Development of a Staphylococcus aureus reporter strain with click beetle red luciferase for enhanced in vivo imaging of experimental bacteremia and mixed infections

Advances in the Development of Bacterial Bioluminescence Imaging

Bioluminescence imaging (BLI) is a powerful method for visualizing biological processes and tracking cells. Engineered bioluminescent bacteria that utilize luciferase-catalyzed biochemical reactions to generate luminescence have become useful analytical tools for in vitro and in vivo bacterial imaging. Accordingly, this review initially introduces the development of engineered bioluminescent bacteria that use different luciferase-luciferin pairs as analytical tools and their applications for in vivo BLI, including real-time bacterial tracking of infection, probiotic investigation, tumor-targeted therapy, and drug screening. Applications of engineered bioluminescent bacteria as whole-cell biosensors for sensing biological changes in vitro and in vivo are then discussed. Finally, we review the optimizations and future directions of bioluminescent bacteria for imaging. This review aims to provide fundamental insights into bacterial BLI and highlight the potential development of this technique in the future.

Staphylococcus aureus senses human neutrophils via PerR to coordinate the expression of the toxin LukAB

Staphylococcus aureus is a gram-positive pathogen that poses a major health concern, in part due to its large array of virulence factors that allow infection and evasion of the immune system. One of these virulence factors is the bicomponent pore-forming leukocidin LukAB. The regulation of lukAB expression is not completely understood, especially in the presence of immune cells such as human polymorphonuclear neutrophils (hPMNs). Here, we screened for transcriptional regulators of lukAB during the infection of primary hPMNs. We uncovered that PerR, a peroxide sensor, is vital for hPMN-mediated induction of lukAB and that PerR upregulates cytotoxicity during the infection of hPMNs. Exposure of S. aureus to hydrogen peroxide (H2O2) alone also results in increased lukAB promoter activity, a phenotype dependent on PerR. Collectively, our data suggest that S. aureus uses PerR to sense the H2O2 produced by hPMNs to stimulate the expression of lukAB, allowing the bacteria to withstand these critical innate immune cells.IMPORTANCEStaphylococcus aureus utilizes a diverse set of virulence factors, such as leukocidins, to subvert human neutrophils, but how these toxins are regulated is incompletely defined. Here, we identified the peroxide-sensitive repressor, PerR, as a required protein involved in the induction of lukAB in the presence of primary human neutrophils, a phenotype directly linked to the ability of PerR to sense H2O2. Thus, we show that S. aureus coordinates sensing and resistance to oxidative stress with toxin production to promote pathogen survival.

Metabolic diversity of human macrophages: potential influence on Staphylococcus aureus intracellular survival

Staphylococcus aureus is a leading cause of medical device-associated biofilm infections. This is influenced by the ability of S. aureus biofilm to evade the host immune response, which is partially driven by the anti-inflammatory cytokine interleukin-10 (IL-10). Here, we show that treatment of human monocyte-derived macrophages (HMDMs) with IL-10 enhanced biofilm formation, suggesting that macrophage anti-inflammatory programming likely plays an important role during the transition from planktonic to biofilm growth. To identify S. aureus genes that were important for intracellular survival in HMDMs and how this was affected by IL-10, transposon sequencing was performed. The size of the S. aureus essential genome was similar between unstimulated HMDMs and the outgrowth control (18.5% vs 18.4%, respectively, with 54.4% overlap) but increased to 22.5% in IL-10-treated macrophages, suggesting that macrophage polarization status exerts differential pressure on S. aureus. Essential genes for S. aureus survival within IL-10-polarized HMDMs were dominated by negative regulatory pathways, including nitrogen and RNA metabolism, whereas S. aureus essential genes within untreated HMDMs were enriched in biosynthetic pathways such as purine and pyrimidine biosynthesis. To explore how IL-10 altered the macrophage intracellular metabolome, targeted metabolomics was performed on HMDMs from six individual donors. IL-10 treatment led to conserved alterations in distinct metabolites that were increased (dihydroxyacetone phosphate, glyceraldehyde-3-phosphate, and acetyl-CoA) or reduced (fructose-6-phosphate, aspartic acid, and ornithine) across donors, whereas other metabolites were variable. Collectively, these findings highlight an important aspect of population-level heterogeneity in human macrophage responsiveness that should be considered when translating results to a patient population.IMPORTANCEOne mechanism that Staphylococcus aureus biofilm elicits in the host to facilitate infection persistence is the production of the anti-inflammatory cytokine interleukin-10 (IL-10). Here, we show that exposure of human monocyte-derived macrophages (HMDMs) to IL-10 promotes S. aureus biofilm formation and programs intracellular bacteria to favor catabolic pathways. Examination of intracellular metabolites in HMDMs revealed heterogeneity between donors that may explain the observed variability in essential genes for S. aureus survival based on nutrient availability for bacteria within the intracellular compartment. Collectively, these studies provide novel insights into how IL-10 polarization affects S. aureus intracellular survival in HMDMs and the importance of considering macrophage heterogeneity between human donors as a variable when examining effector mechanisms.

