PSMs of hypervirulent Staphylococcus aureus act as intracellular toxins that kill infected osteoblasts

Structure of biofilm-forming functional amyloid PSMα1 from Staphylococcus aureus

Biofilm-protected pathogenic Staphylococcus aureus causes chronic infections that are difficult to treat. An essential building block of these biofilms are functional amyloid fibrils that assemble from phenol-soluble modulins (PSMs). PSMα1 cross-seeds other PSMs into cross-β amyloid folds and is therefore a key element in initiating biofilm formation. However, the paucity of high-resolution structures hinders efforts to prevent amyloid assembly and biofilm formation. Here, we present a 3.5 Å resolution density map of the major PSMα1 fibril form revealing a left-handed cross-β fibril composed of two C2-symmetric U-shaped protofilaments whose subunits are unusually tilted out-of-plane. Monomeric α-helical PSMα1 is extremely cytotoxic to cells, despite the moderate toxicity of the cross-β fibril. We suggest mechanistic insights into the PSM functional amyloid formation and conformation transformation on the path from monomer-to-fibril formation. Details of PSMα1 assembly and fibril polymorphism suggest how S. aureus utilizes functional amyloids to form biofilms and establish a framework for developing therapeutics against infection and antimicrobial resistance.

Non-biofilm-forming Staphylococcus epidermidis planktonic cell supernatant induces alterations in osteoblast biological function

Staphylococcal biofilms significantly contribute to prosthetic joint infection (PJI). However, 40% of S. epidermidis PJI isolates do not produce biofilms, which does not explain the role of biofilms in these cases. We studied whether the supernatant from planktonic S. epidermidis alters osteoblast function. Non-biofilm-forming S. epidermidis supernatants (PJI- clinical isolate, healthy skin isolate (HS), and ATCC12228 reference strain) and biofilm-forming supernatants (PJI+ clinical isolate, ATCC35984 reference strain, and Staphylococcus aureus USA300 reference strain) were included. Osteoblasts stimulated with supernatants from non-biofilm-forming isolates for 3, 7, and 14 days showed significantly reduced cellular DNA content compared with unstimulated osteoblasts, and apoptosis was induced in these osteoblasts. Similar results were obtained for biofilm-forming isolates, but with a greater reduction in DNA content and higher apoptosis. Alkaline phosphatase activity and mineralization were significantly reduced in osteoblasts treated with supernatants from non-biofilm-forming isolates compared to the control at the same time points. However, the supernatants from biofilm-forming isolates had a greater effect than those from non-biofilm-forming isolates. A significant decrease in the expression of ATF4, RUNX2, ALP, SPARC, and BGLAP, and a significant increase in RANK-L expression were observed in osteoblasts treated with both supernatants. These results demonstrate that the supernatants of the S. epidermidis isolate from the PJI- and HS (commensal) with a non-biofilm-forming phenotype alter the function of osteoblasts (apoptosis induction, failure of cell differentiation, activation of osteoblasts, and induction of bone resorption), similar to biofilm-forming isolates (PJI+, ATCC35984, and S. aureus USA300), suggesting that biofilm status contributes to impaired osteoblast function and that the planktonic state can do so independently of biofilm production.

Staphylococcus aureus-derived virulent phenol-soluble modulin α triggers alarmin release to drive IL-36-dependent corneal inflammation

Methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients with keratitis produces substantial amounts of phenol-soluble modulin α (PSMα). However, the role of PSMα in S. aureus keratitis remains unclear. We observed that PSMα-producing and PSMα-deficient strains could infect the cornea in our experimental mouse keratitis model; however, only the PSMα-producing strain delayed epithelial wound healing and induced stromal inflammation. PSMα induced damage to the epithelium, the release of alarmins IL-1α and IL-36α, and the expression of inflammatory chemokines by resident corneal cells in the mouse corneal organ culture. The IL-36 (but not IL-1) receptor antagonist attenuated mouse keratitis induced by PSMα-containing bacterial culture supernatants, as well as by infection with PSMα-producing S. aureus, suggesting that the corneal inflammations were dependent on IL-36. Recombinant PSMα elicited IL-36-dependent corneal inflammation in mice. Thus, PSMα and the subsequently released IL-36 are critical factors triggering inflammation during S. aureus keratitis.

Molecular Characteristics and Pathogenicity of Staphylococcus aureus Exotoxins

Staphylococcus aureus stands as one of the most pervasive pathogens given its morbidity and mortality worldwide due to its roles as an infectious agent that causes a wide variety of diseases ranging from moderately severe skin infections to fatal pneumonia and sepsis. S. aureus produces a variety of exotoxins that serve as important virulence factors in S. aureus-related infectious diseases and food poisoning in both humans and animals. For example, staphylococcal enterotoxins (SEs) produced by S. aureus induce staphylococcal foodborne poisoning; toxic shock syndrome toxin-1 (TSST-1), as a typical superantigen, induces toxic shock syndrome; hemolysins induce cell damage in erythrocytes and leukocytes; and exfoliative toxin induces staphylococcal skin scalded syndrome. Recently, Panton-Valentine leucocidin, a cytotoxin produced by community-associated methicillin-resistant S. aureus (CA-MRSA), has been reported, and new types of SEs and staphylococcal enterotoxin-like toxins (SEls) were discovered and reported successively. This review addresses the progress of and novel insights into the molecular structure, biological activities, and pathogenicity of both the classic and the newly identified exotoxins produced by S. aureus.

Solving polymicrobial puzzles: evolutionary dynamics and future directions

Polymicrobial infections include various microorganisms, often necessitating different treatment methods than a monomicrobial infection. Scientists have been puzzled by the complex interactions within these communities for generations. The presence of specific microorganisms warrants a chronic infection and impacts crucial factors such as virulence and antibiotic susceptibility. Game theory is valuable for scenarios involving multiple decision-makers, but its relevance to polymicrobial infections is limited. Eco-evolutionary dynamics introduce causation for multiple proteomic interactions like metabolic syntropy and niche segregation. The review culminates both these giants to form evolutionary dynamics (ED). There is a significant amount of literature on inter-bacterial interactions that remain unsynchronised. Such raw data can only be moulded by analysing the ED involved. The review culminates the inter-bacterial interactions in multiple clinically relevant polymicrobial infections like chronic wounds, CAUTI, otitis media and dental carries. The data is further moulded with ED to analyse the niche colonisation of two notoriously competitive bacteria: S.aureus and P.aeruginosa. The review attempts to develop a future trajectory for polymicrobial research by following recent innovative strategies incorporating ED to curb polymicrobial infections.

Phenol-Soluble Modulin α3 Stimulates Autophagy in HaCaT Keratinocytes

BACKGROUND

Phenol-soluble modulins (PSMs) are pore-forming toxins (PFTs) produced by staphylococci. PSMs exert diverse cellular effects, including lytic, pro-apoptotic, pro-inflammatory and antimicrobial actions. Since the effects of PSMs on autophagy have not yet been reported, we evaluated the autophagic activity in HaCaT keratinocytes treated with recombinant PSMα3.

METHODS

The autophagic flux and levels of autophagic marker proteins were determined using Western blot analysis. Subcellular localization of LC3B and Beclin-1 was investigated using an indirect immunofluorescence assay. The ultrastructural features of control and PSMα3-treated cells were evaluated via transmission electron microscopy. Cytoplasmic acidification was measured via acridine orange staining. Phosphorylation levels of protein kinases, implicated in autophagy regulation, were studied using a phospho-kinase array and Western blot analysis.

RESULTS

PSMα3 facilitated the intracellular redistribution of LC3B, increased the average number of autophagosomes per cell, promoted the development of acidic vesicular organelles, elevated the levels of LC3B-II, stimulated autophagic flux and triggered a significant decrease in the net autophagic turnover rate. PSMα3 induced the accumulation of autophagosomes/autolysosomes, amphisomes and multilamellar bodies at the 0.5, 6 and 24 h time points, respectively. The phospho-Akt1/2/3 (T308 and S473), and phospho-mTOR (S2448) levels were decreased, whereas the phospho-Erk1/2 (T202/Y204 and T185/Y187) level was increased in PSMα3-treated cells.

