Combinatory antibiotic therapy increases rate of bacterial kill but not final outcome in a novel mouse model of Staphylococcus aureus spinal implant infection

Background

Management of spine implant infections (SII) are challenging. Explantation of infected spinal hardware can destabilize the spine, but retention can lead to cord compromise and biofilm formation, complicating management. While vancomycin monotherapy is commonly used, in vitro studies have shown reduced efficacy against biofilm compared to combination therapy with rifampin. Using an established in vivo mouse model of SII, we aim to evaluate whether combination therapy has increased efficacy compared to both vancomycin alone and infected controls.

Methods

An L-shaped, Kirschner-wire was transfixed into the L4 spinous process of 12-week-old C57BL/6 mice, and inoculated with bioluminescent Staphylococcus aureus. Mice were randomized into a vancomycin group, a combination group with vancomycin plus rifampin, or a control group receiving saline. Treatment began on post-operative day (POD) 7 and continued through POD 14. In vivo imaging was performed to monitor bioluminescence for 35 days. Colony-forming units (CFUs) were cultured on POD 35.

Results

Bioluminescence peaked around POD 7 for all groups. The combination group had a 10-fold decrease in signal by POD 10. The vancomycin and control groups reached similar levels on POD 17 and 21, respectively. On POD 25 the combination group dropped below baseline, but rebounded to the same level as the other groups, demonstrating a biofilm-associated infection by POD 35. Quantification of CFUs on POD 35 confirmed an ongoing infection in all three groups.

Conclusions

Although both therapies were initially effective, they were not able to eliminate implant biofilm bacteria, resulting in a rebound infection after antibiotic cessation. This model shows, for the first time, why histologic-based, static assessments of antimicrobials can be misleading, and the importance of longitudinal tracking of infection. Future studies can use this model to test combinations of antibiotic therapies to see if they are more effective in eliminating biofilm prior to human trials.

Application of Fluorescent Protein Expressing Strains to Evaluation of Anti-Tuberculosis Therapeutic Efficacy In Vitro and In Vivo

The slow growth of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), hinders development of new diagnostics, therapeutics and vaccines. Using non-invasive real-time imaging technologies to monitor the disease process in live animals would facilitate TB research in all areas. We developed fluorescent protein (FP) expressing Mycobacterium bovis BCG strains for in vivo imaging, which can be used to track bacterial location, and to quantify bacterial load in live animals. We selected an optimal FP for in vivo imaging, by first cloning six FPs: tdTomato, mCherry, mPlum, mKate, Katushka and mKeima, into mycobacteria under either a mycobacterial Hsp60 or L5 promoter, and compared their fluorescent signals in vitro and in vivo. Fluorescence from each FP-expressing strain was measured with a multimode reader using the optimal excitation and emission wavelengths for the FP. After normalizing bacterial numbers with optical density, the strain expressing L5-tdTomato displayed the highest fluorescence. We used the tdTomato-labeled M. bovis BCG to obtain real-time images of pulmonary infections in living mice and rapidly determined the number of bacteria present. Further comparison between L5-tdTomato and Hsp60-tdTomato revealed that L5-tdTomato carried four-fold more tdTomato gene copies than Hsp60-tdTomato, which eventually led to higher protein expression of tdTomato. Evaluating anti-TB efficacy of rifampicin and isoniazid therapy in vitro and in vivo using the L5-tdTomato strain demonstrated that this strain can be used to identify anti-TB therapeutic efficacy as quickly as 24 h post-treatment. These M. bovis BCG reporter strains represent a valuable new tool for evaluation of therapeutics, vaccines and virulence.

