Adaptive Upregulation of Clumping Factor A (ClfA) by Staphylococcus aureus in the Obese, Type 2 Diabetic Host Mediates Increased Virulence

Geranium essential oil downregulates virulence genes and mitigates MRSA wound infections through a multifunctional hydrogel

This study presents the development of a multifunctional hydrogel system that incorporates a natural therapeutic agent to promote wound healing and combat bacterial infections. A geranium essential oil (GEO)-loaded carboxymethyl chitosan-pullulan (CMCS-PLN-GEO) hydrogel was formulated for the treatment of MRSA-infected wounds. GEO exhibited potent antibiofilm and antivirulence properties by significantly downregulating key virulence-associated genes such as sarA, agrA, icaA, icaD, and clfA, indicating its potential as an effective therapeutic agent against MRSA. Integration of GEO into the CMCS-PLN hydrogel matrix enhanced its mechanical strength, reduced pore size, and enabled sustained GEO release. In a zebrafish wound model, the CMCS-PLN-GEO hydrogel markedly accelerated wound closure and suppressed MRSA colonization. Furthermore, it demonstrated strong anti-inflammatory activity through the downregulation of interleukin-1β (IL-1β), while promoting tissue regeneration via upregulation of transforming growth factor-β (TGF-β) and inhibition of matrix metalloproteinases MMP-9 and MMP-13. The synergistic integration of GEO with the CMCS-PLN polymer matrix offers dual antibacterial and anti-inflammatory effects, presenting a holistic therapeutic approach for the effective management of MRSA-infected wounds. The present study suggests that GEO-loaded hydrogel is a promising biomaterial for treating MRSA-infected wounds, potentially advancing toward clinical applications.

Addressing the challenges of infectious bone defects: a review of recent advances in bifunctional biomaterials

Infectious bone defects present a substantial clinical challenge due to the complex interplay between infection control and bone regeneration. These defects often result from trauma, autoimmune diseases, infections, or tumors, requiring a nuanced approach that simultaneously addresses infection and promotes tissue repair. Recent advances in tissue engineering and materials science, particularly in nanomaterials and nano-drug formulations, have led to the development of bifunctional biomaterials with combined osteogenic and antibacterial properties. These materials offer an alternative to traditional bone grafts, minimizing complications such as multiple surgeries, high antibiotic dosages, and lengthy recovery periods. This review examines the repair mechanisms in the infectious microenvironment and highlights various bifunctional biomaterials that foster both anti-infective and osteogenic processes. Emerging design strategies are also discussed to provide a forward-looking perspective on treating infectious bone defects with clinically significant outcomes.

γ Irradiation Alters the Staphylococcus aureus Proteome and Enhances Pathogenicity

Staphylococcus aureus (S. aureus) infection has become one of the most common and severe complications among cancer patients. The impact of γ radiation from radiotherapy on S. aureus's growth and virulence is not yet fully understood. In this study, S. aureus was exposed to γ radiation at a dose of 100 Gy, and its descendants were cultured under normal conditions. Proteome alternations of unirradiated, irradiated, and descendants of irradiated S. aureus were identified by using data-independent acquisition (DIA) proteomic technology. To investigate the consequences of proteome alternations induced by γ irradiation in S. aureus, functional enrichment analysis, pathway enrichment analysis, and protein-protein interaction network analysis were performed. Differentially expressed proteins (DEPs) in the irradiated S. aureus and its descendants were primarily enriched in lipoteichoic acid biosynthesis, S. aureus infection, two-component system, and cationic antimicrobial peptide resistance, suggesting an enhanced infection ability. A strong infection ability is typically associated with increased biofilm formation. Both the proteome study and the biofilm assay indicate that γ irradiation enhances the infection ability of S. aureus, likely resulting in increased pathogenicity.

Antimicrobial resistance: Biofilms, small colony variants, and intracellular bacteria

Soft tissue and bone infections continue to be a serious complication in orthopedic and trauma surgery. Both can lead to a high burden for the patients and the healthcare system. Musculoskeletal infections can be induced by intraoperative contamination, bacterial contamination of open wounds or hematogenous bacterial spread. During the recent decades, advances were achieved in the understanding of pathogenesis and antibiotic resistance. Despite some progress in the diagnosis and advancing of therapeutic concepts, groundbreaking successful improvement of treatment concepts is still missing. Current therapy concepts are based on the two pillars consisting of surgical debridement with joint or bone reconstruction as well as prolonged antibiotic therapy. An improved understanding of both host and pathogen-related factors leading to treatment failure is essential in musculoskeletal infections. Therefore, this review aims to give an overview of pathogen-related pathophysiology in musculoskeletal infections. It describes defense strategies of pathogens such as (1) biofilm, its development, characteristics, and treatment options. In addition, (2) characteristics of small colony variants and (3) intracellular bacteria are highlighted. Lastly (4) an outlook for potential and promising future therapeutic strategies is provided.

A natural killer cell mimic against intracellular pathogen infections

In the competition between the pathogen infection and the host defense, infectious microorganisms may enter the host cells by evading host defense mechanisms and use the intracellular biomolecules as replication nutrient. Among them, intracellular Staphylococcus aureus relies on the host cells to protect itself from the attacks by antibiotics or immune system to achieve long-term colonization in the host, and the consequent clinical therapeutic failures and relapses after antibiotic treatment. Here, we demonstrate that intracellular S. aureus surviving well even in the presence of vancomycin can be effectively eliminated using an emerging cell-mimicking therapeutic strategy. These cell mimics with natural killer cell-like activity (NKMs) are composed of a redox-responsive degradable carrier, and perforin and granzyme B within the carrier. NKMs perform far more effectivly than clinical antibiotics in treating intracellular bacterial infections, providing a direct evidence of the NK cell-mimicking immune mechanism in the treatment of intracellular S. aureus.