Optimizing a high-sensitivity NanoLuc-based bioluminescence system for in vivo evaluation of antimicrobial treatment

Focal and systemic infections are serious threats to human health. Preclinical models enable the development of new drugs and therapeutic regimens. In vivo, animal bioluminescence (BL) imaging has been used with bacterial reporter strains to evaluate antimicrobial treatment effects. However, high-sensitivity bioluminescent systems are required because of the limited tissue penetration and low brightness of the BL signals of existing approaches. Here, we report that NanoLuc (Nluc) showed better performance than LuxCDABE in bacteria. However, the retention rate of plasmid constructs in bacteria was low. To construct stable Staphylococcus aureus reporter strains, a partner protein enolase (Eno) was identified by screening of S. aureus strain USA300 for fusion expression of Nluc-based luciferases, including Nluc, Teluc, and Antares2. Different substrates, such as hydrofurimazine (HFZ), furimazine (FUR), and diphenylterazine (DTZ), were used to optimize a stable reporter strain/substrate pair for BL imaging. S. aureus USA300/Eno-Antares2/HFZ produced the highest number of photons of orange-red light in vitro and enabled sensitive BL tracking of S. aureus in vivo, with sensitivities of approximately 10 CFU from mouse skin and 750 CFU from mouse kidneys. USA300/Eno-Antares2/HFZ was a powerful combination based on the longitudinal evaluation of the therapeutic efficacy of antibiotics. The optimized S. aureus Eno-Antares2/HFZ pair provides a technological advancement for the in vivo evaluation of antimicrobial treatment.

In Vivo Prevention of Implant-Associated Infections Caused by Antibiotic-Resistant Bacteria through Biofunctionalization of Additively Manufactured Porous Titanium

Additively manufactured (AM) porous titanium implants may have an increased risk of implant-associated infection (IAI) due to their huge internal surfaces. However, the same surface, when biofunctionalized, can be used to prevent IAI. Here, we used a rat implant infection model to evaluate the biocompatibility and infection prevention performance of AM porous titanium against bioluminescent methicillin-resistant Staphylococcus aureus (MRSA). The specimens were biofunctionalized with Ag nanoparticles (NPs) using plasma electrolytic oxidation (PEO). Infection was initiated using either intramedullary injection in vivo or with in vitro inoculation of the implant prior to implantation. Nontreated (NT) implants were compared with PEO-treated implants with Ag NPs (PT-Ag), without Ag NPs (PT) and infection without an implant. After 7 days, the bacterial load and bone morphological changes were evaluated. When infection was initiated through in vivo injection, the presence of the implant did not enhance the infection, indicating that this technique may not assess the prevention but rather the treatment of IAIs. Following in vitro inoculation, the bacterial load on the implant and in the peri-implant bony tissue was reduced by over 90% for the PT-Ag implants compared to the PT and NT implants. All infected groups had enhanced osteomyelitis scores compared to the noninfected controls.

