Targeting Alpha Toxin To Mitigate Its Lethal Toxicity in Ferret and Rabbit Models of Staphylococcus aureus Necrotizing Pneumonia

A review of chemical signaling mechanisms underlying quorum sensing and its inhibition in Staphylococcus aureus

Staphylococcus aureus is a significant bacterium responsible for multiple infections and is a primary cause of fatalities among patients in hospital environments. The advent of pathogenic bacteria such as methicillin-resistant S. aureus revealed the shortcomings of employing antibiotics to treat bacterial infectious diseases. Quorum sensing enhances S. aureus's survivability through signaling processes. Targeting the key components of quorum sensing has drawn much interest nowadays as a promising strategy for combating infections caused by bacteria. Concentrating on the accessory gene regulator quorum-sensing mechanism is the most commonly suggested anti-virulence approach for S.aureus. Quorum quenching is a common strategy for controlling illnesses triggered by microorganisms since it reduces the pathogenicity of bacteria and improves bacterial biofilm susceptibility to antibiotics, thus providing an intriguing prospect for drug discovery. Quorum sensing inhibition reduces selective stresses and constrains the emergence of antibiotic resistance while limiting bacterial pathogenicity. This review examines the quorum sensing mechanisms involved in S. aureus, quorum sensing targets and gene regulation, environmental factors affecting quorum sensing, quorum sensing inhibition, natural products as quorum sensing inhibitory agents and novel therapeutical strategies to target quorum sensing in S. aureus as drug developing technique to augment conventional antibiotic approaches.

Ferrets as a Mammalian Model to Study Influenza Virus-Bacteria Interactions

Ferrets represent an invaluable model for the study of influenza virus pathogenicity and transmissibility. Ferrets are also employed for the study of bacterial pathogens that naturally infect humans at different anatomical sites. While viral and bacterial infection studies in isolation using animal models are important for furthering our understanding of pathogen biology and developing improved therapeutics, it is also critical to extend our knowledge to pathogen coinfections in vivo, to more closely examine interkingdom dynamics that may contribute to overall disease outcomes. We discuss how ferrets have been employed to study a diverse range of both influenza viruses and bacterial species and summarize key studies that have utilized the ferret model for primary influenza virus challenge followed by secondary bacterial infection. These copathogenesis studies have provided critical insight into the dynamic interplay between these pathogens, underscoring the utility of ferrets as a model system for investigating influenza virus-bacteria interactions.

Development and validation of a rabbit model of Pseudomonas aeruginosa non-ventilated pneumonia for preclinical drug development

Background

New drugs targeting antimicrobial resistant pathogens, including Pseudomonas aeruginosa, have been challenging to evaluate in clinical trials, particularly for the non-ventilated hospital-acquired pneumonia and ventilator-associated pneumonia indications. Development of new antibacterial drugs is facilitated by preclinical animal models that could predict clinical efficacy in patients with these infections.

Methods

We report here an FDA-funded study to develop a rabbit model of non-ventilated pneumonia with Pseudomonas aeruginosa by determining the extent to which the natural history of animal disease reproduced human pathophysiology and conducting validation studies to evaluate whether humanized dosing regimens of two antibiotics, meropenem and tobramycin, can halt or reverse disease progression.

Results

In a rabbit model of non-ventilated pneumonia, endobronchial challenge with live P. aeruginosa strain 6206, but not with UV-killed Pa6206, caused acute respiratory distress syndrome, as evidenced by acute lung inflammation, pulmonary edema, hemorrhage, severe hypoxemia, hyperlactatemia, neutropenia, thrombocytopenia, and hypoglycemia, which preceded respiratory failure and death. Pa6206 increased >100-fold in the lungs and then disseminated from there to infect distal organs, including spleen and kidneys. At 5 h post-infection, 67% of Pa6206-challenged rabbits had PaO2 <60 mmHg, corresponding to a clinical cut-off when oxygen therapy would be required. When administered at 5 h post-infection, humanized dosing regimens of tobramycin and meropenem reduced mortality to 17-33%, compared to 100% for saline-treated rabbits (P<0.001 by log-rank tests). For meropenem which exhibits time-dependent bactericidal activity, rabbits treated with a humanized meropenem dosing regimen of 80 mg/kg q2h for 24 h achieved 100% T>MIC, resulting in 75% microbiological clearance rate of Pa6206 from the lungs. For tobramycin which exhibits concentration-dependent killing, rabbits treated with a humanized tobramycin dosing regimen of 8 mg/kg q8h for 24 h achieved Cmax/MIC of 9.8 ± 1.4 at 60 min post-dose, resulting in 50% lung microbiological clearance rate. In contrast, rabbits treated with a single tobramycin dose of 2.5 mg/kg had Cmax/MIC of 7.8 ± 0.8 and 8% (1/12) microbiological clearance rate, indicating that this rabbit model can detect dose-response effects.

Conclusion

The rabbit model may be used to help predict clinical efficacy of new antibacterial drugs for the treatment of non-ventilated P. aeruginosa pneumonia.

Monoclonal antibodies neutralizing alpha-hemolysin, bicomponent leukocidins, and clumping factor A protected against Staphylococcus aureus-induced acute circulatory failure in a mechanically ventilated rabbit model of hyperdynamic septic shock

Background

Patients with septic shock caused by Staphylococcus aureus have mortality rates exceeding 50%, despite appropriate antibiotic therapy. Our objectives were to establish a rabbit model of S. aureus septic shock and to determine whether a novel immunotherapy can prevent or halt its natural disease progression.

Methods

Anesthetized rabbits were ventilated with lung-protective low-tidal volume, instrumented for advanced hemodynamic monitoring, and characterized for longitudinal changes in acute myocardial dysfunction by echocardiography and sepsis-associated biomarkers after S. aureus intravenous challenge. To demonstrate the potential utility of this hyperdynamic septic shock model for preclinical drug development, rabbits were randomized for prophylaxis with anti-Hla/Luk/ClfA monoclonal antibody combination that neutralizes alpha-hemolysin (Hla), the bicomponent pore-forming leukocidins (Luk) including Panton-Valentine leukocidin, leukocidin ED, and gamma-hemolysin, and clumping factor A (ClfA), or an irrelevant isotype-matched control IgG (c-IgG), and then challenged with S. aureus.