Type 2 diabetes mediates the causal relationship between obesity and osteomyelitis: A Mendelian randomization study

Mendelian randomization (MR) analysis was used to determine the causal relationship between Type 2 diabetes (T2D) and osteomyelitis (OM). We performed MR analysis using pooled data from different large-scale genome-wide association studies (GWAS). Instrumental variables were selected based on genome-wide significance, instrumental strength was assessed using F-values, and thresholds for the number of exposed phenotypes were further adjusted by Bonferroni correction. univariable and multivariable MR analyses were performed to assess causal effects and proportions mediated by T2D. IVW (inverse variance weighting) showed a significant genetic effect of osteomyelitis on the following: After correction by Bonferroni, univariable analyses showed that childhood body mass index (BMI) was not significantly associated with genetic susceptibility to OM [odds ratio (OR), 1.26; 95% confidence interval (CI), 1.02, 1.55; P = .030], not significantly associated with adulthood BMI (OR, 1.28; 95% CI, 1.02, 1.61; P = .034), significantly associated with waist circumference (OR, 1.84; 95% CI, 1.51, 2.24; P < .001), and significantly associated with hip circumference (OR, 1.52; 95% CI, 1.31, 1.76; P < .001). Meanwhile, multivariable analyses showed no significant effect of childhood BMI on OM (OR, 1.16; 95% CI, 0.84, 1.62; P = .370), no significant effect of adulthood BMI on OM (OR, 0.42; 95% CI, 0.21, 0.84; P = .015), a significant association between waist circumference and OM (OR, 4.30; 95% CI, 1.89, 9.82; P = .001), T2D mediated 10% (95% CI, 0.02, 0.14), and no significant association between hip circumference and OM (OR, 1.01; 95% CI, 0.54, 1.90; P = .968). Our study provides evidence for a genetically predicted causal relationship among obesity, T2D, and OM. We demonstrate that increased waist circumference is positively associated with an increased risk of OM and that T2D mediates this relationship. Clinicians should be more cautious in the perioperative management of osteomyelitis surgery in obese patients with T2D. In addition, waist circumference may be a more important criterion to emphasize and strictly control than other measures of obesity.

Staphylococcus aureus Panton-Valentine Leukocidin worsens acute implant-associated osteomyelitis in humanized BRGSF mice

Staphylococcus aureus is the most common pathogen that causes implant-associated osteomyelitis, a clinically incurable disease. Immune evasion of S. aureus relies on various mechanisms to survive within the bone niche, including the secretion of leukotoxins such as Panton-Valentine leukocidin (PVL). PVL is a pore-forming toxin exhibiting selective human tropism for C5a receptors (C5aR1 and C5aR2) and CD45 on neutrophils, monocytes, and macrophages. PVL is an important virulence determinant in lung, skin and soft tissue infections. The involvement of PVL in S. aureus pathogenesis during bone infections has not been studied extensively yet. To investigate this, humanized BALB/c Rag2-/-Il2rg-/-SirpaNODFlk2-/- (huBRGSF) mice were subjected to transtibial implant-associated osteomyelitis with community-acquired methicillin-resistant S. aureus (CA-MRSA) USA300 wild type strain (WT), an isogenic mutant lacking lukF/S-PV (Δpvl), or complemented mutant (Δpvl+pvl). Three days post-surgery, Δpvl-infected huBRGSF mice had a less severe infection compared to WT-infected animals as characterized by 1) improved clinical outcomes, 2) lower ex vivo bacterial bone burden, 3) absence of staphylococcal abscess communities (SACs) in their bone marrow, and 4) compromised MRSA dissemination to internal organs (liver, kidney, spleen, heart). Interestingly, Δpvl-infected huBRGSF mice had fewer human myeloid cells, neutrophils, and HLA-DR+ monocytes in the bone niche compared to WT-infected animals. Expectedly, a smaller fraction of human myeloid cells were apoptotic in the Δpvl-infected huBRGSF animals. Taken together, our study highlights the pivotal role of PVL during acute implant-associated osteomyelitis in humanized mice.

Occurrence, Antibiotic Susceptibility, Biofilm Formation and Molecular Characterization of Staphylococcus aureus Isolated from Raw Shrimp in China

The aim of this study was to determine the prevalence and characterization of Staphylococcus aureus isolated from 145 shrimp samples from 39 cities in China. The results show that 41 samples (28%) from 24 cities were positive, and most of the positive samples (39/41, 95.1%) were less than 110 MPN/g. Antimicrobial susceptibility testing showed that only seven isolates were susceptible to all 24 antibiotics, whereas 65.1% were multidrug-resistant. Antibiotic resistance genes that confer resistance to β-lactams, aminoglycosides, tetracycline, macrolides, lincosamides and streptogramin B (MLSB), trimethoprim, fosfomycin and streptothricin antibiotics were detected. All S. aureus isolates had the ability to produce biofilm and harbored most of the biofilm-related genes. Genes encoding one or more of the important virulence factors staphylococcal enterotoxins (sea, seb and sec), toxic shock syndrome toxin 1 (tsst-1) and Panton-Valentine leukocidin (PVL) were detected in 47.6% (30/63) of the S. aureus isolates. Molecular typing showed that ST15-t085 (27.0%, 17/63), ST1-t127 (14.3%, 9/63) and ST188-t189 (11.1%, 7/63) were the dominant genetic types. The finding of this study provides the first comprehensive surveillance on the incidence of S. aureus in raw shrimp in China. Some retained genotypes found in this food have been linked to human infections around the world.

Hyperglycemia Increases Severity of Staphylococcus aureus Osteomyelitis and Influences Bacterial Genes Required for Survival in Bone

Hyperglycemia, or elevated blood glucose, renders individuals more prone to developing severe Staphylococcus aureus infections. S. aureus is the most common etiological agent of musculoskeletal infection, which is a common manifestation of disease in hyperglycemic patients. However, the mechanisms by which S. aureus causes severe musculoskeletal infection during hyperglycemia are incompletely characterized. To examine the influence of hyperglycemia on S. aureus virulence during invasive infection, we used a murine model of osteomyelitis and induced hyperglycemia with streptozotocin. We discovered that hyperglycemic mice exhibited increased bacterial burdens in bone and enhanced dissemination compared to control mice. Furthermore, infected hyperglycemic mice sustained increased bone destruction relative to euglycemic controls, suggesting that hyperglycemia exacerbates infection-associated bone loss. To identify genes contributing to S. aureus pathogenesis during osteomyelitis in hyperglycemic animals relative to euglycemic controls, we used transposon sequencing (TnSeq). We identified 71 genes uniquely essential for S. aureus survival in osteomyelitis in hyperglycemic mice and another 61 mutants with compromised fitness. Among the genes essential for S. aureus survival in hyperglycemic mice was the gene encoding superoxide dismutase A (sodA), one of two S. aureus superoxide dismutases involved in detoxifying reactive oxygen species (ROS). We determined that a sodA mutant exhibits attenuated survival in vitro in high glucose and in vivo during osteomyelitis in hyperglycemic mice. SodA therefore plays an important role during growth in high glucose and promotes S. aureus survival in bone. Collectively, these studies demonstrate that hyperglycemia increases the severity of osteomyelitis and identify genes contributing to S. aureus survival during hyperglycemic infection.

