Staphylococcus aureus α-hemolysin mediates virulence in a murine model of severe pneumonia through activation of the NLRP3 inflammasome

Aerobic spore-forming bacteria associated with ropy bread: Identification, characterization and spoilage potential assessment

Aerobic spore-forming (ASF) bacteria have been reported to cause ropiness in bread. Sticky and stringy degradation, discoloration, and an odor reminiscent of rotting fruit are typical characteristics of ropy bread spoilage. In addition to economic losses, ropy bread spoilage may lead to health risks, as virulent strains of ASF bacteria are not uncommon. However, the lack of systematic approaches to quantify physicochemical spoilage characteristics makes it extremely difficult to assess rope formation in bread. To address this problem, the aim of this study was to identify, characterize and objectively assess the spoilage potential of ASF bacteria associated with ropy bread. Hence, a set of 82 ASF bacteria, including isolates from raw materials and bakery environments as well as strains from international culture collections, were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and their species identity confirmed by 16S rRNA and gyrA or panC gene sequencing. A standardized approach supported by objective colorimetric measurements was developed to assess the rope-inducing potential (RIP) of a strain by inoculating autoclaved bread slices with bacterial spores. In addition, the presence of potential virulence factors such as swarming motility or hemolysis was investigated. This study adds B. velezensis, B. inaquosorum and B. spizizenii to the species potentially implicated of causing ropy bread spoilage. Most importantly, this study introduces a standardized classification protocol for assessing the RIP of a bacterial strain. Colorimetric measurements are used to objectively quantify the degree of breadcrumb discoloration. Furthermore, our results indicate that strains capable of inducing rope spoilage in bread often exhibit swarming motility and virulence factors such as hemolysis, raising important food quality considerations.

HDAC6 inhibitor promotes reactive oxygen species-meditated clearance of Staphylococcus aureus in macrophage

Staphylococcus aureus (S. aureus) pneumonia has become an increasingly important public health problem. Recent evidence suggests that epigenetic modifications are critical in the host immune defence against pathogen infection. In this study, we found that S. aureus infection induces the expression of histone deacetylase 6 (HDAC6) in a dose-dependent manner. Furthermore, by using a S. aureus pneumonia mouse model, we showed that the HDAC6 inhibitor, tubastatin A, demonstrates a protective effect in S. aureus pneumonia, decreasing the mortality and destruction of lung architecture, reducing the bacterial burden in the lungs and inhibiting inflammatory responses. Mechanistic studies in primary bone marrow-derived macrophages demonstrated that the HDAC6 inhibitors, tubastatin A and tubacin, reduced the intracellular bacterial load by promoting bacterial clearance rather than regulating phagocytosis. Finally, N-acetyl-L- cysteine, a widely used reactive oxygen species (ROS) scavenger, antagonized ROS production and significantly inhibited tubastatin A-induced S. aureus clearance. These findings demonstrate that HDAC6 inhibitors promote the bactericidal activity of macrophages by inducing ROS, an important host factor for S. aureus clearance and production. Our study identified HDAC6 as a suitable epigenetic modification target for preventing S. aureus infection, and tubastatin A as a useful compound in treating S. aureus pneumonia.

Strategies of bacterial detection by inflammasomes

Mammalian innate immunity is regulated by pattern-recognition receptors (PRRs) and guard proteins, which use distinct strategies to detect infections. PRRs detect bacterial molecules directly, whereas guards detect host cell manipulations by microbial virulence factors. Despite sensing infection through different mechanisms, both classes of innate immune sensors can activate the inflammasome, an immune complex that can mediate cell death and inflammation. Inflammasome-mediated immune responses are crucial for host defense against many bacterial pathogens and prevent invasion by non-pathogenic organisms. In this review, we discuss the mechanisms by which inflammasomes are stimulated by PRRs and guards during bacterial infection, and the strategies used by virulent bacteria to evade inflammasome-mediated immunity.

Network pharmacology and experimental verification of the mechanism of licochalcone A against Staphylococcus aureus pneumonia

Staphylococcus aureus strains cause the majority of pneumonia cases and are resistant to various antibiotics. Given this background, it is very important to discover novel host-targeted therapies. Licochalcone A (LAA), a natural plant product, has various biological activities, but its primary targets in S. aureus pneumonia remain unclear. Therefore, the purpose of this study was to identify its molecular target against S. aureus pneumonia. Network pharmacology analysis, histological assessment, enzyme-linked immunosorbent assays, and Western blotting were used to confirm the pharmacological effects. Network pharmacology revealed 33 potential targets of LAA and S. aureus pneumonia. Enrichment analysis revealed that these potential genes were enriched in the Toll-like receptor and NOD-like receptor signaling pathways. The results were further verified by experiments in which LAA alleviated histopathological changes, inflammatory infiltrating cells and inflammatory cytokines (TNF, IL-6, and IL-1β) in the serum and bronchoalveolar lavage fluid in vivo. Moreover, LAA treatment effectively reduced the expression levels of NF-κB, p-JNK, p-p38, NLRP3, ASC, caspase 1, IL-1β, and IL-18 in lung tissue. The in vitro experimental results were consistent with the in vivo results. Thus, our findings demonstrated that LAA exerts anti-infective effects on S. aureus-induced lung injury via suppression of the Toll-like receptor and NOD-like receptor signaling pathways, which provides a theoretical basis for understanding the function of LAA against S. aureus pneumonia and implies its potential clinical application.

Antivirulence Effects of Trans-Resveratrol and Curcumin on Methicillin-Resistant Staphylococcus aureus (MRSA) from Saudi Arabia

Methicillin-resistant Staphylococcus aureus (MRSA) is a common resistant bacterium, whose resistance has expanded to commonly used antibiotics. It is crucial to create novel treatments to tackle bacterial resistance. Trans-resveratrol and curcumin are naturally occurring phenolic compounds, whose effects on MRSA virulence are the subject of this investigation. Sub-MICs of trans-resveratrol and curcumin were tested on the virulence factors of 50 MRSA clinical isolates (CIs), including biofilm, hemolysin, hemagglutination, protease, and lecithinase. The distribution of the virulence factors of the CIs was as follows: hemolysin: 98%, hemagglutination: 70%, protease: 62%, biofilm: 56%, and lecithinase: 52%. The sub-MIC that could reduce the effect of the tested virulence factors by 50% or more (IC50) was observed in the strains that showed susceptibility to the individual administration of trans-resveratrol at 50 µg/mL and curcumin at 20 µg/mL. Hemagglutination and hemolysin activity were inhibited by at least 50% in the majority of CIs (57-94%). Meanwhile, the IC50 for protease and biofilm was observed in 6.5-17.8% of the CIs. A few of the CIs were susceptible to lecithinase inhibition, but all showed a full inhibition. This research supports the possibility of the use of these compounds to reduce the bacterial virulence that can reduce antibiotic utilization, and eventually, they can become a potential alternative treatment in combating bacterial resistance.

The role of Staphylococcus aureus quorum sensing in cutaneous and systemic infections

BACKGROUND

Staphylococcus aureus is a leading cause of human bacterial infections worldwide. It is the most common causative agent of skin and soft tissue infections, and can also cause various other infections, including pneumonia, osteomyelitis, as well as life-threatening infections, such as sepsis and infective endocarditis. The pathogen can also asymptomatically colonize human skin, nasal cavity, and the intestine. S. aureus colonizes approximately 20-30% of human nostrils, being an opportunistic pathogen for subsequent infection. Its strong ability to silently spread via human contact makes it difficult to eradicate S. aureus. A major concern with S. aureus is its capacity to develop antibiotic resistance and adapt to diverse environmental conditions. The variability in the accessory gene regulator (Agr) region of the genome contributes to a spectrum of phenotypes within the bacterial population, enhancing the likelihood of survival in different environments. Agr functions as a central quorum sensing (QS) system in S. aureus, allowing bacteria to adjust gene expression in response to population density. Depending on Agr expression, S. aureus secretes various toxins, contributing to virulence in infectious diseases. Paradoxically, expressing Agr may be disadvantageous in certain situations, such as in hospitals, causing S. aureus to generate Agr mutants responsible for infections in healthcare settings.

