Intra-cellular Staphylococcus aureus alone causes infection in vivo

Emerging Concepts in Periprosthetic Joint Infection Research: Intracellular Organisms

Despite advances in the treatment of periprosthetic joint infection (PJI), recurrence is a concern for both patients and orthopaedic surgeons. Staphylococcal species are commonly implicated in PJI and are associated with more instances of treatment failure and recalcitrance relative to other organisms. A possible explanation for this is the ability of staphylococci to undergo phenotypic transformation into a quasidormant small colony variant (SCV) phenotype. These SCVs are difficult to culture and demonstrate increased resistance to targeted antibiotic therapy. Most importantly, SCVs are capable of intracellular invasion and may reside and proliferate within a variety of host cell types, including osteoblasts, osteocytes, fibroblasts, and circulating immune cells, such as neutrophils. In doing so, virulent organisms may interfere with normal host cell function while also converting these cells into reservoirs of bacteria that are sequestered from the extracellular environment and thus less vulnerable to standard antibiotic regimens. Additionally, these intracellular organisms may emerge from dormancy under certain conditions, including dysregulation or suppression of the host immune system, to return to their virulent state and cause recurrence of an infection. This phenomenon has been observed clinically, with reports in the literature describing cases of recurrent PJI and osteomyelitis, often many years later, caused by organisms previously thought to be eradicated. Furthermore, circulating immune cells containing intracellular organisms may also transport bacteria to other body sites before undergoing cellular lysis, leading to metastatic infection via a "Trojan Horse" mechanism. Previous reports have demonstrated intraosteoblastic infection with Staphylococcus aureus in clinical cases of recurrent osteomyelitis, while a recent study identified the same pathogen in osteocytes obtained from patients who have culture-positive PJI. While the translational application of these findings warrants further exploration, a better understanding of the clinical relevance of intracellular invasion could lead to the development of more optimal treatment strategies for eradicating infection and improving patient outcomes.

Implant-Derived S. aureus Isolates Drive Strain-Specific Invasion Dynamics and Bioenergetic Alterations in Osteoblasts

Background: Implants are integral to modern orthopedic surgery. The outcomes are good, but infections remain a serious issue. Staphylococcus aureus (S. aureus), along with Staphylococcus epidermidis, are predominant pathogens responsible for implant-associated infections, as conventional antibiotic treatments often fail due to biofilm formation or the pathogens' ability to invade cells and to persist intracellularly. Objectives: This study therefore focused on interactions of S. aureus isolates from infected implants with MG63 and SaOS2 osteoblasts by investigating the adhesion, invasion, and the impact on the bioenergetics of osteoblasts. Methods and Results: We found that the ability of S. aureus to adhere to osteoblasts depends on the isolate and was not associated with a single gene or expression pattern of characteristic adhesion proteins, and further, was not correlated with invasion. However, analysis of invasion capabilities identified better invasion conditions for S. aureus isolates with the SaOS2 osteoblastic cells. Interestingly, metabolic activity of osteoblasts remained unaffected by S. aureus infection, indicating cell survival. In contrast, respiration assays revealed an altered mitochondrial bioenergetic turnover in infected cells. While basal as well as maximal respiration in MG63 osteoblasts were not influenced statistically by S. aureus infections, we found increased non-mitochondrial respiration and enhanced glycolytic activity in the osteoblasts, which was again, more pronounced in the SaOS2 osteoblastic cells. Conclusions: Our findings highlight the complexity of S. aureus-host interactions, where both the pathogen and the host cell contribute to intracellular persistence and survival, representing a major factor for therapeutic failures.

Antibacterial Activity of Zinc-Doped Hydroxyapatite and Vancomycin-Loaded Gelatin Nanoparticles against Intracellular Staphylococcus aureus in Human THP-1 Derived Macrophages

Treating bone infections with common antibiotics is challenging, since pathogens like Staphylococcus aureus can reside inside macrophages. To target these intracellular bacteria, we have proposed nanoparticles (NPs) as drug carriers. This study aims to investigate the efficacy of hydroxyapatite and gelatin NPs, selected in view of their bone mimicry and potential for targeted delivery, as carriers for the antibacterial agents zinc and vancomycin. Therefore, two distinct NPs are fabricated: zinc-doped hydroxyapatite (ZnHA) and vancomycin-loaded gelatin (VGel) NPs. The NPs are characterized based on morphology, size, chemical composition, cellular internalization, and intracellular bactericidal efficacy. Specifically, the intracellular bactericidal efficacy is tested using a validated coculture model of human THP-1 derived macrophages and phagocytosed S. aureus bacteria. Scanning electron microscopy (SEM) and Fourier transform-infrared spectroscopy (FTIR) results show that the spherical NPs are synthesized successfully. These NPs are internalized by THP-1 cells and show >75% colocalization with lysosomes without compromising the viability of the THP-1 cells. Both ZnHA and VGel NPs substantially reduce the intracellular survival of S. aureus compared to the direct addition of dissolved zinc and vancomycin. Concluding, our NPs are highly effective drug delivery vehicles to kill intracellular S. aureus, which stress the potential of these NPs for future clinical translation.

Clinical and in vitro models identify distinct adaptations enhancing Staphylococcus aureus pathogenesis in human macrophages

Staphylococcus aureus is a facultative intracellular pathogen of human macrophages, which facilitates chronic infection. The genotypes, pathways, and mutations influencing that phenotype remain incompletely explored. Here, we used two distinct strategies to ascertain S. aureus gene mutations affecting pathogenesis in macrophages. First, we analyzed isolates collected serially from chronic cystic fibrosis (CF) respiratory infections. We found that S. aureus strains evolved greater macrophage invasion capacity during chronic human infection. Bacterial genome-wide association studies (GWAS) identified 127 candidate genes for which mutation was significantly associated with macrophage pathogenesis in vivo. In parallel, we passaged laboratory S. aureus strains in vitro to select for increased infection of human THP-1 derived macrophages, which identified 15 candidate genes by whole-genome sequencing. Functional validation of candidate genes using isogenic transposon mutant knockouts and CRISPR interference (CRISPRi) knockdowns confirmed virulence contributions from 37 of 39 tested genes (95%) implicated by in vivo studies and 7 of 10 genes (70%) ascertained from in vitro selection, with one gene in common to the two strategies. Validated genes included 17 known virulence factors (39%) and 27 newly identified by our study (61%), some encoding functions not previously associated with macrophage pathogenesis. Most genes (80%) positively impacted macrophage invasion when disrupted, consistent with the phenotype readily arising from loss-of-function mutations in vivo. This work reveals genes and mechanisms that contribute to S. aureus infection of macrophages, highlights differences in mutations underlying convergent phenotypes arising from in vivo and in vitro systems, and supports the relevance of S. aureus macrophage pathogenesis during chronic respiratory infection in CF. Additional studies will be needed to illuminate the exact mechanisms by which implicated mutations affect their phenotypes.

