP3 inflammasome recognizes alpha-2 and alpha-7.3 giardins and decreases the pathogenicity of Giardia duodenalis in mice

BACKGROUND

Giardia duodenalis is a parasitic organism that can cause giardiasis, an intestinal infection, particularly prevalent in young children, with clinical symptoms of diarrhea. We previously reported that extracellular G. duodenalis triggers intracellular nucleotide-binding oligomerization-like receptor 3 (NLRP3) inflammasome activation and regulates the host inflammatory response by secreting extracellular vesicles (EVs). However, the exact pathogen-associated molecular patterns in G. duodenalis EVs (GEVs) involved in this process and the role of the NLRP3 inflammasome in giardiasis remain to be elucidated.

METHODS

Recombinant eukaryotic expression plasmids of pcDNA3.1(+)-alpha-2 and alpha-7.3 giardins in GEVs were constructed, transfected into primary mouse peritoneal macrophages and screened by measuring the expression levels of the inflammasome target molecule caspase-1 p20. The preliminary identification of G. duodenalis alpha-2 and alpha-7.3 giardins was further verified by measuring the protein expression levels of key molecules of the NLRP3 inflammasome (NLRP3, pro-interleukin-1 beta [IL-1β], pro-caspase-1, and caspase-1 p20), the secretion levels of IL-1β, the level of apoptosis speck-like protein (ASC) oligomerization and the immunofluorescence localization of NLRP3 and ASC. The roles of the NLRP3 inflammasome in G. duodenalis pathogenicity were then evaluated using mice in which NLRP3 activation was blocked (NLRP3-blocked mice), and body weight, parasite burden in the duodenum and histopathological changes in the duodenum were monitored. In addition, we explored whether alpha-2 and alpha-7.3 giardins triggered IL-1β secretion in vivo through the NLRP3 inflammasome and determined the roles of these molecules in G. duodenalis pathogenicity in mice.

RESULTS

Alpha-2 and alpha-7.3 giardins triggered NLRP3 inflammasome activation in vitro. This led to caspase-1 p20 activation, upregulation of the protein expression levels of NLRP3, pro-IL-1β and pro-caspase-1, significant enhancement of IL-1β secretion, ASC speck formation in the cytoplasm and also induction of ASC oligomerization. Deletion of the NLRP3 inflammasome aggravated G. duodenalis pathogenicity in mice. Compared to wild-type mice gavaged with cysts, mice gavaged with cysts in NLRP3-blocked mice displayed increased trophozoite loads and severe duodenal villus damage, characterized by necrotic crypts with atrophy and branching. In vivo assays revealed that alpha-2 and alpha-7.3 giardins could induce IL-1β secretion through the NLRP3 inflammasome and that immunization with alpha-2 and alpha-7.3 giardins decreased G. duodenalis pathogenicity in mice.

CONCLUSIONS

Overall, the results of the present study revealed that alpha-2 and alpha-7.3 giardins trigger host NLRP3 inflammasome activation and decrease G. duodenalis infection ability in mice, which are promising targets for the prevention of giardiasis.

Interactions between gut microbes and NLRP3 inflammasome in the gut-brain axis

The role of the gut-brain axis in maintaining the brain's and gut's homeostasis has been gradually recognized in recent years. The connection between the gut and the brain takes center stage. In this scenario, the nucleotide-binding oligomerization domain leucine-rich repeat and pyrin domain-containing protein 3 (NLRP3) inflammasome promotes inflammatory cell recruitment. It plays a crucial role in coordinating host physiology and immunity. Recent evidence shows how vital the gut-brain axis is for maintaining brain and gut homeostasis. However, more research is needed to determine the precise causal link between changed gut microbiota structure and NLRP3 activation in pathogenic circumstances. This review examines the connection between gut microbiota and the NLRP3 inflammasome. We describe how both dynamically vary in clinical cases and the external factors affecting both. Finally, we suggest that the crosstalk between the gut microbiota and NLRP3 is involved in signaling in the gut-brain axis, which may be a potential pathological mechanism for CNS diseases and intestinal disorders.

Differential activation of NLRP3 inflammasome by Acinetobacter baumannii strains

Acinetobacter baumannii is an emerging nosocomial, opportunistic pathogen with growing clinical significance globally. A. baumannii has an exceptional ability to rapidly develop drug resistance. It is frequently responsible for ventilator-associated pneumonia in clinical settings and inflammation resulting in severe sepsis. The inflammatory response is mediated by host pattern-recognition receptors and the inflammasomes. Inflammasome activation triggers inflammatory responses, including the secretion of the pro-inflammatory cytokines IL-1β and IL-18, the recruitment of innate immune effectors against A. baumannii infection, and the induction programmed cell death by pyroptosis. An important knowledge gap is how variation among clinical isolates affects the host’s innate response and activation of the inflammasome during A. baumannii infection. In this study, we compared nine A. baumannii strains, including clinical locally-acquired isolates, in their ability to induce activation of the inflammasome and programmed cell death in primary macrophages, epithelial lung cell line and mice. We found a variation in survival outcomes of mice and bacterial dissemination in organs among three commercially available A. baumannii strains, likely due to the differences in virulence between strains. Interestingly, we found variability among A. baumannii strains in activation of the NLRP3 inflammasome, non-canonical Caspase-11 pathway, plasmatic secretion of the pro-inflammatory cytokine IL-1β and programmed cell death. Our study highlights the importance of utilising multiple bacterial strains and clinical isolates with different virulence to investigate the innate immune response to A. baumannii infection.