Results

Rabbits challenged with S. aureus, but not those with saline, developed a hyperdynamic state of septic shock characterized by elevated cardiac output (CO), increased stroke volume (SV) and reduced systemic vascular resistance (SVR), which was followed by a lethal hypodynamic state characterized by rapid decline in mean arterial pressure (MAP), increased central venous pressure, reduced CO, reduced SV, elevated SVR, and reduced left-ventricular ejection fraction, thereby reproducing the hallmark clinical features of human staphylococcal septic shock. In this model, rabbits pretreated with anti-Hla/Luk/ClfA mAb combination had 69% reduction in mortality when compared to those pretreated with c-IgG (P<0.001). USA300-induced acute circulatory failure-defined as >70% decreased in MAP from pre-infection baseline-occurred in only 20% (2/10) of rabbits pretreated with anti-Hla/Luk/ClfA mAb combination compared to 100% (9/9) of those pretreated with c-IgG. Prophylaxis with anti-Hla/Luk/ClfA mAb combination halted progression to lethal hypodynamic shock, as evidenced by significant protection against the development of hyperlactatemia, hypocapnia, hyperkalemia, leukopenia, neutropenia, monocytopenia, lymphopenia, as well as biomarkers associated with acute myocardial injury.

Conclusion

These results demonstrate the potential utility of a mechanically ventilated rabbit model that reproduced hallmark clinical features of hyperdynamic septic shock and the translational potential of immunotherapy targeting S. aureus virulence factors for the prevention of staphylococcal septic shock.

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.

Immunotargeting of Gram-Positive Pathogens via a Cell Wall Binding Tick Antifreeze Protein

Immunological agents that supplement or modulate the host immune response have proven to have powerful therapeutic potential, although this modality is less explored against bacterial pathogens. We describe the application of a bacterial binding protein to re-engage the immune system toward pathogenic bacteria. More specifically, a hapten was conjugated to a protein expressed by Ixodes scapularis ticks, called I. scapularis antifreeze glycoprotein (IAFGP), that has high affinity for the d-alanine residue on the bacterial peptidoglycan. We showed that a fragment of this protein retained high surface binding affinity. Moreover, conjugation of a hapten to this peptide led to the display of haptens on the cell surface of vancomycin-resistant Enterococcus faecalis. Hapten display then induced the recruitment of antibodies and promoted uptake of bacterial pathogens by immune cells. These results demonstrate the feasibility in using cell wall binding agents as the basis of a class of bacterial immunotherapies.

A novel fully human recombinant antibody neutralizing the α-hemolysin of Staphylococcus aureus

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to almost all β-lactam antibiotics. Hence, new ways to control MRSA infection, such as antibacterial antibodies, need to be explored. α-hemolysin is the most important virulence factor widely expressed in S. aureus. This study aimed to develop a new fully human antibody against α-hemolysin of S. aureus and research its neutralizing effect.

RESULTS

The single-chain antibody fragments(scFvs)against S. aureus were screened from a fully human scFv library using phage display technology. The selected scFvs had good binding affinities to α-hemolysin and S. aureus. The IgG-like scFv-Fc inserted into the pcDNA3.1 or pMH3 vector was expressed in HEK293F suspension cells to extend the half-life and restore Fc function. The size of purified scFv-Fc was about 55 kDa. The functions of expressed scFv-Fcs against α-hemolysin were validated. The cytotoxicity assays showed that scFv555-Fc had better protective effects on A549 cells than other scFv-Fcs. The results of anti-rabbit erythrocyte lysis and A549 cell apoptosis assay confirmed that scFv555-Fc had a significant neutralizing effect on α-hemolysin. The scFv555-Fc was used to construct the docking model of antigen-antibody complexes using Discovery Studio software. It predicted that the key binding sites of α-hemolysin were TYR28, LYS37, PHE39, ARG56, and LYS58, which might be the key toxic sites of α-hemolysin.

CONCLUSION

A novel fully human scFv-Fc antibody neutralizing the α-hemolysin toxin of S. aureus was successfully developed. The findings might provide a new theoretical basis and treatment method for preventing MRSA infection.

Pathogenicity and virulence of Staphylococcus aureus

Staphylococcus aureus is one of the most frequent worldwide causes of morbidity and mortality due to an infectious agent. This pathogen can cause a wide variety of diseases, ranging from moderately severe skin infections to fatal pneumonia and sepsis. Treatment of S. aureus infections is complicated by antibiotic resistance and a working vaccine is not available. There has been ongoing and increasing interest in the extraordinarily high number of toxins and other virulence determinants that S. aureus produces and how they impact disease. In this review, we will give an overview of how S. aureus initiates and maintains infection and discuss the main determinants involved. A more in-depth understanding of the function and contribution of S. aureus virulence determinants to S. aureus infection will enable us to develop anti-virulence strategies to counteract the lack of an anti-S. aureus vaccine and the ever-increasing shortage of working antibiotics against this important pathogen.

Increased Risk of Thrombocytopenia and Death in Patients with Bacteremia Caused by High Alpha Toxin-Producing Methicillin-Resistant Staphylococcus aureus

Alpha toxin (Hla) is a major virulence factor of Staphylococcus aureus that targets platelets but clinical data on Hla pathogenesis in bacteremia (SAB) is limited. We examined the link between in vitro Hla activity and outcome. Study isolates obtained from 100 patients with SAB (50 survivors; 50 non-survivors) were assessed for in vitro Hla production by Western immunoblotting in a subset of isolates and Hla activity by hemolysis assay in all isolates. Relevant demographics, laboratory and clinical data were extracted from patients' medical records to correlate Hla activity of the infecting isolates with outcome. Hla production strongly correlated with hemolytic activity (r_s = 0.93) in vitro. A trend towards higher hemolytic activity was observed for MRSA compared to MSSA and with high-risk source infection. Significantly higher hemolytic activity was noted for MRSA strains isolated from patients who developed thrombocytopenia (median 52.48 vs. 16.55 HU/mL in normal platelet count, p = 0.012) and from non survivors (median 30.96 vs. 14.87 HU/mL in survivors, p = 0.014) but hemolytic activity of MSSA strains did not differ between patient groups. In vitro Hla activity of MRSA strains obtained from patients with bacteremia is significantly associated with increased risk for thrombocytopenia and death which supports future studies to evaluate feasibility of bedside phenotyping and therapeutic targeting.