Practical Guidance for Clinical Microbiology Laboratories: Microbiologic diagnosis of implant-associated infections

SUMMARYImplant-associated infections (IAIs) pose serious threats to patients and can be associated with significant morbidity and mortality. These infections may be difficult to diagnose due, in part, to biofilm formation on device surfaces, and because even when microbes are found, their clinical significance may be unclear. Despite recent advances in laboratory testing, IAIs remain a diagnostic challenge. From a therapeutic standpoint, many IAIs currently require device removal and prolonged courses of antimicrobial therapy to effect a cure. Therefore, making an accurate diagnosis, defining both the presence of infection and the involved microorganisms, is paramount. The sensitivity of standard microbial culture for IAI diagnosis varies depending on the type of IAI, the specimen analyzed, and the culture technique(s) used. Although IAI-specific culture-based diagnostics have been described, the challenge of culture-negative IAIs remains. Given this, molecular assays, including both nucleic acid amplification tests and next-generation sequencing-based assays, have been used. In this review, an overview of these challenging infections is presented, as well as an approach to their diagnosis from a microbiologic perspective.

Global Transcriptomic Study of Circular-RNA Expression Profile in Osteoclasts Infected by Intracellular Staphylococcus aureus

Osteomyelitis is difficult to cure, and the rapidly rising morbidity is a thorny problem accompanied by a large number of joint replacement applications. Staphylococcus aureus is the main pathogen of osteomyelitis. Circular RNAs (circRNAs), as emerging noncoding RNAs, play important roles in multiple physiopathological processes which could provide novel insights into osteomyelitis. However, little is known about the roles of circRNAs in the pathogenesis of osteomyelitis. Osteoclasts, considered bone sentinels, are the resident macrophages in bone and may play the immune defense roles in osteomyelitis. It has been reported that S. aureus can survive in osteoclasts, but the function of osteoclast circRNAs in response to intracellular S. aureus infection remains unclear. In this study, we investigated the profile of circRNAs in osteoclasts infected by intracellular S. aureus through high-throughput RNA sequencing. In total, 24 upregulated and 62 downregulated differentially expressed circRNAs were identified and subsequently analyzed to demonstrate their potential functions. On this basis, three circRNAs (chr4:130718154-130728164+, chr8:77409548-77413627-, and chr1:190871592-190899571-) were confirmed as potential novel biomarkers for the diagnosis of osteomyelitis through the murine model of osteomyelitis. Most importantly, we verified that the circRNA chr4:130718154-130728164+ named circPum1 could regulate the host autophagy to affect the intracellular infection of S. aureus through miR-767. In addition, circPum1 could serve as a promising serum biomarker in osteomyelitis patients caused by S. aureus infection. Taken together, this study provided the first global transcriptomic profile analysis of circRNAs in osteoclasts infected by intracellular S. aureus and first proposed a novel perspective for the pathogenesis and immunotherapy of S. aureus-induced osteomyelitis from the term of circRNAs.

Evaluation of silver bio-functionality in a multicellular in vitro model: towards reduced animal usage in implant-associated infection research

Background

Despite the extensive use of silver ions or nanoparticles in research related to preventing implant-associated infections (IAI), their use in clinical practice has been debated. This is because the strong antibacterial properties of silver are counterbalanced by adverse effects on host cells. One of the reasons for this may be the lack of comprehensive in vitro models that are capable of analyzing host-bacteria and host-host interactions.

Methods and results

In this study, we tested silver efficacy through multicellular in vitro models involving macrophages (immune system), mesenchymal stem cells (MSCs, bone cells), and S. aureus (pathogen). Our model showed to be capable of identifying each element of culture as well as tracking the intracellular survival of bacteria. Furthermore, the model enabled to find a therapeutic window for silver ions (AgNO₃) and silver nanoparticles (AgNPs) where the viability of host cells was not compromised, and the antibacterial properties of silver were maintained. While AgNO₃ between 0.00017 and 0.017 µg/mL retained antibacterial properties, host cell viability was not affected. The multicellular model, however, demonstrated that those concentrations had no effect on the survival of S. aureus, inside or outside host cells. Similarly, treatment with 20 nm AgNPs did not influence the phagocytic and killing capacity of macrophages or prevent S. aureus from invading MSCs. Moreover, exposure to 100 nm AgNPs elicited an inflammatory response by host cells as detected by the increased production of TNF-α and IL-6. This was visible only when macrophages and MSCs were cultured together.

Conclusions

Multicellular in vitro models such as the one used here that simulate complex in vivo scenarios can be used to screen other therapeutic compounds or antibacterial biomaterials without the need to use animals.

Mechanisms of promoting the differentiation and bone resorption function of osteoclasts by Staphylococcus aureus infection

Bone infection is a common and serious complication in the field of orthopedics, which frequently leads to excessive bone destruction and fracture nonunion. Staphylococcus aureus (S. aureus) infection affects bone cell function which, in turn, causes bone destruction. Bone is mainly regulated by osteoblasts and osteoclasts. Osteoclasts are the only cell type with bone resorptive function. Their over-activation is closely associated with excessive bone loss. Understanding how S. aureus changes the functional state of osteoclasts is the key to effective treatment. By reviewing the literature, this paper summarizes several mechanisms of bone destruction caused by S. aureus influencing osteoclasts, thereby stimulating new ideas for the treatment of bone infection.