Theranostics applications of quantum dots in regenerative medicine, cancer medicine, and infectious diseases

Quantum dots (QDs) have attracted attention for their application and commercialization in all industrial fields, including communications, displays, and solar cells, due to their excellent optical properties based on the quantum size effect. In recent years, the development of QDs that do not contain cadmium which is toxic to cells and living organisms, has progressed, and they have attracted considerable attention in the bio-imaging field for targeting molecules and cells. Furthermore, recently, the need for diagnostics and treatment at the single molecule and single cell level in the medical field has been increasing, and the application of QDs in the medical field is also accelerating. Therefore, this paper outlines the frontiers of diagnostic and therapeutic applications (theranostics) of QDs, especially in advanced medical fields such as regenerative medicine, oncology, and infectious diseases.

γδ T cell-intrinsic IL-1R promotes survival during Staphylococcus aureus bacteremia

Staphylococcus aureus is a leading cause of bacteremia, further complicated by the emergence of antibiotic-resistant strains such as methicillin-resistant S. aureus (MRSA). A better understanding of host defense mechanisms is needed for the development of host-directed therapies as an alternative approach to antibiotics. The levels of IL-1, IL-17, and TNF-α cytokines in circulation have been associated with predictive outcomes in patients with S. aureus bacteremia. However, their causative role in survival and the cell types involved in these responses during bacteremia is not entirely clear. Using a mouse model of S. aureus bacteremia, we demonstrated that IL-17A/F and TNF-α had no significant impact on survival, whereas IL-1R signaling was critical for survival during S. aureus bacteremia. Furthermore, we identified that T cells, but not neutrophils, monocytes/macrophages, or endothelial cells were the crucial cell type for IL-1R-mediated survival against S. aureus bacteremia. Finally, we determined that the expression of IL-1R on γδ T cell, but not CD4+ or CD8+ T cells was responsible for survival against the S. aureus bacteremia. Taken together, we uncovered a role for IL-1R, but not IL-17A/F and TNF-α in protection against S. aureus bacteremia. Importantly, γδ T cell-intrinsic expression of IL-1R was crucial for survival, but not on other immune cells or endothelial cells. These findings reveal potential cellular and immunological targets for host-directed therapies for improved outcomes against S. aureus bacteremia.

Neutrophil-intrinsic TNF receptor signaling orchestrates host defense against Staphylococcus aureus

Staphylococcus aureus is the leading cause of skin and soft tissue infections and is a major health burden due to the emergence of antibiotic-resistant strains. To address the unmet need of alternative treatments to antibiotics, a better understanding of the protective immune mechanisms against S. aureus skin infection is warranted. Here, we report that tumor necrosis factor (TNF) promoted protection against S. aureus in the skin, which was mediated by bone marrow-derived immune cells. Furthermore, neutrophil-intrinsic TNF receptor (TNFR) signaling directed immunity against S. aureus skin infections. Mechanistically, TNFR1 promoted neutrophil recruitment to the skin, whereas TNFR2 prevented systemic bacterial dissemination and directed neutrophil antimicrobial functions. Treatment with a TNFR2 agonist showed therapeutic efficacy against S. aureus and Pseudomonas aeruginosa skin infections, which involved increased neutrophil extracellular trap formation. Our findings revealed nonredundant roles for TNFR1 and TNFR2 in neutrophils for immunity against S. aureus and can be therapeutically targeted for protection against bacterial skin infections.

Gene expression of S100a8/a9 predicts Staphylococcus aureus-induced septic arthritis in mice