In Vivo Bioluminescence Imaging of Intratumoral Bacteria

This chapter describes the use of whole-body bioluminescent imaging (BLI) for the study of bacterial trafficking in live mice, with an emphasis on the use of bacteria in therapy of cancer. Bacteria present an attractive class of vector for cancer therapy, possessing a natural ability to grow preferentially within tumors following systemic administration. Bacteria engineered to express the lux gene cassette permit BLI detection of the bacteria and tumor sites concurrently. The location and levels of bacteria within tumors over time can be readily examined, visualized in two or three dimensions. The method is applicable to a wide range of bacterial species and tumor xenograft types. This article describes the protocol for analysis of bioluminescent bacteria within subcutaneous tumor-bearing mice. This powerful, and inexpensive, real-time imaging strategy represents an ideal method for the study of bacteria in vivo in the context of cancer research. This protocol outlines the procedure for studying lux-tagged Escherichia coli and Bifidobacterium breve in mice, demonstrating the spatial and temporal readout from 2D and 3D BLI achievable with whole-body in vivo luminescence imaging.

Multi-Modal Imaging with a Toolbox of Influenza A Reporter Viruses

Reporter viruses are useful probes for studying multiple stages of the viral life cycle. Here we describe an expanded toolbox of fluorescent and bioluminescent influenza A reporter viruses. The enhanced utility of these tools enabled kinetic studies of viral attachment, infection, and co-infection. Multi-modal bioluminescence and positron emission tomography–computed tomography (PET/CT) imaging of infected animals revealed that antiviral treatment reduced viral load, dissemination, and inflammation. These new technologies and applications will dramatically accelerate in vitro and in vivo influenza virus studies.

Development of a Bioluminescent Nitroreductase Probe for Preclinical Imaging

Bacterial nitroreductases (NTRs) have been widely utilized in the development of novel antibiotics, degradation of pollutants, and gene-directed enzyme prodrug therapy (GDEPT) of cancer that reached clinical trials. In case of GDEPT, since NTR is not naturally present in mammalian cells, the prodrug is activated selectively in NTR-transformed cancer cells, allowing high efficiency treatment of tumors. Currently, no bioluminescent probes exist for sensitive, non-invasive imaging of NTR expression. We therefore developed a "NTR caged luciferin" (NCL) probe that is selectively reduced by NTR, producing light proportional to the NTR activity. Here we report successful application of this probe for imaging of NTR in vitro, in bacteria and cancer cells, as well as in vivo in mouse models of bacterial infection and NTR-expressing tumor xenografts. This novel tool should significantly accelerate the development of cancer therapy approaches based on GDEPT and other fields where NTR expression is important.

Targeted imaging of bacterial infections: advances, hurdles and hopes

Bacterial infections represent an increasing problem in modern health care, in particular due to ageing populations and accumulating bacterial resistance to antibiotics. Diagnosis is rarely straightforward and consequently treatment is often delayed or indefinite. Therefore, novel tools that can be clinically implemented are urgently needed to accurately and swiftly diagnose infections. Especially, the direct imaging of infections is an attractive option. The challenge of specifically imaging bacterial infections in vivo can be met by targeting bacteria with an imaging agent. Here we review the current status of targeted imaging of bacterial infections, and we discuss advantages and disadvantages of the different approaches. Indeed, significant progress has been made in this field and the clinical implementation of targeted imaging of bacterial infections seems highly feasible. This was recently highlighted by the use of so-called smart activatable probes and a fluorescently labelled derivative of the antibiotic vancomycin. A major challenge remains the selection of the best imaging probes, and we therefore present a set of target selection criteria for clinical implementation of targeted bacterial imaging. Altogether, we conclude that the spectrum of potential applications for targeted bacterial imaging is enormous, ranging from fundamental research on infectious diseases to diagnostic and therapeutic applications.

Combined in vivo optical and µCT imaging to monitor infection, inflammation, and bone anatomy in an orthopaedic implant infection in mice