Single Point Mutation and Its Role in Specific Pathogenicity to Reveal the Mechanism of Related Protein Families

Pyolysin (PLO) is secreted by Trueperella pyogenes as a water-soluble monomer after forming transmembrane β-barrel channels in the cell membrane by binding cholesterol. Two significantly conserved residues at domain 1 of PLO are mutated, which provides novel evidence of a relationship between conformational change and interaction with the cell membrane and uncovers the pore formation mechanism of the cholesterol-dependent cytolysin (CDC) family. Moreover, PLO is a special member of the CDCs, which the percentage of sequence identities between PLO and other CDC members is from 31% to 45%, while others are usually from 40% to 70%. It is important to understand that at very low sequence identities, models can be different in the pathogenic mechanisms of these CDC members, which are dedicated to a large number of Gram-positive bacterial pathogens. Our studies, for the first time, located and mutated two different highly conserved structural sites in the primary structure critical for PLO structure and function that proved the importance of these sites. Together, novel and repeatable observations into the pore formation mechanism of CDCs are provided by our findings. IMPORTANCE Postpartum disease of dairy cows caused by persistent bacterial infection is a global disease, which has a serious impact on the development of the dairy industry and brings huge economic losses. As one of the most relevant pathogenic bacteria for postpartum diseases in dairy cows, Trueperella pyogenes can secrete pyolysin (PLO), a member of the cholesterol-dependent cytolysin (CDC) family and recognized as the most important toxin of T. pyogenes. However, the current research work on PLO is still insufficient. The pathogenic mechanism of this toxin can be fully explored by changing the local structure and overall function of the toxin by a previously unidentified single point mutation. These studies lay the groundwork for future studies that will explore the contribution of this large family of CDC proteins to microbial survival and human disease.

Lactobacillus rhamnosus GR-1 attenuates foodborne Bacillus cereus-induced NLRP3 inflammasome activity in bovine mammary epithelial cells by protecting intercellular tight junctions

Background

Bacillus cereus is an important pathogen that causes human food poisoning, specifically diarrhea and vomiting. B. cereus can also induce mastitis in dairy cows and has a strong survival ability in milk, as it cannot be inactivated by high-temperature short-time pasteurization. Therefore, B. cereus can enter the market through pasteurized milk and other dairy products, imposing enormous hidden dangers on food safety and human health.

Results

In this study, B. cereus 2101 (BC) was isolated from milk samples of cows with mastitis. BC grew rapidly with strong hemolysis, making it difficult to prevent mastitis and ensure food security. MAC-T cells were treated with BC and/or Lactobacillus rhamnosus GR-1 (LGR-1). Pretreatment with LGR-1 protected the integrity of tight junctions and the expression of zonula occludens-1 (ZO-1) and occludin destroyed by BC. Furthermore, LGR-1 pretreatment reduced the expression of NOD-like receptor family member pyrin domain-containing protein 3 (NLRP3), caspase recruitment and activation domain (ASC), Caspase-1 p20, gasdermin D (GSDMD) p30, inflammatory factors (interleukin (IL)-1β and IL-18), and cell death induced by BC. Moreover, LGR-1 pretreatment reduced NLRP3 inflammasome activity and increased expressions of ZO-1 and occludin induced by lipopolysaccharides (LPS) + ATP stimulation. MAC-T cells were transfected with NLRP3 siRNA or MCC950 and/or treated with BC and/or LGR-1. NLRP3-siRNA transfection and MCC950 attenuated BC-induced NLRP3 inflammasome activity. Expression of inflammatory cytokines and cell death suggested that the inflammatory pathway might play an important role in the induction of the NLRP3 inflammasome by BC and the protection of LGR-1.

Conclusions

These results suggest that LGR-1 might be a probiotic alternative to antibiotics and could be administered to prevent mastitis in dairy cows, thus ensuring food security.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00752-w.

Pathogen-selective killing by guanylate-binding proteins as a molecular mechanism leading to inflammasome signaling

Inflammasomes are cytosolic signaling complexes capable of sensing microbial ligands to trigger inflammation and cell death responses. Here, we show that guanylate-binding proteins (GBPs) mediate pathogen-selective inflammasome activation. We show that mouse GBP1 and GBP3 are specifically required for inflammasome activation during infection with the cytosolic bacterium Francisella novicida. We show that the selectivity of mouse GBP1 and GBP3 derives from a region within the N-terminal domain containing charged and hydrophobic amino acids, which binds to and facilitates direct killing of F. novicida and Neisseria meningitidis, but not other bacteria or mammalian cells. This pathogen-selective recognition by this region of mouse GBP1 and GBP3 leads to pathogen membrane rupture and release of intracellular content for inflammasome sensing. Our results imply that GBPs discriminate between pathogens, confer activation of innate immunity, and provide a host-inspired roadmap for the design of synthetic antimicrobial peptides that may be of use against emerging and re-emerging pathogens.

Guanylate-binding proteins (GBP) have a function in inflammasome formation and pathogen defence. Here the authors show that these GBP proteins are able to kill certain bacteria and promote selective inflammasome activation and that this is mediated by specific GBP protein regions.