MAIN BODY

This review aims to demonstrate the molecular mechanisms governing the diverse phenotypes of S. aureus, ranging from a harmless colonizer to an organism capable of infecting various human organs. Emphasis will be placed on QS and its role in orchestrating S. aureus behavior across different contexts.

SHORT CONCLUSION

The pathophysiology of S. aureus infection is substantially influenced by phenotypic changes resulting from factors beyond Agr. Future studies are expected to give the comprehensive understanding of S. aureus overall profile in various settings.

"Other Than NLRP3" Inflammasomes: Multiple Roles in Brain Disease

Human neuroinflammatory and neurodegenerative diseases, whose prevalence keeps rising, are still unsolved pathobiological/therapeutical problems. Among others, recent etiology hypotheses stressed as their main driver a chronic neuroinflammation, which is mediated by innate immunity-related protein oligomers: the inflammasomes. A panoply of exogenous and/or endogenous harmful agents activates inflammasomes' assembly, signaling, and IL-1β/IL-18 production and neural cells' pyroptotic death. The underlying concept is that inflammasomes' chronic activation advances neurodegeneration while their short-lasting operation restores tissue homeostasis. Hence, from a therapeutic standpoint, it is crucial to understand inflammasomes' regulatory mechanisms. About this, a deluge of recent studies focused on the NLRP3 inflammasome with suggestions that its pharmacologic block would hinder neurodegeneration. Yet hitherto no evidence proves this view. Moreover, known inflammasomes are numerous, and the mechanisms regulating their expression and function may vary with the involved animal species and strains, as well as organs and cells, and the harmful factors triggered as a result. Therefore, while presently leaving out some little-studied inflammasomes, this review focuses on the "other than NLRP3" inflammasomes that participate in neuroinflammation's complex mechanisms: NLRP1, NLRP2, NLRC4, and AIM2. Although human-specific data about them are relatively scant, we stress that only a holistic view including several human brain inflammasomes and other potential pathogenetic drivers will lead to successful therapies for neuroinflammatory and neurodegenerative diseases.

The pathogenicity and immune effects of different generations of Mycoplasma synoviae on chicken embryos

1. Mycoplasma synoviae (MS) is the primary causative agent of synovitis in avian species. In order to investigate the pathogenicity and immunological responses associated with MS in specific pathogen-free chicken embryos, a series of generations (F1, F95, F120, F160 and F200) of MS were introduced into 7-day-old SPF chicken embryos and subsequent mortality rates were recorded and analysed2. Reverse transcription-quantitative polymerase chain reaction was performed to detect expression of heat shock proteins HSP27, HSP40, HSP60, HSP70 and HSP90 and inflammatory factors interleukin (IL)-1β, caspase-1 and IL-18 in the tracheal tissue.3. The results showed that the mortality rate of SPF chicken embryos decreased with an increase in the number of passages, with the highest being 80% (8/10) for F1 generation and the lowest being 10% (1/10) for F200. The expression of HSP27, IL-1β, HSP40, caspase-1, HSP70 and HSP90 showed a significant downregulation trend with an increase in the generation (except IL-18; P < 0.05). The HSP60 expression was significantly upregulated with increasing generations (P < 0.05).4. A relationship between pathogenicity and the number of passages was observed and the decrease in pathogenicity appeared to be associated with HSP and genes related to inflammatory factors. The present work offers a scientific foundation for screening potential MS strains that might be employed to develop attenuated vaccines.

Investigation of Staphylococcus aureus Biofilm-Associated Toxin as a Potential Squamous Cell Carcinoma Therapeutic

Cancer therapies developed using bacteria and their components have been around since the 19th century. Compared to traditional cancer treatments, the use of bacteria-derived compounds as cancer therapeutics could offer a higher degree of specificity, with minimal off-target effects. Here, we explored the use of soluble bacteria-derived toxins as a potential squamous cell carcinoma (SCC) therapeutic. We optimized a protocol to generate Staphylococcus aureus biofilm-conditioned media (BCM), where soluble bacterial products enriched in the development of biofilms were isolated from a bacterial culture and applied to SCC cell lines. Bioactive components of S. aureus ATCC 29213 (SA29213) BCM display selective toxicity towards cancerous human skin SCC-12 at low doses, while non-cancerous human keratinocyte HaCaT and fibroblast BJ-5ta are minimally affected. SA29213 BCM treatment causes DNA damage to SCC-12 and initiates Caspase 3-dependent-regulated cell death. The use of the novel SA29213 bursa aurealis transposon mutant library led to the identification of S. aureus alpha hemolysin as the main bioactive compound responsible for the observed SCC-12-specific toxicity. The antibody neutralisation of Hla eradicates the cytotoxicity of SA29213 BCM towards SCC-12. Hla displays high SCC-12-specific toxicity, which is exerted primarily through Hla-ADAM10 interaction, Hla oligomerisation, and pore formation. The high target specificity and potential to cause cell death in a controlled manner highlight SA29213 Hla as a good candidate as an alternative SCC therapeutic.

TLR4 sensing of IsdB of Staphylococcus aureus induces a proinflammatory cytokine response via the NLRP3-caspase-1 inflammasome cascade

IMPORTANCE

The prevalence of multidrug-resistant Staphylococcus aureus is of global concern, and vaccines are urgently needed. The iron-regulated surface determinant protein B (IsdB) of S. aureus was investigated as a vaccine candidate because of its essential role in bacterial iron acquisition but failed in clinical trials despite strong immunogenicity. Here, we reveal an unexpected second function for IsdB in pathogen-host interaction: the bacterial fitness factor IsdB triggers a strong inflammatory response in innate immune cells via Toll-like receptor 4 and the inflammasome, thus acting as a novel pathogen-associated molecular pattern of S. aureus. Our discovery contributes to a better understanding of how S. aureus modulates the immune response, which is necessary for vaccine development against the sophisticated pathogen.

Staphylococcus aureus enhances osteoclast differentiation and bone resorption by stimulating the NLRP3 inflammasome pathway

BACKGROUND

Osteomyelitis is one of the most challenging infectious diseases and is mainly caused by Staphylococcus aureus (S. aureus). In this study, we analyzed the effect of S. aureus on osteoclast differentiation and its possible molecular mechanism.

METHODS

We cultured RAW 264.7 cells with live S. aureus for 5 days. We assessed cell viability and the formation of resorption pits. We tested the NLRP3 inflammasome signaling pathways and measured the mRNA expression levels of osteoclastspecific genes, including TRAP, MMP9, cathepsin K, calcitonin receptor and ATP6V0d2. Furthermore, we analyzed the protein expression levels of the protein in the NF-κB and p38 MAPK signaling pathways to clarify the signaling pathways by which S. aureus promotes osteoclast differentiation.