Can intracellular Staphylococcus aureus in osteomyelitis be treated using current antibiotics? A systematic review and narrative synthesis

Approximately 40% of treatments of chronic and recurrent osteomyelitis fail in part due to bacterial persistence. Staphylococcus aureus, the predominant pathogen in human osteomyelitis, is known to persist by phenotypic adaptation as small-colony variants (SCVs) and by formation of intracellular reservoirs, including those in major bone cell types, reducing susceptibility to antibiotics. Intracellular infections with S. aureus are difficult to treat; however, there are no evidence-based clinical guidelines addressing these infections in osteomyelitis. We conducted a systematic review of the literature to determine the demonstrated efficacy of all antibiotics against intracellular S. aureus relevant to osteomyelitis, including protein biosynthesis inhibitors (lincosamides, streptogramins, macrolides, oxazolidines, tetracyclines, fusidic acid, and aminoglycosides), enzyme inhibitors (fluoroquinolones and ansamycines), and cell wall inhibitors (beta-lactam inhibitors, glycopeptides, fosfomycin, and lipopeptides). The PubMed and Embase databases were screened for articles related to intracellular S. aureus infections that compared the effectiveness of multiple antibiotics or a single antibiotic together with another treatment, which resulted in 34 full-text articles fitting the inclusion criteria. The combined findings of these studies were largely inconclusive, most likely due to the plethora of methodologies utilized. Therefore, the reported findings in the context of the models employed and possible solutions for improved understanding are explored here. While rifampicin, oritavancin, linezolid, moxifloxacin and oxacillin were identified as the most effective potential intracellular treatments, the scientific evidence for these is still relatively weak. We advocate for more standardized research on determining the intracellular effectiveness of antibiotics in S. aureus osteomyelitis to improve treatments and patient outcomes.

Role of Animal Models to Advance Research of Bacterial Osteomyelitis

Osteomyelitis is an inflammatory bone disease typically caused by infectious microorganisms, often bacteria, which causes progressive bone destruction and loss. The most common bacteria associated with chronic osteomyelitis is Staphylococcus aureus. The incidence of osteomyelitis in the United States is estimated to be upwards of 50,000 cases annually and places a significant burden upon the healthcare system. There are three general categories of osteomyelitis: hematogenous; secondary to spread from a contiguous focus of infection, often from trauma or implanted medical devices and materials; and secondary to vascular disease, often a result of diabetic foot ulcers. Independent of the route of infection, osteomyelitis is often challenging to diagnose and treat, and the effect on the patient's quality of life is significant. Therapy for osteomyelitis varies based on category and clinical variables in each case. Therapeutic strategies are typically reliant upon protracted antimicrobial therapy and surgical interventions. Therapy is most successful when intensive and initiated early, although infection may recur months to years later. Also, treatment is accompanied by risks such as systemic toxicity, selection for antimicrobial drug resistance from prolonged antimicrobial use, and loss of form or function of the affected area due to radical surgical debridement or implant removal. The challenges of diagnosis and successful treatment, as well as the negative impacts on patient's quality of life, exemplify the need for improved strategies to combat bacterial osteomyelitis. There are many in vitro and in vivo investigations aimed toward better understanding of the pathophysiology of bacterial osteomyelitis, as well as improved diagnostic and therapeutic strategies. Here, we review the role of animal models utilized for the study of bacterial osteomyelitis and their critically important role in understanding and improving the management of bacterial osteomyelitis.

A Human Osteocyte Cell Line Model for Studying Staphylococcus aureus Persistence in Osteomyelitis

Infectious osteomyelitis associated with periprosthetic joint infections is often recalcitrant to treatment and has a high rate of recurrence. In the case of Staphylococcus aureus, the most common pathogen in all forms of osteomyelitis, this may be attributed in part to residual intracellular infection of host cells, yet this is not generally considered in the treatment strategy. Osteocytes represent a unique cell type in this context due to their abundance, their formation of a syncytium throughout the bone that could facilitate bacterial spread and their relative inaccessibility to professional immune cells. As such, there is potential value in studying the host-pathogen interactions in the context of this cell type in a replicable and scalable in vitro model. Here, we examined the utility of the human osteosarcoma cell line SaOS2 differentiated to an osteocyte-like stage (SaOS2-OY) as an intracellular infection model for S. aureus. We demonstrate that S. aureus is capable of generating stable intracellular infections in SaOS2-OY cells but not in undifferentiated, osteoblast-like SaOS2 cells (SaOS2-OB). In SaOS2-OY cells, S. aureus transitioned towards a quasi-dormant small colony variant (SCV) growth phenotype over a 15-day post-infection period. The infected cells exhibited changes in the expression of key immunomodulatory mediators that are consistent with the infection response of primary osteocytes. Thus, SaOS2-OY is an appropriate cell line model that may be predictive of the interactions between S. aureus and human osteocytes, and this will be useful for studying mechanisms of persistence and for testing the efficacy of potential antimicrobial strategies.

More Is Not Always Better-the Double-Headed Role of Fibronectin in Staphylococcus aureus Host Cell Invasion

While Staphylococcus aureus has classically been considered an extracellular pathogen, these bacteria are also capable of being taken up by host cells, including nonprofessional phagocytes such as endothelial cells, epithelial cells, or osteoblasts. The intracellular S. aureus lifestyle contributes to infection development. The predominant recognition and internalization pathway appears to be the binding of the bacteria via a fibronectin bridge to the α5β1-integrin on the host cell membrane, followed by phagocytosis. Although osteoblasts showed high expression of α5β1-integrin and fibronectin, and bacteria adhered to osteoblasts to a high proportion, here we demonstrate by internalization assays and immunofluorescence microscopy that S. aureus was less engulfed in osteoblasts than in epithelial cells. The addition of exogenous fibronectin during the infection of cells with S. aureus resulted in an increased uptake by epithelial cells but not by osteoblasts. This contrasts with the previous conception of the uptake mechanism, where high expression of integrin and fibronectin would promote the bacterial uptake into host cells. Extracellular fibronectin surrounding osteoblasts, but not epithelial cells, is organized in a fibrillary network. The inhibition of fibril formation, the short interfering RNA-mediated reduction of fibronectin expression, and the disruption of the fibronectin-fibril meshwork all resulted in a significant increase in S. aureus uptake by osteoblasts. Thus, the network of fibronectin fibrils appears to strongly reduce the uptake of S. aureus into a given host cell, indicating that the supramolecular structure of fibronectin determines the capacity of particular host cells to internalize the pathogen. IMPORTANCE Traditionally, Staphylococcus aureus has been considered an extracellular pathogen. However, among other factors, the frequent failure of antimicrobial therapy and the ability of the pathogen to cause recurrent disease have established the concept of eukaryotic invasion of the pathogen, thereby evading the host's immune system. In the current model of host cell invasion, bacteria initially bind to α5β1 integrin on the host cell side via a fibronectin bridge, which eventually leads to phagocytosis of S. aureus by host cells. However, in this study, we demonstrate that not the crude amount but the supramolecular structure of fibronectin molecules deposited on the eukaryotic cell surface plays an essential role in bacterial uptake by host cells. Our findings explain the large differences of S. aureus uptake efficacy in different host cell types as well as in vivo differences between courses of bacterial infections and the localization of bacteria in different clinical settings.