Septic arthritis is the most aggressive joint disease associated with high morbidity and mortality. The interplay of the host immune system with the invading pathogens impacts the pathophysiology of septic arthritis. Early antibiotic treatment is crucial for a better prognosis to save the patients from severe bone damage and later joint dysfunction. To date, there are no specific predictive biomarkers for septic arthritis. Transcriptome sequencing analysis identified S100a8/a9 genes to be highly expressed in septic arthritis compared to non-septic arthritis at the early course of infection in an Staphylococcus aureus septic arthritis mouse model. Importantly, downregulation of S100a8/a9 mRNA expression at the early course of infection was noticed in mice infected with the S. aureus Sortase A/B mutant strain totally lacking arthritogenic capacity compared with the mice infected with parental S. aureus arthritogenic strain. The mice infected intra-articularly with the S. aureus arthritogenic strain significantly increased S100a8/a9 protein expression levels in joints over time. Intriguingly, the synthetic bacterial lipopeptide Pam2CSK4 was more potent than Pam3CSK4 in inducing S100a8/a9 release upon intra-articular injection of these lipopeptides into the mouse knee joints. Such an effect was dependent on the presence of monocytes/macrophages. In conclusion, S100a8/a9 gene expression may serve as a potential biomarker to predict septic arthritis, enabling the development of more effective treatment strategies.

SarS Is a Repressor of Staphylococcus aureus Bicomponent Pore-Forming Leukocidins

Staphylococcus aureus is a successful pathogen that produces a wide range of virulence factors that it uses to subvert and suppress the immune system. These include the bicomponent pore-forming leukocidins. How the expression of these toxins is regulated is not completely understood. Here, we describe a screen to identify transcription factors involved in the regulation of leukocidins. The most prominent discovery from this screen is that SarS, a known transcription factor which had previously been described as a repressor of alpha-toxin expression, was found to be a potent repressor of leukocidins LukED and LukSF-PV. We found that inactivating sarS resulted in increased virulence both in an ex vivo model using primary human neutrophils and in an in vivo infection model in mice. Further experimentation revealed that SarS represses leukocidins by serving as an activator of Rot, a critical repressor of toxins, as well as by directly binding and repressing the leukocidin promoters. By studying contemporary clinical isolates, we identified naturally occurring mutations in the sarS promoter that resulted in overexpression of sarS and increased repression of leukocidins in USA300 bloodstream clinical isolates. Overall, these data establish SarS as an important repressor of leukocidins and expand our understanding of how these virulence factors are being regulated in vitro and in vivo by S. aureus.

Complete Genome Sequences of Bioluminescent Staphylococcus aureus Strains Xen31 and Xen36, Derived from Two Clinical Isolates

Here, we report complete genome sequences of two clinical isolates of Staphylococcus aureus, namely, Xen31 and Xen36, which have been genetically modified to express an optimized Photorhabdus luminescens luciferase operon. Xen31 and Xen36 are bioluminescent strains used widely for investigation of bacterial pathogenesis, drug discovery, and development of novel therapies.

Evaluating the Targeting of a Staphylococcus-aureus-Infected Implant with a Radiolabeled Antibody In Vivo

Implant infections caused by Staphylococcus aureus are difficult to treat due to biofilm formation, which complicates surgical and antibiotic treatment. We introduce an alternative approach using monoclonal antibodies (mAbs) targeting S. aureus and provide evidence of the specificity and biodistribution of S.-aureus-targeting antibodies in a mouse implant infection model. The monoclonal antibody 4497-IgG1 targeting wall teichoic acid in S. aureus was labeled with indium-111 using CHX-A"-DTPA as a chelator. Single Photon Emission Computed Tomography/computed tomographyscans were performed at 24, 72 and 120 h after administration of the ¹¹¹In-4497 mAb in Balb/cAnNCrl mice with a subcutaneous implant that was pre-colonized with S. aureus biofilm. The biodistribution of this labelled antibody over various organs was visualized and quantified using SPECT/CT imaging, and was compared to the uptake at the target tissue with the implanted infection. Uptake of the ¹¹¹In-4497 mAbs at the infected implant gradually increased from 8.34 %ID/cm³ at 24 h to 9.22 %ID/cm³ at 120 h. Uptake at the heart/blood pool decreased over time from 11.60 to 7.58 %ID/cm³, whereas the uptake in the other organs decreased from 7.26 to less than 4.66 %ID/cm³ at 120 h. The effective half-life of ¹¹¹In-4497 mAbs was determined to be 59 h. In conclusion, ¹¹¹In-4497 mAbs were found to specifically detect S. aureus and its biofilm with excellent and prolonged accumulation at the site of the colonized implant. Therefore, it has the potential to serve as a drug delivery system for the diagnostic and bactericidal treatment of biofilm.