RESULTS

Staphylococcus aureus induced NLRP3 inflammasome activation. S. aureus promoted bone resorption and enhanced the expression of osteoclastspecific genes, such as TRAP, MMP9, cathepsin K, calcitonin receptor and ATP6V0d2. MCC950 was used to inhibit NLRP3 inflammasome activity. Osteoclast differentiation and the expression of osteoclastspecific genes induced by S. aureus were inhibited by MCC950 pretreatment. The degradation of IκBα and phosphorylation of P65 were increased under the induction of S. aureus, but proteins in the p38 MAPK signaling pathway did not change significantly.

CONCLUSION

Staphylococcus aureus induces osteoclast differentiation and promotes bone resorption in vitro, and the NLRP3 inflammasome signaling pathway plays a significant role in this process. S. aureus-induced NLRP3 inflammasome activation was mainly dependent on the NF-κB signaling pathway during osteoclastogenesis.

Inflammasome-mediated glucose limitation induces antibiotic tolerance in Staphylococcus aureus

Staphylococcus aureus is a leading human pathogen that frequently causes relapsing infections. The failure of antibiotics to eradicate infection contributes to infection relapse. Host-pathogen interactions have a substantial impact on antibiotic susceptibility and the formation of antibiotic tolerant cells. In this study, we interrogate how a major S. aureus virulence factor, α-toxin, interacts with macrophages to alter the microenvironment of the pathogen, thereby influencing its susceptibility to antibiotics. We find α-toxin-mediated activation of the NLRP3 inflammasome induces antibiotic tolerance. Induction of tolerance is driven by increased glycolysis in the host cells, resulting in glucose limitation and ATP depletion in S. aureus. Additionally, inhibition of NLRP3 activation improves antibiotic efficacy in vitro and in vivo, suggesting that this strategy has potential as a host-directed therapeutic to improve outcomes. Our findings identify interactions between S. aureus and the host that result in metabolic crosstalk that can determine the outcome of antimicrobial therapy.

Role of toll-like receptors and nod-like receptors in acute lung infection

The respiratory system exposed to microorganisms continuously, and the pathogenicity of these microbes not only contingent on their virulence factors, but also the host's immunity. A multifaceted innate immune mechanism exists in the respiratory tract to cope with microbial infections and to decrease tissue damage. The key cell types of the innate immune response are macrophages, neutrophils, dendritic cells, epithelial cells, and endothelial cells. Both the myeloid and structural cells of the respiratory system sense invading microorganisms through binding or activation of pathogen-associated molecular patterns (PAMPs) to pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The recognition of microbes and subsequent activation of PRRs triggers a signaling cascade that leads to the activation of transcription factors, induction of cytokines/5chemokines, upregulation of cell adhesion molecules, recruitment of immune cells, and subsequent microbe clearance. Since numerous microbes resist antimicrobial agents and escape innate immune defenses, in the future, a comprehensive strategy consisting of newer vaccines and novel antimicrobials will be required to control microbial infections. This review summarizes key findings in the area of innate immune defense in response to acute microbial infections in the lung. Understanding the innate immune mechanisms is critical to design host-targeted immunotherapies to mitigate excessive inflammation while controlling microbial burden in tissues following lung infection.

New insight into the virulence and inflammatory response of Staphylococcus aureus strains isolated from diabetic foot ulcers

Staphylococcus aureus strains isolated from diabetic foot ulcers (DFUs) have less virulence, but still cause severe infections. Furthermore, hypovirulent S. aureus strains appear to be localized in the deep tissues of diabetic foot osteomyelitis, indicating that the unique environment within DFUs affects the pathogenicity of S. aureus. In this study, the cell-free culture medium (CFCM) of S. aureus strains isolated from DFUs exhibited higher cytotoxicity to human erythrocytes than those isolated from non-diabetic patients with sepsis or wounds. Among these S. aureus strains isolated from DFUs, β-toxin negative strains have less virulence than β-toxin positive strains, but induced a higher expression of inflammatory cytokines. Our study and previous studies have shown that the synergistic effect of phenol-soluble modulin α and β-toxin contributes to the higher hemolytic activity of β-toxin positive strains. However, lysis of human erythrocytes by the CFCM of β-toxin negative strains was greatly inhibited by an autolysin inhibitor, sodium polyanethole sulfonate (SPS). A high level of glucose greatly reduced the hemolytic activity of S. aureus, but promoted the expression of interleukin-6 (IL-6) in human neutrophils. However, 5 mM glucose or glucose-6-phosphate (G6P) increased the hemolytic activity of SA118 (a β-toxin negative strain) isolated from DFUs. Additionally, patients with DFUs with growth of S. aureus had lower level of serum IL-6 than those with other bacteria, and the CFCM of S. aureus strains significantly reduced lipopolysaccharide-induced IL-6 expression in human neutrophils. Therefore, the virulence and inflammatory response of S. aureus strains isolated from DFUs are determined by the levels of glucose and its metabolites, which may explain why it is the predominant bacteria isolated from DFUs.

Monoclonal antibodies for prophylaxis and therapy of respiratory syncytial virus, SARS-CoV-2, human immunodeficiency virus, rabies and bacterial infections: an update from the World Association of Infectious Diseases and Immunological Disorders and the Italian Society of Antinfective Therapy

Monoclonal antibodies (mABs) are safe and effective proteins produced in laboratory that may be used to target a single epitope of a highly conserved protein of a virus or a bacterial pathogen. For this purpose, the epitope is selected among those that play the major role as targets for prevention of infection or tissue damage. In this paper, characteristics of the most important mABs that have been licensed and used or are in advanced stages of development for use in prophylaxis and therapy of infectious diseases are discussed. We showed that a great number of mABs effective against virus or bacterial infections have been developed, although only in a small number of cases these are licensed for use in clinical practice and have reached the market. Although some examples of therapeutic efficacy have been shown, not unlike more traditional antiviral or antibacterial treatments, their efficacy is significantly greater in prophylaxis or early post-exposure treatment. Although in many cases the use of vaccines is more effective and cost-effective than that of mABs, for many infectious diseases no vaccines have yet been developed and licensed. Furthermore, in emergency situations, like in epidemics or pandemics, the availability of mABs can be an attractive adjunct to our armament to reduce the impact. Finally, the availability of mABs against bacteria can be an important alternative, when multidrug-resistant strains are involved.

Necrotizing Pneumonia in Children: Early Recognition and Management

Necrotizing pneumonia (NP) is an uncommon complicated pneumonia with an increasing incidence. Early recognition and timely management can bring excellent outcomes. The diagnosis of NP depends on chest computed tomography, which has radiation damage and may miss the optimal treatment time. The present review aimed to elaborate on the reported predictors for NP. The possible pathogenesis of Streptococcus pneumoniae, Staphylococcus aureus, Mycoplasma pneumoniae and coinfection, clinical manifestations and management were also discussed. Although there is still a long way for these predictors to be used in clinical, it is necessary to investigate early predictors for NP in children.

Neutrophilic Pleuritis Is a Severe Complication of Klebsiella pneumoniae Pneumonia in Old Mice

The pathomechanisms underlying the frequently observed fatal outcome of Klebsiella pneumoniae pneumonia in elderly patients are understudied. In this study, we examined the early antibacterial immune response in young mice (age 2-3 mo) as compared with old mice (age 18-19 mo) postinfection with K. pneumoniae. Old mice exhibited significantly higher bacterial loads in lungs and bacteremia as early as 24 h postinfection compared with young mice, with neutrophilic pleuritis nearly exclusively developing in old but not young mice. Moreover, we observed heavily increased cytokine responses in lungs and pleural spaces along with increased mortality in old mice. Mechanistically, Nlrp3 inflammasome activation and caspase-1-dependent IL-1β secretion contributed to the observed hyperinflammation, which decreased upon caspase-1 inhibitor treatment of K. pneumoniae-infected old mice. Irradiated old mice transplanted with the bone marrow of young mice did not show hyperinflammation or early bacteremia in response to K. pneumoniae. Collectively, the accentuated lung pathology observed in K. pneumoniae-infected old mice appears to be due to regulatory defects of the bone marrow but not the lung, while involving dysregulated activation of the Nlrp3/caspase-1/IL-1β axis.

