Real time noninvasive monitoring of contaminating bacteria in a soft tissue implant infection model

Engineered live bacteria as disease detection and diagnosis tools

Sensitive and minimally invasive medical diagnostics are essential to the early detection of diseases, monitoring their progression and response to treatment. Engineered bacteria as live sensors are being developed as a new class of biosensors for sensitive, robust, noninvasive, and in situ detection of disease onset at low cost. Akin to microrobotic systems, a combination of simple genetic rules, basic logic gates, and complex synthetic bioengineering principles are used to program bacterial vectors as living machines for detecting biomarkers of diseases, some of which cannot be detected with other sensing technologies. Bacterial whole-cell biosensors (BWCBs) can have wide-ranging functions from detection only, to detection and recording, to closed-loop detection-regulated treatment. In this review article, we first summarize the unique benefits of bacteria as living sensors. We then describe the different bacteria-based diagnosis approaches and provide examples of diagnosing various diseases and disorders. We also discuss the use of bacteria as imaging vectors for disease detection and image-guided surgery. We conclude by highlighting current challenges and opportunities for further exploration toward clinical translation of these bacteria-based systems.

Role of Resident Microbial Communities in Biofilm-Related Implant Infections: Recent Insights and Implications

The use of medical implants continues to grow as the population ages. Biofilm-related implant infection is the leading cause of medical implant failure and remains difficult to diagnose and treat. Recent technologies have enhanced our understanding of the composition and complex functions of microbiota occupying various body site niches. In this review, we leverage data from molecular sequencing technologies to explore how silent changes in microbial communities from various sites can influence the development of biofilm-related infections. Specifically, we address biofilm formation and recent insights of the organisms involved in biofilm-related implant infections; how composition of microbiomes from skin, nasopharyngeal, and nearby tissue can impact biofilm-formation, and infection; the role of the gut microbiome in implant-related biofilm formation; and therapeutic strategies to mitigate implant colonization.

Surface-Decorated Graphene Oxide Sheets with Copper Nanoderivatives for Bone Regeneration: An In Vitro and In Vivo Study Regarding Molecular Mechanisms, Osteogenesis, and Anti-infection Potential

It has been previously reported that graphene oxide/copper nanoderivative (GO/Cu)-incorporated chitosan/hyaluronic acid scaffolds might be promising wound dressings for the management of infected wound healing. The aim of the present research is to deeply explore the potential antimicrobial mechanisms and synergistic osteogenic activity, as well as the in vivo anti-infective behavior of GO/Cu nanocomposites, making them possible candidates for establishing implantable biomaterials for the repair of infected bone defects. The antibacterial mechanisms of the nanocomposites were explored through the examination of membrane integrity, oxidative stress, and metabolic enzyme activities. Then, the cytocompatibility with bone mesenchymal stem cells (rBMSCs) and osteogenic potential were confirmed, and a subcutaneous bacterial infection model in rats was also established to verify the in vivo anti-infective property and biosafety of the nanocomposites. It was found that leakage of adenosine triphosphate, proteins, and reducing sugars from the bacterial cells, indicative of damaged permeability of bacterial membranes, and promoted production of reactive oxygen species and disordered metabolic enzyme activities in response to oxidative stress were possible molecular mechanisms responsible for the synergistic antibacterial effects of the GO/Cu nanocomposites. Additionally, good cytocompatibility with rBMSCs and promoted osteogenic differentiation were found in GO/Cu nanocomposites (mass ratio = 2:1), which also demonstrated satisfactory in vivo anti-infective performance, reduced inflammation, and acceptable biosafety. Based on our results, damaged bacterial membranes, increased ROS production, and disorders of crucial enzyme metabolism were the main antibacterial mechanisms involved in the bacterium-killing events caused by the GO/Cu nanocomposites, which also showed enhanced osteogenic activity, in vivo anti-infective capability, and acceptable cytocompatibility and biosafety. Therefore, GO/Cu (2:1) nanocomposites are a potential strategy for improving the biological performance of current bone substitutes used for combating bacterial-contaminated bone defects.