CONCLUSIONS

In HaCaT keratinocytes, PSMα3 stimulates autophagy. The increased autophagic activity elicited by sub-lytic PSM concentrations might be an integral part of the cellular defense mechanisms protecting skin homeostasis.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Quinoxaline-based membrane-targeting therapeutic material: Implications in rejuvenating antibiotic and curb MRSA invasion in an in vitro bone cell infection model

Manifestation of resistance in methicillin-resistant Staphylococcus aureus (MRSA) against multiple antibiotics demands an effective strategy to counter the menace of the pathogen. To address this challenge, the current study explores quinoxaline-based synthetic ligands as an adjuvant material to target MRSA in a combination therapy regimen. Amongst the tested ligands (C1-C4), only C2 was bactericidal against the MRSA strain S. aureus 4 s, with a minimum inhibitory concentration (MIC) of 32 μM. C2 displayed a membrane-directed activity and could effectively hinder MRSA biofilm formation. A quantitative real-time polymerase chain reaction (qRT-PCR) analysis indicated that C2 downregulated expression of the regulator gene agrC and reduced the fold change in the expression of adhesin genes fnbA and cnbA in MRSA in a dose-dependent manner. C2 enabled a 4-fold reduction in the MIC of ciprofloxacin (CPX) and in presence of 10 μM C2 and 8.0 μM CPX, growth of MRSA was arrested. Furthermore, a combination of 10 μM C2 and 12 μM CPX could strongly inhibit MRSA biofilm formation and reduce biofilm metabolic activity. The minimum biofilm inhibitory concentration (MBIC) of CPX against S. aureus 4 s biofilm was reduced and a synergy resulted between C2 and CPX. In a combinatorial treatment regimen, C2 could prevent emergence of CPX resistance and arrest growth of MRSA till 360 generations. C2 could also be leveraged in combination treatment (12 μM CPX and 10 μM C2) to target MRSA in an in vitro bone cell infection model, wherein MRSA cell adhesion and invasion onto cultured MG-63 cells was only ~17 % and ~ 0.37 %, respectively. The combinatorial treatment regimen was also biocompatible as the viability of MG-63 cells was high (~ 91 %). Thus, C2 is a promising adjuvant material to counter antibiotic-refractory therapy and mitigate MRSA-mediated bone cell infection.

Genotypic and Phenotypic Characterization of Some psms Hypervirulent Clinical Isolates of Staphylococcus aureus in a Tertiary Hospital in Hefei, Anhui

Background

Staphylococcus aureus is a highly successful pathogen that can cause various infectious diseases, from relatively mild skin infections to life-threatening severe systemic diseases. The widespread pathogenicity of S. aureus is mainly due to its ability to produce many virulence factors that help destroy various host cells, causing disease. Our primary goal in this study was to explore the genes of highly virulent strains, to identify genes closely associated with high virulence, and to provide ideas for the treatment of infection by highly virulent clinical strains.

Results

This study collected 221 clinical strains from The First Affiliated Hospital Of The University of Science and Technology of China (USTC); their hemolytic abilities were tested. Eight isolates were selected based on their highly hemolytic ability and tested their hemolytic activity again; their phenotypes and gene sequences were also explored. Whole-genome sequencing (WGS) showed six plasmids (pN315, pNE131, pSJH901, pSJH101, SAP106B, and MSSA476), eight antibiotic resistance genes [blaR1, blaI, blaZ, mecA, erm(C), erm(T), tet(38), and fosB-Saur] and seventy-two virulence related genes. Three highly virulent strains, namely X21111206, 21092239, and 21112607, were found according the Galleria mellonella infection model. Therefore, we selected 10 representative virulence genes for qRT-PCR: psmα, psmβ, hlgA, hlgB, hlgC, hla, clfA, clfB, spa, and sak. Among them, the expression levels of psmα and psmβ, the three isolates, were significantly higher than the positive control NCTC8325.

Conclusion

Significant differences appear in the expression of virulence genes in the highly virulent strains, particularly the psmα and psmβ, It may be that the high expression of psm gene is the cause of the high virulence of Staphylococcus aureus. We can reduce the pathogenicity of Staphylococcus aureus by inhibiting the expression of psm gene, which may provide a strong basis for psm as a new target for clinical treatment of S. aureus infection.

In Silico Genome-Scale Analysis of Molecular Mechanisms Contributing to the Development of a Persistent Infection with Methicillin-Resistant Staphylococcus aureus (MRSA) ST239

The increasing frequency of isolation of methicillin-resistant Staphylococcus aureus (MRSA) limits the chances for the effective antibacterial therapy of staphylococcal diseases and results in the development of persistent infection such as bacteremia and osteomyelitis. The aim of this study was to identify features of the MRSA_ST239 0943-1505-2016 (SA943) genome that contribute to the formation of both acute and chronic musculoskeletal infections. The analysis was performed using comparative genomics data of the dominant epidemic S. aureus lineages, namely ST1, ST8, ST30, ST36, and ST239. The SA943 genome encodes proteins that provide resistance to the host's immune system, suppress immunological memory, and form biofilms. The molecular mechanisms of adaptation responsible for the development of persistent infection were as follows: amino acid substitution in PBP2 and PBP2a, providing resistance to ceftaroline; loss of a large part of prophage DNA and restoration of the nucleotide sequence of beta-hemolysin, that greatly facilitates the escape of phagocytosed bacteria from the phagosome and formation of biofilms; dysfunction of the AgrA system due to the presence of psm-mec and several amino acid substitutions in the AgrC; partial deletion of the nucleotide sequence in genomic island vSAβ resulting in the loss of two proteases of Spl-operon; and deletion of SD repeats in the SdrE amino acid sequence.

Dual-functional composite scaffolds for inhibiting infection and promoting bone regeneration

The treatment of infected bone defects is an intractable problem in orthopedics. It comprises two critical parts, namely that of infection control and bone defect repair. According to these two core tasks during treatment, the ideal approach of simultaneously controlling infection and repairing bone defects is promising treatment strategy. Several engineered biomaterials and drug delivery systems with dual functions of anti-bacterial action and ostogenesis-promotion have been developed and demonstrated excellent therapeutic effects. Compared with the conventional treatment method, the dual-functional composite scaffold can provide one-stage treatment avoiding multiple surgeries, thereby remarkably simplifying the treatment process and reducing the treatment time, overcoming the disadvantages of conventional bone transplantation. In this review, the impaired bone repair ability and its specific mechanisms in the microenvironment of pathogen infection and excessive inflammation were analyzed, providing a theoretical basis for the treatment of infectious bone defects. Furthermore, we discussed the composite dual-functional scaffold composed of a combination of antibacterial and osteogenic material. Finally, a series of advanced drug delivery systems with antibacterial and bone-promoting capabilities were summarized and discussed. This review provides a comprehensive understanding for the microenvironment of infectious bone defects and leading-edge design strategies for the antibacterial and bone-promoting dual-function scaffold, thus providing clinically significant treatment methods for infectious bone defects.

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Antibacterial and bone-promoting dual-function scaffolds are ideal strategies for treatment of infectious bone defects.

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The effect of infection on bone repair was summarized in detail from four important aspects.

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A variety of dual-function scaffolds based on antibacterial and osteogenic materials were discussed.

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Dual-function drug delivery systems promoting repair of infectious bone defects by locally releasing functional agents.

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Leading-edge design strategies, challenges and prospects for dual-functional biomaterials were provided.