Multimodality imaging has emerged as a common technological approach used in both preclinical and clinical research. Advanced techniques that combine in vivo optical and μCT imaging allow the visualization of biological phenomena in an anatomical context. These imaging modalities may be especially useful to study conditions that impact bone. In particular, orthopaedic implant infections are an important problem in clinical orthopaedic surgery. These infections are difficult to treat because bacterial biofilms form on the foreign surgically implanted materials, leading to persistent inflammation, osteomyelitis and eventual osteolysis of the bone surrounding the implant, which ultimately results in implant loosening and failure. Here, a mouse model of an infected orthopaedic prosthetic implant was used that involved the surgical placement of a Kirschner-wire implant into an intramedullary canal in the femur in such a way that the end of the implant extended into the knee joint. In this model, LysEGFP mice, a mouse strain that has EGFP-fluorescent neutrophils, were employed in conjunction with a bioluminescent Staphylococcus aureus strain, which naturally emits light. The bacteria were inoculated into the knee joints of the mice prior to closing the surgical site. In vivo bioluminescent and fluorescent imaging was used to quantify the bacterial burden and neutrophil inflammatory response, respectively. In addition, μCT imaging was performed on the same mice so that the 3D location of the bioluminescent and fluorescent optical signals could be co-registered with the anatomical μCT images. To quantify the changes in the bone over time, the outer bone volume of the distal femurs were measured at specific time points using a semi-automated contour based segmentation process. Taken together, the combination of in vivo bioluminescent/fluorescent imaging with μCT imaging may be especially useful for the noninvasive monitoring of the infection, inflammatory response and anatomical changes in bone over time.

Azithromycin inhibits nuclear factor-κB activation during lung inflammation: an in vivo imaging study

We studied in vivo the potential involvement of nuclear factor-κB (NF-κB) pathway in the molecular mechanism of the anti-inflammatory and immunomodulatory activity of azithromycin in the lung. Mice transiently transfected with the luciferase gene under the control of a NF-κB responsive element were used to assess in vivo NF-κB activation by bioluminescence imaging. Bioluminescence as well as inflammatory cells and concentrations of proinflammatory cytokines in bronchoalveolar lavage fluids, were monitored in an acute model of pulmonary inflammation resulting from intratracheal instillation of lipopolysaccharide. Lipopolysaccharide (LPS) instillation induced a marked increase in lung bioluminescence in mice transiently transfected with the luciferase gene under the control of an NF-κB responsive element, with significant luciferase expression in resident cells such as endothelial and epithelial cells, as assessed by duoplex immunofluorescence staining. Activation of NF-κB and inflammatory cell lung infiltration linearly correlated when different doses of bortezomib were used to inhibit NF-κB activation. Pretreatment with azithromycin significantly decreased lung bioluminescence and airways cell infiltration induced by LPS, also reducing proinflammatory cytokines concentrations in bronchoalveolar lavages and inhibiting NF-κB nuclear translocation. The results obtained using a novel approach to monitor NF-κB activation, provided, for the first time, in vivo evidence that azithromycin treatment results in pulmonary anti-inflammatory activity associated with the inhibition of NF-κB activation in the lung.

Efficacy of enrofloxacin in a mouse model of sepsis

We examined the efficacy of enrofloxacin administered by 2 different routes in a mouse model of sepsis. Male CD1 mice were infected with a bioluminescent strain of enteropathogenic Escherichia coli and treated with enrofloxacin either by injection or in drinking water. Peak serum levels were evaluated by using HPLC. Mice were monitored for signs of clinical disease, and infections were monitored by using bioluminescence imaging. Serum levels of enrofloxacin and the active metabolite ciprofloxacin were greater in the group treated by injection than in controls or the groups treated by administration in drinking water. Survival of the group treated with enrofloxacin injection was greater than that of controls and groups treated with enrofloxacin in the drinking water. Bioluminescence in the group treated with enrofloxacin injection was less than that in the groups treated with oral administration at 12 h and in the groups treated orally and the control group at 16 h. According to these findings, we recommend the use of injectable enrofloxacin at 5 mg/kg SC for mice with systemic infections.