Micrococcal Nuclease stimulates Staphylococcus aureus Biofilm Formation in a Murine Implant Infection Model

Advancements in contemporary medicine have led to an increasing life expectancy which has broadened the application of biomaterial implants. As each implant procedure has an innate risk of infection, the number of biomaterial-associated infections keeps rising. Staphylococcus aureus causes 34% of such infections and is known as a potent biofilm producer. By secreting micrococcal nuclease S. aureus is able to escape neutrophil extracellular traps by cleaving their DNA-backbone. Also, micrococcal nuclease potentially limits biofilm growth and adhesion by cleaving extracellular DNA, an important constituent of biofilms. This study aimed to evaluate the impact of micrococcal nuclease on infection persistence and biofilm formation in a murine biomaterial-associated infection-model with polyvinylidene-fluoride mesh implants inoculated with bioluminescent S. aureus or its isogenic micrococcal nuclease deficient mutant. Supported by results based on in-vivo bioluminescence imaging, ex-vivo colony forming unit counts, and histological analysis it was found that production of micrococcal nuclease enables S. aureus bacteria to evade the immune response around an implant resulting in a persistent infection. As a novel finding, histological analysis provided clear indications that the production of micrococcal nuclease stimulates S. aureus to form biofilms, the presence of which extended neutrophil extracellular trap formation up to 13 days after mesh implantation. Since micrococcal nuclease production appeared vital for the persistence of S. aureus biomaterial-associated infection, targeting its production could be a novel strategy in preventing biomaterial-associated infection.

Accepting higher morbidity in exchange for sacrificing fewer animals in studies developing novel infection-control strategies

Preventing bacterial infections from becoming the leading cause of death by the year 2050 requires the development of novel, infection-control strategies, building heavily on biomaterials science, including nanotechnology. Pre-clinical (animal) studies are indispensable for this development. Often, animal infection outcomes bear little relation to human clinical outcome. Here, we review conclusions from pathogen-inoculum dose-finding pilot studies for evaluation of novel infection-control strategies in murine models. Pathogen-inoculum doses are generally preferred that produce the largest differences in quantitative infection outcome parameters between a control and an experimental group, without death or termination of animals due to having reached an inhumane end-point during the study. However, animal death may represent a better end-point for evaluation than large differences in outcome parameters or number of days over which infection persists. The clinical relevance of lower pre-clinical outcomes, such as bioluminescence, colony forming units (CFUs) retrieved or more rapid clearance of infection is unknown, as most animals cure infection without intervention, depending on pathogen-species and pathogen-inoculum dose administered. In human clinical practice, patients suffering from infection present to hospital emergency wards, frequently in life-threatening conditions. Animal infection-models should therefore use prevention of death and recurrence of infection as primary efficacy targets to be addressed by novel strategies. To compensate for increased animal morbidity and mortality, animal experiments should solely be conducted for pre-clinical proof of principle and safety. With the advent of sophisticated in vitro models, we advocate limiting use of animal models when exploring pathogenesis or infection mechanisms.

Comparison of the immune response during acute and chronic Staphylococcus aureus infection

Staphylococcus aureus bacteria are able to grow in a planktonic state that is associated with acute infections and in biofilms that are associated with chronic infections. Acute infections, such as skin infections, are often self-limiting. However, chronic infections, such as implant infections, can be difficult to clear and may require surgical intervention. The host immune response may contribute to the different outcomes often associated with these two disease types. We used proteomic arrays and two murine models for an initial, descriptive characterization of the contribution of the host immune response to outcomes of acute versus chronic S. aureus disease. We compared the immune responses between a model of self-limiting skin and soft tissue infection caused by the planktonic form of S. aureus versus a model of surgical mesh implant infection, which we show to be caused by a bacterial biofilm. The significantly altered host cytokines and chemokines were largely different in the two models, with responses diminished by 21 days post-implantation in surgical mesh infection. Because bacterial levels remained constant during the 21 days that the surgical mesh infection was followed, those cytokines that are significantly increased during chronic infection are not likely effective in eradicating biofilm. Comparison of the levels of cytokines and chemokines in acute versus chronic S. aureus infection can provide a starting point for evaluation of the role of specific immune factors that are present in one disease manifestation but not the other.