Human Osteoblast-Conditioned Media Can Influence Staphylococcus aureus Biofilm Formation

Osteoblasts are bone-forming and highly active cells participating in bone homeostasis. In the case of osteomyelitis and more specifically prosthetic joint infections (PJI) for which Staphylococcus aureus (S. aureus) is mainly involved, the interaction between osteoblasts and S. aureus results in impaired bone homeostasis. If, so far, most of the studies of osteoblasts and S. aureus interactions were focused on osteoblast response following direct interactions with co-culture and/or internalization models, less is known about the effect of osteoblast factors on S. aureus biofilm formation. In the present study, we investigated the effect of human osteoblast culture supernatant on methicillin sensitive S. aureus (MSSA) SH1000 and methicillin resistant S. aureus (MRSA) USA300. Firstly, Saos-2 cell line was incubated with either medium containing TNF-α to mimic the inflammatory periprosthetic environment or with regular medium. Biofilm biomass was slightly increased for both strains in the presence of culture supernatant collected from Saos-2 cells, stimulated or not with TNF-α. In such conditions, SH1000 was able to develop microcolonies, suggesting a rearrangement in biofilm organization. However, the biofilm matrix and regulation of genes dedicated to biofilm formation were not substantially changed. Secondly, culture supernatant obtained from primary osteoblast culture induced varied response from SH1000 strain depending on the different donors tested, whereas USA300 was only slightly affected. This suggested that the sensitivity to bone cell secretions is strain dependent. Our results have shown the impact of osteoblast secretions on bacteria and further identification of involved factors will help to manage PJI.

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.

Functional amyloids from bacterial biofilms - structural properties and interaction partners

Protein aggregation and amyloid formation have historically been linked with various diseases such as Alzheimer's and Parkinson's disease, but recently functional amyloids have gained a great deal of interest in not causing a disease and having a distinct function in vivo. Functional bacterial amyloids form the structural scaffold in bacterial biofilms and provide a survival strategy for the bacteria along with antibiotic resistance. The formation of functional amyloids happens extracellularly which differs from most disease related amyloids. Studies of functional amyloids have revealed several distinctions compared to disease related amyloids including primary structures designed to optimize amyloid formation while still retaining a controlled assembly of the individual subunits into classical cross-β-sheet structures, along with a unique cross-α-sheet amyloid fold. Studies have revealed that functional amyloids interact with components found in the extracellular matrix space such as lipids from membranes and polymers from the biofilm. Intriguingly, a level of complexity is added as functional amyloids also interact with several disease related amyloids and a causative link has even been established between functional amyloids and neurodegenerative diseases. It is hence becoming increasingly clear that functional amyloids are not inert protein structures found in bacterial biofilms but interact with many different components including human proteins related to pathology. Gaining a clear understanding of the factors governing the interactions will lead to improved strategies to combat biofilm associated infections and the correlated antibiotic resistance. In the current review we summarize the current state of the art knowledge on this exciting and fast growing research field of biofilm forming bacterial functional amyloids, their structural features and interaction partners.

Osteomyelitis, Oxidative Stress and Related Biomarkers

Bone is a very dynamic tissue, subject to continuous renewal to maintain homeostasis through bone remodeling, a process promoted by two cell types: osteoblasts, of mesenchymal derivation, are responsible for the deposition of new material, and osteoclasts, which are hematopoietic cells, responsible for bone resorption. Osteomyelitis (OM) is an invasive infectious process, with several etiological agents, the most common being Staphylococcus aureus, affecting bone or bone marrow, and severely impairing bone homeostasis, resulting in osteolysis. One of the characteristic features of OM is a strong state of oxidative stress (OS) with severe consequences on the delicate balance between osteoblastogenesis and osteoclastogenesis. Here we describe this, analyzing the effects of OS in bone remodeling and discussing the need for new, easy-to-measure and widely available OS biomarkers that will provide valid support in the management of the disease.

In vitro antibiotic activity against intraosteoblastic Staphylococcus aureus: a narrative review of the literature

Staphylococcus aureus – a major aetiological agent of bone and joint infection (BJI) – is associated with a high risk of relapse and chronicity, in part due to its ability to invade and persist in non-professional phagocytic bone cells such as osteoblasts. This intracellular reservoir protects S. aureus from the action of the immune system and most antibiotics. To date, the choice of antimicrobial strategies for BJI treatment mostly relies on standard susceptibility testing, bone penetration of antibiotics and their ‘antibiofilm’ activity. Despite the role of intracellular persistent S. aureus in the development of chronic infection, the ability of antibiotics to target the S. aureus intraosteoblastic reservoir is not considered in therapeutic choices but might represent a key determinant of treatment outcome. This review provides an overview of the intracellular pharmacokinetics of antistaphylococcal drugs used in the treatment of BJI and of their ability to target intraosteoblastic S. aureus. Thirteen studies focusing on the intraosteoblastic activity of antibiotics against S. aureus were reviewed, all relying on in vitro models of osteoblast infection. Despite varying incubation times, multiplicities of infection, bacterial strains, and the types of infected cell lines, rifamycins and fluoroquinolones remain the two most potent antimicrobial classes for intraosteoblastic S. aureus eradication, consistent with clinical data showing a superiority of this combination therapy in S. aureus orthopaedic device-related infections.

Intracellular Staphylococcus aureus employs the cysteine protease staphopain A to induce host cell death in epithelial cells

Staphylococcus aureus is a major human pathogen, which can invade and survive in non-professional and professional phagocytes. Uptake by host cells is thought to contribute to pathogenicity and persistence of the bacterium. Upon internalization by epithelial cells, cytotoxic S. aureus strains can escape from the phagosome, replicate in the cytosol and induce host cell death. Here, we identified a staphylococcal cysteine protease to induce cell death after translocation of intracellular S. aureus into the host cell cytoplasm. We demonstrated that loss of staphopain A function leads to delayed onset of host cell death and prolonged intracellular replication of S. aureus in epithelial cells. Overexpression of staphopain A in a non-cytotoxic strain facilitated intracellular killing of the host cell even in the absence of detectable intracellular replication. Moreover, staphopain A contributed to efficient colonization of the lung in a mouse pneumonia model. In phagocytic cells, where intracellular S. aureus is exclusively localized in the phagosome, staphopain A did not contribute to cytotoxicity. Our study suggests that staphopain A is utilized by S. aureus to exit the epithelial host cell and thus contributes to tissue destruction and dissemination of infection.

Staphylococcus aureus is an antibiotic-resistant pathogen that emerges in hospital and community settings and can cause a variety of diseases ranging from skin abscesses to lung inflammation and blood poisoning. The bacterium can asymptomatically colonize the upper respiratory tract and skin of humans and take advantage of opportune conditions, like immunodeficiency or breached barriers, to cause infection. Although S. aureus was not regarded as intracellular bacterium, it can be internalized by human cells and subsequently exit the host cells by induction of cell death, which is considered to cause tissue destruction and spread of infection. The bacterial virulence factors and underlying molecular mechanisms involved in the intracellular lifestyle of S. aureus remain largely unknown. We identified a bacterial cysteine protease to contribute to host cell death of epithelial cells mediated by intracellular S. aureus. Staphopain A induced killing of the host cell after translocation of the pathogen into the cell cytosol, while bacterial proliferation was not required. Further, the protease enhanced survival of the pathogen during lung infection. These findings reveal a novel, intracellular role for the bacterial protease staphopain A.