Regulation of Enterotoxins Associated with Bacillus cereus Sensu Lato Toxicoinfection

Bacillus cereus sensu lato (s.l.) includes foodborne pathogens, as well as beneficial microorganisms, such as bioinsecticides. Some of the beneficial and commercially used B. cereus s.l. strains have been shown to carry enterotoxin genes, the products of which can cause toxicoinfection in humans. Furthermore, recent epidemiological reports indicated that some bioinsecticidal strains have been linked with foodborne illness outbreaks. This demonstrates the need for improved surveillance of B. cereus s.l., which includes characterization of isolates' virulence capacity. However, the prediction of virulence capacity of B. cereus s.l. strains is challenging. Genetic screening for enterotoxin gene presence has proven to be insufficient for accurate discrimination between virulent and avirulent strains, given that nearly all B. cereus s.l. strains carry at least one enterotoxin gene. Furthermore, complex regulatory networks governing the expression of enterotoxins, and potential synergistic interactions between enterotoxins and other virulence factors make the prediction of toxicoinfection based on isolates' genome sequences challenging. In this review, we summarize and synthesize the current understanding of the regulation of enterotoxins associated with the B. cereus s.l. toxicoinfection and identify gaps in the knowledge that need to be addressed to facilitate identification of genetic markers predictive of cytotoxicity and toxicoinfection.

Bacillus cereus cytotoxin K triggers gasdermin D-dependent pyroptosis

Bacillus cereus is well known as a causative agent of foodborne gastrointestinal diseases and systemic non-gastrointestinal diseases. We have recently identified a pathogenic B. cereus (named H2) from a deep-sea cold-seep. H2 possesses the pyroptosis-inducing capacity and contains a number of enterotoxins including cytotoxin K (CytK). In the present work, we examined the cytotoxicity of the CytK of H2 to human macrophages. CytK bound macrophages by interaction with the plasma membrane and caused cellular structure damage. CytK−cell interaction triggered rapid pyroptosis mediated by caspase 1-activated gasdermin D (GSDMD). CytK-induced pyroptosis required NLRP3 inflammasome activation, K⁺ efflux, and intracellular Ca²⁺ accumulation. CytK exhibited apparent binding to several cytomembrane lipids, in particular phosphatidic acid, which proved to be essential to CytK-elicited cell death. Together, these results add new insights into the cytotoxic mechanism of CytK.

Clostridium septicum α-toxin activates the NLRP3 inflammasome by engaging GPI-anchored proteins

Clostridium species are a group of Gram-positive bacteria that cause diseases in humans, such as food poisoning, botulism, and tetanus. Here, we analyzed 10 different Clostridium species and identified that Clostridium septicum, a pathogen that causes sepsis and gas gangrene, activates the mammalian cytosolic inflammasome complex in mice and humans. Mechanistically, we demonstrate that α-toxin secreted by C. septicum binds to glycosylphosphatidylinositol (GPI)-anchored proteins on the host plasma membrane, oligomerizing and forming a membrane pore that is permissive to efflux of magnesium and potassium ions. Efflux of these cytosolic ions triggers the activation of the innate immune sensor NLRP3, inducing activation of caspase-1 and gasdermin D, secretion of the proinflammatory cytokines interleukin-1β and interleukin-18, pyroptosis, and plasma membrane rupture via ninjurin-1. Furthermore, α-toxin of C. septicum induces rapid inflammasome-mediated lethality in mice and pharmacological inhibition of the NLRP3 inflammasome using MCC950 prevents C. septicum-induced lethality. Overall, our results reveal that cytosolic innate sensing of α-toxin is central to the recognition of C. septicum infection and that therapeutic blockade of the inflammasome pathway may prevent sepsis and death caused by toxin-producing pathogens.

Bacterial pore-forming toxins

Pore-forming toxins (PFTs) are widely distributed in both Gram-negative and Gram-positive bacteria. PFTs can act as virulence factors that bacteria utilise in dissemination and host colonisation or, alternatively, they can be employed to compete with rival microbes in polymicrobial niches. PFTs transition from a soluble form to become membrane-embedded by undergoing large conformational changes. Once inserted, they perforate the membrane, causing uncontrolled efflux of ions and/or nutrients and dissipating the protonmotive force (PMF). In some instances, target cells intoxicated by PFTs display additional effects as part of the cellular response to pore formation. Significant progress has been made in the mechanistic description of pore formation for the different PFTs families, but in several cases a complete understanding of pore structure remains lacking. PFTs have evolved recognition mechanisms to bind specific receptors that define their host tropism, although this can be remarkably diverse even within the same family. Here we summarise the salient features of PFTs and highlight where additional research is necessary to fully understand the mechanism of pore formation by members of this diverse group of protein toxins.

New Insights into the Potential Cytotoxic Role of Bacillus cytotoxicus Cytotoxin K-1

The thermotolerant representative of the Bacillus cereus group, Bacillus cytotoxicus, reliably harbors the coding gene of cytotoxin K-1 (CytK-1). This protein is a highly cytotoxic variant of CytK toxin, initially recovered from a diarrheal foodborne outbreak that caused the death of three people. In recent years, the cytotoxicity of B. cytotoxicus has become controversial, with some strains displaying a high cytotoxicity while others show no cytotoxicity towards cell lines. In order to better circumscribe the potential pathogenic role of CytK-1, knockout (KO) mutants were constructed in two B. cytotoxicus strains, E8.1 and E28.3. The complementation of the cytK-1 KO mutation was implemented in a mutant strain lacking in the cytK-1 gene. Using the tetrazolium salt (MTT) method, cytotoxicity tests of the cytK-1 KO and complemented mutants, as well as those of their wild-type strains, were carried out on Caco-2 cells. The results showed that cytK-1 KO mutants were significantly less cytotoxic than the parental wild-type strains. However, the complemented mutant was as cytotoxic as the wild-type, suggesting that CytK-1 is the major cytotoxicity factor in B. cytotoxicus.