Viral and Bacterial Co-Infections in the Lungs: Dangerous Liaisons

Respiratory tract infections constitute a significant public health problem, with a therapeutic arsenal that remains relatively limited and that is threatened by the emergence of antiviral and/or antibiotic resistance. Viral–bacterial co-infections are very often associated with the severity of these respiratory infections and have been explored mainly in the context of bacterial superinfections following primary influenza infection. This review summarizes our current knowledge of the mechanisms underlying these co-infections between respiratory viruses (influenza viruses, RSV, and SARS-CoV-2) and bacteria, at both the physiological and immunological levels. This review also explores the importance of the microbiome and the pathological context in the evolution of these respiratory tract co-infections and presents the different in vitro and in vivo experimental models available. A better understanding of the complex functional interactions between viruses/bacteria and host cells will allow the development of new, specific, and more effective diagnostic and therapeutic approaches.

Multimechanistic Monoclonal Antibody Combination Targeting Key Staphylococcus aureus Virulence Determinants in a Rabbit Model of Prosthetic Joint Infection

In a rabbit model of methicillin-resistant Staphylococcus aureus prosthetic joint infection (PJI), prophylaxis with AZD6389*-a combination of three monoclonal antibodies targeting alpha-hemolysin, bicomponent cytotoxins (LukSF/LukED/HlgAB/HlgCB), and clumping factor A-resulted in significant reductions in joint swelling, erythema, intra-articular pus, and bacterial burden in synovial tissues and biofilm-associated prosthetic implants compared with isotype-matched control IgG. Targeting specific staphylococcal virulence factors may thus have potential clinical utility for prevention of PJI.

Staphylococcus aureus, Antibiotic Resistance, and the Interaction with Human Neutrophils

Significance:Staphylococcus aureus is among the leading causes of bacterial infections worldwide. The high burden of S. aureus among human and animal hosts, which includes asymptomatic carriage and infection, is coupled with a notorious ability of the microbe to become resistant to antibiotics. Notably, S. aureus has the ability to produce molecules that promote evasion of host defense, including the ability to avoid killing by neutrophils. Recent Advances: Significant progress has been made to better understand S. aureus-host interactions. These discoveries include elucidation of the role played by numerous S. aureus virulence molecules during infection. Based on putative functions, a number of these virulence molecules, including S. aureus alpha-hemolysin and protein A, have been identified as therapeutic targets. Although it has not been possible to develop a vaccine that can prevent S. aureus infections, monoclonal antibodies specific for S. aureus virulence molecules have the potential to moderate the severity of disease. Critical Issues: Therapeutic options for treatment of methicillin-resistant S. aureus (MRSA) are limited, and the microbe typically develops resistance to new antibiotics. New prophylactics and/or therapeutics are needed. Future Directions: Research that promotes an enhanced understanding of S. aureus-host interaction is an important step toward developing new therapeutic approaches directed to moderate disease severity and facilitate treatment of infection. This research effort includes studies that enhance our view of the interaction of S. aureus with human neutrophils. Antioxid. Redox Signal. 34, 452-470.

Therapeutic Antibodies for the Treatment of Respiratory Tract Infections-Current Overview and Perspectives

Respiratorytract infections (RTIs) are frequent and life-threatening diseases, accounting for several millions of deaths worldwide. RTIs implicate microorganisms, including viruses (influenza virus, coronavirus, respiratory syncytial virus (RSV)), bacteria (Pseudomonas aeruginosa, Streptococcus pneumoniae, Staphylococcus aureus and Bacillus anthracis) and fungi (Pneumocystis spp., Aspergillus spp. and very occasionally Candida spp.). The emergence of new pathogens, like the coronavirus SARS-CoV-2, and the substantial increase in drug resistance have highlighted the critical necessity to develop novel anti-infective molecules. In this context, antibodies (Abs) are becoming increasingly important in respiratory medicine and may fulfill the unmet medical needs of RTIs. However, development of Abs for treating infectious diseases is less advanced than for cancer and inflammatory diseases. Currently, only three Abs have been marketed for RTIs, namely, against pulmonary anthrax and RSV infection, while several clinical and preclinical studies are in progress. This article gives an overview of the advances in the use of Abs for the treatment of RTIs, based on the analysis of clinical studies in this field. It describes the Ab structure, function and pharmacokinetics, and discusses the opportunities offered by the various Ab formats, Ab engineering and co-treatment strategies. Including the most recent literature, it finally highlights the strengths, weaknesses and likely future trends of a novel anti-RTI Ab armamentarium.

Exploring Virulence Factors and Alternative Therapies against Staphylococcus aureus Pneumonia

Pneumonia is an acute pulmonary infection associated with high mortality and an immense financial burden on healthcare systems. Staphylococcus aureus is an opportunistic pathogen capable of inducing S. aureus pneumonia (SAP), with some lineages also showing multidrug resistance. Given the high level of antibiotic resistance, much research has been focused on targeting S. aureus virulence factors, including toxins and biofilm-associated proteins, in an attempt to develop effective SAP therapeutics. Despite several promising leads, many hurdles still remain for S. aureus vaccine research. Here, we review the state-of-the-art SAP therapeutics, highlight their pitfalls, and discuss alternative approaches of potential significance and future perspectives.