In-vivo monitoring of infectious diseases in living animals using bioluminescence imaging

Traditional methods of localizing and quantifying the presence of pathogenic microorganisms in living experimental animal models of infections have mostly relied on sacrificing the animals, dissociating the tissue and counting the number of colony forming units. However, the discovery of several varieties of the light producing enzyme, luciferase, and the genetic engineering of bacteria, fungi, parasites and mice to make them emit light, either after administration of the luciferase substrate, or in the case of the bacterial lux operon without any exogenous substrate, has provided a new alternative. Dedicated bioluminescence imaging (BLI) cameras can record the light emitted from living animals in real time allowing non-invasive, longitudinal monitoring of the anatomical location and growth of infectious microorganisms as measured by strength of the BLI signal. BLI technology has been used to follow bacterial infections in traumatic skin wounds and burns, osteomyelitis, infections in intestines, Mycobacterial infections, otitis media, lung infections, biofilm and endodontic infections and meningitis. Fungi that have been engineered to be bioluminescent have been used to study infections caused by yeasts (Candida) and by filamentous fungi. Parasitic infections caused by malaria, Leishmania, trypanosomes and toxoplasma have all been monitored by BLI. Viruses such as vaccinia, herpes simplex, hepatitis B and C and influenza, have been studied using BLI. This rapidly growing technology is expected to continue to provide much useful information, while drastically reducing the numbers of animals needed in experimental studies.

Enhanced Anti-Infective Efficacy of ZnO Nanoreservoirs through a Combination of Intrinsic Anti-Biofilm Activity and Reinforced Innate Defense

The increasing prevalence of implant-associated infections (IAIs) imposes a heavy burden on patients and medical providers. Bacterial biofilms are recalcitrant to antiseptic drugs and local immune defense and can attenuate host proinflammatory response to interfere with bacterial clearance. Zinc oxide nanoparticles (ZnO NPs) play a dual role in antibacterial and immunomodulatory activities but compromise the cytocompatibility because of their intracellular uptake. Here, ZnO NPs were immobilized on titanium to form homogeneous nanofilms (from discontinuous to continuous) through magnetron sputtering, and the possible antimicrobial activity and immunomodulatory effect of nano-ZnO films were investigated. Nano-ZnO films were found to prohibit sessile bacteria more than planktonic bacteria in vitro, and the antibacterial effect occurred in a dose-dependent manner. Using a novel mouse soft tissue IAI model, the in vivo results revealed that nano-ZnO films possessed outstanding antimicrobial efficacy, which could not be ascribed solely to the intrinsic anti-infective activity of nano-ZnO films observed in vitro. Macrophages and polymorphonuclear leukocytes (PMNs), two important factors in innate immune response, were cocultured with nano-ZnO and bacteria/lipopolysaccharide in vitro, and the nano-ZnO films could enhance the antimicrobial efficacy of macrophages and PMNs through promoting phagocytosis and secretion of inflammatory cytokines. This study provides insights into the anti-infective activity and mechanism of ZnO and consolidates the theoretical basis for future clinical applications of ZnO.

Microbial biofilm in human health - an updated theoretical and practical insight

The term biofilm designates an aggregate of microorganisms belonging to one or more species which adhere to various surfaces but also to each another. These microbial communities are included and interconnected within an organic structure known as slime, composed of protein substances, polysaccharides, and DNA.

The Center for Disease prevention and control considers infections with bacteria in biofilms among the 7 most important challenges which must be overcome in order to improve the safety of health services. The risk of microbial biofilm development exists for a long list of medical devices and equipment, as well as in certain diseases such as cystic fibrosis. An aggravating aspect is represented by the almost 1,000 times higher antimicrobial resistance of bacteria growing and multiplying within biofilms. Thus, in case of biofilm-infected medical devices, the resistance to antimicrobial treatments requires the removal of the device which essentially means the failure of the exploratory or therapeutic intervention in question.

The role of microbial biofilms in medical pathology is a subject that raises interest for both researchers and clinicians in order to establish new methods for prevention and treatment of biofilms. This paper is intended as an overview in the management of microbial biofilms, presenting future insights, with technological progress in microscopy, molecular genetics, and genome analysis. Therefore the present paper will focus on describing the mechanisms involved in biofilm development, biofilm related infections, methods of detection and quantification of microbial communities and therapeutical approaches.