α-Intercalated cells defend the urinary system from bacterial infection

α-Intercalated cells (A-ICs) within the collecting duct of the kidney are critical for acid-base homeostasis. Here, we have shown that A-ICs also serve as both sentinels and effectors in the defense against urinary infections. In a murine urinary tract infection model, A-ICs bound uropathogenic E. coli and responded by acidifying the urine and secreting the bacteriostatic protein lipocalin 2 (LCN2; also known as NGAL). A-IC-dependent LCN2 secretion required TLR4, as mice expressing an LPS-insensitive form of TLR4 expressed reduced levels of LCN2. The presence of LCN2 in urine was both necessary and sufficient to control the urinary tract infection through iron sequestration, even in the harsh condition of urine acidification. In mice lacking A-ICs, both urinary LCN2 and urinary acidification were reduced, and consequently bacterial clearance was limited. Together these results indicate that A-ICs, which are known to regulate acid-base metabolism, are also critical for urinary defense against pathogenic bacteria. They respond to both cystitis and pyelonephritis by delivering bacteriostatic chemical agents to the lower urinary system.

Transfusion of stored blood impairs host defenses against Gram-negative pathogens in mice

BACKGROUND

Although human red blood cell (RBC) units may be refrigerator stored for up to 42 days, transfusion of older RBCs acutely delivers a large bolus of iron to mononuclear phagocytes. Similarly, iron dextran circulates in plasma for hours to days and is progressively cleared by mononuclear phagocytes, which return iron to plasma. Finally, malaria infection continuously delivers iron to macrophages by intra- and extravascular hemolysis. Studies suggest that iron administration increases infectious risk.

STUDY DESIGN AND METHODS

To assess the effects of increased iron availability on susceptibility to infection, we infected mice with model Gram-negative intracellular or extracellular pathogens (Salmonella typhimurium or Escherichia coli, respectively), accompanied by RBC transfusion, iron dextran administration, or malarial coinfection.

RESULTS

In our mouse models, transfusion of older RBCs exacerbates infection with both Gram-negative pathogens. Although iron dextran exacerbates E. coli infection to a similar extent as transfusion of corresponding amounts of iron, higher iron doses are required to produce comparable effects with S. typhimurium. Coinfection of mice with Plasmodium yoelii and S. typhimurium produces overwhelming Salmonella sepsis. Finally, treating mice with antibiotics abrogates the enhancing effect on E. coli infection of both older RBC transfusion and iron dextran administration.

CONCLUSIONS

Transfusion of older RBCs exacerbates Gram-negative infection to a similar extent as malaria coinfection or iron dextran administration. Appropriate antibiotic therapy abrogates the effect of older RBC transfusions on infection with E. coli. Iron delivery to macrophages may be an underappreciated mechanism mediating, at least some, adverse effects of RBC transfusions.

Opening up the optical imaging window using nano-luciferin

PURPOSE

The objective of this study was to formulate nanoparticles of D-luciferin (Nano-Luc), DiR (Nano-DiR) and dual functional nanoparticles with DiR and luciferin (Nano-LucDiR) for in-vivo imaging as well as tracking of the nanoparticles in tumors.

METHODS

Nano-Luc and Nano-LucDiR were prepared using different lipids, and subsequently characterized for loading and entrapment efficiency, physical properties, release profile, toxicity and stability. We utilized Response Surface Methodology (RSM) to optimize the nanoparticles using design of experiment (DOE Vr.8.0). Nano-Luc was evaluated against free luciferin to establish its pharmacokinetic parameters in mice. In-vivo imaging of tumors and tracking of nanoparticles was carried out with an IVIS® Spectrum-CT (Caliper) using xenograft, orthotopic and metastatic tumor models in BALB/c nude mice with different cell lines and different routes of nanoparticle administration (subcutaneous, intraperitoneal and intravenous).

RESULTS

Particle size of both Nano-Luc and Nano-LucDiR were found to be <200 nm. Nano-Luc formulation showed a slow and controlled release upto 72 h (90%) in vitro. The optimized Nano-Luc had loading efficiency of 5.0 mg/ml with 99% encapsulation efficiency. Nano-Luc and Nano-LucDiR formulations had good shelf stability. Nano-Luc and Nano-LucDiR enhanced plasma half-life of luciferin compared to free luciferin thus providing longer circulation of luciferin in plasma enabling imaging of tumors for more than 24 h. Nano-LucDiR allowed simultaneous bioluminescent and fluorescent imaging to be conducted, with three-dimensional reconstruct of tumors without losing either signal during the acquisition time.