Probiotic induced synthesis of microbiota polyamine as a nutraceutical for metabolic syndrome and obesity-related type 2 diabetes

The gut microbiota regulates multiple facets of host metabolism and immunity through the production of signaling metabolites, such as polyamines which are small organic compounds that are essential to host cell growth and lymphocyte activation. Polyamines are most abundant in the intestinal lumen, where their synthesis by the gut microbiota is influenced by microbiome composition and host diet. Disruption of the host gut microbiome in metabolic syndrome and obesity-related type 2 diabetes (obesity/T2D) results in potential dysregulation of polyamine synthesis. A growing body of evidence suggests that restoration of the dysbiotic gut microbiota and polyamine synthesis is effective in ameliorating metabolic syndrome and strengthening the impaired immune responses of obesity/T2D. In this review, we discuss existing studies on gut microbiome determinants of polyamine synthesis, polyamine production in obesity/T2D, and evidence that demonstrates the potential of polyamines as a nutraceutical in obesity/T2D hosts.

Fracture-related infection

Musculoskeletal trauma leading to broken and damaged bones and soft tissues can be a life-threating event. Modern orthopaedic trauma surgery, combined with innovation in medical devices, allows many severe injuries to be rapidly repaired and to eventually heal. Unfortunately, one of the persisting complications is fracture-related infection (FRI). In these cases, pathogenic bacteria enter the wound and divert the host responses from a bone-healing course to an inflammatory and antibacterial course that can prevent the bone from healing. FRI can lead to permanent disability, or long courses of therapy lasting from months to years. In the past 5 years, international consensus on a definition of these infections has focused greater attention on FRI, and new guidelines are available for prevention, diagnosis and treatment. Further improvements in understanding the role of perioperative antibiotic prophylaxis and the optimal treatment approach would be transformative for the field. Basic science and engineering innovations will be required to reduce infection rates, with interventions such as more efficient delivery of antibiotics, new antimicrobials, and optimizing host defences among the most likely to improve the care of patients with FRI.

Skeletal infections: microbial pathogenesis, immunity and clinical management

Osteomyelitis remains one of the greatest risks in orthopaedic surgery. Although many organisms are linked to skeletal infections, Staphylococcus aureus remains the most prevalent and devastating causative pathogen. Important discoveries have uncovered novel mechanisms of S. aureus pathogenesis and persistence within bone tissue, including implant-associated biofilms, abscesses and invasion of the osteocyte lacuno-canalicular network. However, little clinical progress has been made in the prevention and eradication of skeletal infection as treatment algorithms and outcomes have only incrementally changed over the past half century. In this Review, we discuss the mechanisms of persistence and immune evasion in S. aureus infection of the skeletal system as well as features of other osteomyelitis-causing pathogens in implant-associated and native bone infections. We also describe how the host fails to eradicate bacterial bone infections, and how this new information may lead to the development of novel interventions. Finally, we discuss the clinical management of skeletal infection, including osteomyelitis classification and strategies to treat skeletal infections with emerging technologies that could translate to the clinic in the future.

Neutralizing Staphylococcus aureus Virulence with AZD6389, a Three mAb Combination, Accelerates Closure of a Diabetic Polymicrobial Wound

Nonhealing diabetic foot ulcers (DFU), a major complication of diabetes, are associated with high morbidity and mortality despite current standard of care. Since Staphylococcus aureus is the most common pathogen isolated from nonhealing and infected DFU, we hypothesized that S. aureus virulence factors would damage tissue, promote immune evasion and alter the microbiome, leading to bacterial persistence and delayed wound healing. In a diabetic mouse polymicrobial wound model with S. aureus, Pseudomonas aeruginosa, and Streptococcus pyogenes, we report a rapid bacterial proliferation, prolonged pro-inflammatory response and large necrotic lesions unclosed for up to 40 days. Treatment with AZD6389, a three-monoclonal antibody combination targeting S. aureus alpha toxin, 4 secreted leukotoxins, and fibrinogen binding cell-surface adhesin clumping factor A resulted in full skin re-epithelization 21 days after inoculation. By neutralizing multiple virulence factors, AZD6389 effectively blocked bacterial agglutination and S. aureus-mediated cell killing, abrogated S. aureus-mediated immune evasion and targeted the bacteria for opsonophagocytic killing. Neutralizing S. aureus virulence not only facilitated S. aureus clearance in lesions, but also reduced S. pyogenes and P. aeruginosa numbers, damaging inflammatory mediators and markers for neutrophil extracellular trap formation 14 days post initiation. Collectively, our data suggest that AZD6389 holds promise as an immunotherapeutic approach against DFU complications.

IMPORTANCE Diabetic foot ulcers (DFU) represent a major complication of diabetes and are associated with poor quality of life and increased morbidity and mortality despite standard of care. They have a complex pathogenesis starting with superficial skin lesions, which often progress to deeper tissue structures up to the bone and ultimately require limb amputation. The skin microbiome of diabetic patients has emerged as having an impact on DFU occurrence and chronicity. DFU are mostly polymicrobial, and the Gram-positive bacterium Staphylococcus aureus detected in more than 95% of cases. S. aureus possess a collection of virulence factors which participate in disease progression and may facilitate growth of other pathogens. Here we show in a diabetic mouse wound model that targeting some specific S. aureus virulence factors with a multimechanistic antibody combination accelerated wound closure and promoted full skin re-epithelization. This work opens promising new avenues for the treatment of DFU.