Intracellular Pathogen Detection Based on Dual-Recognition Units Constructed Fluorescence Resonance Energy Transfer Nanoprobe

The intracellular invasion and survival of a pathogen like Staphylococcus aureus (S. aureus) within host cells enable them to resist antibiotic treatment and colonize long-term in the host, which leads to a series of clinical issues. Rapid and specific detection of intracellular bacteria is important in diagnosis of infection and guiding antibiotic administration. Herein, this work reports a simple one-step fluorescence resonance energy transfer (FRET) platform-based strategy to achieve specific and rapid detection of S. aureus in specimens of phagocytic cells. The aptamer modified quantum dots (Aptamer-QDs) and antibiotic molecule of Teicoplanin functionalized-gold nanoparticles (Teico-AuNPs) dual-recognition units to S. aureus are employed as energy donor and acceptor, respectively. Based on the "off" to "on" signal readout mode, when in the presence of target S. aureus, the donor and acceptor are close to each other and bring high FRET efficiency, which is suitable for analysis of intracellular S. aureus. After it was incubated with the sample for 2 h, the as-prepared FRET sensor showed selectivity to the target S. aureus, and the changed fluorescence signal shows an obvious variation with increasing concentration of S. aureus in pure buffer. When the FRET strategy was further applied to assay intracellular S. aureus, there was an obvious fluorescence signal change obtained both by spectrum analysis and visual fluorescence microscope observation when the average number of S. aureus in one host cell (N_{S. aureus/cell}) was as low as 1, which can be attributed to the high fluorescence quenching efficiency of about 41.3%. It could be envisioned that this FRET nanoprobe with high fluorescence quenching efficiency may provide a simple approach for the facile, selective, and rapid diagnosis of an intracellular bacterial infection.

Silver nanoparticles present high intracellular and extracellular killing against Staphylococcus aureus

OBJECTIVES

Bone and joint infections caused by Staphylococcus aureus are becoming increasingly difficult to treat due to rising antibiotic resistance, resilient biofilms and intracellular survival of S. aureus. It has been challenging to identify and develop antimicrobial agents that can be used to kill extracellular and intracellular bacteria while having limited toxicity towards host cells. In addressing this challenge, this study investigates the antimicrobial efficacy and toxicity of silver nanoparticles (AgNPs).

METHODS

Intracellular bacteria were generated using a co-culture model of human osteoblast cells and S. aureus. Extracellular and intracellular S. aureus were treated with AgNPs, antibiotics and their combinations, and numbers of colonies were quantified. Toxicity of AgNPs against human osteoblast cells was determined by quantifying the number of viable cells after treatment.

RESULTS

AgNPs demonstrated excellent antimicrobial activity against extracellular S. aureus with a 100% killing efficacy at concentrations as low as 56 μM, along with a high intracellular killing efficacy of 76% at 371 μM. AgNPs were non-toxic or slightly toxic towards human osteoblasts at the concentrations studied (up to 927 μM). Moreover, smaller-sized (40 nm) AgNPs were more efficacious in killing bacteria compared with their larger-sized (100 nm) counterparts and synergistic antimicrobial effects against extracellular bacteria were observed when AgNPs were combined with gentamicin.

CONCLUSIONS

AgNPs and their combination with antibiotics have demonstrated high extracellular and intracellular bacterial killing and presented unique aspects for potential clinical applications, especially for chronic and recurrent infections where intracellular bacteria may be the cause.

EGFR/FAK and c-Src signalling pathways mediate the internalisation of Staphylococcus aureus by osteoblasts

Internalisation of Staphylococcus aureus in osteoblasts plays a critical role in the persistence and recurrence of osteomyelitis, the mechanisms involved in this process remain largely unknown. In the present study, evidence of internalised S. aureus in osteoblasts was found in long bone of haematogenous osteomyelitis in mice after 2 weeks of infection. Meanwhile, eliminating extracellular S. aureus by gentamicin can partially rescue bone loss, whereas the remaining intracellular S. aureus in osteoblasts may be associated with continuous bone destruction. In osteoblastic MC3T3 cells, intracellular S. aureus was detectable as early as 15 min after infection, and the internalisation rates increased with the extension of infection time. Additionally, S. aureus invasion stimulated the expression of phosphor-focal adhesion kinase (FAK), phosphor-epidermal growth factor receptor (EGFR) and phosphor-c-Src in a time-dependent way, and blocking EGFR/FAK or c-Src signalling significantly reduced the internalisation rate of S. aureus in osteoblasts. Our findings provide new insights into the mechanism of S. aureus internalisation in osteoblast and raise the potential of targeting EGFR/FAK and c-Src as adjunctive therapeutics for treating chronic S. aureus osteomyelitis.

Is intracellular Staphylococcus aureus associated with recurrent infection in a rat model of open fracture?

Aims

The purpose of this study was to determine whether intracellular Staphylococcus aureus is associated with recurrent infection in a rat model of open fracture.

Methods

After stabilizing with Kirschner wire, we created a midshaft femur fracture in Sprague-Dawley rats and infected the wound with green fluorescent protein (GFP)-tagged S. aureus. After repeated debridement and negative swab culture was achieved, the isolation of GFP-containing cells from skin, bone marrow, and muscle was then performed. The composition and viability of intracellular S. aureus in isolated GFP-positive cells was assessed. We suppressed the host immune system and observed whether recurrent infection would occur. Finally, rats were assigned to one of six treatment groups (a combination of antibiotic treatment and implant removal/retention). The proportion of successful eradication was determined.