Bavachin Suppresses Alpha-Hemolysin Expression and Protects Mice from Pneumonia Infection by Staphylococcus aureus

Staphylococcus aureus (S. aureus) infection causes dramatic harm to human health as well as to livestock development. As an important virulence factor, alpha-hemolysin (hla) is critical in the process of S. aureus infection. In this report, we found that bavachin, a natural flavonoid, not only efficiently inhibited the hemolytic activity of hla, but was also capable of inhibiting it on transcriptional and translational levels. Moreover, further data revealed that bavachin had no neutralizing activity on hla, which did not affect the formation of hla heptamers and exhibited no effects on the hla thermal stability. In vitro assays showed that bavachin was able to reduce the S. aureus-induced damage of A549 cells. Thus, bavachin repressed the lethality of pneumonia infection, lung bacterial load and lung tissue inflammation in mice, providing potent protection to mice models in vivo. Our results indicated that bavachin has the potential for development as a candidate hla inhibitor against S. aureus.

A Dual-Action Molecule Suppresses S. aureus Infection as an Inhibitor Targeting Hla Pore Formation and TLR2 Signaling

Antibiotic resistance is the greatest challenge for the treatment of Staphylococcus aureus (S. aureus) infection under the global antibiotic resistance crisis. With the bottleneck period of the development of new antibiotics, novel alternative agents are urgently in need. In this study, the small molecule amentoflavone is identified as a dual-action inhibitor of Hla, a pore-forming virulence determinant particularly important for S. aureus pathogenicity and Toll-like receptor 2 (TLR2) signaling, which triggers inflammation response upon recognizing pathogen-associated molecular patterns. Amentoflavone treatment conferred effective protection against S. aureus pneumonia through this dual-action mechanism. Mechanically, amentoflavone effectively inhibited Hla pore formation, thereby reducing Hla-mediated cytotoxicity and tissue damage; at the same time, amentoflavone suppressed TLR2-mediated inflammatory response by blocking the interaction between TLR2 and its adapter myeloid differentiation primary response gene 88 (MyD88). Surprisingly, TLR2 signaling induced by synthetic bacterial TLR2 agonists and other heat-killed gram-positive bacteria is also blocked by amentoflavone. In summary, these results presented amentoflavone as a potential antibiotic alternative that curbed S. aureus infection by simultaneously suppressing host-damaging virulence determinants derived from bacteria and the detrimental effect of excessive inflammation derived from the host rather than bacteria viability.

7,8-Dihydroxyflavone attenuates the virulence of Staphylococcus aureus by inhibiting alpha-hemolysin

Staphylococcus aureus (S. aureus), a Gram-positive bacteria, is an incurable cause of hospital and community-acquired infections. Inhibition bacterial virulence is a viable strategy against S. aureus infections based on the multiple virulence factors secreted by S. aureus. Alpha-hemolysin (Hla) plays a crucial role in bacteria virulence without affecting bacterial viability. Here, we identified that 7,8-Dihydroxyflavone (7,8-DHF), a natural compound, was able to decrease the expression of and did not affect the in vitro growth of S. aureus USA300 at a concentration of 32 μg/mL. It was verified by western blot and RT-qPCR that the natural compound could inhibit the transcription and translation of Hla. Further mechanism studies revealed that 7,8-DHF has a negative effect on transcriptional regulator agrA and RNAIII, preventing the upregulation of virulence gene. Cytotoxicity assays showed that 7,8-DHF did not produce significant cytotoxicity to A549 cells. Animal experiments showed that the combination of 7,8-DHF and vancomycin had a more significant therapeutic effect on S. aureus infection, reflecting the synergistic effect of 7,8-DHF with antibiotics. In conclusion, 7,8-DHF was able to target Hla to protect host cells from hemolysis while limiting the development of bacterial resistance.

Characterization of the Basal and mTOR-Dependent Acute Pulmonary and Systemic Immune Response in a Murine Model of Combined Burn and Inhalation Injury

Severe burn injury leads to a cascade of local and systemic immune responses that trigger an extreme state of immune dysfunction, leaving the patient highly susceptible to acute and chronic infection. When combined with inhalation injury, burn patients have higher mortality and a greater chance of developing secondary respiratory complications including infection. No animal model of combined burn and inhalation injury (B+I) exists that accurately mirrors the human clinical picture, nor are there any effective immunotherapies or predictive models of the risk of immune dysfunction. Our earlier work showed that the mechanistic/mammalian target of rapamycin (mTOR) pathway is activated early after burn injury, and its chemical blockade at injury reduced subsequent chronic bacterial susceptibility. It is unclear if mTOR plays a role in the exacerbated immune dysfunction seen after B+I injury. We aimed to: (1) characterize a novel murine model of B+I injury, and (2) investigate the role of mTOR in the immune response after B+I injury. Pulmonary and systemic immune responses to B+I were characterized in the absence or presence of mTOR inhibition at the time of injury. Data describe a murine model of B+I with inhalation-specific immune phenotypes and implicate mTOR in the acute immune dysfunction observed.

Exploring the Role of Staphylococcus aureus in Inflammatory Diseases

Staphylococcus aureus is a very common Gram-positive bacterium, and S. aureus infections play an extremely important role in a variety of diseases. This paper describes the types of virulence factors involved, the inflammatory cells activated, the process of host cell death, and the associated diseases caused by S. aureus. S. aureus can secrete a variety of enterotoxins and other toxins to trigger inflammatory responses and activate inflammatory cells, such as keratinocytes, helper T cells, innate lymphoid cells, macrophages, dendritic cells, mast cells, neutrophils, eosinophils, and basophils. Activated inflammatory cells can express various cytokines and induce an inflammatory response. S. aureus can also induce host cell death through pyroptosis, apoptosis, necroptosis, autophagy, etc. This article discusses S. aureus and MRSA (methicillin-resistant S. aureus) in atopic dermatitis, psoriasis, pulmonary cystic fibrosis, allergic asthma, food poisoning, sarcoidosis, multiple sclerosis, and osteomyelitis. Summarizing the pathogenic mechanism of Staphylococcus aureus provides a basis for the targeted treatment of Staphylococcus aureus infection.

Selenium Ameliorates S. aureus-Induced Inflammation in Bovine Mammary Epithelial Cells by Regulating ROS-Induced NLRP3 Inflammasome

In this study, the regulation effects of selenium (Se) on the expression of pyrin domain-containing protein (NLRP) 3 inflammasome and reactive oxygen species (ROS) in bovine mammary epithelial cells (bMECs) infected by Staphylococcus aureus (S. aureus) were detected. bMECs were treated with 8 μmol/L Na₂SeO₃ for 12 h before infection with S. aureus for 2 h. Through flow cytometry, Western blot, and qRT-PCR analysis, the expression of ROS and NLRP3 imflammasome was detected. Results showed Se significantly reduced the ROS level in bMECs; at the same time, the expressions of NLRP3, ASC, caspase-1, Pro-IL-1β, and IL-1β were also decreased. In conclusion, Se inhibits S. aureus-induced inflammation by suppressing the activation of NLRP3 inflammasome and ROS in bMECs.