Modulation of Gut Microbiota Metabolism in Obesity-Related Type 2 Diabetes Reduces Osteomyelitis Severity

Staphylococcus aureus is an opportunistic pathogen causing osteomyelitis through hematogenous seeding or contamination of implants and open wounds following orthopedic surgeries. The severity of S. aureus-mediated osteomyelitis is enhanced in obesity-related type 2 diabetes (obesity/T2D) due to chronic inflammation impairing both adaptive and innate immunity. Obesity-induced inflammation is linked to gut dysbiosis, with modification of the gut microbiota by high-fiber diets leading to a reduction in the symptoms and complications of obesity/T2D. However, our understanding of the mechanisms by which modifications of the gut microbiota alter host infection responses is limited. To address this gap, we monitored tibial S. aureus infections in obese/T2D mice treated with the inulin-like fructan fiber oligofructose. Treatment with oligofructose significantly decreased S. aureus colonization and lowered proinflammatory signaling postinfection in obese/T2D mice, as observed by decreased circulating inflammatory cytokines (tumor necrosis factor-α [TNF-α]) and chemokines (interferon-γ-induced protein 10 kDa [IP-10], keratinocyte-derived chemokine [KC], monokine induced by interferon-γ [MIG], monocyte chemoattractant protein-1 [MCP-1], and regulated upon activation, normal T cell expressed and presumably secreted [RANTES]), indicating partial reduction in inflammation. Oligofructose markedly shifted diversity in the gut microbiota of obese/T2D mice, with notable increases in the anti-inflammatory bacterium Bifidobacterium pseudolongum. Analysis of the cecum and plasma metabolome suggested that polyamine production was increased, specifically spermine and spermidine. Oral administration of these polyamines to obese/T2D mice resulted in reduced infection severity similar to oligofructose supplementation, suggesting that polyamines can mediate the beneficial effects of fiber on osteomyelitis severity. These results demonstrate the contribution of gut microbiota metabolites to the control of bacterial infections distal to the gut and polyamines as an adjunct therapeutic for osteomyelitis in obesity/T2D. IMPORTANCE Individuals with obesity-related type 2 diabetes (obesity/T2D) are at a five times increased risk for invasive Staphylococcus aureus osteomyelitis (bone infection) following orthopedic surgeries. With increasing antibiotic resistance and limited discoveries of novel antibiotics, it is imperative that we explore other avenues for therapeutics. In this study, we demonstrated that the dietary fiber oligofructose markedly reduced osteomyelitis severity and hyperinflammation following acute prosthetic joint infections in obese/T2D mice. Reduced infection severity was associated with changes in gut microbiota composition and metabolism, as indicated by increased production of natural polyamines in the gut and circulating plasma. This work identifies a novel role for the gut microbiome in mediating control of bacterial infections and polyamines as beneficial metabolites involved in improving the obesity/T2D host response to osteomyelitis. Understanding the impact of polyamines on host immunity and mechanisms behind decreasing susceptibility to severe implant-associated osteomyelitis is crucial to improving treatment strategies for this patient population.

Interactions between the foreign body reaction and Staphylococcus aureus biomaterial-associated infection. Winning strategies in the derby on biomaterial implant surfaces

Biomaterial-associated infections (BAIs) are an increasing problem where antibiotic therapies are often ineffective. The design of novel strategies to prevent or combat infection requires a better understanding of how an implanted foreign body prevents the immune system from eradicating surface-colonizing pathogens. The objective of this review is to chart factors resulting in sub-optimal clearance of Staphylococcus aureus bacteria involved in BAIs. To this end, we first describe three categories of bacterial mechanisms to counter the host immune system around foreign bodies: direct interaction with host cells, modulation of intercellular communication, and evasion of the immune system. These mechanisms take place in a time frame that differentiates sterile foreign body reactions, BAIs, and soft tissue infections. In addition, we identify experimental interventions in S. aureus BAI that may impact infectious mechanisms. Most experimental treatments modulate the host response to infection or alter the course of BAI through implant surface modulation. In conclusion, the first week after implantation and infection is crucial for the establishment of an S. aureus biofilm that resists the local immune reaction and antibiotic treatment. Although established and chronic S. aureus BAI is still treatable and manageable, the focus of interventions should lie on this first period.

Influence of Nisin-Biogel at Subinhibitory Concentrations on Virulence Expression in Staphylococcus aureus Isolates from Diabetic Foot Infections

A new approach to diabetic foot infections (DFIs) has been investigated, using a nisin-biogel combining the antimicrobial peptide (AMP) nisin with the natural polysaccharide guar-gum. Since in in vivo conditions bacteria may be exposed to decreased antimicrobial concentrations, known as subinhibitory concentrations (sub-MICs), effects of nisin-biogel sub-MIC values corresponding to 1/2, 1/4 and 1/8 of nisin's minimum inhibitory concentration (MIC) on virulence expression by six Staphylococcus aureus DFI isolates was evaluated by determining bacteria growth rate; expression of genes encoding for staphylococcal protein A (spA), coagulase (coa), clumping factor A (clfA), autolysin (atl), intracellular adhesin A (icaA), intracellular adhesin D (icaD), and the accessory gene regulator I (agrI); biofilm formation; Coa production; and SpA release. Nisin-biogel sub-MICs decreased bacterial growth in a strain- and dose-dependent manner, decreased agrI, atl and clfA expression, and increased spA, coa, icaA and icaD expression. Biofilm formation increased in the presence of nisin-biogel at 1/4 and 1/8 MIC, whereas 1/2 MIC had no effect. Finally, nisin-biogel at sub-MICs did not affect coagulase production, but decreased SpA production in a dose-dependent manner. Results highlight the importance of optimizing nisin-biogel doses before proceeding to in vivo trials, to reduce the risk of virulence factor's up-regulation due to the presence of inappropriate antimicrobial concentrations.

Functional Insights of MraZ on the Pathogenicity of Staphylococcus aureus

Introduction

In recent years, multidrug-resistant methicillin-resistant Staphylococcus aureus has become increasingly prevalent, which raised a huge challenge to antibiotic treatment of infectious diseases. The anti-virulence strategy targeting virulent factors is a promising novel therapy for S. aureus infection. The virulence mechanism of S. aureus was needed to explore deeply to develop more targets and improve the effectiveness of anti-virulence strategies.

Results

In this study, we found mraZ was highly conserved in S. aureus, and its production is homologous with the MraZ of Escherichia coli, a transcriptional regulator involved in the growth and cell division of E. coli. To investigate the function of mraZ in S. aureus, we constructed a MW2 mraZ deletion mutant and its complementary mutant for virulence comparison. Although no remarkable influence on the growth, the mraZ deletion mutant led to significantly reduced resistance to human neutrophils and decreased virulence in Galleria mellonella model as well as mouse skin and soft tissue infection models, indicating its essential contribution to virulence and immune evasion to support the pathogenicity of S. aureus infection. RNA-Seq and quantitative RT-qPCR revealed that MraZ is a multi-functional regulator; it involves in diverse biological processes and can up-regulate the expression of various virulence genes by agr and sarA.