CONCLUSION

Nano-Luc and Nano-LucDiR allowed prolonged reproducible in-vivo imaging of tumors, especially during multimodality 3D imaging.

Real-time in vivo imaging of invasive- and biomaterial-associated bacterial infections using fluorescently labelled vancomycin

Invasive and biomaterial-associated infections in humans are often difficult to diagnose and treat. Here, guided by recent advances in clinically relevant optical imaging technologies, we explore the use of fluorescently labelled vancomycin (vanco-800CW) to specifically target and detect infections caused by Gram-positive bacteria. The application potential of vanco-800CW for real-time in vivo imaging of bacterial infections is assessed in a mouse myositis model and a human post-mortem implant model. We show that vanco-800CW can specifically detect Gram-positive bacterial infections in our mouse myositis model, discriminate bacterial infections from sterile inflammation in vivo and detect biomaterial-associated infections in the lower leg of a human cadaver. We conclude that vanco-800CW has a high potential for enhanced non-invasive diagnosis of infections with Gram-positive bacteria and is a promising candidate for early-phase clinical trials.

4D multimodality imaging of Citrobacter rodentium infections in mice

This protocol outlines the steps required to longitudinally monitor a bioluminescent bacterial infection using composite 3D diffuse light imaging tomography with integrated μCT (DLIT-μCT) and the subsequent use of this data to generate a four dimensional (4D) movie of the infection cycle. To develop the 4D infection movies and to validate the DLIT-μCT imaging for bacterial infection studies using an IVIS Spectrum CT, we used infection with bioluminescent C. rodentium, which causes self-limiting colitis in mice. In this protocol, we outline the infection of mice with bioluminescent C. rodentium and non-invasive monitoring of colonization by daily DLIT-μCT imaging and bacterial enumeration from feces for 8 days. The use of the IVIS Spectrum CT facilitates seamless co-registration of optical and μCT scans using a single imaging platform. The low dose μCT modality enables the imaging of mice at multiple time points during infection, providing detailed anatomical localization of bioluminescent bacterial foci in 3D without causing artifacts from the cumulative radiation. Importantly, the 4D movies of infected mice provide a powerful analytical tool to monitor bacterial colonization dynamics in vivo.

Use of optical imaging to progress novel therapeutics to the clinic

There is an undisputed need for employment and improvement of robust technology for real-time analyses of therapeutic delivery and responses in clinical translation of gene and cell therapies. Over the past decade, optical imaging has become the in vivo imaging modality of choice for many preclinical laboratories due to its efficiency, practicality and affordability, while more recently, the clinical potential for this technology is becoming apparent. This review provides an update on the current state of the art in in vivo optical imaging and discusses this rapidly improving technology in the context of it representing a translation enabler or indeed a future clinical imaging modality in its own right.

Bioluminescent bacterial imaging in vivo

This video describes the use of whole body bioluminesce imaging (BLI) for the study of bacterial trafficking in live mice, with an emphasis on the use of bacteria in gene and cell therapy for cancer. Bacteria present an attractive class of vector for cancer therapy, possessing a natural ability to grow preferentially within tumors following systemic administration. Bacteria engineered to express the lux gene cassette permit BLI detection of the bacteria and concurrently tumor sites. The location and levels of bacteria within tumors over time can be readily examined, visualized in two or three dimensions. The method is applicable to a wide range of bacterial species and tumor xenograft types. This article describes the protocol for analysis of bioluminescent bacteria within subcutaneous tumor bearing mice. Visualization of commensal bacteria in the Gastrointestinal tract (GIT) by BLI is also described. This powerful, and cheap, real-time imaging strategy represents an ideal method for the study of bacteria in vivo in the context of cancer research, in particular gene therapy, and infectious disease. This video outlines the procedure for studying lux-tagged E. coli in live mice, demonstrating the spatial and temporal readout achievable utilizing BLI with the IVIS system.