Staphylococcus aureus internalization impairs osteoblastic activity and early differentiation process

Staphylococcus aureus is the most frequent aetiology of bone and joint infections (BJI) and can cause relapsing and chronic infections. One of the main factors involved in the chronicization of staphylococcal BJIs is the internalization of S. aureus into osteoblasts, the bone-forming cells. Previous studies have shown that S. aureus triggers an impairment of osteoblasts function that could contribute to bone loss. However, these studies focused mainly on the extracellular effects of S. aureus. Our study aimed at understanding the intracellular effects of S. aureus on the early osteoblast differentiation process. In our in vitro model of osteoblast lineage infection, we first observed that internalized S. aureus 8325-4 (a reference lab strain) significantly impacted RUNX2 and COL1A1 expression compared to its non-internalized counterpart 8325-4^∆fnbAB (with deletion of fnbA and fnbB). Then, in a murine model of osteomyelitis, we reported no significant effect for S. aureus 8325-4 and 8325-4^∆fnbAB on bone parameters at 7 days post-infection whereas S. aureus 8325-4 significantly decreased trabecular bone thickness at 14 days post-infection compared to 8325-4^∆fnbAB. When challenged with two clinical isogenic strains isolated from initial and relapse phase of the same BJI, significant impairments of bone parameters were observed for both initial and relapse strain, without differences between the two strains. Finally, in our in vitro osteoblast infection model, both clinical strains impacted alkaline phosphatase activity whereas the expression of bone differentiation genes was significantly decreased only after infection with the relapse strain. Globally, we highlighted that S. aureus internalization into osteoblasts is responsible for an impairment of the early differentiation in vitro and that S. aureus impaired bone parameters in vivo in a strain-dependent manner.

Quorum-sensing regulation of virulence factors in bacterial biofilm

Chronic polymicrobial wound infections are often characterized by the presence of bacterial biofilms. They show considerable structural and functional heterogeneity, which influences the choice of antimicrobial therapy and wound healing dynamics. The hallmarks of biofilm-associated bacterial infections include elevated antibiotic resistance and extreme pathogenicity. Biofilm helps bacteria to evade the host defense mechanisms and persist longer in the host. Quorum-sensing (QS)-mediated cell signaling primarily regulates biofilm formation in chronic infections and plays a major role in eliciting virulence. This review focuses on the QS mechanisms of two major bacterial pathogens, Staphylococcus aureus and Pseudomonas aeruginosa and explains how they interact in the wound microenvironment to regulate biofilm development and virulence. The review also provides an insight into the treatment modalities aimed at eradicating polymicrobial biofilms. This information will help us develop better diagnostic modalities and devise effective treatment regimens to successfully manage and overcome severe life-threatening bacterial infections.

3D Cocultures of Osteoblasts and Staphylococcus aureus on Biomimetic Bone Scaffolds as a Tool to Investigate the Host-Pathogen Interface in Osteomyelitis

Osteomyelitis (OM) is an infectious disease of the bone primarily caused by the opportunistic pathogen Staphylococcus aureus (SA). This Gram-positive bacterium has evolved a number of strategies to evade the immune response and subvert bone homeostasis, yet the underlying mechanisms remain poorly understood. OM has been modeled in vitro to challenge pathogenetic hypotheses in controlled conditions, thus providing guidance and support to animal experimentation. In this regard, traditional 2D models of OM inherently lack the spatial complexity of bone architecture. Three-dimensional models of the disease overcome this limitation; however, they poorly reproduce composition and texture of the natural bone. Here, we developed a new 3D model of OM based on cocultures of SA and murine osteoblastic MC3T3-E1 cells on magnesium-doped hydroxyapatite/collagen I (MgHA/Col) scaffolds that closely recapitulate the bone extracellular matrix. In this model, matrix-dependent effects were observed in proliferation, gene transcription, protein expression, and cell–matrix interactions both of the osteoblastic cell line and of bacterium. Additionally, these had distinct metabolic and gene expression profiles, compared to conventional 2D settings, when grown on MgHA/Col scaffolds in separate monocultures. Our study points to MgHA/Col scaffolds as biocompatible and bioactive matrices and provides a novel and close-to-physiology tool to address the pathogenetic mechanisms of OM at the host–pathogen interface.

Validation of the Osteomyelitis Induced by Methicillin-Resistant Staphylococcus aureus (MRSA) on Rat Model with Calvaria Defect

BACKGROUND

Osteomyelitis resulting from bacterial strains, such as methicillin-resistant Staphylococcus aureus (MRSA) that are resistant to multiple drugs, brings further clinical challenges. There is currently no model of osteomyelitis induced by MRSA using rats with calvaria defects. So, We induced osteomyelitis in rat models with the calvaria bone defect.

METHODS

The rats were randomly divided into six groups according to inoculation dose levels, which ranged from 6 × 10⁰ to 6 × 10⁵ CFU/5 µl. Bone tissues were retrieved from all rats used in the study and assessed using histology, microbiology, and radiobiology 4 weeks after surgery to evaluate the relationship between inoculation dose and infectivity.

RESULTS

In Histological results, high levels of inflammatory responses, bone necrosis, and bacteria were observed in treatment groups G3 to G5. In IHC staining, high levels of cox-2 expression were observed in treatment groups G3. Microbiological observations also indicated that significantly higher numbers of CFUs were found in G3 to G5. In radiography results, the bone mineral density in G3 to G5 was significantly higher than in the control group, G1, and G2. Our results indicate that an inoculating dose of 6 × 10³ CFU/5 μl is sufficient to induce the development of osteomyelitis in rat models.

CONCLUSION

This study suggests that the minimum dose (6 × 10³ CFU/5 µl) can induce osteomyelitis in calvaria rat model. This can offer information and ability of more accurately modeling osteomyelitis and simulating the challenge of osteomyelitis treat.

Infectious Osteomyelitis: Marrying Bone Biology and Microbiology to Shed New Light on a Persistent Clinical Challenge

Infections of bone occur in a variety of clinical settings, ranging from spontaneous isolated infections arising from presumed hematogenous spread to those associated with skin and soft tissue wounds or medical implants. The majority are caused by the ubiquitous bacterium Staphyloccocus (S.) aureus, which can exist as a commensal organism on human skin as well as an invasive pathogen, but a multitude of other microbes are also capable of establishing bone infections. While studies of clinical isolates and small animal models have advanced our understanding of the role of various pathogen and host factors in infectious osteomyelitis (iOM), many questions remain unaddressed. Thus, there are many opportunities to elucidate host-pathogen interactions that may be leveraged toward treatment or prevention of this troublesome problem. Herein, we combine perspectives from bone biology and microbiology and suggest that interdisciplinary approaches will bring new insights to the field. © 2021 American Society for Bone and Mineral Research (ASBMR).

Staphylococcus aureus Internalization in Osteoblast Cells: Mechanisms, Interactions and Biochemical Processes. What Did We Learn from Experimental Models?

Bacterial internalization is a strategy that non-intracellular microorganisms use to escape the host immune system and survive inside the human body. Among bacterial species, Staphylococcus aureus showed the ability to interact with and infect osteoblasts, causing osteomyelitis as well as bone and joint infection, while also becoming increasingly resistant to antibiotic therapy and a reservoir of bacteria that can make the infection difficult to cure. Despite being a serious issue in orthopedic surgery, little is known about the mechanisms that allow bacteria to enter and survive inside the osteoblasts, due to the lack of consistent experimental models. In this review, we describe the current knowledge about S. aureus internalization mechanisms and various aspects of the interaction between bacteria and osteoblasts (e.g., best experimental conditions, bacteria-induced damages and immune system response), focusing on studies performed using the MG-63 osteoblastic cell line, the best traditional (2D) model for the study of this phenomenon to date. At the same time, as it has been widely demonstrated that 2D culture systems are not completely indicative of the dynamic environment in vivo, and more recent 3D models—representative of bone infection—have also been investigated.