Genomics and proteomics combined analysis revealed the toxicity response of silkworm Bombyx mori to the environmental pathogen Bacillus cereus ZJ-4

Bacterial contamination has caused a major public health problem worldwide. Bacillus cereus is a conditional environmental pathogenic bacteria that can cause food poisoning. Whether environmental pathogens can cause widespread transmission in the insect kingdom is unclear. In this study, a Bacillus cereus ZJ-4 was isolated from the hospital environment of Zhenjiang City, Jiangsu Province, China. It was fatal by injection into the silkworm hemolymph. To investigated the potential toxic factors of ZJ-4 and clarified the toxicity response mechanism of silkworm by the ZJ-4 infection. Then, the whole genome of ZJ-4 was sequenced, and the immune mechanism of silkworm fat body to ZJ-4 pathogen was studied by HE pathological section and proteomics. Bacterial genome sequencing indicated that ZJ-4 had 352 drug resistance genes and 6 virulence genes. After 36 h of subcutaneous puncture with ZJ-4 suspension, the pathological changes were obviously found in HE pathological sections of fat body tissue. Comparative proteomic results indicated that differentially expressed proteins are mainly involved in stress reactions, biological regulation, and innate immunity. The qRT-PCR analysis showed that the expressions of β-GRP, Spaetzle, MyD88, Tube and Dorsal genes in Toll pathway were up-regulated, while Pell and Cactus genes were down-regulated; in the antimicrobial peptide pathway, Glv2, Lzm, Mor, and Leb3 genes were up-regulated, while attacin1 and defensin genes were down-regulated; Sod gene was up-regulated, while Cat gene was down-regulated in the antioxidant pathway; Ldh, Sdh, and Mdh genes were down-regulated in glucose metabolism pathway. These results indicated that ZJ-4 can damage the innate immune pathway of silkworm, and also affect the normal immune function of fat body cells.

Pore-Forming Toxins During Bacterial Infection: Molecular Mechanisms and Potential Therapeutic Targets

Bacterial infections are predominantly treated with antibiotics, and resistance to antibiotics is becoming an increasing threat to our health. Pore-forming toxins (PFTs) are virulence factors secreted by many pathogenic bacterial strains, both in acute and chronic infections. They are special membrane-targeting proteins that exert toxic effects by forming pores in the cell membrane. Recent studies have elucidated the structure of PFTs and the detailed molecular mechanisms of their pathogenicity. Here, we discuss recent findings that highlight the regulatory mechanisms and important roles of two types of PFTs, α-PFTs and β-PFTs, in mediating the virulence of bacteria, and the therapeutic potential of targeting PFTs for antibacterial treatment. Therapeutic strategies based on PFTs are highly specific and may alleviate the issue of increasing resistance to antibiotics.

Imaging Infection Across Scales of Size: From Whole Animals to Single Molecules

Infectious diseases are a leading cause of global morbidity and mortality, and the threat of infectious diseases to human health is steadily increasing as new diseases emerge, existing diseases reemerge, and antimicrobial resistance expands. The application of imaging technology to the study of infection biology has the potential to uncover new factors that are critical to the outcome of host-pathogen interactions and to lead to innovations in diagnosis and treatment of infectious diseases. This article reviews current and future opportunities for the application of imaging to the study of infectious diseases, with a particular focus on the power of imaging objects across a broad range of sizes to expand the utility of these approaches. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Soluble total antigen derived from Toxoplasma gondii tachyzoites increased the expression levels of NLRP1, NLRP3, NLRC4, AIM2, and the release of mature form of IL1β, but downregulated the expression of IL1β and IL18 genes in THP-1cell line

Toxoplasma gondii (T. gondii) is an intracellular parasitic protozoan infecting homoeothermic animals and about a third of the world's population. Inflammasomes are intracellular multi-protein complex, which are activated by many factors. Inflammasomes are activated during toxoplasmosis; however, there are a lot of obscure aspects. THP-1 monocyte cells were converted to M0 macrophages by PMA and treated by 100 μg/mL soluble total Ag (STAg) derived from T. gondii strain RH for two time points 3 h and 24 h. After total RNA extraction and cDNA synthesis, the expression pattern of NLRP1, NLRP3, NLRC4, AIM2, IL1β, and IL18 was evaluated by relative real-time PCR. In addition, the cytokine release of IL1β and TNFα was evaluated in the supernatant of each well. The results showed statistically significant time-dependent overexpression of inflammasomes. NLRP1 and NLRP3 showed the higher and lower expression, respectively, during 3 h and 24 h after exposure. Both IL1β and IL18 downregulated 3 h after exposure. IL18 presented statistically significant upregulation after 24 h, but IL1β showed statistically significant downregulation after 24 h. The release of IL1β increased after 3 h, but it slightly decreased during 24 h after exposure. The concentration of TNFα showed an insignificant decrease compared to control, while it increased during 24 h after exposure. Taken together, this study suggested that T. gondii STAg induces NLRP1 more than NLRP3, NLRC4, and AIM2. Our findings also proposed that T. gondii STAg downregulates the gene expression of IL1β, but increases the release of this cytokine. It seems that Toxoplasma STAg probably increase the release of IL1β via activating NLRPs and AIM2 to cleave pro-caspase 1 to caspase 1 that leads to conversion of pro IL1β to mature IL1β.