Monitoring bacterial burden, inflammation and bone damage longitudinally using optical and μCT imaging in an orthopaedic implant infection in mice

BACKGROUND

Recent advances in non-invasive optical, radiographic and μCT imaging provide an opportunity to monitor biological processes longitudinally in an anatomical context. One particularly relevant application for combining these modalities is to study orthopaedic implant infections. These infections are characterized by the formation of persistent bacterial biofilms on the implanted materials, causing inflammation, periprosthetic osteolysis, osteomyelitis, and bone damage, resulting in implant loosening and failure.

METHODOLOGY/PRINCIPAL FINDINGS

An orthopaedic implant infection model was used in which a titanium Kirshner-wire was surgically placed in femurs of LysEGFP mice, which possess EGFP-fluorescent neutrophils, and a bioluminescent S. aureus strain (Xen29; 1×10(3) CFUs) was inoculated in the knee joint before closure. In vivo bioluminescent, fluorescent, X-ray and μCT imaging were performed on various postoperative days. The bacterial bioluminescent signals of the S. aureus-infected mice peaked on day 19, before decreasing to a basal level of light, which remained measurable for the entire 48 day experiment. Neutrophil EGFP-fluorescent signals of the S. aureus-infected mice were statistically greater than uninfected mice on days 2 and 5, but afterwards the signals for both groups approached background levels of detection. To visualize the three-dimensional location of the bacterial infection and neutrophil infiltration, a diffuse optical tomography reconstruction algorithm was used to co-register the bioluminescent and fluorescent signals with μCT images. To quantify the anatomical bone changes on the μCT images, the outer bone volume of the distal femurs were measured using a semi-automated contour based segmentation process. The outer bone volume increased through day 48, indicating that bone damage continued during the implant infection.

CONCLUSIONS/SIGNIFICANCE

Bioluminescent and fluorescent optical imaging was combined with X-ray and μCT imaging to provide noninvasive and longitudinal measurements of the dynamic changes in bacterial burden, neutrophil recruitment and bone damage in a mouse orthopaedic implant infection model.

Bacterial vectors for imaging and cancer gene therapy: a review

The significant burden of resistance to conventional anticancer treatments in patients with advanced disease has prompted the need to explore alternative therapeutic strategies. The challenge for oncology researchers is to identify a therapy which is selective for tumors with limited toxicity to normal tissue. Engineered bacteria have the unique potential to overcome traditional therapies' limitations by specifically targeting tumors. It has been shown that bacteria are naturally capable of homing to tumors when systemically administered resulting in high levels of replication locally, either external to (non-invasive species) or within tumor cells (pathogens). Pre-clinical and clinical investigations involving bacterial vectors require relevant means of monitoring vector trafficking and levels over time, and development of bacterial-specific real-time imaging modalities are key for successful development of clinical bacterial gene delivery. This review discusses the currently available imaging technologies and the progress to date exploiting these for monitoring of bacterial gene delivery in vivo.

Pre-treatment with Bifidobacterium breve UCC2003 modulates Citrobacter rodentium-induced colonic inflammation and organ specificity

Citrobacter rodentium, which colonizes the gut mucosa via formation of attaching and effacing (A/E) lesions, causes transmissible colonic hyperplasia. The aim of this study was to evaluate whether prophylactic treatment with Bifidobacterium breve UCC2003 can improve the outcome of C. rodentium infection. Six-week-old albino C57BL/6 mice were pre-treated for 3 days with B. breve, challenged with bioluminescent C. rodentium and administered B. breve or PBS-C for 8 days post-infection; control mice were either administered B. breve and mock-infected with PBS, or mock-treated with PBS-C and mock-infected with PBS. C. rodentium colonization was monitored by bacterial enumeration from faeces and by a combination of both 2D bioluminescence imaging (BLI) and composite 3D diffuse light imaging tomography with µCT imaging (DLIT-µCT). At day 8 post-infection, colons were removed and assessed for crypt hyperplasia, histology by light microscopy, bacterial colonization by immunofluorescence, and A/E lesion formation by electron microscopy. Prophylactic administration of B. breve did not prevent C. rodentium colonization or A/E lesion formation. However, this treatment did alter C. rodentium distribution within the large intestine and significantly reduced colonic crypt hyperplasia at the peak of bacterial infection. These results show that B. breve could not competitively exclude C. rodentium, but reduced pathogen-induced colonic inflammation.