Efficacy of Active Immunization With Attenuated α-Hemolysin and Panton-Valentine Leukocidin in a Rabbit Model of Staphylococcus aureus Necrotizing Pneumonia

Staphylococcus aureus is a common pathogen causing infections in humans with various degrees of severity, with pneumonia being one of the most severe infections. In as much as staphylococcal pneumonia is a disease driven in large part by α-hemolysin (Hla) and Panton-Valentine leukocidin (PVL), we evaluated whether active immunization with attenuated forms of Hla (HlaH35L/H48L) alone, PVL components (LukS-PVT28F/K97A/S209A and LukF-PVK102A) alone, or combination of all 3 toxoids could prevent lethal challenge in a rabbit model of necrotizing pneumonia caused by the USA300 community-associated methicillin-resistant S. aureus (MRSA). Rabbits vaccinated with Hla toxoid alone or PVL components alone were only partially protected against lethal pneumonia, whereas those vaccinated with all 3 toxoids had 100% protection against lethality. Vaccine-mediated protection correlated with induction of polyclonal antibody response that neutralized not only α-hemolysin and PVL, but also other related toxins, produced by USA300 and other epidemic MRSA clones.

The antibacterial activity and toxin production control of bee venom in mouse MRSA pneumonia model

Background

The current antimicrobial therapy is still important for the treatment of pneumonia due to MRSA infection, but there are some limitations, including the route of administration, side effect profile, and increased microbial resistance patterns. Therefore, we investigated whether BV, which shows a strong antimicrobial effect against MRSA, would be effective in a pneumonia model.

Methods

In vitro, we checked MIC, qRT-PCR, western blot, ELISA, LDH-assay. In vivo, we checked survival rate, gross pathological change, histopathology, lung bacterial clearance assay, and the expression of inflammatory related gene.

Results

The minimum inhibitory concentration of BV against MRSA is 15.6 μg/ml by broth dilution method. The production of toxins and related gene were reduced by BV in MRSA. The secretion of cytokines were decreased by treatment with BV in 264.7 RAW macrophages stimulated by MRSA Also, BV protected A549 from pathogenicity of MRSA. Bee venom reduced the number of bacteria in the lungs and alleviated the symptoms of MRSA-induced pneumonia in mouse.

Conclusion

BV inhibited the virulence of the bacterium and the number of bacterial cells present in lung tissue, thereby alleviating the symptoms of pneumonia in mice. This study suggested that BV may be a candidate substance for the treatment of pneumonia caused by MRSA infection.

Innovative therapies for acute bacterial skin and skin-structure infections (ABSSSI) caused by methicillin-resistant Staphylococcus aureus: advances in phase I and II trials

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) is among the most frequent causative agents of acute bacterial skin and skin-structure infections (ABSSSI) and has been associated with increased risks of invasive disease and of treatment failure.

AREAS COVERED

In this review, we focus on those novel anti-MRSA agents currently in phase I or II of clinical development that may enrich the armamentarium against ABSSSI caused by MRSA in the future.

EXPERT OPINION

Promising agents belonging to either old or novel antibiotic classes are currently in early phases of clinical development and may become available in the future for the effective treatment of ABSSSI caused by MRSA. In particular, the future availability of agents belonging to novel classes will be important for guaranteeing an effective treatment and for allowing outpatient treatment/early discharge, with a consequent reduced impact on healthcare resources. However, this does not mean that we can relax our efforts directed toward improving the responsible use of already available agents. Indeed, preserving their activity in the long term is crucial for optimizing the use of healthcare resources.

Monoclonal Antibodies as an Antibacterial Approach Against Bacterial Pathogens

In the beginning of the 21st century, the frequency of antimicrobial resistance (AMR) has reached an apex, where even 4th and 5th generation antibiotics are becoming useless in clinical settings. In turn, patients are suffering from once-curable infections, with increases in morbidity and mortality. The root cause of many of these infections are the ESKAPEE pathogens (Enterococcus species, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter species, and Escherichia coli), which thrive in the nosocomial environment and are the bacterial species that have seen the largest rise in the acquisition of antibiotic resistance genes. While traditional small-molecule development still dominates the antibacterial landscape for solutions to AMR, some researchers are now turning to biological approaches as potential game changers. Monoclonal antibodies (mAbs)—more specifically, human monoclonal antibodies (Hu-mAbs)—have been highly pursued in the anti-cancer, autoimmune, and antiviral fields with many success stories, but antibody development for bacterial infection is still just scratching the surface. The untapped potential for Hu-mAbs to be used as a prophylactic or therapeutic treatment for bacterial infection is exciting, as these biologics do not have the same toxicity hurdles of small molecules, could have less resistance as they often target virulence proteins rather than proteins required for survival, and are narrow spectrum (targeting just one pathogenic species), therefore avoiding the disruption of the microbiome. This mini-review will highlight the current antibacterial mAbs approved for patient use, the success stories for mAb development, and new Hu-mAb products in the antibacterial pipeline.

Protective Efficacy of Monoclonal Antibodies Neutralizing Alpha-Hemolysin and Bicomponent Leukocidins in a Rabbit Model of Staphylococcus aureus Necrotizing Pneumonia

Staphylococcus aureus is a major human pathogen that causes a wide range of infections by producing an arsenal of cytotoxins. We found that passive immunization with either a monoclonal antibody (MAb) neutralizing alpha-hemolysin or a broadly cross-reactive MAb neutralizing Panton-Valentine leukocidin, leukocidin ED, and gamma-hemolysins HlgAB and HlgCB conferred only partial protection, whereas the combination of those two MAbs conferred significant protection in a rabbit model of necrotizing pneumonia caused by the USA300 methicillin-resistant S. aureus epidemic clone.