Bacteria and Host Interplay in Staphylococcus aureus Septic Arthritis and Sepsis

Staphylococcus aureus (S. aureus) infections are a major healthcare challenge and new treatment alternatives are needed. S. aureus septic arthritis, a debilitating joint disease, causes permanent joint dysfunction in almost 50% of the patients. S. aureus bacteremia is associated with higher mortalities than bacteremia caused by most other microbes and can develop to severe sepsis and death. The key to new therapies is understanding the interplay between bacterial virulence factors and host immune response, which decides the disease outcome. S. aureus produces numerous virulence factors that facilitate bacterial dissemination, invasion into joint cavity, and cause septic arthritis. Monocytes, activated by several components of S. aureus such as lipoproteins, are responsible for bone destructions. In S. aureus sepsis, cytokine storm induced by S. aureus components leads to the hyperinflammatory status, DIC, multiple organ failure, and later death. The immune suppressive therapies at the very early time point might be protective. However, the timing of treatment is crucial, as late treatment may aggravate the immune paralysis and lead to uncontrolled infection and death.

Multifunctional Amyloids in the Biology of Gram-Positive Bacteria

Since they were discovered, amyloids have proven to be versatile proteins able to participate in a variety of cellular functions across all kingdoms of life. This multitask trait seems to reside in their ability to coexist as monomers, aggregates or fibrillar entities, with morphological and biochemical peculiarities. It is precisely this common molecular behaviour that allows amyloids to cross react with one another, triggering heterologous aggregation. In bacteria, many of these functional amyloids are devoted to the assembly of biofilms by organizing the matrix scaffold that keeps cells together. However, consistent with their notion of multifunctional proteins, functional amyloids participate in other biological roles within the same organisms, and emerging unprecedented functions are being discovered. In this review, we focus on functional amyloids reported in gram-positive bacteria, which are diverse in their assembly mechanisms and remarkably specific in their biological functions that they perform. Finally, we consider cross-seeding between functional amyloids as an emerging theme in interspecies interactions that contributes to the diversification of bacterial biology.

Neutrophil Recruitment to Noninvasive MRSA at the Stratum Corneum of Human Skin Mediates Transient Colonization

Staphylococcus aureus is a leading cause of skin and soft issue infection, but paradoxically, it also transiently, and often harmlessly, colonizes human skin. An obstacle to understanding this contradiction has been a shortage of in vivo models reproducing the unique structure and immunology of human skin. In this work, we developed a humanized model to study how healthy adult human skin responds to colonizing methicillin-resistant S. aureus (MRSA). We demonstrate the importance of the outer stratum corneum as the major site of bacterial colonization and how noninvasive MRSA adhesion to corneocytes induces a local inflammatory response in underlying skin layers. This signaling recruits neutrophils to the skin, where they control bacterial numbers, mediating transiency in colonization. This work highlights the spatiotemporal aspects of human skin colonization and demonstrates a subclinical inflammatory response to noninvasive MRSA that allows human skin to regulate the bacterial population at its outer surface.

Quorum Sensing and Toxin Production in Staphylococcus aureus Osteomyelitis: Pathogenesis and Paradox

Staphylococcus aureus is a Gram-positive pathogen capable of infecting nearly every vertebrate organ. Among these tissues, invasive infection of bone (osteomyelitis) is particularly common and induces high morbidity. Treatment of osteomyelitis is notoriously difficult and often requires debridement of diseased bone in conjunction with prolonged antibiotic treatment to resolve infection. During osteomyelitis, S. aureus forms characteristic multicellular microcolonies in distinct niches within bone. Virulence and metabolic responses within these multicellular microcolonies are coordinated, in part, by quorum sensing via the accessory gene regulator (agr) locus, which allows staphylococcal populations to produce toxins and adapt in response to bacterial density. During osteomyelitis, the Agr system significantly contributes to dysregulation of skeletal homeostasis and disease severity but may also paradoxically inhibit persistence in the host. Moreover, the Agr system is subject to complex crosstalk with other S. aureus regulatory systems, including SaeRS and SrrAB, which can significantly impact the progression of osteomyelitis. The objective of this review is to highlight Agr regulation, its implications on toxin production, factors that affect Agr activation, and the potential paradoxical influences of Agr regulation on disease progression during osteomyelitis.

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.

Necrotizing Soft Tissue Infection Staphylococcus aureus but not S. pyogenes Isolates Display High Rates of Internalization and Cytotoxicity Toward Human Myoblasts

BACKGROUND

Necrotizing soft tissue infections (NSTIs) caused by group A Streptococcus (GAS) and occasionally by Staphylococcus aureus (SA) frequently involve the deep fascia and often lead to muscle necrosis.

METHODS

To assess the pathogenicity of GAS and S. aureus for muscles in comparison to keratinocytes, adhesion and invasion of NSTI-GAS and NSTI-SA isolates were assessed in these cells. Bloodstream infections (BSI-SA) and noninvasive coagulase-negative staphylococci (CNS) isolates were used as controls.

RESULTS

NSTI-SA and BSI-SA exhibited stronger internalization into human keratinocytes and myoblasts than NSTI-GAS or CNS. S. aureus internalization reached over 30% in human myoblasts due to a higher percentage of infected myoblasts (>11%) as compared to keratinocytes (<3%). Higher cytotoxicity for myoblasts of NSTI-SA as compared to BSI-SA was attributed to higher levels of psmα and RNAIII transcripts in NSTI-SA. However, the 2 groups were not discriminated at the genomic level. The cellular basis of high internalization rate in myoblasts was attributed to higher expression of α5β1 integrin in myoblasts. Major contribution of FnbpAB-integrin α5β1 pathway to internalization was confirmed by isogenic mutants.

CONCLUSIONS

Our findings suggest a factor in NSTI-SA severity is the strong invasiveness of S. aureus in muscle cells, a property not shared by NSTI-GAS isolates.

Evaluation of the ability of linezolid and tedizolid to eradicate intraosteoblastic and biofilm-embedded Staphylococcus aureus in the bone and joint infection setting

OBJECTIVES

Prolonged use of linezolid for bone and joint infection (BJI) is limited by its long-term toxicity. The better safety profile of tedizolid, a recently developed oxazolidinone, could offer an alternative. However, its efficacy against biofilm-embedded and intracellular Staphylococcus aureus, the two main bacterial reservoirs associated with BJI chronicity, is unknown.

METHODS

Using three S. aureus strains (6850 and two clinical BJI isolates), linezolid and tedizolid were compared regarding their ability: (i) to target the S. aureus intracellular reservoir in an in vitro model of osteoblast infection, using three concentrations increasing from the bone concentration reached with standard therapeutic doses (Cbone = 2.5 × MIC; Cplasm = 10 × MIC; Cmax = 40 × MIC); (ii) to eradicate mature biofilm [minimal biofilm eradication concentration (MBEC)]; and (iii) to prevent biofilm formation [biofilm MIC (bMIC) and confocal microscopy].

RESULTS

Linezolid and tedizolid weakly reduced the intracellular inoculum of S. aureus in a strain-dependent manner despite the similar MICs for the tested strains, but improved cell viability even in the absence of an intracellular bactericidal effect. Conversely, linezolid and tedizolid were ineffective in eradicating mature biofilm formed in vitro, with MBEC >2000 and >675 mg/L, respectively. bMICs of tedizolid were 4-fold lower than those of linezolid for all strains.

CONCLUSIONS

Linezolid and tedizolid alone are not optimal candidates to target bacterial phenotypes associated with chronic forms of BJI. Despite weak intracellular activity, they both reduce infection-related cytotoxicity, suggesting a role in modulating intracellular expression of staphylococcal virulence factors. Although inactive against biofilm-embedded S. aureus, both-but particularly tedizolid-are able to prevent biofilm formation.

Antibiofilm and intraosteoblastic activities of rifamycins against Staphylococcus aureus: promising in vitro profile of rifabutin

Background

Targeting biofilm-embedded and intraosteoblastic Staphylococcus aureus, rifampicin gained a pivotal role in bone and joint infection (BJI) treatment. Two other rifamycins, rifabutin and rifapentine, may represent better-tolerated alternatives, but their activity against bacterial reservoirs associated with BJI chronicity has never been evaluated.