Results

Green fluorescent protein-containing cells were successfully isolated after the swab culture was negative from skin (n = 0, 0%), muscle (n = 10, 100%), and bone marrow (n = 10, 100%) of a total of ten rats. The phagocytes were predominant in GFP-positive cells from muscle (73%) and bone marrow (81%) with a significantly higher viability of intracellular S. aureus (all p-values < 0.001). The recurrent infection occurred in up to 75% of rats after the immunosuppression. The proportion of successful eradication was not associated with implant retention or removal, and the efficacy of linezolid in eradicating intracellular S. aureus is significantly higher than that of vancomycin.

Conclusion

Intracellular S. aureus is associated with recurrent infection in the rat model of open fracture. Usage of linezolid, a membrane-permeable antibiotic, is an effective strategy against intracellular S. aureus.

Cite this article:Bone Joint Res. 2020;9(2):71–76.

Identification and Characterization of Staphylococcus delphini Internalization Pathway in Nonprofessional Phagocytic Cells

The intracellular lifestyle of bacteria is widely acknowledged to be an important mechanism in chronic and recurring infection. Among the Staphylococcus genus, only Staphylococcus aureus and Staphylococcus pseudintermedius have been clearly identified as intracellular in nonprofessional phagocytic cells (NPPCs), for which the mechanism is mainly fibronectin-binding dependent. Here, we used bioinformatics tools to search for possible new fibronectin-binding proteins (FnBP-like) in other Staphylococcus species. We found a protein in Staphylococcus delphini called Staphylococcus delphini surface protein Y (SdsY). This protein shares 68% identity with the Staphylococcus pseudintermedius surface protein D (SpsD), 36% identity with S. aureus FnBPA, and 39% identity with S. aureus FnBPB. The SdsY protein possesses the typical structure of FnBP-like proteins, including an N-terminal signal sequence, an A domain, a characteristic repeated pattern, and an LPXTG cell wall anchor motif. The level of adhesion to immobilized fibronectin was significantly higher in all S. delphini strains tested than in the fibronectin-binding-deficient S. aureus DU5883 strain. By using a model of human osteoblast infection, the level of internalization of all strains tested was significantly higher than with the invasive-incompetent S. aureus DU5883. These findings were confirmed by phenotype restoration after transformation of DU5883 by a plasmid expression vector encoding the SdsY repeats. Additionally, using fibronectin-depleted serum and murine osteoblast cell lines deficient for the β₁ integrin, the involvement of fibronectin and β₁ integrin was demonstrated in S. delphini internalization. The present study demonstrates that additional staphylococcal species are able to invade NPPCs and proposes a method to identify FnBP-like proteins.

Staphylococcus aureus Infects Osteoclasts and Replicates Intracellularly

The inflammation of bone tissue is called osteomyelitis, and most cases are caused by an infection with the bacterium Staphylococcus aureus. To date, the bone-building cells, osteoblasts, have been implicated in the progression of these infections, but not much is known about how the bone-resorbing cells, osteoclasts, participate. In this study, we show that S. aureus can infect osteoclasts and proliferate inside these cells, whereas bone-residing macrophages, immune cells related to osteoclasts, destroy the bacteria. These findings elucidate a unique role for osteoclasts to harbor bacteria during infection, providing a possible mechanism by which bacteria could evade destruction by the immune system.

Osteomyelitis (OM), or inflammation of bone tissue, occurs most frequently as a result of bacterial infection and severely perturbs bone structure. OM is predominantly caused by Staphylococcus aureus, and even with proper treatment, OM has a high rate of recurrence and chronicity. While S. aureus has been shown to infect osteoblasts, it remains unclear whether osteoclasts (OCs) are also a target of intracellular infection. Here, we demonstrate the ability of S. aureus to intracellularly infect and divide within OCs. OCs were differentiated from bone marrow macrophages (BMMs) by exposure to receptor activator of nuclear factor kappa-B ligand (RANKL). By utilizing an intracellular survival assay and flow cytometry, we found that at 18 h postinfection the intracellular burden of S. aureus increased dramatically in cells with at least 2 days of RANKL exposure, while the bacterial burden decreased in BMMs. To further explore the signals downstream of RANKL, we manipulated factors controlling OC differentiation, NFATc1 and alternative NF-κB, and found that intracellular bacterial growth correlates with NFATc1 levels in RANKL-treated cells. Confocal and time-lapse microscopy in mature OCs showed a range of intracellular infection that correlated inversely with S. aureus-phagolysosome colocalization. The propensity of OCs to become infected, paired with their diminished bactericidal capacity compared to BMMs, could promote OM progression by allowing S. aureus to evade initial immune regulation and proliferate at the periphery of lesions where OCs are most abundant.

Impact of Bacterial Infections on Osteogenesis: Evidence From In Vivo Studies

The clinical impact of bacterial infections on bone regeneration has been incompletely quantified and documented. As a result, controversy exists about the optimal treatment strategy to maximize healing of a contaminated defect. Animal models are extremely useful in this respect, as they can elucidate how a bacterial burden influences quantitative healing of various types of defects relative to non-infected controls. Moreover, they may demonstrate how antibacterial treatment and/or bone grafting techniques facilitate the osteogenic response in the harsh environment of a bacterial infection. Finally, it a well-known contradiction that osteomyelitis is characterized by uncontrolled bone remodeling and bone loss, but at the same time, it can be associated with excessive new bone apposition. Animal studies can provide a better understanding of how osteolytic and osteogenic responses are related to each other during infection. This review discusses the in vivo impact of bacterial infection on osteogenesis by addressing the following questions (i) How does osteomyelitis affect the radiographic bone appearance? (ii) What is the influence of bacterial infection on histological bone healing? (iii) How do bacterial infections affect quantitative bone healing? (iv) What is the effect of antibacterial treatment on the healing outcome during infection? (v) What is the efficacy of osteoinductive proteins in infected bones? (vi) What is the balance between the osteoclastic and osteoblastic response during bacterial infections? (vii) What is the mechanism of the observed pro-osteogenic response as observed in osteomyelitis? © 2019 The Authors. Journal of Orthopaedic Research^© published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:2067-2076, 2019.