Methicillin-Resistant Staphylococcus aureus in Hospitals: Latest Trends and Treatments Based on Bacteriophages

Even though antibiotic resistance in bacteria is a natural phenomenon, the alarming increase in pathogenic bacteria refractory to a wide range of antimicrobials is attracting attention worldwide. Indeed, the World Health Organization (WHO) has recently published a list of priority pathogens for which new antimicrobial alternatives are urgently needed. Among these pathogens, methicillin-resistant Staphylococcus aureus (MRSA) strains are perhaps the best known by the general public. In addition to its potential to acquire antibiotic resistance, S. aureus can produce a large number of virulence factors, such as hemolysins, enterotoxins, and proteases, and exhibits the ability to form biofilms as well as to evolve into different clones that can spread and colonize new environments. This review provides a brief overview of the latest options in antibacterial therapies, mainly focusing on phage therapy. In this regard, the current stage of research about antimicrobial compounds based on bacteriophages and endolysins against MRSA infections is shown and discussed.

Linezolid Attenuates Lethal Lung Damage during Postinfluenza Methicillin-Resistant Staphylococcus aureus Pneumonia

Postinfluenza methicillin-resistant Staphylococcus aureus (MRSA) infection can quickly develop into severe, necrotizing pneumonia, causing over 50% mortality despite antibiotic treatments. In this study, we investigated the efficacy of antibiotic therapies and the impact of S. aureus alpha-toxin in a model of lethal influenza virus and MRSA coinfection. We demonstrate that antibiotics primarily attenuate alpha-toxin-induced acute lethality, even though both alpha-toxin-dependent and -independent mechanisms significantly contribute to animal mortality after coinfection. Furthermore, we found that the protein synthesis-suppressing antibiotic linezolid has an advantageous therapeutic effect on alpha-toxin-induced lung damage, as measured by protein leak and lactate dehydrogenase (LDH) activity. Importantly, using a Panton-Valentine leucocidin (PVL)-negative MRSA isolate from patient sputum, we show that linezolid therapy significantly improves animal survival from postinfluenza MRSA pneumonia compared with vancomycin treatment. Rather than improved viral or bacterial control, this advantageous therapeutic effect is associated with a significantly attenuated proinflammatory cytokine response and acute lung damage in linezolid-treated mice. Together, our findings not only establish a critical role of alpha-toxin in the extreme mortality of secondary MRSA pneumonia after influenza but also provide support for the possibility that linezolid could be a more effective treatment than vancomycin to improve disease outcomes.

Randomized, Double-Blind, Placebo-Controlled, Single-Ascending-Dose Study of the Penetration of a Monoclonal Antibody Combination (ASN100) Targeting Staphylococcus aureus Cytotoxins in the Lung Epithelial Lining Fluid of Healthy Volunteers

ASN100 is a novel antibody combination of two fully human IgG1(κ) monoclonal antibodies (MAbs), ASN-1 and ASN-2, which neutralize six Staphylococcus aureus cytotoxins, alpha-hemolysin (Hla) and five bicomponent leukocidins. We assessed the safety, tolerability, and serum and lung pharmacokinetics of ASN100 in a randomized, double-blind, placebo-controlled single-dose-escalation first-in-human study.

ASN100 is a novel antibody combination of two fully human IgG1(κ) monoclonal antibodies (MAbs), ASN-1 and ASN-2, which neutralize six Staphylococcus aureus cytotoxins, alpha-hemolysin (Hla) and five bicomponent leukocidins. We assessed the safety, tolerability, and serum and lung pharmacokinetics of ASN100 in a randomized, double-blind, placebo-controlled single-dose-escalation first-in-human study. Fifty-two healthy volunteers were enrolled and randomized to receive either ASN-1, ASN-2, a combination of both MAbs (ASN100), or a corresponding placebo. Thirty-two subjects in the double-blind dose escalation portion of the study received ASN-1 or ASN-2 at a 200-, 600-, 1,800-, or 4,000-mg dose, or placebo. Eight subjects received both MAbs simultaneously in a 1:1 ratio (ASN100) at 3,600 or 8,000 mg, or they received placebos. Twelve additional subjects received open-label ASN100 at 3,600 or 8,000 mg to assess the pharmacokinetics of ASN-1 and ASN-2 in epithelial lining fluid (ELF) by bronchoalveolar lavage fluid sampling. Subjects were monitored for 98 days (double-blind cohorts) or 30 days (open-label cohorts) for safety assessment. No dose-limiting toxicities were observed, and all adverse events were mild and transient, with only two adverse events considered possibly related to the investigational product. ASN100 exhibited linear serum pharmacokinetics with a half-life of approximately 3 weeks and showed detectable penetration into the ELF. No treatment-emergent anti-drug antibody responses were detected. The toxin neutralizing potency of ASN100 in human serum was confirmed up to 58 days postdosing. The favorable safety profile, ELF penetration, and maintained functional activity in serum supported the further clinical development of ASN100.

Candida albicans Augments Staphylococcus aureus Virulence by Engaging the Staphylococcal agr Quorum Sensing System

Relatively little is known about the complex interactions and signaling events that occur between microbes and even less so about how microbial “cross talk” shapes human health and disease. Candida albicans (a fungus) and Staphylococcus aureus (a bacterium) are formidable human nosocomial pathogens, causing severe morbidity and mortality. Moreover, they are frequently coisolated from central venous catheters and deep-seated infections, including intra-abdominal sepsis. In this work, we have shown that coinfection with C. albicans and S. aureus is highly lethal, leading to >80% mortality by day 1 postinfection, whereas monoinfection with C. albicans or S. aureus does not cause mortality. This infectious synergism is dependent on the expression of staphylococcal alpha-toxin, and secretion of this potent virulence factor is actually augmented by C. albicans via an agr-dependent mechanism. Moreover, prophylactic neutralization of alpha-toxin with a monoclonal antibody is sufficient to elicit protection during coinfection. Therefore, we have demonstrated that a pathogenic fungus can enhance virulence determinants of a bacterium in vivo with devastating consequences to the host. These results have important implications in the surveillance and treatment of polymicrobial disease and highlight the dynamic intersection of environment, pathogens, and host.