Nano-layered magnesium fluoride reservoirs on biomaterial surfaces strengthen polymorphonuclear leukocyte resistance to bacterial pathogens

Biomaterial-related bacterial infections cause patient suffering, mortality and extended periods of hospitalization, imposing a substantial burden on medical systems. In this context, understanding of nanomaterials-bacteria-cells interactions is of both fundamental and clinical significance. Herein, nano-MgF₂ films were deposited on titanium substrate via magnetron sputtering. Using this platform, the antibacterial behavior and mechanism of the nano-MgF₂ films were investigated in vitro and in vivo. It was found that, for S. aureus (CA-MRSA, USA300) and S. epidermidis (RP62A), the nano-MgF₂ films possessed excellent anti-biofilm activity, but poor anti-planktonic bacteria activity in vitro. Nevertheless, both the traditional SD rat osteomyelitis model and the novel stably luminescent mouse infection model demonstrated that nano-MgF₂ films exerted superior anti-infection effect in vivo, which cannot be completely explained by the antibacterial activity of the nanomaterial itself. Further, using polymorphonuclear leukocytes (PMNs), the critical immune cells of innate immunity, a complementary investigation of MgF₂-bacteria-PMNs co-culturing revealed that the nano-MgF₂ films improved the antibacterial effect of PMNs through enhancing their phagocytosis and stability. To our knowledge, this is the first time of exploring the antimicrobial mechanism of nano-MgF₂ from the perspective of innate immunity both in vitro and in vivo. Based on the research results, a plausible mechanism is put forward for the predominant antibacterial effect of nano-MgF₂in vivo, which may originate from the indirect immune enhancement effect of nano-MgF₂ films. In summary, this study of surface antibacterial design using MgF₂ nanolayer is a meaningful attempt, which can promote the host innate immune response to bacterial pathogens. This may give us a new understanding towards the antibacterial behavior and mechanism of nano-MgF₂ films and pave the way towards their clinical applications.

Anti-infective efficacy, cytocompatibility and biocompatibility of a 3D-printed osteoconductive composite scaffold functionalized with quaternized chitosan

Contaminated or infected bone defects remain serious challenges in clinical trauma and orthopaedics, and a bone substitute with both osteoconductivity and antibacterial properties represents an improvement for treatment strategy. In this study, quaternized chitosan (hydroxypropyltrimethyl ammonium chloride chitosan, HACC) was grafted to 3D-printed scaffolds composed of polylactide-co-glycolide (PLGA) and hydroxyapatite (HA), in order to design bone engineering scaffolds endowed with antibacterial and osteoconductive properties. We found that both the PLGA/HA/HACC and PLGA/HACC composite scaffolds decreased bacterial adhesion and biofilm formation under in vitro and in vivo conditions. Additionally, ATP leakage assay indicated that immobilizing HACC on the scaffolds could effectively disrupt microbial membranes. Using human bone marrow-derived mesenchymal stem cells (hBMSCs), we demonstrated that HA incorporated scaffolds, including PLGA/HA and PLGA/HA/HACC, favoured cell attachment, proliferation, spreading and osteogenic differentiation compared to HA-free PLGA or PLGA/HACC scaffolds. Finally, an in vivo biocompatibility assay conducted on rats, showed that HA incorporated scaffolds (including PLGA/HA and PLGA/HA/HACC scaffolds) exhibited good neovascularization and tissue integration. Taken together, our findings support the approach for developing porous PLGA/HA/HACC composite scaffold with potential clinical application in the treatment of infected bone.

STATEMENT OF SIGNIFICANCE

Although plenty of conductive scaffold biomaterials have been exploited to improve bone regeneration under infection, potential tissue toxicity under high concentration and antibiotic-resistance are their main deficiencies. This study indicated that HACC-grafted PLGA/HA composite scaffold prepared using an innovative 3D-printing technique and covalent grafting strategy showed significantly enhanced antibacterial activities, especially against the antibiotic-resistant strains, together with good osteogenic activity and biocompatibility. Therefore, it provides an effective porous composite scaffold to combat the infected bone defect in clinic with decreased risks of bacterial resistance and open a feasible strategy for the modification of scaffold interfaces involved in the bone regeneration and anti-infection.

Developing a Clinically Representative Model of Periprosthetic Joint Infection

➤The poor treatment outcomes for periprosthetic joint infection (PJI) reflect the limited understanding that currently exists regarding the pathogenesis of this devastating clinical problem.➤Current animal models of PJI are limited in their translational nature primarily because of their inability to recreate the periprosthetic environment.➤A greater mechanistic understanding of the musculoskeletal and immune systems of small animals, such as mice and rats, provides a more robust platform for modeling and examining the pathogenesis of PJI.➤A clinically representative PJI model must involve an implant that recreates the periprosthetic space and be amenable to methodologies that identify implant biofilm as well as quantify the peri-implant bacterial load.