The pro-inflammatory effect of Staphylokinase contributes to community-associated Staphylococcus aureus pneumonia

Pneumonia caused by community-associated Staphylococcus aureus (CA-SA) has high morbidity and mortality, but its pathogenic mechanism remains to be further investigated. Herein, we identify that staphylokinase (SAK) is significantly induced in CA-SA and inhibits biofilm formation in a plasminogen-dependent manner. Importantly, SAK can enhance CA-SA-mediated pneumonia in both wild-type and cathelicidins-related antimicrobial peptide knockout (CRAMP^−/−) mice, suggesting that SAK exacerbates pneumonia in a CRAMP-independent manner. Mechanistically, SAK induces pro-inflammatory effects, especially in the priming step of NLRP3 inflammasome activation. Moreover, we demonstrate that SAK can increase K⁺ efflux, production of reactive oxygen species production, and activation of NF-κB signaling. Furthermore, the NLRP3 inflammasome inhibitor can counteract the effective of SAK induced CA-SA lung infection in mice. Taken together, we speculate that SAK exacerbates CA-SA-induced pneumonia by promoting NLRP3 inflammasome activation, providing new insights into the pathogenesis of highly virulent CA-SA and emphasizes the importance of controlling inflammation in acute pneumonia.

Staphylokinase (Sak) is highly prevalent in human-adapted S. aureus strains, with increased expression in community-associated (CA-SA) strains, promoting lung infection and activation of the NLRP3 inflammasome.

Impact of IsaA Gene Disruption: Decreasing Staphylococcal Biofilm and Alteration of Transcriptomic and Proteomic Profiles

Staphylococcus aureus expresses diverse proteins at different stages of growth. The immunodominant staphylococcal antigen A (IsaA) is one of the proteins that is constitutively produced by S. aureus during colonisation and infection. SACOL2584 (or isaA) is the gene that encodes this protein. It has been suggested that IsaA can hydrolyse cell walls, and there is still need to study isaA gene disruption to analyse its impact on staphylococcal phenotypes and on alteration to its transcription and protein profiles. In the present study, the growth curve in RPMI medium (which mimics human plasma), autolytic activity, cell wall morphology, fibronectin and fibrinogen adhesion and biofilm formation of S. aureus SH1000 (wildtype) was compared to that of S. aureus MS001 (isaA mutant). RNA sequencing and liquid chromatography–tandem mass spectrometry were carried out on samples of both S. aureus strains taken during the exponential growth phase, followed by bioinformatics analysis. Disruption of isaA had no obvious effect on the growth curve and autolysis ability or thickness of cell walls, but this study revealed significant strength of fibronectin adherence in S. aureus MS001. In particular, the isaA mutant formed less biofilm than S. aureus SH1000. In addition, proteomics and transcriptomics showed that the adhesin/biofilm-related genes and hemolysin genes, such as sasF, sarX and hlgC, were consistently downregulated with isaA gene disruption. The majority of the upregulated genes or proteins in S. aureus MS001 were pur genes. Taken together, this study provides insight into how isaA disruption changes the expression of other genes and has implications regarding biofilm formation and biological processes.

Inhibition of Staphylococcus aureus α-Hemolysin Production Using Nanocurcumin Capped Au@ZnO Nanocomposite

Nanoparticles of gold with zinc oxide (Au@ZnO NPs) were prepared by laser ablation and then capped with curcumin nanoparticles (Cur-Au@ZnO NPs). The synthesized NPs were characterized using different techniques, including transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), UV-visible spectroscopy, and X-ray diffraction. In addition, the ability of NPs as a promising antibacterial agent was tested against Staphylococcus aureus through the agar well diffusion method and AO/EtBr staining assay. The results showed that the prepared nanoparticles (Cur-Au@ZnO) served as an antibacterial agent and can destroy the bacterial cells by losing the cell wall integrity and penetrating the cytoplasmic membrane. Moreover, the findings confirmed the role of the formed NPs in attenuation of the adherence and invasion of S. aureus to rat embryonic fibroblast (REF) cells. Furthermore, the activity of Cur-Au@ZnO NPs against the S. aureus α-hemolysin toxin was evaluated using the western blot technique, using human alveolar epithelial cells (A549), and through histopathology examination in a mouse model. In conclusion, the built Cur-Au@ZnO NPs can be used as a potential antibacterial agent and an inhibitor of α-hemolysin toxin secreted by S. aureus. These NPs may offer a new strategy in combating pathogen infections and in the future for biomedical and pharmaceutical applications.

Detecting DNA: An Overview of DNA Recognition by Inflammasomes and Protection against Bacterial Respiratory Infections

The innate immune system plays a key role in modulating host immune defense during bacterial disease. Upon sensing pathogen-associated molecular patterns (PAMPs), the multi-protein complex known as the inflammasome serves a protective role against bacteria burden through facilitating pathogen clearance and bacteria lysis. This can occur through two mechanisms: (1) the cleavage of pro-inflammatory cytokines IL-1β/IL-18 and (2) the initiation of inflammatory cell death termed pyroptosis. In recent literature, AIM2-like Receptor (ALR) and Nod-like Receptor (NLR) inflammasome activation has been implicated in host protection following recognition of bacterial DNA. Here, we review current literature synthesizing mechanisms of DNA recognition by inflammasomes during bacterial respiratory disease. This process can occur through direct sensing of DNA or indirectly by sensing pathogen-associated intracellular changes. Additionally, DNA recognition may be assisted through inflammasome–inflammasome interactions, specifically non-canonical inflammasome activation of NLRP3, and crosstalk with the interferon-inducible DNA sensors Stimulator of Interferon Genes (STING) and Z-DNA Binding Protein-1 (ZBP1). Ultimately, bacterial DNA sensing by inflammasomes is highly protective during respiratory disease, emphasizing the importance of inflammasome involvement in the respiratory tract.

Potent Activity of Ertapenem Plus Cefazolin Within Staphylococcal Biofilms: A Contributing Factor in the Treatment of Methicillin-Susceptible Staphylococcus aureus Endocarditis

Background

Besides antistaphylococcal beta-lactams and source control, there are limited validated antimicrobial salvage options in patients with prolonged methicillin-susceptible Staphylococcus aureus (MSSA) bacteremia, including infective endocarditis (IE).

Methods

MSSA IE cases treated with ertapenem (ETP) plus cefazolin (CZ) were compared with matched IE cases treated with standard beta-lactam monotherapy. The bactericidal activity of ETP plus CZ was also compared with nafcillin (NAF), CZ, and ETP alone using an in vitro MSSA biofilm model.

Results

The median duration of bacteremia experienced by patients (n = 12) while on CZ or NAF was 4 days (range 1-16 days) compared with 1 day (range 1-3 days) for patients (n = 5) treated with ETP + CZ (P = .01, Mann-Whitney U test). Cefazolin and NAF alone or in combination did not achieve biofilm eradication at clinically relevant concentrations. However, the addition of ETP to CZ led to bactericidal eradication within biofilms at standard dosing.

Conclusions

Ertapenem reduces CZ concentrations required to eradicate MSSA biofilms to those achievable in vivo by standard dosing, translating into shorter bacteremia duration in patients with MSSA endocarditis. Larger studies are needed to investigate ETP plus CZ therapy in the treatment of biofilm-related MSSA infections such as endocarditis.