Implant infections: adhesion, biofilm formation and immune evasion

Medical device-associated infections account for a large proportion of hospital-acquired infections. A variety of opportunistic pathogens can cause implant infections, depending on the type of the implant and on the anatomical site of implantation. The success of these versatile pathogens depends on rapid adhesion to virtually all biomaterial surfaces and survival in the hostile host environment. Biofilm formation on implant surfaces shelters the bacteria and encourages persistence of infection. Furthermore, implant-infecting bacteria can elude innate and adaptive host defences as well as biocides and antibiotic chemotherapies. In this Review, we explore the fundamental pathogenic mechanisms underlying implant infections, highlighting orthopaedic implants and Staphylococcus aureus as a prime example, and discuss innovative targets for preventive and therapeutic strategies.

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.

Capsule-Integrated Polypeptide Multilayer Films for Effective pH-Responsive Multiple Drug Co-Delivery

Many applications using drug-carrying biomedical materials require on-demand, localized delivery of multiple therapeutic agents in precisely controlled and patient-specific time sequences, especially after assembly of the delivery vehicles; however, creating such materials has proven extremely challenging. Here, we report a novel strategy to create polypeptide multilayer films integrated with capsules as vehicles for co-delivery of multiple drugs using layer-by-layer self-assembly technology. Our approach allows the multilayered polypeptide nanofilms and preimpregnated capsules to assemble into innovative biomedical materials with high and controllable loading of multiple drugs at any time postpreparation and to achieve pH-responsive and sustained release. The resulting capsule-integrated polypeptide multilayer films effectively co-deliver various drugs with very different properties, including proteins (e.g., growth factors) and nanoparticles, achieving bovine serum albumin loading of 80 μg cm-2 and release of 2 weeks, and histone loading of 100 μg cm-2 and release of 6 weeks; which also enable Staphylococcus aureus killing efficacy of 83% while maintaining osteoblast viability of >85% with silver nanoparticle delivery; and >5-fold cell adhesion and proliferation capability with live cell percentage of >90% via human recombinant bone morphogenetic protein 2 delivery. The successful development of such fascinating materials can not only function as advanced nanocoatings to reduce two major complications of orthopedic bone injuries (i.e., infection and delayed bone regeneration) but also provide new insights into the design and development of multifunctional materials for various other biomedical applications.

Targeting intracellular Staphylococcus aureus to lower recurrence of orthopaedic infection

Staphylococcus aureus is often found in orthopaedic infections and may be protected from commonly prescribed antibiotics by forming biofilms or growing intracellularly within osteoblasts. To investigate the effect of non-antibiotic compounds in conjunction with antibiotics to clear intracellular and biofilm forming S. aureus causing osteomyelitis. SAOS-2 osteoblast-like cell lines were infected with S. aureus BB1279. Antibiotics (vancomycin, VAN; and dicloxacillin, DICLOX), bacterial efflux pump inhibitors (piperine, PIP; carbonyl cyanide m-chlorophenyl hydrazone, CCCP), and bone morphogenetic protein (BMP-2) were evaluated individually and in combination to kill intracellular bacteria. We present direct evidence that after gentamicin killed extracellular planktonic bacteria and antibiotics had been stopped, seeding from the infected osteoblasts grew as biofilms. VAN was ineffective in treating the intracellular bacteria even at 10× MIC; however in presence of PIP or CCCP the intracellular S. aureus was significantly reduced. Bacterial efflux pump inhibitors (PIP and CCCP) were effective in enhancing permeability of antibiotics within the osteoblasts and facilitated killing of intracellular S. aureus. Confocal laser scanning microscopy (CLSM) showed increased uptake of propidium iodide within osteoblasts in presence of PIP and CCCP. BMP-2 had no effect on growth of S. aureus either alone or in combination with antibiotics. Combined application of antibiotics and natural agents could help in the treatment of osteoblast infected intracellular bacteria and biofilms associated with osteomyelitis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1086-1092, 2018.

Cellular Pharmacokinetics and Intracellular Activity of Gepotidacin against Staphylococcus aureus Isolates with Different Resistance Phenotypes in Models of Cultured Phagocytic Cells

Gepotidacin (GSK2140944), a novel triazaacenaphthylene bacterial topoisomerase inhibitor, is currently in clinical development for the treatment of bacterial infections. This study examined in vitro its activity against intracellular Staphylococcus aureus (involved in the persistent character of skin and skin structure infections) by use of a pharmacodynamic model and in relation to cellular pharmacokinetics in phagocytic cells. Compared to oxacillin, vancomycin, linezolid, daptomycin, azithromycin, and moxifloxacin, gepotidacin was (i) more potent intracellularly (the apparent bacteriostatic concentration [C_s] was reached at an extracellular concentration about 0.7× its MIC and was not affected by mechanisms of resistance to the comparators) and (ii) caused a maximal reduction of the intracellular burden (maximum effect) of about −1.6 log₁₀ CFU (which was better than that caused by linezolid, macrolides, and daptomycin and similar to that caused by moxifloxacin). After 24 h of incubation of infected cells with antibiotics at 100× their MIC, the intracellular persisting fraction was <0.1% with moxifloxacin, 0.5% with gepotidacin, and >1% with the other drugs. The accumulation and efflux of gepotidacin in phagocytes were very fast (k_in and k_out, ∼0.3 min⁻¹; the plateau was reached within 15 min) but modest (intracellular concentration-to-extracellular concentration ratio, ∼1.6). In cell fractionation studies, about 40 to 60% of the drug was recovered in the soluble fraction and ∼40% was associated with lysosomes in uninfected cells. In infected cells, about 20% of cell-associated gepotidacin was recovered in a sedimentable fraction that also contained bacteria. This study highlights the potential for further study of gepotidacin to fight infections where intracellular niches may play a determining role in bacterial persistence and relapses.

Pharmacodynamics of ceftazidime/avibactam against extracellular and intracellular forms of Pseudomonas aeruginosa

Objectives

When tested in broth, avibactam reverses ceftazidime resistance in many Pseudomonas aeruginosa that express ESBLs. We examined whether similar reversal is observed against intracellular forms of P. aeruginosa .

Methods

Strains: reference strains; two engineered strains with basal non-inducible expression of AmpC and their isogenic mutants with stably derepressed AmpC; and clinical isolates with complete, partial or no resistance to reversion with avibactam. Pharmacodynamic model: 24 h concentration-response to ceftazidime [0.01-200 mg/L alone or with avibactam (4 mg/L)] of bacteria in broth or bacteria phagocytosed by THP-1 monocytes, with calculation of ceftazidime relative potency ( C s : concentration yielding a static effect) and maximal relative effect [ E max : cfu decrease at infinitely large antibiotic concentrations (efficacy in the model)] using the Hill equation. Cellular content of avibactam: quantification by LC-MS/MS.