Conclusion

mraZ plays vital roles in the pathyogenicity of S. aureus via regulating many virulence genes. It may be an attractive target for anti-virulence therapy of S. aureus.

In Vitro 3D Staphylococcus aureus Abscess Communities Induce Bone Marrow Cells to Expand into Myeloid-Derived Suppressor Cells

Staphylococcus aureus is the main causative pathogen of subcutaneous, bone, and implant-related infections, forming structures known as staphylococcal abscess communities (SACs) within tissues that also contain immunosuppressive myeloid-derived suppressor cells (MDSCs). Although both SACs and MDSCs are present in chronic S. aureus infections, it remains unknown whether SACs directly trigger MDSC expansion. To investigate this, a previously developed 3D in vitro SAC model was co-cultured with murine and human bone marrow cells. Subsequently, it was shown that SAC-exposed human CD11b^low/− myeloid cells or SAC-exposed murine CD11b⁺ Gr-1⁺ cells were immunosuppressive mainly by reducing absolute CD4⁺ and CD8α⁺ T cell numbers, as shown in T cell proliferation assays and with flow cytometry. Monocytic MDSCs from mice with an S. aureus bone infection also strongly reduced CD4⁺ and CD8α⁺ T cell numbers. Using protein biomarker analysis and an immunoassay, we detected in SAC–bone marrow co-cultures high levels of GM-CSF, IL-6, VEGF, IL-1β, TNFα, IL-10, and TGF-β. Furthermore, SAC-exposed neutrophils expressed Arg-1 and SAC-exposed monocytes expressed Arg-1 and iNOS, as shown via immunofluorescent stains. Overall, this study showed that SACs cause MDSC expansion from bone marrow cells and identified possible mediators to target as an additional strategy for treating chronic S. aureus infections.

An Enzybiotic Regimen for the Treatment of Methicillin-Resistant Staphylococcus aureus Orthopaedic Device-Related Infection

Orthopaedic device-related infection (ODRI) presents a significant challenge to the field of orthopaedic and trauma surgery. Despite extensive treatment involving surgical debridement and prolonged antibiotic therapy, outcomes remain poor. This is largely due to the unique abilities of Staphylococcus aureus, the most common causative agent of ODRI, to establish and protect itself within the host by forming biofilms on implanted devices and staphylococcal abscess communities (SACs). There is a need for novel antimicrobials that can readily target such features. Enzybiotics are a class of antimicrobial enzymes derived from bacteria and bacteriophages, which function by enzymatically degrading bacterial polymers essential to bacterial survival or biofilm formation. Here, we apply an enzybiotic-based combination regimen to a set of in vitro models as well as in a murine ODRI model to evaluate their usefulness in eradicating established S. aureus infection, compared to classical antibiotics. We show that two chimeric endolysins previously selected for their functional efficacy in human serum in combination with a polysaccharide depolymerase reduce bacterial CFU numbers 10,000-fold in a peg model and in an implant model of biofilm. The enzyme combination also completely eradicates S. aureus in a SAC in vitro model where classical antibiotics are ineffective. In an in vivo ODRI model in mice, the antibiofilm effects of this enzyme regimen are further enhanced when combined with a classical gentamicin/vancomycin treatment. In a mouse model of methicillin-resistant S. aureus (MRSA) ODRI following a fracture repair, a combined local enzybiotic/antibiotic treatment regimen showed a significant CFU reduction in the device and the surrounding soft tissue, as well as significant prevention of weight loss. These outcomes were superior to treatment with antibiotics alone. Overall, this study demonstrates that the addition of enzybiotics, which are distinguished by their extremely rapid killing efficacy and antibiofilm activities, can enhance the treatment of severe MRSA ODRI.

Staphylococcus aureus Cell Wall Biosynthesis Modulates Bone Invasion and Osteomyelitis Pathogenesis

Staphylococcus aureus invasion of the osteocyte lacuno-canalicular network (OLCN) is a novel mechanism of bacterial persistence and immune evasion in chronic osteomyelitis. Previous work highlighted S. aureus cell wall transpeptidase, penicillin binding protein 4 (PBP4), and surface adhesin, S. aureus surface protein C (SasC), as critical factors for bacterial deformation and propagation through nanopores in vitro, representative of the confined canaliculi in vivo. Given these findings, we hypothesized that cell wall synthesis machinery and surface adhesins enable durotaxis- and haptotaxis-guided invasion of the OLCN, respectively. Here, we investigated select S. aureus cell wall synthesis mutants (Δpbp3, Δatl, and ΔmreC) and surface adhesin mutants (ΔclfA and ΔsasC) for nanopore propagation in vitro and osteomyelitis pathogenesis in vivo. In vitro evaluation in the microfluidic silicon membrane-canalicular array (μSiM-CA) showed pbp3, atl, clfA, and sasC deletion reduced nanopore propagation. Using a murine model for implant-associated osteomyelitis, S. aureus cell wall synthesis proteins were found to be key modulators of S. aureus osteomyelitis pathogenesis, while surface adhesins had minimal effects. Specifically, deletion of pbp3 and atl decreased septic implant loosening and S. aureus abscess formation in the medullary cavity, while deletion of surface adhesins showed no significant differences. Further, peri-implant osteolysis, osteoclast activity, and receptor activator of nuclear factor kappa-B ligand (RANKL) production were decreased following pbp3 deletion. Most notably, transmission electron microscopy (TEM) imaging of infected bone showed that pbp3 was the only gene herein associated with decreased submicron invasion of canaliculi in vivo. Together, these results demonstrate that S. aureus cell wall synthesis enzymes are critical for OLCN invasion and osteomyelitis pathogenesis in vivo.