Bacillus cereus food intoxication and toxicoinfection

Bacillus cereus is one of the leading etiological agents of toxin-induced foodborne diseases. Its omnipresence in different environments, spore formation, and its ability to adapt to varying conditions and produce harmful toxins make this pathogen a health hazard that should not be underestimated. Food poisoning by B. cereus can manifest itself as an emetic or diarrheal syndrome. The former is caused by the release of the potent peptide toxin cereulide, whereas the latter is the result of proteinaceous enterotoxins (e.g., hemolysin BL, nonhemolytic enterotoxin, and cytotoxin K). The final harmful effect is not only toxin and strain dependent, but is also affected by the stress responses, accessory virulence factors, and phenotypic properties under extrinsic, intrinsic, and explicit food conditions and host-related environment. Infamous portrait of B. cereus as a foodborne pathogen, as well as a causative agent of nongastrointestinal infections and even nosocomial complications, has inspired vast volumes of multidisciplinary research in food and clinical domains. As a result, extensive original data became available asking for a new, both broad and deep, multifaceted look into the current state-of-the art regarding the role of B. cereus in food safety. In this review, we first provide an overview of the latest knowledge on B. cereus toxins and accessory virulence factors. Second, we describe the novel taxonomy and some of the most pertinent phenotypic characteristics of B. cereus related to food safety. We link these aspects to toxin production, overall pathogenesis, and interactions with its human host. Then we reflect on the prevalence of different toxinotypes in foods opening the scene for epidemiological aspects of B. cereus foodborne diseases and methods available to prevent food poisoning including overview of the different available methods to detect B. cereus and its toxins.

Pharmacological Inhibition of the Nod-Like Receptor Family Pyrin Domain Containing 3 Inflammasome with MCC950

Activation of the Nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome drives release of the proinflammatory cytokines interleukin (IL)-1β and IL-18 and induces pyroptosis (lytic cell death). These events drive chronic inflammation, and as such, NLRP3 has been implicated in a large number of human diseases. These range from autoimmune conditions, the simplest of which is NLRP3 gain-of-function mutations leading to an orphan disease, cryopyrin-associated period syndrome, to large disease burden indications, such as atherosclerosis, heart failure, stroke, neurodegeneration, asthma, ulcerative colitis, and arthritis. The potential clinical utility of NLRP3 inhibitors is substantiated by an expanding list of indications in which NLRP3 activation has been shown to play a detrimental role. Studies of pharmacological inhibition of NLRP3 in nonclinical models of disease using MCC950 in combination with human genetics, epigenetics, and analyses of the efficacy of biologic inhibitors of IL-1β, such as anakinra and canakinumab, can help to prioritize clinical trials of NLRP3-directed therapeutics. Although MCC950 shows excellent (nanomolar) potency and high target selectivity, its pharmacokinetic and toxicokinetic properties limited its therapeutic development in the clinic. Several improved, next-generation inhibitors are now in clinical trials. Hence the body of research in a plethora of conditions reviewed herein may inform analysis of the potential translational value of NLRP3 inhibition in diseases with significant unmet medical need. SIGNIFICANCE STATEMENT: The nod-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is one of the most widely studied and best validated biological targets in innate immunity. Activation of NLRP3 can be inhibited with MCC950, resulting in efficacy in more than 100 nonclinical models of inflammatory diseases. As several next-generation NLRP3 inhibitors are entering proof-of-concept clinical trials in 2020, a review of the pharmacology of MCC950 is timely and significant.

A virulent Bacillus cereus strain from deep-sea cold seep induces pyroptosis in a manner that involves NLRP3 inflammasome, JNK pathway, and lysosomal rupture

Recent studies indicate that the Bacillus species is distributed in deep-sea environments. However, no specific studies on deep-sea Bacillus cereus have been documented. In the present work, we isolated a B. cereus strain, H2, from the deep-sea cold seep in South China Sea. We characterized the pathogenic potential of H2 and investigated H2-induced death of different types of cells. We found that H2 was capable of tissue dissemination and causing acute mortality in mice and fish following intraperitoneal/intramuscular injection. In vitro studies revealed that H2 infection of macrophages induced pyroptosis and activation of the NLRP3 inflammasome pathway that contributed partly to cell death. H2 infection activated p38, JNK, and ERK, but only JNK proved to participate in H2-triggered cell death. Reactive oxygen species (ROS) and intracellular Ca²⁺ were essential to H2-induced activation of JNK and NLRP3 inflammasome. In contrast, lysosomal rupture and cathepsins were required for H2-induced NLRP3 inflammasome activation but not for JNK activation. This study revealed for the first time the virulence characteristics of deep-sea B. cereus and provided new insights into the mechanism of B. cereus infection.

Innate Immune Interference Attenuates Inflammation In Bacillus Endophthalmitis

Purpose

To explore the consequences of innate interference on intraocular inflammatory responses during Bacillus endophthalmitis.