"Race for the Surface": Eukaryotic Cells Can Win

With an aging population and the consequent increasing use of medical implants, managing the possible infections arising from implant surgery remains a global challenge. Here, we demonstrate for the first time that a precise nanotopology provides an effective intervention in bacterial cocolonization enabling the proliferation of eukaryotic cells on a substratum surface, preinfected by both live Gram-negative, Pseudomonas aeruginosa, and Gram-positive, Staphylococcus aureus, pathogenic bacteria. The topology of the model black silicon (bSi) substratum not only favors the proliferation of eukaryotic cells but is biocompatible, not triggering an inflammatory response in the host. The attachment behavior and development of filopodia when COS-7 fibroblast cells are placed in contact with the bSi surface are demonstrated in the dynamic study, which is based on the use of real-time sequential confocal imaging. Bactericidal nanotopology may enhance the prospect for further development of inherently responsive antibacterial nanomaterials for bionic applications such as prosthetics and implants.

Bacterial adherence and biofilm formation on medical implants: a review

Biofilms are a complex group of microbial cells that adhere to the exopolysaccharide matrix present on the surface of medical devices. Biofilm-associated infections in the medical devices pose a serious problem to the public health and adversely affect the function of the device. Medical implants used in oral and orthopedic surgery are fabricated using alloys such as stainless steel and titanium. The biological behavior, such as osseointegration and its antibacterial activity, essentially depends on both the chemical composition and the morphology of the surface of the device. Surface treatment of medical implants by various physical and chemical techniques are attempted in order to improve their surface properties so as to facilitate bio-integration and prevent bacterial adhesion. The potential source of infection of the surrounding tissue and antimicrobial strategies are from bacteria adherent to or in a biofilm on the implant which should prevent both biofilm formation and tissue colonization. This article provides an overview of bacterial biofilm formation and methods adopted for the inhibition of bacterial adhesion on medical implants.

Infected animal models for tissue engineering

Infection is one of the most common complications associated with medical interventions and implants. As tissue engineering strategies to replace missing or damaged tissue advance, the focus on prevention and treatment of concomitant infection has also begun to emerge as an important area of research. Because the in vivo environment is a complex interaction between host tissue, implanted materials, and native immune system that cannot be replicated in vitro, animal models of infection are integral in evaluating the safety and efficacy of experimental treatments for infection. In this review, considerations for selecting an animal model, established models of infection, and areas that require further model development are discussed with regard to cutaneous, fascial, and orthopedic infections.

In Vivo MicroCT Monitoring of Osteomyelitis in a Rat Model

Infection associated with orthopedic implants often results in bone loss and requires surgical removal of the implant. The aim of this study was to evaluate morphological changes of bone adjacent to a bacteria-colonized implant, with the aim of identifying temporal patterns that are characteristic of infection. In an in vivo study with rats, bone changes were assessed using in vivo microCT at 7 time points during a one-month postoperative period. The rats received either a sterile or Staphylococcus aureus-colonized polyetheretherketone screw in the tibia. Bone-implant contact, bone fraction, and bone changes (quiescent, resorbed, and new bone) were calculated from consecutive scans and validated against histomorphometry. The screw pullout strength was estimated from FE models and the results were validated against mechanical testing. In the sterile group, bone-implant contact, bone fraction, and mechanical fixation increased steadily until day 14 and then plateaued. In the infected group, they decreased rapidly. Bone formation was reduced while resorption was increased, with maximum effects observed within 6 days. In summary, the model presented is capable of evaluating the patterns of bone changes due to implant-related infections. The combined use of longitudinal in vivo microCT imaging and image-based finite element analysis provides characteristic signs of infection within 6 days.

Tn5/7-lux: a versatile tool for the identification and capture of promoters in gram-negative bacteria

BACKGROUND

The combination of imaging technologies and luciferase-based bioluminescent bacterial reporter strains provide a sensitive and simple non-invasive detection method (photonic bioimaging) for the study of diverse biological processes, as well as efficacy of therapeutic interventions, in live animal models of disease. The engineering of bioluminescent bacteria required for photonic bioimaging is frequently hampered by lack of promoters suitable for strong, yet stable luciferase gene expression.