Streptomyces sp.-A Treasure Trove of Weapons to Combat Methicillin-Resistant Staphylococcus aureus Biofilm Associated with Biomedical Devices

Biofilms formed by methicillin-resistant S. aureus (MRSA) are among the most frequent causes of biomedical device-related infection, which are difficult to treat and are often persistent and recurrent. Thus, new and effective antibiofilm agents are urgently needed. In this article, we review the most relevant literature of the recent years reporting on promising anti-MRSA biofilm agents derived from the genus Streptomyces bacteria, and discuss the potential contribution of these newly reported antibiofilm compounds to the current strategies in preventing biofilm formation and eradicating pre-existing biofilms of the clinically important pathogen MRSA. Many efforts are evidenced to address biofilm-related infections, and some novel strategies have been developed and demonstrated encouraging results in preclinical studies. Nevertheless, more in vivo studies with appropriate biofilm models and well-designed multicenter clinical trials are needed to assess the prospects of these strategies.

A gut dysbiotic microbiota-based hypothesis of human-to-human transmission of non-communicable diseases

Non-communicable diseases (NCDs) have replaced communicable diseases as the leading cause of premature death worldwide over the past century. Increasing numbers of studies have reported a link between NCDs and dysbiotic gut microbiota. Some gut microbiota, such as Helicobacter pylori, have been implicated in person-to-person transmission. Based on these reports, we develop a hypothesis regarding dysbiotic microbiota-associated NCDs, and explore how the presence of communicable NCDs could be confirmedexperimentally. We have also reviewed reports on environmental factors, including a high-fat diet, alcohol, smoking, exercise, radiation and air pollution, which have been associated with dysbiotic microbiota, and determined whether any of these parameters were also associated with NCDs. This review discusses the potential mechanism by which dysbiotic microbiota induced by environmental factors are directly or indirectly involved in person-to-person transmission. The hypothetical interplay between the environment, gut microbiota and host can be tested through high-throughput sequencing, animal models, and cell studies, although each of these modalities presents specific challenges. Confirmation of a causative association of dysbiotic microbiota with NCDs would represent a paradigm shift in efforts to prevent and control these diseases, and should stimulate additional studies on the associations among environmental factors, gut microbiota, and NCDs.

Three-Dimensional In Vitro Staphylococcus aureus Abscess Communities Display Antibiotic Tolerance and Protection from Neutrophil Clearance

Staphylococcus aureus is a prominent human pathogen in bone and soft-tissue infections. Pathophysiology involves abscess formation, which consists of central staphylococcal abscess communities (SACs), surrounded by a fibrin pseudocapsule and infiltrating immune cells. Protection against the ingress of immune cells such as neutrophils, or tolerance to antibiotics, remains largely unknown for SACs and is limited by the lack of availability of in vitro models. We describe a three-dimensional in vitro model of SACs grown in a human plasma-supplemented collagen gel. The in vitro SACs reached their maximum size by 24 h and elaborated a fibrin pseudocapsule, as confirmed by electron and immunofluorescence microscopy. The in vitro SACs tolerated 100× the MIC of gentamicin alone and in combination with rifampin, while planktonic controls and mechanically dispersed SACs were efficiently killed. To simulate a host response, SACs were exposed to differentiated PLB-985 neutrophil-like (dPLB) cells and to primary human neutrophils at an early stage of SAC formation or after maturation at 24 h. Both cell types were unable to clear mature in vitro SACs, but dPLB cells prevented SAC growth upon early exposure before pseudocapsule maturation. Neutrophil exposure after plasmin pretreatment of the SACs resulted in a significant decrease in the number of bacteria within the SACs. The in vitro SAC model mimics key in vivo features, offers a new tool to study host-pathogen interactions and drug efficacy assessment, and has revealed the functionality of the S. aureus pseudocapsule in protecting the bacteria from host phagocytic responses and antibiotics.

Identification of Penicillin Binding Protein 4 (PBP4) as a critical factor for Staphylococcus aureus bone invasion during osteomyelitis in mice

Staphylococcus aureus infection of bone is challenging to treat because it colonizes the osteocyte lacuno-canalicular network (OLCN) of cortical bone. To elucidate factors involved in OLCN invasion and identify novel drug targets, we completed a hypothesis-driven screen of 24 S. aureus transposon insertion mutant strains for their ability to propagate through 0.5 μm-sized pores in the Microfluidic Silicon Membrane Canalicular Arrays (μSiM-CA), developed to model S. aureus invasion of the OLCN. This screen identified the uncanonical S. aureus transpeptidase, penicillin binding protein 4 (PBP4), as a necessary gene for S. aureus deformation and propagation through nanopores. In vivo studies revealed that Δpbp4 infected tibiae treated with vancomycin showed a significant 12-fold reduction in bacterial load compared to WT infected tibiae treated with vancomycin (p<0.05). Additionally, Δpbp4 infected tibiae displayed a remarkable decrease in pathogenic bone-loss at the implant site with and without vancomycin therapy. Most importantly, Δpbp4 S. aureus failed to invade and colonize the OLCN despite high bacterial loads on the implant and in adjacent tissues. Together, these results demonstrate that PBP4 is required for S. aureus colonization of the OLCN and suggest that inhibitors may be synergistic with standard of care antibiotics ineffective against bacteria within the OLCN.

Staphylococcus aureus Osteomyelitis: Bone, Bugs, and Surgery

Osteomyelitis, or inflammation of bone, is most commonly caused by invasion of bacterial pathogens into the skeleton. Bacterial osteomyelitis is notoriously difficult to treat, in part because of the widespread antimicrobial resistance in the preeminent etiologic agent, the Gram-positive bacterium Staphylococcus aureus Bacterial osteomyelitis triggers pathological bone remodeling, which in turn leads to sequestration of infectious foci from innate immune effectors and systemically delivered antimicrobials. Treatment of osteomyelitis therefore typically consists of long courses of antibiotics in conjunction with surgical debridement of necrotic infected tissues. Even with these extreme measures, many patients go on to develop chronic infection or sustain disease comorbidities. A better mechanistic understanding of how bacteria invade, survive within, and trigger pathological remodeling of bone could therefore lead to new therapies aimed at prevention or treatment of osteomyelitis as well as amelioration of disease morbidity. In this minireview, we highlight recent developments in our understanding of how pathogens invade and survive within bone, how bacterial infection or resulting innate immune responses trigger changes in bone remodeling, and how model systems can be leveraged to identify new therapeutic targets. We review the current state of osteomyelitis epidemiology, diagnostics, and therapeutic guidelines to help direct future research in bacterial pathogenesis.