Objectives

To evaluate the activities of rifampicin, rifabutin and rifapentine in osteoblast infection models.

Methods

Using three S. aureus isolates, rifamycins were compared regarding: (i) their intracellular activity in ‘acute’ (24 h) and ‘chronic’ (7 days) osteoblast infection models at 0.1× MIC, 1× MIC, 10× MIC and 100× MIC, while impacting infection-induced cytotoxicity (MTT assay), intracellular phenol-soluble modulin (PSM) secretion (RT–PCR), resistance selection and small colony variant (SCV) emergence; and (ii) their minimal biofilm eradication concentration (MBEC) and their MIC to prevent biofilm formation (bMIC).

Results

At 0.1× MIC, only rifabutin significantly reduced intracellular inoculum and PSM secretion. All rifamycins allowed a 50% reduction of intraosteoblastic inoculum at higher concentrations, with no difference between acute and chronic infection models, while reducing infection-induced cytotoxicity and PSM secretion. Dose-dependent emergence of intracellular SCVs was observed for all molecules. No intracellular emergence of resistance was detected. bMICs were equivalent for all molecules, but MBEC90s of rifapentine and rifabutin were 10- to 100-fold lower than those of rifampicin, respectively.

Conclusions

All rifamycins are efficient in reducing the S. aureus intraosteoblastic reservoir while limiting infection-induced cytotoxicity, with a higher activity of rifabutin at low concentrations. All molecules prevent biofilm formation, but only rifapentine and rifabutin consistently reduce formed biofilm-embedded bacteria for all isolates. The activity of rifabutin at lower doses highlights its therapeutic potential.

The Process of Osteoblastic Infection by Staphylococcus Aureus

Bone infection is difficult to cure, and relapse frequently occurs, which is a major treatment problem. One of the main reasons for the refractory and recurrent nature of bone infection is that bacteria, such as Staphylococcus aureus (S. aureus), can be internalized into osteoblasts after infecting bone tissue, thereby avoiding attack by the immune system and antibiotics. Understanding how bacteria (such as S. aureus) are internalized into osteoblasts is key to effective treatment. S. aureus is the most common pathogenic bacterium that causes bone infection. This paper reviews the literature, analyzes the specific process of osteoblastic S. aureus infection, and summarizes specific treatment strategies to improve bone infection treatment.

Toxin-Triggered Interleukin-1 Receptor Signaling Enables Early-Life Discrimination of Pathogenic versus Commensal Skin Bacteria

The host must develop tolerance to commensal microbes and protective responses to infectious pathogens, yet the mechanisms enabling a privileged relationship with commensals remain largely unknown. Skin colonization by commensal Staphylococcus epidermidis facilitates immune tolerance preferentially in neonates via induction of antigen-specific regulatory T cells (Tregs). Here, we demonstrate that this tolerance is not indiscriminately extended to all bacteria encountered in this early window. Rather, neonatal colonization by Staphylococcus aureus minimally enriches for antigen-specific Tregs and does not prevent skin inflammation upon later-life exposure. S. aureus α-toxin contributes to this response by stimulating myeloid cell production of IL-1β, which limits S. aureus-specific Tregs. Loss of α-toxin or the IL-1 receptor increases Treg enrichment, whereas topical application of IL-1β or α-toxin diminishes tolerogenic responses to S. epidermidis. Thus, the preferential activation of a key alarmin pathway facilitates early discrimination of microbial "foe" from "friend," thereby preventing tolerance to a common skin pathogen.

Staphylococcus aureus Small Colony Variants (SCVs): News From a Chronic Prosthetic Joint Infection

Small colony variants (SCV) of Staphylococcus aureus have been reported as implicated in chronic infections. Here, we investigated the genomic and transcriptomic changes involved in the evolution from a wild-type to a SCV from in a patient with prosthetic joint infection relapse. The SCV presented a stable phenotype with no classical auxotrophy and the emergence of rifampicin resistance. Whole Genome Sequencing (WGS) analysis showed only the loss of a 42.5 kb phage and 3 deletions, among which one targeting the rpoB gene, known to be the target of rifampicin and to be associated to SCV formation in the context of a constitutively active stringent response. Transcriptomic analysis highlighted a specific signature in the SCV strain including a complex, multi-level strategy of survival and adaptation to chronicity within the host including a protection from the inflammatory response, an evasion of the immune response, a constitutively activated stringent response and a scavenging of iron sources.

In vivo exposure of insect AMP resistant Staphylococcus aureus to an insect immune system

Antimicrobial peptides (AMPs) are important immune effectors in insects. Bacteria have a limited number of ways to resist AMPs, and AMP-resistance is often costly. Recently, it has become clear that AMP activities in vitro and in vivo differ. Although some studies have followed the in vivo survival of AMP resistant pathogens, studying a pathogen resistant to the AMPs of that particular host has never been reported. Here, we infected the mealworm beetle Tenebrio molitor with Staphylococcus aureus strains that were evolved in vitro in the presence of one or two antimicrobial peptides from T. molitor. We found that the Tenebrio immune system could clear mutant Tenecin resistant strains at least as efficiently as sensitive controls. The bacterial load of Tenecin resistant S. aureus segregated by mutation. Strains with mutations in both the pmt and rpo operons showed the highest in vivo survival and therefore showed the lowest fitness cost amongst the evolved resistance mutations. In contrast, Tenecin resistant strains with mutations in the nsa and rpo operons showed much lower survival within the hosts. Our study shows that Tenecin resistant strains are phagocytosed at a lower rate. The nsa/rpo mutants were phagocytosed at a higher rate than other Tenecin resistant S. aureus strains. The differences in resistance against AMPs and phagocytosis did not translate into changes in virulence. AMP resistance, while a prerequisite for an infection in vertebrates, does not provide a survival advantage to S. aureus in a host environment that is dominated by AMPs.

The Role of Streptococcal and Staphylococcal Exotoxins and Proteases in Human Necrotizing Soft Tissue Infections

Necrotizing soft tissue infections (NSTIs) are critical clinical conditions characterized by extensive necrosis of any layer of the soft tissue and systemic toxicity. Group A streptococci (GAS) and Staphylococcus aureus are two major pathogens associated with monomicrobial NSTIs. In the tissue environment, both Gram-positive bacteria secrete a variety of molecules, including pore-forming exotoxins, superantigens, and proteases with cytolytic and immunomodulatory functions. The present review summarizes the current knowledge about streptococcal and staphylococcal toxins in NSTIs with a special focus on their contribution to disease progression, tissue pathology, and immune evasion strategies.

Microbiome Imbalances: An Overlooked Potential Mechanism in Chronic Nonhealing Wounds

Chronic nonhealing wounds are a severe burden to health care systems worldwide, causing millions of patients to have lengthy hospital stays, high health care costs, periods of unemployment, and reduced quality of life. Moreover, treating chronic nonhealing wounds effectively and reasonably in countries with limited medical resources can be extremely challenging. With many outstanding questions surrounding chronic nonhealing wounds, in this review, we offer changes to the microbiome as a potentially ignored mechanism important in the formation and treatment of chronic wounds. Our analysis helps bring a whole new understanding to wound formation and healing and provides a potential breakthrough in the treatment of chronic nonhealing wounds in the future.