RESULTS

We devised a novel method for identification of constitutive native promoters in Gram-negative bacteria. The method is based on a Tn5/7 transposon that exploits the unique features of Tn5 (random transposition) and Tn7 (site-specific transposition). The transposons are designed such that Tn5 transposition will allow insertion of a promoter-less bacterial luxCDABE operon downstream of a bacterial gene promoter. Cloning of DNA fragments from luminescent isolates results in a plasmid that replicates in pir (+) hosts. Sequencing of the lux-chromosomal DNA junctions on the plasmid reveals transposon insertion sites within genes or operons. The plasmid is also a mini-Tn7-lux delivery vector that can be used to introduce the promoter-lux operon fusion into other derivatives of the bacterium of interest in an isogenic fashion. Alternatively, promoter-containing sequences can be PCR-amplified from plasmid or chromosomal DNA and cloned into a series of accompanying mini-Tn7-lux vectors. The mini-Tn5/7-lux and mini-Tn7-lux vectors are equipped with diverse selection markers and thus applicable in numerous Gram-negative bacteria. Various mini-Tn5/7-lux vectors were successfully tested for transposition and promoter identification by imaging in Acinetobacter baumannii, Escherichia coli, and Burkholderia pseudomallei. Strong promoters were captured for lux expression in E. coli and A. baumannii. Some mini-Tn7-lux vectors are also equipped with attB sites for swapping of the lux operon with other reporter genes using Gateway technology.

CONCLUSIONS

Although mini-Tn5-lux and mini-Tn7-lux elements have previously been developed and used for bacterial promoter identification and chromosomal insertion of promoter-lux gene fusions, respectively, the newly developed mini-Tn5/7-lux and accompanying accessory plasmids streamline and accelerate the promoter discovery and bioluminescent strain engineering processes. Availability of vectors with diverse selection markers greatly extend the host-range of promoter probe and lux gene fusion vectors.

Scaffold-based anti-infection strategies in bone repair

Bone fractures and non-union defects often require surgical intervention where biomaterials are used to correct the defect, and approximately 10% of these procedures are compromised by bacterial infection. Currently, treatment options are limited to sustained, high doses of antibiotics and surgical debridement of affected tissue, leaving a significant, unmet need for the development of therapies to combat device-associated biofilm and infections. Engineering implants to prevent infection is a desirable material characteristic. Tissue engineered scaffolds for bone repair provide a means to both regenerate bone and serve as a base for adding antimicrobial agents. Incorporating anti-infection properties into regenerative medicine therapies could improve clinical outcomes and reduce the morbidity and mortality associated with biomaterial implant-associated infections. This review focuses on current animal models and technologies available to assess bone repair in the context of infection, antimicrobial agents to fight infection, the current state of antimicrobial scaffolds, and future directions in the field.

A novel mouse model of soft-tissue infection using bioluminescence imaging allows noninvasive, real-time monitoring of bacterial growth

Musculoskeletal infections, including surgical-site and implant-associated infections, often cause progressive inflammation and destroy areas of the soft tissue. Treating infections, especially those caused by multi-antibiotic resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) remains a challenge. Although there are a few animal models that enable the quantitative evaluation of infection in soft tissues, these models are not always reproducible or sustainable. Here, we successfully established a real-time, in vivo, quantitative mouse model of soft-tissue infection in the superficial gluteus muscle (SGM) using bioluminescence imaging. A bioluminescent strain of MRSA was inoculated into the SGM of BALB/c adult male mice, followed by sequential measurement of bacterial photon intensity and serological and histological analyses of the mice. The mean photon intensity in the mice peaked immediately after inoculation and remained stable until day 28. The serum levels of interleukin-6, interleukin-1 and C-reactive protein at 12 hours after inoculation were significantly higher than those prior to inoculation, and the C-reactive protein remained significantly elevated until day 21. Histological analyses showed marked neutrophil infiltration and abscesses containing necrotic and fibrous tissues in the SGM. With this SGM mouse model, we successfully visualized and quantified stable bacterial growth over an extended period of time with bioluminescence imaging, which allowed us to monitor the process of infection without euthanizing the experimental animals. This model is applicable to in vivo evaluations of the long-term efficacy of novel antibiotics or antibacterial implants.