Methods

Bacillus endophthalmitis was induced in mice. Innate immune pathway activation was interfered by injecting S layer protein-deficient (∆slpA) B. thuringiensis or by treating wild-type (WT)–infected mice with a TLR2/4 inhibitor (WT+OxPAPC). At 10 hours postinfection, eyes were harvested and RNA was purified. A NanoString murine inflammation panel was used to compare gene expression in WT-infected, WT+OxPAPC, ∆slpA-infected, and uninfected eyes.

Results

In WT-infected eyes, 56% of genes were significantly upregulated compared to uninfected controls. Compared to WT-infected eyes, the expression of 27% and 50% of genes were significantly reduced in WT+OxPAPC and ∆slpA-infected eyes, respectively. Expression of 61 genes that were upregulated in WT-infected eyes was decreased in WT+OxPAPC and ∆slpA-infected eyes. Innate interference resulted in blunted expression of complement factors (C3, Cfb, and C6) and several innate pathway genes (TLRs 2, 4, 6, and 8, MyD88, Nod2, Nlrp3, NF-κB, STAT3, RelA, RelB, and Ptgs2). Innate interference also reduced the expression of several inflammatory cytokines (CSF2, CSF3, IL-6, IL-1β, IL-1α, TNFα, IL-23α, TGFβ1, and IL-12β) and chemokines (CCL2, CCL3, and CXCLs 1, 2, 3, 5, 9, and 10). All of the aforementioned genes were significantly upregulated in WT-infected eyes.

Conclusions

These results suggest that interfering with innate activation significantly reduced the intraocular inflammatory response in Bacillus endophthalmitis. This positive clinical outcome could be a strategy for anti-inflammatory therapy of an infection typically refractory to corticosteroid treatment.

Keeping up with the Bacillus cereus group: taxonomy through the genomics era and beyond

The Bacillus cereus group, also known as B. cereus sensu lato (s.l.), is a species complex that contains numerous closely related lineages, which vary in their ability to cause illness in humans and animals. The classification of B. cereus s.l. isolates into species-level taxonomic units is thus essential for informing public health and food safety efforts. However, taxonomic classification of these organisms is challenging. Numerous-often conflicting-taxonomic changes to the group have been proposed over the past two decades, making it difficult to remain up to date. In this review, we discuss the major nomenclatural changes that have accumulated in the B. cereus s.l. taxonomic space prior to 2020, particularly in the genomic sequencing era, and outline the resulting problems. We discuss several contemporary taxonomic frameworks as applied to B. cereus s.l., including (i) phenotypic, (ii) genomic, and (iii) hybrid nomenclatural frameworks, and we discuss the advantages and disadvantages of each. We offer suggestions as to how readers can avoid B. cereus s.l. taxonomic ambiguities, regardless of the nomenclatural framework(s) they choose to employ. Finally, we discuss future directions and open problems in the B. cereus s.l. taxonomic realm, including those that cannot be solved by genomic approaches alone.

Pore-forming toxins of foodborne pathogens

Pore-forming toxins (PFTs) are water-soluble molecules that have been identified as the most crucial virulence factors during bacterial pathogenesis. PFTs disrupt the host cell membrane to internalize or to deliver other bacterial or virulence factors for establishing infections. Disruption of the host cell membrane by PFTs can lead to uncontrollable exchanges between the extracellular and the intracellular matrix, thereby disturbing the cellular homeostasis. Recent studies have provided insights into the molecular mechanism of PFTs during pathogenesis. Evidence also suggests the activation of several signal transduction pathways in the host cell on recognition of PFTs. Additionally, numerous distinctive host defense mechanisms as well as membrane repair mechanisms have been reported; however, studies reveal that PFTs aid in host immune evasion of the bacteria through numerous pathways. PFTs have been primarily associated with foodborne pathogens. Infection and death from diseases by consuming contaminated food are a constant threat to public health worldwide, affecting socioeconomic development. Moreover, the emergence of new foodborne pathogens has led to the rise of bacterial antimicrobial resistance affecting the population. Hence, this review focuses on the role of PFTs secreted by foodborne pathogens. The review highlights the molecular mechanism of foodborne bacterial PFTs, assisting bacterial survival from the host immune responses and understanding the downstream mechanism in the activation of various signaling pathways in the host upon PFT recognition. PFT research is a remarkable and an important field for exploring novel and broad applications of antimicrobial compounds as therapeutics.

Paeoniflorin Derivative in Paeoniae Radix Aqueous Extract Suppresses Alpha-Toxin of Staphylococcus aureus

The emergence and dissemination of bacterial infections is paralyzing our public health systems worldwide. Worse still, there are no effective antibiotics against bacterial toxins, which facilitate the infection. Natural herbs that target bacterial toxins may be a better choice for therapy of infectious diseases. However, most natural drugs present unknown compositions and unclear mechanisms. Here we demonstrated that the Chinese herb Paeoniae Radix aqueous extract (PRAE) could suppress alpha-toxin (α-toxin) of Staphylococcus aureus. We observed that the paeoniflorin derivative (PRAE-a) derivative in PRAE significantly abolished the hemolytic activity of S. aureus α-toxin. The analyses of high-performance liquid chromatography (HPLC), mass spectrometer (MS), Fourier transform infrared spectrometer (FTIR), and nuclear magnetic resonance (NMR) showed that PRAE-a was a glycoside compound with a paeoniflorin nucleus. We further found that PRAE-a disrupted the pore-forming ability of α-toxin by prevention of the dimer to heptamer. Therefore, PRAE-a proved to be an effective therapy for S. aureus lung infections in mice by inhibiting α-toxin. Collectively, these results highlighted that PRAE-a can be used as an antibacterial agent to attenuate S. aureus virulence by targeting α-toxin.