Staphylococcus aureus Secreted Toxins and Extracellular Enzymes

Staphylococcus aureus is a formidable pathogen capable of causing infections in different sites of the body in a variety of vertebrate animals, including humans and livestock. A major contribution to the success of S. aureus as a pathogen is the plethora of virulence factors that manipulate the host's innate and adaptive immune responses. Many of these immune modulating virulence factors are secreted toxins, cofactors for activating host zymogens, and exoenzymes. Secreted toxins such as pore-forming toxins and superantigens are highly inflammatory and can cause leukocyte cell death by cytolysis and clonal deletion, respectively. Coagulases and staphylokinases are cofactors that hijack the host's coagulation system. Exoenzymes, including nucleases and proteases, cleave and inactivate various immune defense and surveillance molecules, such as complement factors, antimicrobial peptides, and surface receptors that are important for leukocyte chemotaxis. Additionally, some of these secreted toxins and exoenzymes can cause disruption of endothelial and epithelial barriers through cell lysis and cleavage of junction proteins. A unique feature when examining the repertoire of S. aureus secreted virulence factors is the apparent functional redundancy exhibited by the majority of the toxins and exoenzymes. However, closer examination of each virulence factor revealed that each has unique properties that have important functional consequences. This chapter provides a brief overview of our current understanding of the major secreted virulence factors critical for S. aureus pathogenesis.

Staphylococcus aureus: setting its sights on the human innate immune system

Staphylococcus aureus has colonized humans for at least 10 000 years, and today inhabits roughly a third of the population. In addition, S. aureus is a major pathogen that is responsible for a significant disease burden, ranging in severity from mild skin and soft-tissue infections to life-threatening endocarditis and necrotizing pneumonia, with treatment often hampered by resistance to commonly available antibiotics. Underpinning its versatility as a pathogen is its ability to evade the innate immune system. S. aureus specifically targets innate immunity to establish and sustain infection, utilizing a large repertoire of virulence factors to do so. Using these factors, S. aureus can resist phagosomal killing, impair complement activity, disrupt cytokine signalling and target phagocytes directly using proteolytic enzymes and cytolytic toxins. Although most of these virulence factors are well characterized, their importance during infection is less clear, as many display species-specific activity against humans or against animal hosts, including cows, horses and chickens. Several staphylococcal virulence factors display species specificity for components of the human innate immune system, with as few as two amino acid changes reducing binding affinity by as much as 100-fold. This represents a major issue for studying their roles during infection, which cannot be examined without the use of humanized infection models. This review summarizes the major factors S. aureus uses to impair the innate immune system, and provides an in-depth look into the host specificity of S. aureus and how this problem is being approached.

Survival of Staphylococcus epidermidis in Fibroblasts and Osteoblasts

Staphylococcus epidermidis is a leading cause of infections associated with indwelling medical devices, including prosthetic joint infection. While biofilm formation is assumed to be the main mechanism underlying the chronic infections S. epidermidis causes, we hypothesized that S. epidermidis also evades immune killing, contributing to its pathogenesis. Here, we show that prosthetic joint-associated S. epidermidis isolates can persist intracellularly within human fibroblasts and inside human and mouse osteoblasts. We also show that the intracellularly persisting bacteria reside primarily within acidic phagolysosomes and that over the course of infection, small-colony variants are selected for. Moreover, upon eukaryotic cell death, these bacteria, which can outlive their host, can escape into the extracellular environment, providing them an opportunity to form biofilms on implant surfaces at delayed time points in implant-associated infection. In summary, the acidic phagolysosomes of fibroblasts and osteoblasts serve as reservoirs for chronic or delayed S. epidermidis infection.

Acrylic microparticles increase daptomycin intracellular and in vivo anti-biofilm activity against Staphylococcus aureus

Daptomycin (DAP) is a cyclic lipopeptide antibiotic with potential clinical application in orthopedic infections caused by staphylococci. However, it failed to eradicate Staphylococcus aureus in vitro, in intracellular infection studies, as well as in vivo in an experimental model of implant-associated biofilm infections. In this study, the antimicrobial effect of DAP encapsulated in poly(methyl methacrylate)-Eudragit (PMMA-EUD) microparticles (DAP-MPs) on intracellular S. aureus was evaluated in human osteoblast cells using fluorescence in situ hybridization (FISH) analysis. Encapsulated DAP was able to reduce the amount of intracellular S. aureus by 73% compared to blank microparticles (MPs). Then, the advantage of treating with DAP-MPs versus free DAP was evaluated in a murine model of implant-associated biofilm infection. Free DAP showed a >3 log₁₀ decrease in planktonic and adherent bacteria but failed to eradicate adherent methicillin-resistant S. aureus (MRSA), whereas DAP-MPs showed a clearance of planktonic MRSA, significantly reduced adherent MRSA by more than 3 log₁₀ and cured the infection in 60%. This was linked to the prolonged higher DAP concentration within the tissue cage fluid compared to free DAP. To our knowledge, this study provides the first evidence for the high intracellular and in vivo anti-biofilm efficacy of DAP-MPs to target staphylococcal infections.

Do amyloid structures formed by Staphylococcus aureus phenol-soluble modulins have a biological function?

Phenol-soluble modulins (PSMs) are alpha-helical, amphipathic peptides that have multiple functions in staphylococcal physiology and virulence. Recent research has suggested that PSMs form amyloid fibrils and amyloids are involved in PSM-mediated phenotypes such as cytolysis and biofilm stability. While we observed PSM amyloid formation using electron microscopy and dye assays, there were no apparent differences in the production of extracellular fibrous material between a PSM-deficient strain and the isogenic wild-type strain. Furthermore, we detected no correlation between cytolytic or pro-inflammatory activities with the propensity of PSM derivatives to form amyloids. In addition, we propose a model based on our finding of non-specific attachment of PSMs to DNA, which we here report results in resistance to DNase digestion, explaining previous findings on PSM-mediated biofilm stability without the necessity to assume amyloid involvement. Collectively, our results indicate that PSM amyloid formation may not be of major relevance for known key biological functions of PSMs. Intriguingly, however, we found that amyloid-forming capacity of PSMalpha3 allows almost no amino acid exchanges, suggesting importance of amyloid formation in possibly yet unknown functions of PSMs.

Antibacterial surface modification of titanium implants in orthopaedics

The use of biomaterials in orthopaedics for joint replacement, fracture healing and bone regeneration is a rapidly expanding field. Infection of these biomaterials is a major healthcare burden, leading to significant morbidity and mortality. Furthermore, the cost to healthcare systems is increasing dramatically. With advances in implant design and production, research has predominately focussed on osseointegration; however, modification of implant material, surface topography and chemistry can also provide antibacterial activity. With the increasing burden of infection, it is vitally important that we consider the bacterial interaction with the biomaterial and the host when designing and manufacturing future implants. During this review, we will elucidate the interaction between patient, biomaterial surface and bacteria. We aim to review current and developing surface modifications with a view towards antibacterial orthopaedic implants for clinical applications.

Understanding the Virulence of Staphylococcus pseudintermedius: A Major Role of Pore-Forming Toxins

Staphylococcus pseudintermedius is responsible for severe and necrotizing infections in humans and dogs. Contrary to S. aureus, the pathophysiological mechanisms involved in this virulence are incompletely understood. We previously showed the intracellular cytotoxicity induced after internalization of S. pseudintermedius. Herein, we aimed to identify the virulence factors responsible for this cytotoxic activity. After addition of filtered S. pseudintermedius supernatants in culture cell media, MG63 cells, used as representative of non-professional phagocytic cells (NPPc), released a high level of LDH, indicating that the cytotoxicity was mainly mediated by secreted factors. Accordingly, we focused our attention on S. pseudintermedius toxins. In silico analysis found the presence of two PSMs (δ-toxin and PSMε) as well as Luk-I leukotoxin, the presence of which was confirmed by PCR in all clinical strains tested (n = 17). Recombinant Luk-I leukotoxin had no cytotoxic activity on NPPc but the ectopic expression of the CXCR2 receptor in U937 cells conferred cytotoxity to Luk-I. This is in agreement with the lack of Luk-I effect on NPPc and the previous report of Luk-I cytoxic activity on immune cells. Contrary to Luk-I, synthetic δ-toxin and PSMε had a strong cytotoxic activity on NPPc. The secretion of δ-toxin and PSMε at cytotoxic concentrations by S. pseudintermedius in culture supernatant was confirmed by HPLC-MS. In addition, the supplementation of such supernatants with human serum, known to inhibit PSM, induced a complete abolition of cytotoxicity which indicates that PSMs are the key players in the cytotoxic phenotype of NPPc. The results suggest that the severity of S. pseudintermedius infections is, at least in part, explained by a combined action of Luk-I that specifically targets immune cells expressing the CXCR2 receptor, and PSMs that disrupt cell membranes whatever the cell types. The present study strengthens the key role of PSMs in virulence of the different species belonging to Staphylococcus genus.