In vivo optical imaging in gene & cell therapy

Integral to the development of all gene therapy technologies is the ability to monitor gene delivery, in terms of distribution, levels and kinetics of vector transgene expression. This can be achieved to some extent at the preclinical level through use of traditional ex vivo analytical methods, but these hold several drawbacks, not least the requirement for death of experimental subjects for such end-point assays. Real-time in vivo analysis of reporter gene expression empowers the investigator with the ability to non-invasively assess gene delivery over time, as well as host responses to vector administration and therapeutic interventions. While there exist several technologies for such small animal monitoring, imaging of light emission from luminescent or fluorescent reporters has become the mainstay of preclinical imaging for gene therapy research. Optical imaging strategies represent powerful yet cost-efficient and convenient systems compared with alternative methods. Through tagging of vector and/or cells or interest with suitable reporter genes, both vector and host responses can be assessed in rapid, high-throughput analyses, providing spatial, temporal and quantitative read-out, without the need for radioactivity. In this review, we discuss the current state-of-the-art for optical technologies, describe related approaches employed in gene therapy research for a wide range of diseases, and outline the potential for this imaging modality in the progression of gene therapy as a medicine.

Mouse model of chronic post-arthroplasty infection: noninvasive in vivo bioluminescence imaging to monitor bacterial burden for long-term study

Post-arthroplasty infections are a devastating problem in orthopaedic surgery. While acute infections can be treated with a single stage washout and liner exchange, chronic infections lead to multiple reoperations, prolonged antibiotic courses, extended disability, and worse clinical outcomes. Unlike previous mouse models that studied an acute infection, this work aimed to develop a model of a chronic post-arthroplasty infection. To achieve this, a stainless steel implant in the knee joints of mice was inoculated with a bioluminescent Staphylococcus aureus strain (1 × 10(2) -1 × 10(4) colony forming units, CFUs) and in vivo imaging was used to monitor the bacterial burden for 42 days. Four different S. aureus strains were compared in which the bioluminescent construct was integrated in an antibiotic selection plasmid (ALC2906), the bacterial chromosome (Xen29 and Xen40), or a stable plasmid (Xen36). ALC2906 had increased bioluminescent signals through day 10, after which the signals became undetectable. In contrast, Xen29, Xen40, and Xen36 had increased bioluminescent signals through 42 days with the highest signals observed with Xen36. ALC2906, Xen29, and Xen40 induced significantly more inflammation than Xen36 as measured by in vivo enhanced green fluorescence protein (EGFP)-neutrophil flourescence of LysEGFP mice. All four strains induced comparable biofilm formation as determined by variable-pressure scanning electron microscopy. Using a titanium implant, Xen36 had higher in vivo bioluminescence signals than Xen40 but had similar biofilm formation and adherent bacteria. In conclusion, Xen29, Xen40, and especially Xen36, which had stable bioluminescent constructs, are feasible for long-term in vivo monitoring of bacterial burden and biofilm formation to study chronic post-arthroplasty infections and potential antimicrobial interventions.

High resolution in vivo bioluminescent imaging for the study of bacterial tumour targeting

The ability to track microbes in real time in vivo is of enormous value for preclinical investigations in infectious disease or gene therapy research. Bacteria present an attractive class of vector for cancer therapy, possessing a natural ability to grow preferentially within tumours following systemic administration. Bioluminescent Imaging (BLI) represents a powerful tool for use with bacteria engineered to express reporter genes such as lux. BLI is traditionally used as a 2D modality resulting in images that are limited in their ability to anatomically locate cell populations. Use of 3D diffuse optical tomography can localize the signals but still need to be combined with an anatomical imaging modality like micro-Computed Tomography (μCT) for interpretation.