Characterisation of a tripartite α-pore forming toxin from Serratia marcescens

Tripartite members of the ClyA family of α-PFTs have recently been identified in a number of pathogenic Gram-negative bacteria, including the human pathogen Serratia marcescens. Structures of a Gram-negative A component and a tripartite α-PFT complete pore are unknown and a mechanism for pore formation is still uncertain. Here we characterise the tripartite SmhABC toxin from S. marcescens and propose a mechanism of pore assembly. We present the structure of soluble SmhA, as well as the soluble and pore forms of SmhB. We show that the β-tongue soluble structure is well conserved in the family and propose two conserved latches between the head and tail domains that are broken on the soluble to pore conformational change. Using the structures of individual components, sequence analysis and docking predictions we illustrate how the A, B and C protomers would assemble on the membrane to produce a complete tripartite α-PFT pore.

The Food Poisoning Toxins of Bacillus cereus

Bacillus cereus is a ubiquitous soil bacterium responsible for two types of food-associated gastrointestinal diseases. While the emetic type, a food intoxication, manifests in nausea and vomiting, food infections with enteropathogenic strains cause diarrhea and abdominal pain. Causative toxins are the cyclic dodecadepsipeptide cereulide, and the proteinaceous enterotoxins hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe) and cytotoxin K (CytK), respectively. This review covers the current knowledge on distribution and genetic organization of the toxin genes, as well as mechanisms of enterotoxin gene regulation and toxin secretion. In this context, the exceptionally high variability of toxin production between single strains is highlighted. In addition, the mode of action of the pore-forming enterotoxins and their effect on target cells is described in detail. The main focus of this review are the two tripartite enterotoxin complexes Hbl and Nhe, but the latest findings on cereulide and CytK are also presented, as well as methods for toxin detection, and the contribution of further putative virulence factors to the diarrheal disease.

Activation mechanisms of inflammasomes by bacterial toxins

Inflammasomes are cytosolic innate immune complexes, which assemble in mammalian cells in response to microbial components and endogenous danger signals. A major family of inflammasome activators is bacterial toxins. Inflammasome sensor proteins, such as the nucleotide-binding oligomerisation domain-like receptor (NLR) family members NLRP1b and NLRP3, and the tripartite motif family member Pyrin⁺ efflux triggered by pore-forming toxins or by other toxin-induced homeostasis-altering events such as lysosomal rupture. Pyrin senses perturbation of host cell functions induced by certain enzymatic toxins resulting in impairment of RhoA GTPase activity. Assembly of the inflammasome complex activates the cysteine protease caspase-1, leading to the proteolytic cleavage of the proinflammatory cytokines IL-1β and IL-18, and the pore-forming protein gasdermin D causing pyroptosis. In this review, we discuss the latest progress in our understanding on the activation mechanisms of inflammasome complexes by bacterial toxins and effector proteins and explore avenues for future research into the relationships between inflammasomes and bacterial toxins.

Interactions between Food Hazards and Intestinal Barrier: Impact on Foodborne Diseases

The intestine is an important digestive organ of the human body, and its barrier is the guardian of the body from the external environment. The impairment of the intestinal barrier is believed to be an important determinant in various foodborne diseases. Food hazards can lead to the occurrence of many foodborne diseases represented by inflammation. Therefore, understanding the mechanisms of the impact of the food hazards on intestinal barriers is essential for promoting human health. This review examined the relationship between food hazards and the intestinal barrier in three aspects: apoptosis, imbalance of gut microbiota, and pro-inflammatory cytokines. The mechanism of dysfunctional gut microbiota caused by food hazards was also discussed. This review discusses the interaction among food hazards, intestinal barrier, and foodborne diseases and, thus, offers a new thought to deal with foodborne disease.

The Bacillus cereus Food Infection as Multifactorial Process

The ubiquitous soil bacterium Bacillus cereus presents major challenges to food safety. It is responsible for two types of food poisoning, the emetic form due to food intoxication and the diarrheal form emerging from food infections with enteropathogenic strains, also known as toxico-infections, which are the subject of this review. The diarrheal type of food poisoning emerges after production of enterotoxins by viable bacteria in the human intestine. Basically, the manifestation of the disease is, however, the result of a multifactorial process, including B. cereus prevalence and survival in different foods, survival of the stomach passage, spore germination, motility, adhesion, and finally enterotoxin production in the intestine. Moreover, all of these processes are influenced by the consumed foodstuffs as well as the intestinal microbiota which have, therefore, to be considered for a reliable prediction of the hazardous potential of contaminated foods. Current knowledge regarding these single aspects is summarized in this review aiming for risk-oriented diagnostics for enteropathogenic B. cereus.

Bacillus cereus: Epidemiology, Virulence Factors, and Host-Pathogen Interactions

The toxin-producing bacterium Bacillus cereus is an important and neglected human pathogen and a common cause of food poisoning. Several toxins have been implicated in disease, including the pore-forming toxins hemolysin BL (HBL) and nonhemolytic enterotoxin (NHE). Recent work revealed that HBL binds to the mammalian surface receptors LITAF and CDIP1 and that both HBL and NHE induce potassium efflux and activate the NLRP3 inflammasome, leading to pyroptosis. These mammalian receptors, in part, contribute to inflammation and pathology. Other putative virulence factors of B. cereus include cytotoxin K, cereulide, metalloproteases, sphingomyelinase, and phospholipases. In this review, we highlight the latest progress in our understanding of B. cereus biology, epidemiology, and pathogenesis, and discuss potential new directions for research in this field.