Candida albicans and Staphylococcus aureus are among the most prevalent nosocomial pathogens that are responsible for severe morbidity and mortality, even with appropriate treatment. Using a murine model of polymicrobial intra-abdominal infection (IAI), we have previously shown that coinfection with these pathogens results in synergistic lethality that is partially dependent on exacerbated prostaglandin signaling, while monomicrobial infection is nonlethal. Therefore, the objective of this study was to identify staphylococcal virulence determinants that drive lethal synergism during polymicrobial IAI. Using the toxigenic S. aureus strain JE2, we observed that coinfection with C. albicans led to a striking 80 to 100% mortality rate within 20 h postinoculation (p.i.) while monomicrobial infections were nonlethal. Use of a green fluorescent protein (GFP)-P3 promoter S. aureus reporter strain revealed enhanced activation of the staphylococcal agr quorum sensing system during in vitro polymicrobial versus monomicrobial growth. Analyses by quantitative real-time PCR (qPCR), Western blot, and toxin functional assays confirmed enhanced agr-associated gene transcription and increases in secreted alpha- and delta-toxins. C. albicans-mediated elevated toxin production and hemolytic activity were determined to be agrA dependent, and genetic knockout and complementation of hla identified alpha-toxin as the key staphylococcal virulence factor driving lethal synergism. Analysis of mono- and polymicrobial infections 8 h p.i. demonstrated equivalent bacterial burdens in the peritoneal cavity but significantly elevated levels of alpha-toxin (3-fold) and the eicosanoid prostaglandin E₂ (PGE₂) (4-fold) during coinfection. Importantly, prophylactic passive immunization using the monoclonal anti-alpha-toxin antibody MEDI4893* led to significantly improved survival rates compared to those following treatment with isotype control antibody. Collectively, these results define alpha-toxin as an essential virulence determinant during C. albicans-S. aureus IAI and describe a novel mechanism by which a human-pathogenic fungus can augment the virulence of a highly pathogenic bacterium in vivo.

Preventing lung pathology and mortality in rabbit Staphylococcus aureus pneumonia models with cytotoxin-neutralizing monoclonal IgGs penetrating the epithelial lining fluid

Staphylococcus aureus pneumonia is associated with high mortality irrespective of antibiotic susceptibility. Both MRSA and MSSA strains produce powerful cytotoxins: alpha-hemolysin(Hla) and up to five leukocidins - LukSF-PV, HlgAB, HlgCB, LukED and LukGH (LukAB) - to evade host innate defense mechanisms. Neutralizing cytotoxins has been shown to provide survival benefit in rabbit S. aureus pneumonia models. We studied the mechanisms of protection of ASN100, a combination of two human monoclonal antibodies (mAbs), ASN-1 and ASN-2, that together neutralize Hla and the five leukocidins, in rabbit MRSA and MSSA pneumonia models. Upon prophylactic passive immunization, ASN100 displayed dose-dependent increase in survival and was fully protective against all S. aureus strains tested at 5 or 20 mg/kg doses. Macroscopic and microscopic lung pathology, edema rate, and bacterial burden were evaluated 12 hours post infection and reduced by ASN100. Pharmacokinetic analysis of ASN100 in bronchoalveolar-lavage fluid from uninfected animals detected efficient penetration to lung epithelial lining fluid reaching peak levels between 24 and 48 hours post dosing that were comparable to the mAb concentration measured in serum. These data confirm that the ASN100 mAbs neutralize the powerful cytotoxins of S. aureus in the lung and prevent damage to the mucosal barrier and innate immune cells.

Alanine Scanning Mutagenesis of the MEDI4893 (Suvratoxumab) Epitope Reduces Alpha Toxin Lytic Activity In Vitro and Staphylococcus aureus Fitness in Infection Models

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development.

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development. As shown previously, the MEDI4893-binding epitope on AT is comprised of the highly conserved amino acid regions 177 to 200 and 261 to 271, suggesting these amino acids are important for AT function. To test this hypothesis and gain insight into the effect of mutations in the epitope on AT neutralization by MEDI4893, nine MEDI4893 contact residues in AT were individually mutated to alanine. Consistent with our hypothesis, 8 out of 9 mutants exhibited >2-fold loss in lytic activity resulting from a defect in cell binding and pore formation. MEDI4893 binding affinity was reduced >2-fold (2- to 27-fold) for 7 out of 9 mutants, and no binding was detected for the W187A mutant. MEDI4893 effectively neutralized all of the lytic mutants in vitro and in vivo. When the defective mutants were introduced into an S. aureus clinical isolate, the mutant-expressing strains exhibited less severe disease in mouse models and were effectively neutralized by MEDI4893. These results indicate the MEDI4893 epitope is highly conserved due in part to its role in AT pore formation and bacterial fitness, thereby decreasing the likelihood for the emergence of MAb-resistant variants.

MEDI3902 Correlates of Protection against Severe Pseudomonas aeruginosa Pneumonia in a Rabbit Acute Pneumonia Model

Pseudomonas aeruginosa is among the most formidable antibiotic-resistant pathogens and is a leading cause of hospital-associated infections. With dwindling options for antibiotic-resistant infections, a new paradigm for treatment and disease resolution is required. MEDI3902, a bispecific antibody targeting the P. aeruginosa type III secretion (T3S) protein PcrV and Psl exopolysaccharide, was previously shown to mediate potent protective activity in murine infection models. With the current challenges associated with the clinical development of narrow-spectrum agents, robust preclinical efficacy data in multiple animal species are desirable. Here, we sought to develop a rabbit P. aeruginosa acute pneumonia model to further evaluate the activity of MEDI3902 intervention. In the rabbit model of acute pneumonia, prophylaxis with MEDI3902 exhibited potent dose-dependent protection, whereas those receiving control IgG developed fatal hemorrhagic necrotizing pneumonia between 12 and 54 h after infection. Blood biomarkers (e.g., partial pressure of oxygen [pO₂], partial pressure of carbon dioxide [pCO₂], base excess, lactate, and creatinine) were grossly deranged for the vast majority of control IgG-treated animals but remained within normal limits for MEDI3902-treated animals. In addition, MEDI3902-treated animals exhibited a profound reduction in P. aeruginosa organ burden and a marked reduction in the expression of proinflammatory mediators from lung tissue, which correlated with reduced lung histopathology. These results confirm that targeting PcrV and Psl via MEDI3902 is a promising candidate for immunotherapy against P. aeruginosa pneumonia.