The role of Photorhabdus-induced bioluminescence and red cadaver coloration on the deterrence of insect scavengers from entomopathogenic nematode-infected cadavers

Photorhabdus spp. and Xenorhabdus spp. bacteria produce a variety of molecules that inhibit bacterial and fungal contamination as well as deter scavenging invertebrates and some vertebrates in soil. Certain Heterorhabditis/Photorhabdus-infected insect cadavers can be bioluminescent in the dark and/or turn red from the production of anthraquinone pigments. The role of these traits remains unresolved. The aim of the present study was to evaluate the role of red color (anthraquinone) and bioluminescence on the deterrence of insect scavengers. Our data shows that scavenger deterrent factor (SDF) is not related to red cadaver coloration or bioluminescence activity as crickets and ants did not consume Galleria mellonella cadavers infected by P. laumondii strain 48-02 and X. bovienii. Both bacteria exhibit SDF activity but do not produce anthraquinone. Also, the insects were not affected by anthraquinone in agar plugs prepared with supernatant from induced P. laumondii Δpptase Pcep-KM-antA (SVS-275) mutant strain, which overproduces anthraquinone. Since bioluminescence and anthraquinone are not responsible for SDF activity against insect scavengers, more studies are needed to elucidate the SDF compound from Xenorhabdus and Photorhabdus bacteria.

Advances in luminescence-based technologies for drug discovery

INTRODUCTION

Luminescence-based technologies, specifically bioluminescence and chemiluminescence, are powerful tools with extensive use in drug discovery. Production of light during chemiluminescence and bioluminescence, unlike fluorescence, doesn't require an excitation light source, resulting in high signal-to-noise ratio, less background interference, and no issues from phototoxicity and photobleaching. These characteristics of luminescence technologies offer unique advantages for experimental designs, allowing for greater flexibility to target a wide range of proteins and biological processes for drug discovery at different stages.

AREAS COVERED

This review provides a basic overview of luciferase-based technologies and details recent advances and use cases of luciferase and luciferin variations and their applicability in the drug discovery toolset. The authors expand upon specific applications of luciferase technologies, including chemiluminescent and bioluminescent-based microscopy. Finally, the authors lay out forward-looking statements on the field of luminescence and how it may shape the translational scientists' work moving forward.

EXPERT OPINION

The demand for improved luciferase and luciferin pairs correlates strongly with efforts to improve the sensitivity and robustness of high-throughput assays. As luminescent reporter systems improve, so will the expansion of use cases for luminescence-based technologies in early-stage drug discovery. With the synthesis of novel, non-enzymatic chemiluminescence-based probes, which previously were restrained to only basic research applications, they may now be readily implemented in drug discovery campaigns.

The Novel Oxazolidinone TBI-223 Is Effective in Three Preclinical Mouse Models of Methicillin-Resistant Staphylococcus aureus Infection