Persistence of Moraxella catarrhalis in Chronic Obstructive Pulmonary Disease and Regulation of the Hag/MID Adhesin

BACKGROUND

Persistence of bacterial pathogens in the airways has profound consequences on the course and pathogenesis of chronic obstructive pulmonary disease (COPD). Patients with COPD continuously acquire and clear strains of Moraxella catarrhalis, a major pathogen in COPD. Some strains are cleared quickly and some persist for months to years. The mechanism of the variability in duration of persistence is unknown.

METHODS

Guided by genome sequences of selected strains, we studied the expression of Hag/MID, hag/mid gene sequences, adherence to human cells, and autoaggregation in longitudinally collected strains of M. catarrhalis from adults with COPD.

RESULTS

Twenty-eight of 30 cleared strains of M. catarrhalis expressed Hag/MID whereas 17 of 30 persistent strains expressed Hag/MID upon acquisition by patients. All persistent strains ceased expression of Hag/MID during persistence. Expression of Hag/MID in human airways was regulated by slipped-strand mispairing. Virulence-associated phenotypes (adherence to human respiratory epithelial cells and autoaggregation) paralleled Hag/MID expression in airway isolates.

CONCLUSIONS

Most strains of M. catarrhalis express Hag/MID upon acquisition by adults with COPD and all persistent strains shut off expression during persistence. These observations suggest that Hag/MID is important for initial colonization by M. catarrhalis and that cessation of expression facilitates persistence in COPD airways.

Mechanisms of Immune Evasion and Bone Tissue Colonization That Make Staphylococcus aureus the Primary Pathogen in Osteomyelitis

PURPOSE OF REVIEW

Staphylococcus aureus is the primary pathogen responsible for osteomyelitis, which remains a major healthcare burden. To understand its dominance, here we review the unique pathogenic mechanisms utilized by S. aureus that enable it to cause incurable osteomyelitis.

RECENT FINDINGS

Using an arsenal of toxins and virulence proteins, S. aureus kills and usurps immune cells during infection, to produce non-neutralizing pathogenic antibodies that thwart adaptive immunity. S. aureus also has specific mechanisms for distinct biofilm formation on implants, necrotic bone tissue, bone marrow, and within the osteocyte lacuno-canicular networks (OLCN) of live bone. In vitro studies have also demonstrated potential for intracellular colonization of osteocytes, osteoblasts, and osteoclasts. S. aureus has evolved a multitude of virulence mechanisms to achieve life-long infection of the bone, most notably colonization of OLCN. Targeting S. aureus proteins involved in these pathways could provide new targets for antibiotics and immunotherapies.

Plasma Fibrin Degradation Product and D-Dimer Are of Limited Value for Diagnosing Periprosthetic Joint Infection

BACKGROUND

Although the Musculoskeletal Infection Society introduced the use of erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) as inflammatory markers for diagnosing periprosthetic joint infection (PJI), no single blood marker reliably detects infection before revision arthroplasty. We therefore posed 2 questions: (1) Are fibrin degradation product (FDP) and D-dimer of value for diagnosing PJI before revision arthroplasty? (2) What are their sensitivity and specificity for that purpose?

METHODS

To answer these questions, we retrospectively enrolled 318 patients (129 with PJI [group A], 189 with aseptic mechanical failure [group B]) who underwent revision arthroplasty during 2013-2018. Receiver operating characteristic curves were used to determine maximum sensitivity and specificity of the 2 markers. Inflammatory and fibrinolytic markers were evaluated based on (1) the Tsukayama-type infection present and (2) the 3 most common PJI-related pathogens.

RESULTS

FDP and D-dimer levels were higher in group A than in group B: 4.97 ± 2.83 vs 4.14 ± 2.67 mg/L and 2.14 ± 2.01 vs 1.51 ± 1.37 mg/L fibrinogen equivalent units (FEU), respectively (both P < .05). Based on the Youden index, 2.95 mg/L and 1.02 mg/L FEU are the optimal FDP and D-dimer predictive cutoffs, respectively, for diagnosing PJI. Sensitivity and specificity, respectively, were 65.12% and 60.33% (FDP) and 68.29% and 50.70% (D-dimer). ESR, CRP, and interleukin-6 values were diagnostically superior to those of FDP and D-dimer.

CONCLUSION

The value of plasma FDP and D-dimer for diagnosing PJI is limited compared with traditional inflammatory markers (ESR, CRP, interleukin-6) before revision arthroplasty.

Evolving concepts in bone infection: redefining "biofilm", "acute vs. chronic osteomyelitis", "the immune proteome" and "local antibiotic therapy"

Osteomyelitis is a devastating disease caused by microbial infection of bone. While the frequency of infection following elective orthopedic surgery is low, rates of reinfection are disturbingly high. Staphylococcus aureus is responsible for the majority of chronic osteomyelitis cases and is often considered to be incurable due to bacterial persistence deep within bone. Unfortunately, there is no consensus on clinical classifications of osteomyelitis and the ensuing treatment algorithm. Given the high patient morbidity, mortality, and economic burden caused by osteomyelitis, it is important to elucidate mechanisms of bone infection to inform novel strategies for prevention and curative treatment. Recent discoveries in this field have identified three distinct reservoirs of bacterial biofilm including: Staphylococcal abscess communities in the local soft tissue and bone marrow, glycocalyx formation on implant hardware and necrotic tissue, and colonization of the osteocyte-lacuno canalicular network (OLCN) of cortical bone. In contrast, S. aureus intracellular persistence in bone cells has not been substantiated in vivo, which challenges this mode of chronic osteomyelitis. There have also been major advances in our understanding of the immune proteome against S. aureus, from clinical studies of serum antibodies and media enriched for newly synthesized antibodies (MENSA), which may provide new opportunities for osteomyelitis diagnosis, prognosis, and vaccine development. Finally, novel therapies such as antimicrobial implant coatings and antibiotic impregnated 3D-printed scaffolds represent promising strategies for preventing and managing this devastating disease. Here, we review these recent advances and highlight translational opportunities towards a cure.

Lactic Acid Produced by Glycolysis Contributed to Staphylococcus aureus Aggregation Induced by Glucose

High concentration of glucose induces Staphylococcus aureus (S. aureus) aggregation, but the mechanism of this is still unclear. In this study, the aggregation of S. aureus strains was induced by high concentration of glucose (>7.8 mM), and which was dose- and time-dependent. In addition, the large amount of lactate acid produced during S. aureus aggregation, induced by glucose, resulted in decreased pH value. Lactic acid, the end product of glycolysis, could quickly induce S. aureus aggregation. Except for lactic acid, acetic acid and HCl also induced S. aureus aggregation. In addition, the aggregation of S. aureus strains induced by glucose or lactic acid was completely inhibited in Tris-HCl buffer (pH 7.5), and inhibition of glycolysis by 2-deoxyglucose significantly decreased S. aureus aggregation. The aggregation induced by glucose was dispersed by periodate and proteinase K. In summary, lactate acid produced by glycolysis contributed to S. aureus aggregation induced by high concentration of glucose.