Treatment of Infections Caused by Extended-Spectrum-Beta-Lactamase-, AmpC-, and Carbapenemase-Producing Enterobacteriaceae

Therapy of invasive infections due to multidrug-resistant Enterobacteriaceae (MDR-E) is challenging, and some of the few active drugs are not available in many countries. For extended-spectrum β-lactamase and AmpC producers, carbapenems are the drugs of choice, but alternatives are needed because the rate of carbapenem resistance is rising. Potential active drugs include classic and newer β-lactam-β-lactamase inhibitor combinations, cephamycins, temocillin, aminoglycosides, tigecycline, fosfomycin, and, rarely, fluoroquinolones or trimethoprim-sulfamethoxazole. These drugs might be considered in some specific situations. AmpC producers are resistant to cephamycins, but cefepime is an option. In the case of carbapenemase-producing Enterobacteriaceae (CPE), only some "second-line" drugs, such as polymyxins, tigecycline, aminoglycosides, and fosfomycin, may be active; double carbapenems can also be considered in specific situations. Combination therapy is associated with better outcomes for high-risk patients, such as those in septic shock or with pneumonia. Ceftazidime-avibactam was recently approved and is active against KPC and OXA-48 producers; the available experience is scarce but promising, although development of resistance is a concern. New drugs active against some CPE isolates are in different stages of development, including meropenem-vaborbactam, imipenem-relebactam, plazomicin, cefiderocol, eravacycline, and aztreonam-avibactam. Overall, therapy of MDR-E infection must be individualized according to the susceptibility profile, type, and severity of infection and the features of the patient.

Evaluation of Activity and Emergence of Resistance of Polymyxin B and ZTI-01 (Fosfomycin for Injection) against KPC-Producing Klebsiella pneumoniae

ZTI-01 (fosfomycin for injection) is a broad-spectrum antibiotic with a novel mechanism of action and is currently under development in the United States for treatment of complicated urinary tract infections. Globally, fosfomycin and polymyxin B are increasingly being used to treat multidrug-resistant Gram-negative infections. The objectives were to evaluate the pharmacodynamic activity of polymyxin B and fosfomycin alone and in combination against KPC-producing Klebsiella pneumoniae and to assess the rate and extent of emergence of resistance to different antibiotic regimens. Two clinical isolates, BRKP26 (MIC of polymyxin B[MIC_PMB], 0.5 mg/liter; MIC of fosfomycin [MIC_FOF], 32 mg/liter) and BRKP67 (MIC_PMB, 8 mg/liter; MIC_FOF, 32 mg/liter) at an initial inoculum of 10⁷ CFU/ml, were evaluated over 168 h in a hollow-fiber infection model simulating clinically relevant polymyxin B (2.5-mg/kg loading dose as a 2 h-infusion followed by 1.5-mg/kg dose every 12 h [q12h] as a 1-h infusion) and fosfomycin (6 g q6h as a 1-h or 3-h infusion) regimens alone and in combination. Population analysis profiles (PAPs) and MIC testing were performed to assess emergence of resistance. Polymyxin B or fosfomycin monotherapy was ineffective and selected for resistance by 24 h. Polymyxin B plus a fosfomycin 1-h infusion demonstrated sustained bactericidal activity by 4 h, with undetectable colony counts beyond 144 h. Polymyxin B plus a fosfomycin 3-h infusion demonstrated bactericidal activity at 4 h, followed by regrowth similar to that of the control by 144 h. PAPs revealed resistant subpopulations by 120 h. The combination of polymyxin B and a fosfomycin 1-h infusion is a promising treatment option for KPC-producing K. pneumoniae and suppresses the emergence of resistance. Further evaluation of novel dosing strategies is warranted to optimize therapy.

Targeting Virulence in Staphylococcus aureus by Chemical Inhibition of the Accessory Gene Regulator System In Vivo

Methicillin-resistant Staphylococcus aureus (MRSA) presents one of the most serious health concerns worldwide. The WHO labeled it as a “high-priority” pathogen in 2017, also citing the more recently emerged vancomycin-intermediate and -resistant strains.

Methicillin-resistant Staphylococcus aureus (MRSA) presents one of the most serious health concerns worldwide. The WHO labeled it as a “high-priority” pathogen in 2017, also citing the more recently emerged vancomycin-intermediate and -resistant strains. With the spread of antibiotic resistance due in large part to the selective pressure exerted by conventional antibiotics, the use of antivirulence strategies has been recurrently proposed as a promising therapeutic approach. In MRSA, virulence is chiefly controlled by quorum sensing (QS); inhibitors of QS are called quorum quenchers (QQ). In S. aureus, the majority of QS components are coded for by the accessory gene regulator (Agr) system. Although much work has been done to develop QQs against MRSA, only a few studies have progressed to in vivo models. Those studies include both prophylactic and curative models of infection as well as combination treatments with antibiotic. For most, high efficacy is seen at attenuating MRSA virulence and pathogenicity, with some studies showing effects such as synergy with antibiotics and antibiotic resensitization. This minireview aims to summarize and derive conclusions from the literature on the in vivo efficacy of QQ agents in MRSA infection models. In vitro data are also summarized to provide sufficient background on the hits discussed. On the whole, the reported in vivo effects of the reviewed QQs against MRSA represent positive progress at this early stage in drug development. Follow-up studies that thoroughly examine in vitro and in vivo activity are needed to propel the field forward and set the stage for lead optimization.