Structural Modeling of Cell Wall Peptidase CwpFM (EntFM) Reveals Distinct Intrinsically Disordered Extensions Specific to Pathogenic Bacillus cereus Strains

The emergence of B. cereus as an opportunistic food-borne pathogen has intensified the need to distinguish strains of public health concern. The heterogeneity of the diseases associated with B. cereus infections emphasizes the versatility of these bacteria strains to colonize their host. Nevertheless, the molecular basis of these differences remains unclear. Several toxins are involved in virulence, particularly in gastrointestinal disorders, but there are currently no biological markers able to differentiate pathogenic from harmless strains. We have previously shown that CwpFM is a cell wall peptidase involved in B. cereus virulence. Here, we report a sequence/structure/function characterization of 39 CwpFM sequences, chosen from a collection of B. cereus with diverse virulence phenotypes, from harmless to highly pathogenic strains. CwpFM is homology-modeled in silico as an exported papain-like endopeptidase, with an N-terminal end composed of three successive bacterial Src Homology 3 domains (SH3b_1–3) likely to control protein–protein interactions in signaling pathways, and a C-terminal end that contains a catalytic NLPC_P60 domain primed to form a competent active site. We confirmed in vitro that CwpFM is an endopeptidase with a moderate peptidoglycan hydrolase activity. Remarkably, CwpFMs from pathogenic strains harbor a specific stretch of twenty residues intrinsically disordered, inserted between the SH3b₃ and the catalytic NLPC_P60 domain. This strongly suggests this linker as a marker of differentiation between B. cereus strains. We believe that our findings improve our understanding of the pathogenicity of B. cereus while advancing both clinical diagnosis and food safety.

Sublethal Levels of Antibiotics Promote Bacterial Persistence in Epithelial Cells

Antibiotic therapy and host cells frequently fail to eliminate invasive bacterial pathogens due to the emergence of antibiotic resistance, resulting in the relapse and recurrence of infections. Bacteria evolve various strategies to persist and survive in epithelial cells, a front-line barrier of host tissues counteracting invasion; however, it remains unclear how bacteria hijack cellular responses to promote cytoplasmic survival under antibiotic therapy. Here, it is demonstrated that extracellular bacteria show invasive behavior and survive in epithelial cells in both in vivo and in vitro models, to increase antibiotic tolerance. In turn, sublethal levels of antibiotics increase bacterial invasion through promoting the production of bacterial virulence factors. Furthermore, antibiotic treatments interrupt lysosomal acidification in autophagy due to the internalized bacteria, using Bacillus cereus and ciprofloxacin as a model. In addition, it is found that sublethal levels of ciprofloxacin cause mitochondrial dysfunction and reactive oxygen species (ROS) accumulation to impair lysosomal vascular tape ATPase (V-ATPase) to further promote bacterial persistence. Collectively, these results highlight the potential of host cells mediated antibiotic tolerance, which markedly compromises antibiotic efficacy and worsens the outcomes of infection.

A benzimidazole inhibitor attenuates sterile inflammation induced in a model of systemic autoinflammation in female mice

Sterile stimuli can trigger inflammatory responses, and in some cases can lead to a variety of acute or chronic diseases. In this study, we hypothesize that a benzimidazole inhibitor may be used as a therapeutic in the treatment of sterile inflammation. In vitro, this inhibitor blocks TLR signalling and inflammatory responses. The benzimidazole inhibitor does not prevent mouse macrophage activation after stimulation with 2,6,10,14-tetramethylpentadecane (TMPD, also known as pristane), a hydrocarbon oil that mimics features of sterile inflammation when injected in vivo. However, C57BL/6J female mice treated with the benzimidazole inhibitor exhibited a significant reduction of pristane-dependent induction of splenocyte number and weight. Conversely, no significant difference was observed in males. Using mass spectrometry, we found that the urine of pristane-injected mice contained increased levels of putative markers for several inflammatory diseases, which were reduced by the benzimidazole inhibitor. To study the mechanism, we showed that pristane-injected mice had increased cell free DNA in serum, which was not impacted by inhibitor treatment. However, chemokine release (e.g. MCP-1, RANTES and TARC) was significantly reduced in inhibitor-treated mice. Thus, the benzimidazole inhibitor might be used as a new drug to block the recruitment of immune cells during sterile inflammatory diseases in humans.

Characterization of Bacillus cereus in Dairy Products in China

Bacillus cereus is a common and ubiquitous foodborne pathogen with an increasing prevalence rate in dairy products in China. High and unmet demands for such products, particularly milk, raise the risk of B. cereus associated contamination. The presence of B. cereus and its virulence factors in dairy products may cause food poisoning and other illnesses. Thus, this review first summarizes the epidemiological characteristics and analytical assays of B. cereus from dairy products in China, providing insights into the implementation of intervention strategies. In addition, the recent achievements on the cytotoxicity and mechanisms of B. cereus are also presented to shed light on the therapeutic options for B. cereus associated infections.