Cancer Metastases to Bone: Concepts, Mechanisms, and Interactions with Bone Osteoblasts

The skeleton is a unique structure capable of providing support for the body. Bone resorption and deposition are controlled in a tightly regulated balance between osteoblasts and osteoclasts with no net bone gain or loss. However, under conditions of disease, the balance between bone resorption and deposition is upset. Osteoblasts play an important role in bone homeostasis by depositing new bone osteoid into resorption pits. It is becoming increasingly evident that osteoblasts additionally play key roles in cancer cell dissemination to bone and subsequent metastasis. Our laboratory has evidence that when osteoblasts come into contact with disseminated breast cancer cells, the osteoblasts produce factors that initially reduce breast cancer cell proliferation, yet promote cancer cell survival in bone. Other laboratories have demonstrated that osteoblasts both directly and indirectly contribute to dormant cancer cell reactivation in bone. Moreover, we have demonstrated that osteoblasts undergo an inflammatory stress response in late stages of breast cancer, and produce inflammatory cytokines that are maintenance and survival factors for breast cancer cells and osteoclasts. Advances in understanding interactions between osteoblasts, osteoclasts, and bone metastatic cancer cells will aid in controlling and ultimately preventing cancer cell metastasis to bone.

Staphylococcal Osteomyelitis: Disease Progression, Treatment Challenges, and Future Directions

Osteomyelitis is an inflammatory bone disease that is caused by an infecting microorganism and leads to progressive bone destruction and loss. The most common causative species are the usually commensal staphylococci, with Staphylococcus aureus and Staphylococcus epidermidis responsible for the majority of cases. Staphylococcal infections are becoming an increasing global concern, partially due to the resistance mechanisms developed by staphylococci to evade the host immune system and antibiotic treatment. In addition to the ability of staphylococci to withstand treatment, surgical intervention in an effort to remove necrotic and infected bone further exacerbates patient impairment. Despite the advances in current health care, osteomyelitis is now a major clinical challenge, with recurrent and persistent infections occurring in approximately 40% of patients. This review aims to provide information about staphylococcus-induced bone infection, covering the clinical presentation and diagnosis of osteomyelitis, pathophysiology and complications of osteomyelitis, and future avenues that are being explored to treat osteomyelitis.

Staphylococcus aureus triggers a shift from influenza virus-induced apoptosis to necrotic cell death

Superinfections with Staphylococcus aureus are a major complication of influenza disease, causing excessive inflammation and tissue damage. This enhanced cell-damaging effect is also observed in superinfected tissue cultures, leading to a strong decrease in overall cell viability. In our analysis of the underlying molecular mechanisms, we observed that, despite enhanced cell damage in superinfection, S. aureus did not increase but rather inhibited influenza virus (IV)-induced apoptosis in cells on the level of procaspase-8 activation. This apparent contradiction was solved when we observed that S. aureus mediated a switch from apoptosis to necrotic cell death of IV-infected cells, a mechanism that was dependent on the bacterial accessory gene regulator ( agr) locus that promotes bacterial survival and spread. This so far unknown action may be a bacterial strategy to enhance dissemination of intracellular S. aureus and may thereby contribute to increased tissue damage and severity of disease.-Van Krüchten, A., Wilden, J. J., Niemann, S., Peters, G., Löffler, B., Ludwig, S., Ehrhardt, C. Staphylococcus aureus triggers a shift from influenza virus-induced apoptosis to necrotic cell death.

Inside job: Staphylococcus aureus host-pathogen interactions

Staphylococcus aureus is a notorious opportunistic pathogen causing a plethora of diseases. Recent research established that once phagocytosed by neutrophils and macrophages, a certain percentage of S. aureus is able to survive within these phagocytes which thereby even may contribute to dissemination of the pathogen. S. aureus further induces its uptake by otherwise non-phagocytic cells and the ensuing intracellular cytotoxicity is suggested to lead to tissue destruction, whereas bacterial persistence within cells is thought to lead to immune evasion and chronicity of infections. We here review recent work on the S. aureus host pathogen interactions with a focus on the intracellular survival of the pathogen.

Staphylococcus Aureus Induces Osteoclastogenesis via the NF-κB Signaling Pathway

Background

Osteomyelitis is one of the refractory diseases encountered in orthopedics, while Staphylococcus aureus (S. aureus) is the most common causative organism in osteomyelitis. However, the precise mechanisms underlying the bone loss caused by S. aureus infection have not been well defined. Here, we investigated the effect of S. aureus on osteoclast differentiation and the probable molecular mechanism.

Material/Methods

RAW 264.7 cells were treated for 5 days with live S. aureus, inactivated S. aureus, and S. aureus filtrate. Then, the formation of osteoclast-like cells and resorption pits was observed, and the expression of osteoclast-specific genes (TRAP, MMP-9, cathepsin K, CTR and Atp6v0d2) was detected by real-time PCR. Moreover, key proteins in the signaling pathway associated with osteoclast differentiation were detected with Western blot.

Results

The data showed that live S. aureus, inactivated S. aureus, and S. aureus filtrate induced osteoclast formation, promoted bone resorption, and increased the expression of osteoclast-specific genes in a dose-dependent manner in the absence RANKL. In addition, we found that the S. aureus-induced osteoclastogenesis was related to the degradation of IκB-α, phosphorylation of NF-κB p65, and increased expression of NFATc1. Thus, we used JSH-23 to inhibit NF-κB transcriptional activity. The effect of the S. aureus-induced osteoclastogenesis and the expression of osteoclast-specific genes and NFATc1 were inhibited, which indicated that the NF-κB signaling pathway plays a role in S. aureus-induced osteoclastogenesis.

Conclusions

This study demonstrated that S. aureus induces osteoclastogenesis through its cell wall compound and secretion of small soluble molecules, and the NF-κB signaling pathway plays a role in this process.

Staphylococcus aureus Protein A induces osteoclastogenesis via the NF‑κB signaling pathway

Staphylococcus aureus (S. aureus) is the most common organism causing osteomyelitis, and Staphylococcus aureus protein A (SpA) is an important virulence factor anchored in its cell wall. However, the precise mechanisms underlying the bone loss caused by SpA have not been well understood. The present study aimed to investigate the effect of SpA on osteoclast differentiation, and the probable mechanism was investigated. Raw264.7 cells were treated with SpA in the absence or presence of receptor-activated (NF)-κB ligand for 5 days, and morphological and biochemical assays were used to assess osteoclastogenesis and explore the underlying mechanisms. Data demonstrated that SpA induced osteoclast differentiation and promoted bone resorption in a dose-dependent manner in the absence or presence of RANKL. In addition, the expression of osteoclast-specific genes, such as the tartrate resistant acid phosphatase, matrix metalloproteinase-9, cathepsin K, calcitonin receptors and d2 isoform of the vacuolar ATPase Vo domain, were enhanced by SpA. Furthermore, the SpA-induced osteoclast differentiation was associated with the degradation of inhibitor of κB-α, phosphorylation of NF-κB p65 and increased expression of nuclear factor of activated T-cells. However, by treatment with JSH-23, an NF-κB inhibitor, the formation of osteoclast-like cells and resorption pits was significantly reduced, and the expression of osteoclast-specific genes was also inhibited. Collectively, in the present study SpA induced osteoclast differentiation, promoted bone resorption, and the NF-κB signaling pathway was involved in this process.