Disarming Staphylococcus aureus from destroying human cells by simultaneously neutralizing six cytotoxins with two human monoclonal antibodies

Pathogenesis of Staphylococcus aureus is increasingly recognized to be driven by powerful toxins. Staphylococcus aureus employs up to six pore-forming toxins to subvert the human host defense and to promote bacterial invasion: alpha-hemolysin that disrupts epithelial and endothelial barriers and five leukocidins that lyse phagocytes involved in bacterial clearance. Previously, we described two human monoclonal antibodies (mAbs), ASN-1 that neutralizes alpha-hemolysin and four leukocidins (LukSF-PV, LukED, HlgAB, HlgCB), and ASN-2 that inactivates the 5^th leukocidin, LukGH. In this study we tested the individual and combined effects of ASN-1 and ASN-2 in multiple in vitro models employing relevant human target cells. We found that diverse S. aureus isolates with different genetic backgrounds (based on MLST- and spa-typing) and antibiotic sensitivity (both MRSA and MSSA) displayed greatly different cytotoxin expression patterns influenced by the type of growth medium used. Both mAbs were required to fully prevent the lysis of human neutrophils exposed to the mixture of recombinant cytotoxins or native toxins present in the culture supernatants of S. aureus isolates. Flow cytometry confirmed the protective effects of ASN-1 + ASN-2 (known as ASN100) on granulocytes, monocytes, NK-cells and T-lymphocytes. ASN-1 alone preserved the integrity of a 3D-primary culture of human tracheal/bronchial mucociliary epithelial tissue infected with S. aureus. We conclude that simultaneous inhibition of alpha-hemolysin and five leukocidins by ASN100 blocks cytolytic activity of S. aureus towards human target cells in vitro.

Port d'Entrée for Respiratory Infections - Does the Influenza A Virus Pave the Way for Bacteria?

Bacterial and viral co-infections of the respiratory tract are life-threatening and present a global burden to the global community. Staphylococcus aureus, Streptococcus pneumoniae, and Streptococcus pyogenes are frequent colonizers of the upper respiratory tract. Imbalances through acquisition of seasonal viruses, e.g., Influenza A virus, can lead to bacterial dissemination to the lower respiratory tract, which in turn can result in severe pneumonia. In this review, we summarize the current knowledge about bacterial and viral co-infections of the respiratory tract and focus on potential experimental models suitable for mimicking this disease. Transmission of IAV and pneumonia is mainly modeled by mouse infection. Few studies utilizing ferrets, rats, guinea pigs, rabbits, and non-human primates are also available. The knowledge gained from these studies led to important discoveries and advances in understanding these infectious diseases. Nevertheless, mouse and other infection models have limitations, especially in translation of the discoveries to humans. Here, we suggest the use of human engineered lung tissue, human ex vivo lung tissue, and porcine models to study respiratory co-infections, which might contribute to a greater translation of the results to humans and improve both, animal and human health.

A neutralizing antibody that blocks delivery of the enzymatic cargo of Clostridium difficile toxin TcdB into host cells

Clostridium difficile infection is the leading cause of hospital-acquired diarrhea and is mediated by the actions of two toxins, TcdA and TcdB. The toxins perturb host cell function through a multistep process of receptor binding, endocytosis, low pH-induced pore formation, and the translocation and delivery of an N-terminal glucosyltransferase domain that inactivates host GTPases. Infection studies with isogenic strains having defined toxin deletions have established TcdB as an important target for therapeutic development. Monoclonal antibodies that neutralize TcdB function have been shown to protect against C. difficile infection in animal models and reduce recurrence in humans. Here, we report the mechanism of TcdB neutralization by PA41, a humanized monoclonal antibody capable of neutralizing TcdB from a diverse array of C. difficile strains. Through a combination of structural, biochemical, and cell functional studies, involving X-ray crystallography and EM, we show that PA41 recognizes a single, highly conserved epitope on the TcdB glucosyltransferase domain and blocks productive translocation and delivery of the enzymatic cargo into the host cell. Our study reveals a unique mechanism of C. difficile toxin neutralization by a monoclonal antibody, which involves targeting a process that is conserved across the large clostridial glucosylating toxins. The PA41 antibody described here provides a valuable tool for dissecting the mechanism of toxin pore formation and translocation across the endosomal membrane.

Multimechanistic Monoclonal Antibodies (MAbs) Targeting Staphylococcus aureus Alpha-Toxin and Clumping Factor A: Activity and Efficacy Comparisons of a MAb Combination and an Engineered Bispecific Antibody Approach

Secreted alpha-toxin and surface-localized clumping factor A (ClfA) are key virulence determinants in Staphylococcus aureus bloodstream infections. We previously demonstrated that prophylaxis with a multimechanistic monoclonal antibody (MAb) combination against alpha-toxin (MEDI4893*) and ClfA (11H10) provided greater strain coverage and improved efficacy in an S. aureus lethal bacteremia model. Subsequently, 11H10 was found to exhibit reduced affinity and impaired inhibition of fibrinogen binding to ClfA002 expressed by members of a predominant hospital-associated methicillin-resistant S. aureus (MRSA) clone, ST5. Consequently, we identified another anti-ClfA MAb (SAR114) from human tonsillar B cells with >100-fold increased affinity for three prominent ClfA variants, including ClfA002, and potent inhibition of bacterial agglutination by 112 diverse clinical isolates. We next constructed bispecific Abs (BiSAbs) comprised of 11H10 or SAR114 as IgG scaffolds and grafted anti-alpha-toxin (MEDI4893*) single-chain variable fragment to the amino or carboxy terminus of the anti-ClfA heavy chains. Although the BiSAbs exhibited in vitro potencies similar to those of the parental MAbs, only 11H10-BiSAb, but not SAR114-BiSAb, showed protective activity in murine infection models comparable to the respective MAb combination. In vivo activity with SAR114-BiSAb was observed in infection models with S. aureus lacking ClfA. Our data suggest that high-affinity binding to ClfA sequesters the SAR114-BiSAb to the bacterial surface, thereby reducing both alpha-toxin neutralization and protection in vivo These results indicate that a MAb combination targeting ClfA and alpha-toxin is more promising for future development than the corresponding BiSAb.