Persistence of a bioluminescent Staphylococcus aureus strain on and around degradable and non-degradable surgical meshes in a murine model

Biomaterials are increasingly used for the restoration of human function, but can become infected as a result of peri- or early post-operative bacterial contamination, although biomaterial-associated infections (BAIs) can also initiate at any time from hematogenous spreading of bacteria from an infection elsewhere in the body. Infecting bacteria in BAIs not only seek shelter in their own protective biofilm matrix, but also hide in surrounding tissue. This study compares staphylococcal persistence on and around a degradable and non-degradable surgical mesh through the use of longitudinal bioluminescence imaging in a murine model, including histological evaluation of surrounding tissue after sacrifice. Surgical meshes were first contaminated with bioluminescent Staphylococcus aureus Xen29 and subsequently subcutaneously implanted in mice. Bioluminescent staphylococci persisted on and around non-degradable meshes during the 28-day course of the study, whereas bioluminescence returned to control levels and bacteria disappeared from surrounding tissues once a degradable mesh had fully dissolved. Thus the application of degradable biomaterials yields major advantages with respect to the prevention of BAIs, as dissolution of the implant not only is associated with elimination of the protective biofilm mode of growth of the infecting organisms, but also allows the immune system to clear the surrounding tissue from infecting organisms.

A gentamicin-releasing coating for cementless hip prostheses-Longitudinal evaluation of efficacy using in vitro bio-optical imaging and its wide-spectrum antibacterial efficacy

Cementless prostheses are increasingly popular in total hip arthroplasties. Therewith, common prophylactic measures to reduce the risk of postoperative infection like the use of antibiotic-loaded bone cements, will no longer be available. Alternative prophylactic measures may include the use of antibiotic-releasing coatings. Previously, we developed a gentamicin-releasing coating for cementless titanium hip prostheses and derived an appropriate dosing of this coating by adjusting the amount of gentamicin in the coating to match the antibacterial efficacy of clinically employed gentamicin-loaded bone cement. In this manuscript, we investigated two important issues regarding the prophylactic use of this 1 mg cm(-2) bioactive gentamicin-releasing coating in cementless total hip arthroplasty: (1) its ability to prevent bacterial growth in a geometrically relevant set-up and (2) its antibacterial spectrum. A geometrically relevant set-up was developed in which miniature titanium stems were surrounded by agar, contaminated with bioluminescent Staphylococcus aureus. Novel, bio-optical imaging was performed allowing noninvasive, longitudinal monitoring of staphylococcal growth around miniature stems with and without the gentamicin-releasing coating. Furthermore, the antibacterial efficacy of the gentamicin-releasing coating was determined against a wide variety of clinical isolates, including bioluminescent Staphylococcus aureus strains, using traditional zone of inhibition measurements. The gentamicin-releasing coating demonstrated a wide-spectrum of antibacterial efficacy and successfully prevented growth of bioluminescent staphylococci around a miniature stem mounted in bacterially contaminated agar for at least 60 h. This implies that the gentamicin-releasing coating has potential to contribute to the improvement of infection prophylaxis in cementless total hip arthroplasty.

Bioluminescence imaging: progress and applications

Application of bioluminescence imaging has increased tremendously in the past decade and has significantly contributed to core conceptual advances in biomedical research. This technology provides valuable means for monitoring of different biological processes in immunology, oncology, virology and neuroscience. In this review, we discuss current trends in bioluminescence and its application in different fields with an emphasis on cancer research.

Visualization of abscess formation in a murine thigh infection model of Staphylococcus aureus by 19F-magnetic resonance imaging (MRI)

BACKGROUND

During the last years, (19)F-MRI and perfluorocarbon nanoemulsion (PFC) emerged as a powerful contrast agent based MRI methodology to track cells and to visualize inflammation. We applied this new modality to visualize deep tissue abscesses during acute and chronic phase of inflammation caused by Staphylococcus aureus infection.

METHODOLOGY AND PRINCIPAL FINDINGS

In this study, a murine thigh infection model was used to induce abscess formation and PFC or CLIO (cross linked ironoxides) was administered during acute or chronic phase of inflammation. 24 h after inoculation, the contrast agent accumulation was imaged at the site of infection by MRI. Measurements revealed a strong accumulation of PFC at the abscess rim at acute and chronic phase of infection. The pattern was similar to CLIO accumulation at chronic phase and formed a hollow sphere around the edema area. Histology revealed strong influx of neutrophils at the site of infection and to a smaller extend macrophages during acute phase and strong influx of macrophages at chronic phase of inflammation.