Staphylococcus aureus is an important cause of various infections in humans, including bacteremia, skin and soft tissue infections, and infections associated with implanted medical devices. The emergence of hospital- and community-acquired methicillin-resistant Staphylococcus aureus (MRSA) underscores the urgent and unmet need to develop novel, safe, and effective antibiotics against these multidrug-resistant clinical isolates. Oxazolidinone antibiotics such as linezolid have excellent oral bioavailability and provide coverage against MRSA infections. However, their widespread and long-term use is often limited by adverse effects, especially myelosuppression. TBI-223 is a novel oxazolidinone with potentially reduced myelosuppression, compared to linezolid, but its efficacy against MRSA infections is unknown. Therefore, the preclinical efficacy of TBI-223 (80 and 160 mg/kg twice daily) was compared with that of linezolid (40 and 80 mg/kg twice daily) and sham treatment in mouse models of MRSA bacteremia, skin wound infection, and orthopedic-implant-associated infection. The dosage was selected based on mouse pharmacokinetic analysis of both linezolid and TBI-223, as well as measurement of the MICs. In all three models, TBI-223 and linezolid had comparable dose-dependent efficacies in reducing bacterial burden and disease severity, compared with sham-treated control mice. Taken together, these findings indicate that TBI-223 represents a novel oxazolidinone antibiotic that may provide an additional option against MRSA infections. Future studies in larger animal models and clinical trials are warranted to translate these findings to humans. IMPORTANCE Staphylococcus aureus is the predominant cause of bloodstream, skin, and bone infections in humans. Resistance to commonly used antibiotics is a growing concern, making it more difficult to treat staphylococcal infections. Use of the oxazolidinone antibiotic linezolid against resistant strains is hindered by high rates of adverse reactions during prolonged therapy. Here, a new oxazolidinone named TBI-223 was tested against S. aureus in three mouse models of infection, i.e., bloodstream infection, skin infection, and bone infection. We found that TBI-223 was as effective as linezolid in these three models. Previous data suggest that TBI-223 has a better safety profile than linezolid. Taken together, these findings indicate that this new agent may provide an additional option against MRSA infections. Future studies in larger animal models and clinical trials are warranted to translate these findings to humans.

Targeted Antimicrobial Photodynamic Therapy of Biofilm-Embedded and Intracellular Staphylococci with a Phage Endolysin's Cell Binding Domain

Bacterial pathogens are progressively adapting to current antimicrobial therapies with severe consequences for patients and global health care systems. This is critically underscored by the rise of methicillin resistant Staphylococcus aureus (MRSA) and other biofilm-forming staphylococci. Accordingly, alternative strategies have been explored to fight such highly multidrug resistant microorganisms, including antimicrobial photodynamic therapy (aPDT) and phage therapy. aPDT has the great advantage that it does not elicit resistance, while phage therapy allows targeting of specific pathogens. In the present study, we aimed to merge these benefits by conjugating the cell-binding domain (CBD3) of a Staphylococcus aureus phage endolysin to a photoactivatable silicon phthalocyanine (IRDye 700DX) for the development of a Staphylococcus-targeted aPDT approach. We show that, upon red-light activation, the resulting CBD3-700DX conjugate generates reactive oxygen species that effectively kill high loads of planktonic and biofilm-resident staphylococci, including MRSA. Furthermore, CBD3-700DX is readily internalized by mammalian cells, where it allows the targeted killing of intracellular MRSA upon photoactivation. Intriguingly, aPDT with CBD3-700DX also affects mammalian cells with internalized MRSA, but it has no detectable side effects on uninfected cells. Altogether, we conclude that CBD3 represents an attractive targeting agent for Staphylococcus-specific aPDT, irrespective of planktonic, biofilm-embedded, or intracellular states of the bacterium.

IMPORTANCE Antimicrobial resistance is among the biggest threats to mankind today. There are two alternative antimicrobial therapies that may help to control multidrug-resistant bacteria. In phage therapy, natural antagonists of bacteria, lytic phages, are harnessed to fight pathogens. In antimicrobial photodynamic therapy (aPDT), a photosensitizer, molecular oxygen, and light are used to produce reactive oxygen species (ROS) that inflict lethal damage on pathogens. Since aPDT destroys multiple essential components in targeted pathogens, aPDT resistance is unlikely. However, the challenge in aPDT is to maximize target specificity and minimize collateral oxidative damage to host cells. We now present an antimicrobial approach that combines the best features of both alternative therapies, namely, the high target specificity of phages and the efficacy of aPDT. This is achieved by conjugating the specific cell-binding domain from a phage protein to a near-infrared photosensitizer. aPDT with the resulting conjugate shows high target specificity toward MRSA with minimal side effects.