Recommendations for design and conduct of preclinical in vivo studies of orthopedic device-related infection

Orthopedic device-related infection (ODRI), including both fracture-related infection (FRI) and periprosthetic joint infection (PJI), remain among the most challenging complications in orthopedic and musculoskeletal trauma surgery. ODRI has been convincingly shown to delay healing, worsen functional outcome and incur significant socio-economic costs. To address this clinical problem, ever more sophisticated technologies targeting the prevention and/or treatment of ODRI are being developed and tested in vitro and in vivo. Among the most commonly described innovations are antimicrobial-coated orthopedic devices, antimicrobial-loaded bone cements and void fillers, and dual osteo-inductive/antimicrobial biomaterials. Unfortunately, translation of these technologies to the clinic has been limited, at least partially due to the challenging and still evolving regulatory environment for antimicrobial drug-device combination products, and a lack of clarity in the burden of proof required in preclinical studies. Preclinical in vivo testing (i.e. animal studies) represents a critical phase of the multidisciplinary effort to design, produce and reliably test both safety and efficacy of any new antimicrobial device. Nonetheless, current in vivo testing protocols, procedures, models, and assessments are highly disparate, irregularly conducted and reported, and without standardization and validation. The purpose of the present opinion piece is to discuss best practices in preclinical in vivo testing of antimicrobial interventions targeting ODRI. By sharing these experience-driven views, we aim to aid others in conducting such studies both for fundamental biomedical research, but also for regulatory and clinical evaluation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:271-287, 2019.

Staphylococcus aureus Evasion of Host Immunity in the Setting of Prosthetic Joint Infection: Biofilm and Beyond

PURPOSE OF REVIEW

The incidence of complications from prosthetic joint infection (PJI) is increasing, and treatment failure remains high. We review the current literature with a focus on Staphylococcus aureus pathogenesis and biofilm, as well as treatment challenges, and novel therapeutic strategies.

RECENT FINDINGS

S. aureus biofilm creates a favorable environment that increases antibiotic resistance, impairs host immunity, and increases tolerance to nutritional deprivation. Secreted proteins from bacterial cells within the biofilm and the quorum-sensing agr system contribute to immune evasion. Additional immunoevasive properties of S. aureus include the formation of staphylococcal abscess communities (SACs) and canalicular invasion. Novel approaches to target biofilm and increase resistance to implant colonization include novel antibiotic therapy, immunotherapy, and local implant treatments. Challenges remain given the diverse mechanisms developed by S. aureus to alter the host immune responses. Further understanding of these processes should provide novel therapeutic mechanisms to enhance eradication after PJI.

Obesity/type 2 diabetes increases inflammation, periosteal reactive bone formation, and osteolysis during Staphylococcus aureus implant-associated bone infection

Obese and type 2 diabetic (T2D) patients have a fivefold increased rate of infection following placement of an indwelling orthopaedic device. Though implant infections are associated with inflammation, periosteal reactive bone formation, and osteolysis, the effect of obesity/T2D on these complicating factors has not been studied. To address this question, C57BL/6J mice were fed a high fat diet (60% Kcal from fat) to induce obesity/T2D, or a control diet (10% Kcal from fat) for 3 months, and challenged with a transtibial pin coated with a bioluminescent USA300 strain of S. aureus. In the resulting infected bone, obesity/T2D was associated with increased S. aureus proliferation and colony forming units. RNA sequencing of the infected tibiae on days 7 and 14 revealed an increase in 635 genes in obese/T2D mice relative to controls. Pathways associated with ossification, angiogenesis, and immunity were enriched. MicroCT and histology on days 21 and 35 demonstrated significant increased periosteal reactive bone formation in infected obese/T2D mice versus infected controls (p < 0.05). The enhanced periosteal bone formation was associated with increased osteoblastic activity and robust endochondral ossification, with persistant cartilage on day 21 that was only observed in infected obesity/T2D. Osteolysis and osteoclast numbers in obesity/T2D were also significantly increased versus infected controls (p < 0.05). Consistent with an up-regulated immune transcriptome, macrophages were more abundant within both the periosteum and the new reactive bone of obese/T2D mice. In conclusion, we find that implant-associated S. aureus osteomyelitis in obesity/T2D is associated with increased inflammation, reactive bone formation, and osteolysis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1614-1623, 2018.

Exacerbated Staphylococcus aureus Foot Infections in Obese/Diabetic Mice Are Associated with Impaired Germinal Center Reactions, Ig Class Switching, and Humoral Immunity

Obese patients with type 2 diabetes (T2D) are at an increased risk of foot infection, with impaired immune function believed to be a critical factor in the infectious process. In this study, we test the hypothesis that humoral immune defects contribute to exacerbated foot infection in a murine model of obesity/T2D. C57BL/6J mice were rendered obese and T2D by a high-fat diet for 3 mo and were compared with controls receiving a low-fat diet. Following injection of Staphylococcus aureus into the footpad, obese/T2D mice had greater foot swelling and reduced S. aureus clearance than controls. Obese/T2D mice also had impaired humoral immune responses as indicated by lower total IgG levels and lower anti-S. aureus Ab production. Within the draining popliteal lymph nodes of obese/T2D mice, germinal center formation was reduced, and the percentage of germinal center T and B cells was decreased by 40-50%. Activation of both T and B lymphocytes was similarly suppressed in obese/T2D mice. Impaired humoral immunity in obesity/T2D was independent of active S. aureus infection, as a similarly impaired humoral immune response was demonstrated when mice were administered an S. aureus digest. Isolated splenic B cells from obese/T2D mice activated normally but had markedly suppressed expression of Aicda, with diminished IgG and IgE responses. These results demonstrate impaired humoral immune responses in obesity/T2D, including B cell-specific defects in Ab production and class-switch recombination. Together, the defects in humoral immunity may contribute to the increased risk of foot infection in obese/T2D patients.