Results

For both extracellular and intracellular bacteria, ceftazidime C s was always close to its MIC. For ceftazidime-resistant strains, avibactam addition shifted ceftazidime C s to values close to the MIC of the combination in broth. E max was systematically below the detection limit (-5 log 10 ) for extracellular bacteria, but limited to -1.3 log 10 for intracellular bacteria (except for two isolates) with no effect of avibactam. The cellular concentration of avibactam reflected extracellular concentration and was not influenced by ceftazidime (0-160 mg/L).

Conclusions

The potential for avibactam to inhibit β-lactamases does not differ for extracellular and intracellular forms of P. aeruginosa , denoting an unhindered access to its target in both situations. The loss of maximal relative efficacy of ceftazidime against intracellular P. aeruginosa was unrelated to resistance via avibactam-inhibitable β-lactamases.

Toxicity and oxidative stress responses induced by nano- and micro-CoCrMo particles

Particles on the nano- and micro-meter scales present unique cell-specific cellular effects (i.e.cytotoxicity and oxidative stress).

Molecular characterization and antibacterial effect of endophytic actinomycetes Nocardiopsis sp. GRG1 (KT235640) from brown algae against MDR strains of uropathogens

Our study is to evaluate the potential bioactive compound of Nocardiopsis sp. GRG1 (KT235640) and its antibacterial activity against multi drug resistant strains (MDRS) on urinary tract infections (UTIs). Two brown algae samples were collected and were subjected to isolation of endophytic actinomycetes. 100 strains of actinomycetes were isolated from algal samples based on observation of morphology and physiological characters. 40 strains were active in antagonistic activity against various clinical pathogens. Among the strains, 10 showed better antimicrobial activity against MDRS on UTIs. The secondary metabolite of Nocardiopsis sp. GRG1 (KT235640) has showed tremendous antibacterial activity against UTI pathogens compared to other strains. Influence of various growth parameters were used for synthesis of secondary metabolites, such as optimum pH 7, incubation time 5-7 days, temperature (30 °C), salinity (5%), fructose and mannitol as the suitable carbon and nitrogen sources. At 100 μg/ml concentration MIC of Nocardiopsis sp. GRG1 (KT235640) showed highest percentage of inhibition against Proteus mirabilis (85%), and E.coli, Staphylococcus auerues, Psuedomonas aeroginasa, Enterobactor sp and Coagulinase negative staphylococci 78-85% respectively.

Monitoring immune responses in a mouse model of fracture fixation with and without Staphylococcus aureus osteomyelitis

Post-traumatic bone fractures are commonly fixed with implanted devices to restore the anatomical position of bone fragments and aid in the healing process. Bacterial infection in this situation is a challenge for clinicians due to the need for aggressive antibiotic therapy, debridement of infected tissues, and the need to maintain fracture stability. The aim of this study was to monitor immune responses that occur during healing and during Staphylococcus aureus infection, in a clinically relevant murine model of fracture fixation. Skeletally mature C57bl/6 mice received a transverse osteotomy of the femur, which was treated with commercially available titanium fracture fixation plates and screws. In the absence of infection, healing of the fracture was complete within 35days and was characterized by elevated Interleukin (IL)-4 and Interferon-gamma secretion from bone-derived cells and expression of these same genes. In contrast, mice inoculated with S. aureus could not heal the fracture within the observation period and were found to develop typical signs of implant-associated bone infection, including biofilm formation on the implant and osteolysis of surrounding bone. The immune response to infection was characterized by a TH17-led bone response, and a pro-inflammatory cytokine-led Tumor necrosis factor (TNF)-α, Interleukin (IL)-1β) soft tissue response, both of which were ineffectual in clearing implant related bone and soft tissue infections respectively. In this murine model, we characterize the kinetics of pro-inflammatory responses to infection, secondary to bone trauma and surgery. A divergent local immune polarization is evident in the infected versus non-infected animals, with the immune response ultimately unable to clear the S. aureus infection.

Differential induction of inflammatory cytokines and reactive oxygen species in murine peritoneal macrophages and resident fresh bone marrow cells by acute staphylococcus aureus infection: contribution of toll-like receptor 2 (TLR2)

Among the known Toll-like receptors (TLRs), Toll-like receptor 2 (TLR2) is a key sensor for detecting Staphylococcus aureus invasion. But the function of TLR2 during S. aureus infection in different cell populations is unclear. Two different cell subtypes were chosen to study the interaction of S. aureus with TLR2 because macrophages are extremely different from one compartment to another and their capacity to respond to live bacteria or bacterial products differs from one site to another. The contribution of TLR2 to the host innate response against acute live S. aureus infection and heat-killed S. aureus (HKSA) using anti-TLR2 antibody in murine peritoneal macrophages and resident fresh bone marrow cells has been investigated here. TLR2 blocking before infection induces the release of interleukin (IL)-10 by macrophages thereby inhibiting excessive production of oxidants by activating antioxidant enzymes. TLR2-blocked peritoneal macrophages showed impaired release of tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and IL-6 in response to both live and heat-killed S. aureus infection except bone marrow cells. TLR2-mediated free radical production and killing of S. aureus were modulated by TLR2 blocking in peritoneal macrophages and resident bone marrow cells. This study supported that S. aureus persists in resident bone marrow cells in a state of quiescence.

Pathogenic potential of Escherichia coli clinical strains from orthopedic implant infections towards human osteoblastic cells

Escherichia coli is one of the first causes of Gram-negative orthopedic implant infections (OII), but little is known about the pathogenicity of this species in such infections that are increasing due to the ageing of the population. We report how this pathogen interacts with human osteoblastic MG-63 cells in vitro, by comparing 20 OII E. coli strains to two Staphylococcus aureus and two Pseudomonas aeruginosa strains. LDH release assay revealed that 6/20 (30%) OII E. coli induced MG-63 cell lysis whereas none of the four control strains was cytotoxic after 4 h of coculture. This high cytotoxicity was associated with hemolytic properties and linked to hlyA gene expression. We further showed by gentamicin protection assay and confocal microscopy that the non-cytotoxic E. coli were not able to invade MG-63 cells unlike S. aureus strains (internalization rate <0.01% for the non-cytotoxic E. coli versus 8.88 ± 2.31% and 4.60 ± 0.42% for both S. aureus). The non-cytotoxic E. coli also demonstrated low adherence rates (<7%), the most adherent E. coli eliciting higher IL-6 and TNF-α mRNA expression in the osteoblastic cells. Either highly cytotoxic or slightly invasive OII E. coli do not show the same infection strategies as S. aureus towards osteoblasts.