Quercetin Reduces the Virulence of S. aureus by Targeting ClpP to Protect Mice from MRSA-Induced Lethal Pneumonia

The dramatic increase of methicillin-resistant Staphylococcus aureus (MRSA) poses a great challenge to the treatment of Staphylococcus aureus (S. aureus) infections. Therefore, there is an urgent need to identify novel anti-infective agents to attack new targets to overcome antibiotic resistance. Casein hydrolase P (ClpP) is a key virulence factor in S. aureus to maintain cellular homeostasis. We screened from flavonoids and finally determined that quercetin could effectively attenuate the virulence of MRSA. The results of the thermal shift assay showed that quercetin could bind to ClpP and reduce the thermal stability of ClpP, and the KD value between quercetin and ClpP was 197 nM as determined by localized surface plasmon resonance. We found that quercetin exhibited a protective role of a mouse model of MRSA-induced lethal infection in a murine model. Based on the above facts, quercetin, as a ClpP inhibitor, could be further developed as a potential candidate for antivirulence agents to combat S. aureus infections. IMPORTANCE The resistance of Staphylococcus aureus (S. aureus) to various antibiotics has increased dramatically, and thus the development of new anti-infective drugs with new targets is urgently needed to combat resistance. Caseinolytic peptidase P (ClpP) is a casein hydrolase that has been shown to regulate a variety of important virulence factors in S. aureus. Here, we found that quercetin, a small-molecule compound from traditional Chinese herbal flavonoids, effectively inhibits ClpP activity. Quercetin attenuates the expression of multiple virulence factors in S. aureus and effectively protects mice from lethal pneumonia caused by MRSA. In conclusion, we determined that quercetin is a ClpP inhibitor and an effective lead compound for the development of a virulence factor-based treatment for S. aureus infection.

Activation and manipulation of inflammasomes and pyroptosis during bacterial infections

Following detection of pathogen infection and disrupted cellular homeostasis, cells can activate a range of cell death pathways, such as apoptosis, necroptosis and pyroptosis, as part of their defence strategy. The initiation of pro-inflammatory, lytic pyroptosis is controlled by inflammasomes, which respond to a range of cellular perturbations. As is true for many host defence pathways, pathogens have evolved multiple mechanisms to subvert this pathway, many of which have only recently been described. Herein, we will discuss the mechanisms by which inflammasomes sense pathogen invasion and initiate pyroptosis and the effector mechanisms used by pathogens to suppress this pathway and preserve their niche.

Nepetin reduces virulence factors expression by targeting ClpP against MRSA-induced pneumonia infection

The resistance of Staphylococcus aureus (S. aureus) to various antibiotics has increased dramatically due to the misuse of antibiotics, and thus the development of new anti-infective drugs with new targets is urgently needed to combat resistance. Caseinolytic peptidase P is a case in hydrolase that regulates the virulence level of S. aureus. Here, we found that nepetin, a small-molecule compound from traditional Chinese herbal flavonoids, effectively inhibits ClpP activity. Nepetin suppressed the virulence of S. aureus and effectively combated the lethal pneumonia caused by MRSA. The results of cellular thermal shift assay showed that nepetin could bind to ClpP and reduce the thermal stability of ClpP, and the K_D value of 602 nM between them was determined using localized surface plasmon resonance. The binding mode of nepetin and ClpP was further investigated by molecular docking, and it was found that Ser-22 and Gln-47 of ClpP residues were found to be involved in the binding of nepetin to ClpP. In conclusion, we determined that nepetin is a ClpP inhibitor and an effective lead compound for the development of a virulence factor-based treatment for MRSA infection.

Platelets, Bacterial Adhesins and the Pneumococcus

Systemic infections with pathogenic or facultative pathogenic bacteria are associated with activation and aggregation of platelets leading to thrombocytopenia and activation of the clotting system. Bacterial proteins leading to platelet activation and aggregation have been identified, and while platelet receptors are recognized, induced signal transduction cascades are still often unknown. In addition to proteinaceous adhesins, pathogenic bacteria such as Staphylococcus aureus and Streptococcus pneumoniae also produce toxins such as pneumolysin and alpha-hemolysin. They bind to cellular receptors or form pores, which can result in disturbance of physiological functions of platelets. Here, we discuss the bacteria-platelet interplay in the context of adhesin–receptor interactions and platelet-activating bacterial proteins, with a main emphasis on S. aureus and S. pneumoniae. More importantly, we summarize recent findings of how S. aureus toxins and the pore-forming toxin pneumolysin of S. pneumoniae interfere with platelet function. Finally, the relevance of platelet dysfunction due to killing by toxins and potential treatment interventions protecting platelets against cell death are summarized.

Inflammasome Contribution to the Activation of Th1, Th2, and Th17 Immune Responses

Inflammasomes are cytosolic polyprotein complexes formed in response to various external and internal stimuli, including viral and bacterial antigens. The main product of the inflammasome is active caspase 1 which proteolytically cleaves, releasing functional interleukin-1 beta (IL-1β) and interleukin-18 (IL-18). These cytokines play a central role in shaping immune response to pathogens. In this review, we will focus on the mechanisms of inflammasome activation, as well as their role in development of Th1, Th2, and Th17 lymphocytes. The contribution of cytokines IL-1β, IL-18, and IL-33, products of activated inflammasomes, are summarized. Additionally, the role of cytokines released from tissue cells in promoting differentiation of lymphocyte populations is discussed.

Lysoptosis is an evolutionarily conserved cell death pathway moderated by intracellular serpins

Lysosomal membrane permeabilization (LMP) and cathepsin release typifies lysosome-dependent cell death (LDCD). However, LMP occurs in most regulated cell death programs suggesting LDCD is not an independent cell death pathway, but is conscripted to facilitate the final cellular demise by other cell death routines. Previously, we demonstrated that Caenorhabditis elegans (C. elegans) null for a cysteine protease inhibitor, srp-6, undergo a specific LDCD pathway characterized by LMP and cathepsin-dependent cytoplasmic proteolysis. We designated this cell death routine, lysoptosis, to distinguish it from other pathways employing LMP. In this study, mouse and human epithelial cells lacking srp-6 homologues, mSerpinb3a and SERPINB3, respectively, demonstrated a lysoptosis phenotype distinct from other cell death pathways. Like in C. elegans, this pathway depended on LMP and released cathepsins, predominantly cathepsin L. These studies suggested that lysoptosis is an evolutionarily-conserved eukaryotic LDCD that predominates in the absence of neutralizing endogenous inhibitors.

Investigation of Virulence Genes of Staphylococcus aureus Isolated from Sterile Body Fluid Samples and Their Correlation with Clinical Symptoms and Outcomes

Staphylococcus aureus is the major pathogen causing nosocomial human infections and produces a variety of virulence factors that contribute to its ability to colonize and cause diseases. This study was conducted to investigate the virulence genes in S. aureus isolated from sterile body fluid samples and their correlation with clinical symptoms and outcomes. The VITEK 2® Compact system was used to perform biochemical identification and antimicrobial susceptibility tests on 33 S. aureus isolates. Virulence genes were amplified using multiplex PCR. The virulence gene patterns were analyzed by systematic cluster analysis. The frequency of methicillin-resistant S. aureus was 45.45%, and 17 virulence genes were identified. Genes encoding hemolysins showed high frequencies. The frequencies of hla, hlb, hld, and hlgB were 93.94% and that of the luk-F/S-PV was 21.21%. Except for the frequency of splB (51.52%), the remaining genes encoding invasive proteases showed frequencies greater than 81.82%. Among the patients, 100.00% had undergone invasive medical procedures and 24.00% had been treated with more than three types of antibiotic drugs. Invasive medical procedures are the main causes of infection. Resistance to antibiotic drugs and the status of carrying virulence genes were highly related to clinical symptoms and outcomes.