In this study, the non-pathogenic commensal bacteria E.coli K-12 MG1655 and Bifidobacterium breve UCC2003, or Salmonella Typhimurium SL7207 each expressing the luxABCDE operon were intravenously (IV) administered to mice bearing subcutaneous (s.c) FLuc-expressing xenograft tumours. Bacterial lux signal was detected specifically in tumours of mice post IV-administration and bioluminescence correlated with the numbers of bacteria recovered from tissue. Through whole body imaging for both lux and FLuc, bacteria and tumour cells were co-localised. 3D BLI and μCT image analysis revealed a pattern of multiple clusters of bacteria within tumours. Investigation of spatial resolution of 3D optical imaging was supported by ex vivo histological analyses. In vivo imaging of orally-administered commensal bacteria in the gastrointestinal tract (GIT) was also achieved using 3D BLI. This study demonstrates for the first time the potential to simultaneously image multiple BLI reporter genes three dimensionally in vivo using approaches that provide unique information on spatial locations.

Glutathione reductase facilitates host defense by sustaining phagocytic oxidative burst and promoting the development of neutrophil extracellular traps

Glutathione reductase (Gsr) catalyzes the reduction of glutathione disulfide to glutathione, which plays an important role in the bactericidal function of phagocytes. Because Gsr has been implicated in the oxidative burst in human neutrophils and is abundantly expressed in the lymphoid system, we hypothesized that Gsr-deficient mice would exhibit marked defects during the immune response against bacterial challenge. We report in this study that Gsr-null mice exhibited enhanced susceptibility to Escherichia coli challenge, indicated by dramatically increased bacterial burden, cytokine storm, striking histological abnormalities, and substantially elevated mortality. Additionally, Gsr-null mice exhibited elevated sensitivity to Staphylococcus aureus. Examination of the bactericidal functions of the neutrophils from Gsr-deficient mice in vitro revealed impaired phagocytosis and defective bacterial killing activities. Although Gsr catalyzes the regeneration of glutathione, a major cellular antioxidant, Gsr-deficient neutrophils paradoxically produced far less reactive oxygen species upon activation both ex vivo and in vivo. Unlike wild-type neutrophils that exhibited a sustained oxidative burst upon stimulation with phorbol ester and fMLP, Gsr-deficient neutrophils displayed a very transient oxidative burst that abruptly ceased shortly after stimulation. Likewise, Gsr-deficient neutrophils also exhibited an attenuated oxidative burst upon encountering E. coli. Biochemical analysis revealed that the hexose monophosphate shunt was compromised in Gsr-deficient neutrophils. Moreover, Gsr-deficient neutrophils displayed a marked impairment in the formation of neutrophil extracellular traps, a bactericidal mechanism that operates after neutrophil death. Thus, Gsr-mediated redox regulation is crucial for bacterial clearance during host defense against massive bacterial challenge.

Protective role of IL-1β against post-arthroplasty Staphylococcus aureus infection

MyD88 is an adapter molecule that is used by both IL-1R and TLR family members to initiate downstream signaling and promote immune responses. Given that IL-1β is induced after Staphylococcus aureus infections and TLR2 is activated by S. aureus lipopeptides, we hypothesized that IL-1β and TLR2 contribute to MyD88-dependent protective immune responses against post-arthroplasty S. aureus infections. To test this hypothesis, we used a mouse model of a post-arthroplasty S. aureus infection to compare the bacterial burden, biofilm formation and neutrophil recruitment in IL-1β-deficient, TLR2-deficient and wild-type (wt) mice. By using in vivo bioluminescence imaging, we found that the bacterial burden in IL-1β-deficient mice was 26-fold higher at 1 day after infection and remained 3- to 10-fold greater than wt mice through day 42. In contrast, the bacterial burden in TLR2-deficient mice did not differ from wt mice. In addition, implants harvested from IL-1β-deficient mice had more biofilm formation and 14-fold higher adherent bacteria compared with those from wt mice. Finally, IL-1β-deficient mice had ∼50% decreased neutrophil recruitment to the infected postoperative joints than wt mice. Taken together, these findings suggest a mechanism by which IL-1β induces neutrophil recruitment to help control the bacterial burden and the ensuing biofilm formation in a post-surgical joint.