Rifamycin Derivatives Are Effective Against Staphylococcal Biofilms In Vitro and Elutable From PMMA

BACKGROUND

Local antimicrobial delivery through polymethylmethacrylate beads (PMMA), commonly vancomycin, is used for the treatment of contaminated open fractures but has limited activity against Staphylococcus aureus biofilms, which occur commonly in such fractures. Rifamycins have activity against biofilms and are an effective treatment for osteoarticular infections involving staphylococcal biofilms, but there are limited studies evaluating the activity of rifamycin derivatives, other than rifampin, against biofilms of S. aureus and evaluating incorporation of these drugs into PMMA for treatment of contaminated open fractures.

QUESTIONS/PURPOSES

(1) Are rifamycin derivatives effective against established biofilms of clinical isolates of S. aureus? (2) Can PMMA be used as a carrier for rifamycin derivatives?

METHODS

Biofilms were developed and evaluated for susceptibility to a panel of antimicrobials in vitro using the minimum biofilm eradication concentration high-throughput model. Susceptibility was assessed by measuring bacterial recovery at 6 and 24 hours after antimicrobial treatment. Activity of rifamycin derivatives against intracellular bacteria was also evaluated using a gentamicin protection assay. Evaluation of PMMA as a carrier for rifampin and rifamycin derivatives was determined by assessing the curing time subsequent to loading of rifamycins and characterizing the release kinetics of rifamycins at daily intervals for 14 days from PMMA by performing bioassays.

RESULTS

Rifamycin derivatives between 1 and 8 µg/mL reduced bacteria within biofilms 5- to 9-logs and prevented bacterial recovery up to 24 hours post-treatment, indicating near to complete eradication of biofilms. Rifamycin derivatives at 32 µg/mL had activity against intracellular staphylococci, significantly reducing the number of internalized bacteria with limited effects on osteoblast viability. Rifampin was the only rifamycin observed to have a suitable release profile from PMMA, releasing 49% of the total antibiotic and maintaining a sustained released profile up to 14 days at a mean 28 ± 6 μg/mL.

CONCLUSIONS

Rifampin can be incorporated into PMMA and eluted at concentrations effective against biofilms and intracellular staphylococci.

CLINICAL RELEVANCE

Our in vitro findings suggest that local delivery of rifampin may be an effective strategy for the prevention and/or treatment of open fractures where S. aureus biofilms might develop. Clinical studies are needed to characterize what role this approach might have in the prevention and treatment of infections involving biofilms.

Local delivery of the cationic steroid antibiotic CSA-90 enables osseous union in a rat open fracture model of Staphylococcus aureus infection

BACKGROUND

Treatment of infected open fractures remains a major clinical challenge. In this study, we investigated the novel broad-spectrum antibiotic CSA-90 (cationic steroid antibiotic-90) as an antimicrobial agent.

METHODS

CSA-90 was screened in an osteoblast cell culture model for effects on differentiation and mineralization. Local delivery of CSA-90 was then tested alone and in combination with recombinant human bone morphogenetic protein-2 (rhBMP-2) in a mouse ectopic bone formation model (n=40 mice) and in a rat open fracture model inoculated with pathogenic Staphylococcus aureus (n=84 rats).

RESULTS

CSA-90 enhanced matrix mineralization in cultured osteoblasts and increased rhBMP-2-induced bone formation in vivo. All animals in which an open fracture had been inoculated with Staphylococcus aureus and not treated with local CSA-90, including those treated with rhBMP-2, had to be culled prior to the experimental end point (six weeks) because of localized osteolysis and deterioration of overall health, whereas CSA-90 prevented establishment of infection in all open fractures in which it was used (p≤0.012). Increased union rates were seen for the fractures treated with rhBMP-2 or with the combination of rhBMP-2 and CSA-90 compared with that observed for the fractures treated with CSA-90 alone (p=0.04).

CONCLUSIONS

CSA-90 can promote osteogenesis and be used for prevention of Staphylococcus aureus infection in preclinical models.

CLINICAL RELEVANCE

Local delivery of CSA-90 represents a novel strategy for prevention of infection and may have specific benefits in the context of orthopaedic injuries.

Differential responses of osteoblasts and macrophages upon Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus (S. aureus) is one of the primary causes of bone infections which are often chronic and difficult to eradicate. Bacteria like S. aureus may survive upon internalization in cells and may be responsible for chronic and recurrent infections. In this study, we compared the responses of a phagocytic cell (i.e. macrophage) to a non-phagocytic cell (i.e. osteoblast) upon S. aureus internalization.

RESULTS

We found that upon internalization, S. aureus could survive for up to 5 and 7 days within macrophages and osteoblasts, respectively. Significantly more S. aureus was internalized in macrophages compared to osteoblasts and a significantly higher (100 fold) level of live intracellular S. aureus was detected in macrophages compared to osteoblasts. However, the percentage of S. aureus survival after infection was significantly lower in macrophages compared to osteoblasts at post-infection days 1-6. Interestingly, macrophages had relatively lower viability in shorter infection time periods (i.e. 0.5-4 h; significant at 2 h) but higher viability in longer infection time periods (i.e. 6-8 h; significant at 8 h) compared to osteoblasts. In addition, S. aureus infection led to significant changes in reactive oxygen species production in both macrophages and osteoblasts. Moreover, infected osteoblasts had significantly lower alkaline phosphatase activity at post-infection day 7 and infected macrophages had higher phagocytosis activity compared to non-infected cells.

CONCLUSIONS

S. aureus was found to internalize and survive within osteoblasts and macrophages and led to differential responses between osteoblasts and macrophages. These findings may assist in evaluation of the pathogenesis of chronic and recurrent infections which may be related to the intracellular persistence of bacteria within host cells.