Staphylococcal Enterotoxin A Induces Intestinal Barrier Dysfunction and Activates NLRP3 Inflammasome via NF-κB/MAPK Signaling Pathways in Mice

Staphylococcal enterotoxin A (SEA), the toxin protein secreted by Staphylococcus aureus, can cause staphylococcal food poisoning outbreaks and seriously threaten global public health. However, little is known about the pathogenesis of SEA in staphylococcal foodborne diseases. In this study, the effect of SEA on intestinal barrier injury and NLRP3 inflammasome activation was investigated by exposing BALB/c mice to SEA with increasing doses and a potential toxic mechanism was elucidated. Our findings suggested that SEA exposure provoked villi injury and suppressed the expression of ZO-1 and occludin proteins, thereby inducing intestinal barrier dysfunction and small intestinal injury in mice. Concurrently, SEA significantly up-regulated the expression of NLRP3 inflammasome-associated proteins and triggered the mitogen-activated protein kinase (MAPK) and nuclear factor kappa-B (NF-κB) signaling pathways in jejunum tissues. Notably, selective inhibitors of MAPKs and NF-κB p65 ameliorated the activation of NLRP3 inflammasome stimulated by SEA, which further indicated that SEA could activate NLRP3 inflammasome through NF-κB/MAPK pathways. In summary, SEA was first confirmed to induce intestinal barrier dysfunction and activate NLRP3 inflammasome via NF-κB/MAPK signaling pathways. These findings will contribute to a more comprehensive understanding of the pathogenesis of SEA and related drug-screening for the treatment and prevention of bacteriotoxin-caused foodborne diseases via targeting specific pathways.

Immunometabolites Drive Bacterial Adaptation to the Airway

Pseudomonas aeruginosa and Staphylococcus aureus are both opportunistic pathogens that are frequently associated with chronic lung infections. While bacterial virulence determinants are critical in initiating infection, the metabolic flexibility of these bacteria promotes their persistence in the airway. Upon infection, these pathogens induce host immunometabolic reprogramming, resulting in an airway milieu replete with immune-signaling metabolites. These metabolites are often toxic to the bacteria and create a steep selection pressure for the emergence of bacterial isolates adapted for long-term survival in the inflamed lung. In this review, we discuss the main differences in the host immunometabolic response to P. aeruginosa and S. aureus, as well as how these pathogens alter their own metabolism to adapt to airway metabolites and cause persistent lung infections.

Categorizing sequences of concern by function to better assess mechanisms of microbial pathogenesis

To identify sequences with a role in microbial pathogenesis, we assessed the adequacy of their annotation by existing controlled vocabularies and sequence databases. Our goal was to regularize descriptions of microbial pathogenesis for improved integration with bioinformatic applications. Here, we review the challenges of annotating sequences for pathogenic activity. We relate the categorization of more than 2,750 sequences of pathogenic microbes through a controlled vocabulary called Functions of Sequences of Concern (FunSoCs). These allow for an ease of description by both humans and machines. We provide a subset of 220 fully annotated sequences in the supplemental material as examples. The use of this compact (∼30 terms), controlled vocabulary has potential benefits for research in microbial genomics, public health, biosecurity, biosurveillance, and the characterization of new and emerging pathogens.

Epigallocatechin gallate (EGCG) attenuates staphylococcal alpha-hemolysin (Hla)-induced NLRP3 inflammasome activation via ROS-MAPK pathways and EGCG-Hla interactions

Alpha-hemolysin (Hla), the virulence factor secreted by Staphylococcus aureus (S. aureus), plays a critical role in infection and inflammation, which is a severe health burden worldwide. Therefore, it is necessary to develop a drug against Hla. Epigallocatechin gallate (EGCG), a polyphenol extracted from green tea, has excellent anti-inflammatory activity. In this study, we investigated the inhibitory effect of EGCG on Hla-induced NLRP3 inflammasome activation in vitro and in vivo and elucidated the potential molecular mechanism. We found that EGCG attenuated the hemolysis of Hla by inhibiting its secretion. Besides, EGCG significantly decreased overproduction of ROS and activation of MAPK signaling pathway induced by Hla, thereby markedly attenuating the expression of NLRP3 inflammasome-related proteins in THP-1 cells. Notably, EGCG could spontaneously bind to Hla with affinity constant of 1.71 × 10-4 M, thus blocking the formation of the Hla heptamer. Moreover, Hla-induced expression of NLRP3, ASC and caspase-1 protein and generation of IL-1β and IL-18 in the damaged liver tissue of mice were also significantly suppressed by EGCG in a dose-dependent manner. Collectively, EGCG could be a promising candidate for alleviating Hla-induced the activation of NLRP3 inflammasome, depending on ROS mediated MAPK signaling pathway, and inhibition of Hla secretion and heptamer formation. These findings will enlighten the applications of EGCG to reduce the S. aureus infection by targeting Hla in food and related pharmaceutical fields.

Neutrophil-Dependent Immunity During Pulmonary Infections and Inflammations

Rapid recruitment of neutrophils to an inflamed site is one of the hallmarks of an effective host defense mechanism. The main pathway through which this happens is by the innate immune response. Neutrophils, which play an important part in innate immune defense, migrate into lungs through the modulation actions of chemokines to execute a variety of pro-inflammatory functions. Despite the importance of chemokines in host immunity, little has been discussed on their roles in host immunity. A holistic understanding of neutrophil recruitment, pattern recognition pathways, the roles of chemokines and the pathophysiological roles of neutrophils in host immunity may allow for new approaches in the treatment of infectious and inflammatory disease of the lung. Herein, this review aims at highlighting some of the developments in lung neutrophil-immunity by focusing on the functions and roles of CXC/CC chemokines and pattern recognition receptors in neutrophil immunity during pulmonary inflammations. The pathophysiological roles of neutrophils in COVID-19 and thromboembolism have also been summarized. We finally summarized various neutrophil biomarkers that can be utilized as prognostic molecules in pulmonary inflammations and discussed various neutrophil-targeted therapies for neutrophil-driven pulmonary inflammatory diseases.

Diphenyl pyrimidine exhibits protective effect on Staphylococcus aureus pneumonia in rat model by targeting NLRP3 expression

Pneumonia is one of the most frequent disorder induced by S. aureus infection and accounts for 13.3% of the all the infections caused by staphylococcus. In the present study effect of diphenyl pyrimidine was investigated against Staphylococcus aureus (S. aureus) induced pneumonia in the rat model. The results demonstrated that diphenyl pyrimidine treatment of the rats effectively prevented S. aureus induced increase in mortality in dose-dependent manner. Diphenyl pyrimidine treatment inhibited histopathological changes in S. aureus infected rat lungs. Treatment of the rats with 1.25, 2.5, 5 and 10 mg/kg doses of diphenyl pyrimidine significantly (P < 0.05) reversed S. aureus infection induced increase in interleukin (IL)-1β, IL-18 and tumor necrosis factor (TNF)-α levels. Treatment with 1.25, 2.5, 5 and 10 mg/kg doses of diphenyl pyrimidine significantly (P < 0.05) reversed S. aureus infection induced increase nucleotide-binding domain and leucine-rich repeat containing (NLR) protein (NLRP3), apoptosis-associated speck-like protein containing a C-terminal caspase recruitment domain (ASC) and caspase-1 protein expression in rat lungs in dose-dependent manner. The NLRP3, ASC and caspase-1 mRNA level in S. aureus infected rat pulmonary tissues was significantly (P < 0.05) reduced by diphenyl pyrimidine treatment in dose-dependent manner. Thus, diphenyl pyrimidine protects S. aureus-induced pneumonia through suppression of NLRP3 and inflammatory cytokine expression. Therefore, diphenyl pyrimidine can be of therapeutic importance for the treatment of S. aureus induced pneumonia.