Human monoclonal antibodies against Staphylococcus aureus surface antigens recognize in vitro and in vivo biofilm

Implant-associated Staphylococcus aureus infections are difficult to treat because of biofilm formation. Bacteria in a biofilm are often insensitive to antibiotics and host immunity. Monoclonal antibodies (mAbs) could provide an alternative approach to improve the diagnosis and potential treatment of biofilm-related infections. Here, we show that mAbs targeting common surface components of S. aureus can recognize clinically relevant biofilm types. The mAbs were also shown to bind a collection of clinical isolates derived from different biofilm-associated infections (endocarditis, prosthetic joint, catheter). We identify two groups of antibodies: one group that uniquely binds S. aureus in biofilm state and one that recognizes S. aureus in both biofilm and planktonic state. Furthermore, we show that a mAb recognizing wall teichoic acid (clone 4497) specifically localizes to a subcutaneously implanted pre-colonized catheter in mice. In conclusion, we demonstrate the capacity of several human mAbs to detect S. aureus biofilms in vitro and in vivo.

Empowering antimicrobial photodynamic therapy of Staphylococcus aureus infections with potassium iodide

Infections caused by the Gram-positive bacterium Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), impose a great burden on global healthcare systems. Thus, there is an urgent need for alternative approaches to fight staphylococcal infections, such as targeted antimicrobial photodynamic therapy (aPDT). We recently reported that targeted aPDT with the S. aureus-specific immunoconjugate 1D9-700DX can be effectively applied to eradicate MRSA. Nonetheless, the efficacy of aPDT in the human body may be diminished by powerful antioxidant activities. In particular, we observed that the efficacy of aPDT with 1D9-700DX towards MRSA was reduced in human plasma. Here we show that this antagonistic effect can be attributed to human serum albumin, which represents the largest pool of free thiols in plasma for trapping reactive oxygen species. Importantly, we also show that our targeted aPDT approach with 1D9-700DX can be empowered by the non-toxic inorganic salt potassium iodide (KI), which reacts with the singlet oxygen produced upon aPDT, resulting in the formation of free iodine. The targeted iodine formation allows full eradication of MRSA (more than 6-log reduction) without negatively affecting other non-targeted bacterial species or human cells. Altogether, we show that the addition of KI allows a drastic reduction of both the amount of the immunoconjugate 1D9-700DX and the irradiation time needed for effective elimination of MRSA by aPDT in the presence of human serum albumin.

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.

Commensal microbiota regulates skin barrier function and repair via signaling through the aryl hydrocarbon receptor

The epidermis forms a barrier that defends the body from desiccation and entry of harmful substances, while also sensing and integrating environmental signals. The tightly orchestrated cellular changes needed for the formation and maintenance of this epidermal barrier occur in the context of the skin microbiome. Using germ-free mice, we demonstrate the microbiota is necessary for proper differentiation and repair of the epidermal barrier. These effects are mediated by microbiota signaling through the aryl hydrocarbon receptor (AHR) in keratinocytes, a xenobiotic receptor also implicated in epidermal differentiation. Mice lacking keratinocyte AHR are more susceptible to barrier damage and infection, during steady-state and epicutaneous sensitization. Colonization with a defined consortium of human skin isolates restored barrier competence in an AHR-dependent manner. We reveal a fundamental mechanism whereby the microbiota regulates skin barrier formation and repair, which has far-reaching implications for the numerous skin disorders characterized by epidermal barrier dysfunction.

Fighting Staphylococcus aureus infections with light and photoimmunoconjugates

Infections caused by multidrug-resistant Staphylococcus aureus, especially methicillin-resistant S. aureus (MRSA), are responsible for high mortality and morbidity worldwide. Resistant lineages were previously confined to hospitals but are now also causing infections among healthy individuals in the community. It is therefore imperative to explore therapeutic avenues that are less prone to raise drug resistance compared with today’s antibiotics. An opportunity to achieve this ambitious goal could be provided by targeted antimicrobial photodynamic therapy (aPDT), which relies on the combination of a bacteria-specific targeting agent and light-induced generation of ROS by an appropriate photosensitizer. Here, we conjugated the near-infrared photosensitizer IRDye700DX to a fully human mAb, specific for the invariantly expressed staphylococcal antigen immunodominant staphylococcal antigen A (IsaA). The resulting immunoconjugate 1D9-700DX was characterized biochemically and in preclinical infection models. As demonstrated in vitro, in vivo, and in a human postmortem orthopedic implant infection model, targeted aPDT with 1D9-700DX is highly effective. Importantly, combined with the nontoxic aPDT-enhancing agent potassium iodide, 1D9-700DX overcomes the antioxidant properties of human plasma and fully eradicates high titers of MRSA. We show that the developed immunoconjugate 1D9-700DX targets MRSA and kills it upon illumination with red light, without causing collateral damage to human cells.