Reduced pannexin 1-IL-33 axis function in donor livers increases risk of MRSA infection in liver transplant recipients

Liver transplantation patients are at increased risk for methicillin-resistant Staphylococcus aureus (MRSA) infection, but the molecular mechanism remains unclear. We found that genetic predisposition to low pannexin 1 (PANX1) expression in donor livers was associated with MRSA infection in human liver transplantation recipients. Using Panx1 and Il-33-knockout mice for liver transplantation models with MRSA tail vein injection, we demonstrated that Panx1 deficiency increased MRSA-induced liver injury and animal death. We found that decreased PANX1 expression in the liver led to reduced release of adenosine triphosphate (ATP) from hepatocytes, which further reduced the activation of P2X2, an ATP-activating P2X receptor. Reduced P2X2 function further decreased the NLRP3-mediated release of interleukin-33 (IL-33), reducing hepatic recruitment of macrophages and neutrophils. Administration of mouse IL-33 to Panx1^-/- mice significantly (P = 0.011) ameliorated MRSA infection and animal death. Reduced human hepatic IL-33 protein abundance also associated with increased predisposition to MRSA infection. Our findings reveal that genetic predisposition to reduced PANX1 function increases risk for MRSA infection after liver transplantation by decreasing hepatic host innate immune defense, which can be attenuated by IL-33 treatment.

Pore-forming alpha-hemolysin efficiently improves the immunogenicity and protective efficacy of protein antigens

Highly immunogenic exotoxins are used as carrier proteins because they efficiently improve the immunogenicity of polysaccharides. However, their efficiency with protein antigens remains unclear. In the current study, the candidate antigen PA0833 from Pseudomonas aeruginosa was fused to the α-hemolysin mutant HlaH35A from Staphylococcus aureus to form a HlaH35A-PA0833 fusion protein (HPF). Immunization with HPF resulted in increased PA0833-specific antibody titers, higher protective efficacy, and decreased bacterial burden and pro-inflammatory cytokine secretion compared with PA0833 immunization alone. Using fluorescently labeled antigens to track antigen uptake and delivery, we found that HlaH35A fusion significantly improved antigen uptake in injected muscles and antigen delivery to draining lymph nodes. Both in vivo and in vitro studies demonstrated that the increased antigen uptake after immunization with HPF was mainly due to monocyte- and macrophage-dependent macropinocytosis, which was probably the result of HPF binding to ADAM10, the Hla host receptor. Furthermore, a transcriptome analysis showed that several immune signaling pathways were activated by HPF, shedding light on the mechanism whereby HlaH35A fusion improves immunogenicity. Finally, the improvement in immunogenicity by HlaH35A fusion was also confirmed with two other antigens, GlnH from Klebsiella pneumoniae and the model antigen OVA, indicating that HlaH35A could serve as a universal carrier protein to improve the immunogenicity of protein antigens.

Identification of a Human Anti-Alpha-Toxin Monoclonal Antibody Against Staphylococcus aureus Infection

Staphylococcus aureus is a major pathogenic bacterium that causes a variety of clinical infections. The emergence of multi-drug resistant mechanisms requires novel strategies to mitigate S. aureus infection. Alpha-hemolysin (Hla) is a key virulence factor that is believed to play a significant role in the pathogenesis of S. aureus infections. In this study, we screened a naïve human Fab library for identification of monoclonal antibodies targeting Hla by phage display technology. We found that the monoclonal antibody YG1 blocked the Hla-mediated lysis of rabbit red blood cells and inhibited Hla binding to A549 cells in a concentration-dependent manner. YG1 also provided protection against acute peritoneal infection, bacteremia, and pneumonia in murine models. We further characterized its epitope using different Hla variants and found that the amino acids N209 and F210 of Hla were functionally and structurally important for YG1 binding. Overall, these results indicated that targeting Hla with YG1 could serve as a promising protective strategy against S. aureus infection.

Pan-caspase inhibition as a potential host-directed immunotherapy against MRSA and other bacterial skin infections

Staphylococcus aureus causes most skin infections in humans, and the emergence of methicillin-resistant S. aureus (MRSA) strains is a serious public health threat. There is an urgent clinical need for nonantibiotic immunotherapies to treat MRSA infections and prevent the spread of antibiotic resistance. Here, we investigated the pan-caspase inhibitor quinoline-valine-aspartic acid-difluorophenoxymethyl ketone (Q-VD-OPH) for efficacy against MRSA skin infection in mice. A single systemic dose of Q-VD-OPH decreased skin lesion sizes and reduced bacterial burden compared with vehicle-treated or untreated mice. Although Q-VD-OPH inhibited inflammasome-dependent apoptosis-associated speck-like protein containing caspase activation and recruitment domain (ASC) speck formation and caspase-1-mediated interleukin-1β (IL-1β) production, Q-VD-OPH maintained efficacy in mice deficient in IL-1β, ASC, caspase-1, caspase-11, or gasdermin D. Thus, Q-VD-OPH efficacy was independent of inflammasome-mediated pyroptosis. Rather, Q-VD-OPH reduced apoptosis of monocytes and neutrophils. Moreover, Q-VD-OPH enhanced necroptosis of macrophages with concomitant increases in serum TNF and TNF-producing neutrophils, monocytes/macrophages, and neutrophils in the infected skin. Consistent with this, Q-VD-OPH lacked efficacy in mice deficient in TNF (with associated reduced neutrophil influx and necroptosis), in mice deficient in TNF/IL-1R and anti-TNF antibody-treated WT mice. In vitro studies revealed that combined caspase-3, caspase-8, and caspase-9 inhibition reduced apoptosis, and combined caspase-1, caspase-8, and caspase-11 inhibition increased TNF, suggesting a mechanism for Q-VD-OPH efficacy in vivo. Last, Q-VD-OPH also had a therapeutic effect against Streptococcus pyogenes and Pseudomonas aeruginosa skin infections in mice. Collectively, pan-caspase inhibition represents a potential host-directed immunotherapy against MRSA and other bacterial skin infections.

Immunodominance of Epitopes and Protective Efficacy of HI Antigen Are Differentially Altered Using Different Adjuvants in a Mouse Model of Staphylococcus aureus Bacteremia

HI, a fusion protein that consists of the alpha-toxin (Hla) and the N2 domain of iron surface determinant B (IsdB), is one of the antigens in the previously reported S. aureus vaccine rFSAV and has already entered phase II clinical trials. Previous studies revealed that HI is highly immunogenic in both mice and healthy volunteers, and the humoral immune response plays key roles in HI-mediated protection. In this study, we further investigated the protective efficacy of immunization with HI plus four different adjuvants in a mouse bacteremia model. Results showed that HI-mediated protection was altered in response to different adjuvants. Using antisera from immunized mice, we identified seven B-cell immunodominant epitopes on Hla and IsdB, including 6 novel epitopes (Hla_1-18, Hla_84-101, Hla_186-203, IsdB_342-359, IsdB_366-383, and IsdB_384-401). The immunodominance of B-cell epitopes, total IgG titers and the levels of IFN-γ and IL-17A from mice immunized with HI plus different adjuvants were different from each other, which may explain the difference in protective immunity observed in each immunized group. Thus, our results indicate that adjuvants largely affected the immunodominance of epitopes and the protective efficacy of HI, which may guide further adjuvant screening for vaccine development and optimization.