An immunoconjugate for targeted photodynamic therapy of Staphylococcus aureus infections kills MRSA with high efficacy upon illumination with red light.

Intracellular escape strategies of Staphylococcus aureus in persistent cutaneous infections

Pathogenic invasion of Staphylococcus aureus is a major concern in patients with chronic skin diseases like atopic dermatitis (AD), epidermolysis bullosa (EB), or chronic diabetic foot and venous leg ulcers, and can result in persistent and life-threatening chronic non-healing wounds. Staphylococcus aureus is generally recognized as extracellular pathogens. However, S. aureus can also invade, hide and persist in skin cells to contribute to wound chronicity. The intracellular life cycle of S. aureus is currently incompletely understood, although published studies indicate that its intracellular escape strategies play an important role in persistent cutaneous infections. This review provides current scientific knowledge about the intracellular life cycle of S. aureus in skin cells, which can be classified into professional and non-professional antigen-presenting cells, and its strategies to escape adaptive defense mechanisms. First, we discuss phenotypic switch of S. aureus, which affects intracellular routing and degradation. This review also evaluates potential intracellular escape mechanism of S. aureus to avoid intracellular degradation and antigen presentation, preventing an immune response. Furthermore, we discuss potential drug targets that can interfere with the intracellular life cycle of S. aureus. Taken together, this review aimed to increase scientific understanding about the intracellular life cycle of S. aureus into skin cells and its strategies to evade the host immune response, information that is crucial to reduce pathogenic invasion and life-threatening persistence of S. aureus in chronic cutaneous infections.

Who's in control? Regulation of metabolism and pathogenesis in space and time

Bacterial pathogens need to sense and respond to their environments during infection to align cell metabolism and virulence factor production to survive and battle host defenses. Complex regulatory networks including ligand-binding transcription factors, two-component systems, RNA-binding proteins, and small non-coding regulatory RNAs adjust gene expression programs in response to changes in metabolic fluxes, environmental cues, and nutrient availability. Recent studies underlined that these different layers of regulation occur along varying spatial and temporal scales, leading to changes in cell behavior and heterogeneity among the bacterial community. This brief review will highlight current research emphasizing that cell metabolism and pathogenesis are inextricably intertwined in both Gram-positive and Gram-negative bacteria.

Research Techniques Made Simple: Mouse Bacterial Skin Infection Models for Immunity Research

Bacterial skin infections are a major societal health burden and are increasingly difficult to treat owing to the emergence of antibiotic-resistant strains such as community-acquired methicillin-resistant Staphylococcus aureus. Understanding the immunologic mechanisms that provide durable protection against skin infections has the potential to guide the development of immunotherapies and vaccines to engage the host immune response to combat these antibiotic-resistant strains. To this end, mouse skin infection models allow researchers to examine host immunity by investigating the timing, inoculum, route of infection and the causative bacterial species in different wild-type mouse backgrounds as well as in knockout, transgenic, and other types of genetically engineered mouse strains. To recapitulate the various types of human skin infections, many different mouse models have been developed. For example, four models frequently used in dermatological research are based on the route of infection, including (i) subcutaneous infection models, (ii) intradermal infection models, (iii) wound infection models, and (iv) epicutaneous infection models. In this article, we will describe these skin infection models in detail along with their advantages and limitations. In addition, we will discuss how humanized mouse models such as the human skin xenograft on immunocompromised mice might be used in bacterial skin infection research.