CONCLUSION AND SIGNIFICANCE

We introduce (19)F-MRI in combination with PFC nanoemulsions as a new platform to visualize abscess formation in a murine thigh infection model of S. aureus. The possibility to track immune cells in vivo by this modality offers new opportunities to investigate host immune response, the efficacy of antibacterial therapies and the influence of virulence factors for pathogenesis.

In vivo bioluminescent imaging (BLI): noninvasive visualization and interrogation of biological processes in living animals

In vivo bioluminescent imaging (BLI) is increasingly being utilized as a method for modern biological research. This process, which involves the noninvasive interrogation of living animals using light emitted from luciferase-expressing bioreporter cells, has been applied to study a wide range of biomolecular functions such as gene function, drug discovery and development, cellular trafficking, protein-protein interactions, and especially tumorigenesis, cancer treatment, and disease progression. This article will review the various bioreporter/biosensor integrations of BLI and discuss how BLI is being applied towards a new visual understanding of biological processes within the living organism.

Noninvasive biophotonic imaging for studies of infectious disease

According to World Health Organization estimates, infectious organisms are responsible for approximately one in four deaths worldwide. Animal models play an essential role in the development of vaccines and therapeutic agents but large numbers of animals are required to obtain quantitative microbiological data by tissue sampling. Biophotonic imaging (BPI) is a highly sensitive, nontoxic technique based on the detection of visible light, produced by luciferase-catalysed reactions (bioluminescence) or by excitation of fluorescent molecules, using sensitive photon detectors. The development of bioluminescent/fluorescent microorganisms therefore allows the real-time noninvasive detection of microorganisms within intact living animals. Multiple imaging of the same animal throughout an experiment allows disease progression to be followed with extreme accuracy, reducing the number of animals required to yield statistically meaningful data. In the study of infectious disease, the use of BPI is becoming widespread due to the novel insights it can provide into established models, as well as the impact of the technique on two of the guiding principles of using animals in research, namely reduction and refinement. Here, we review the technology of BPI, from the instrumentation through to the generation of a photonic signal, and illustrate how the technique is shedding light on infection dynamics in vivo.

In vivo detection of Staphylococcus aureus in biofilm on vascular prostheses using non-invasive biophotonic imaging

OBJECTIVES

Biophotonic imaging was compared to standard enumeration method both for counting Staphylococcus aureus in biofilm and bacterial susceptibility tests of different graft materials.

DESIGN

Prospective, randomized, controlled animal study.

MATERIAL AND METHODS

Five types of vascular grafts were placed subcutaneously in 35 mice and challenged with bioluminescent S. aureus. The mice were divided into equal groups as follows: group A (polyester), group B (polytetrafluoroethylene), group C and D (two types of silver acetate-coated polyester) and group E (bovine pericardium). Controls were given only the bacteria. The bioluminescence signal of S. aureus, able to predict number of viable bacteria in biofilm without any manipulation, was measured at different time points. Five days postinfection, regular cultures of adherent bacteria on grafts were obtained. Comparative analyses between bioluminescence activity and culture enumeration were performed.

RESULTS

The number of viable bacteria on silver-coated prostheses was the slightest, indicating superior bacterial resistance. The density of bacteria on polytetrafluoroethylene and polyester was comparable, with a non-significant advantage for polytetrafluoroethylene. Moreover, bioluminescence detected the number of viable S. aureus in biofilm more exactly compared to enumeration of bacteria.

CONCLUSION

Bioluminescence imaging can be considered a useful tool to characterize susceptibility of any graft material to bacterial biofilm prior to implantation.

The risk of biomaterial-associated infection after revision surgery due to an experimental primary implant infection

The fate of secondary biomaterial implants was determined by bio-optical imaging and plate counting, after antibiotic treatment of biomaterials-associated-infection (BAI) and surgical removal of an experimentally infected, primary implant. All primary implants and tissue samples from control mice showed bioluminescence and were culture-positive. In an antibiotic treated group, no bioluminescence was detected and only 20% of all primary implants and no tissue samples were culture-positive. After revision surgery, bioluminescence was detected in all control mice. All the implants and 80% of all tissue samples were culture-positive. In contrast, in the antibiotic treated group, 17% of all secondary implants and 33% of all tissue samples were culture-positive, despite antibiotic treatment. The study illustrates that due to the BAI of a primary implant, the infection risk of biomaterial implants is higher in revision surgery than in primary surgery, emphasizing the need for full clearance of the infection, as well as from surrounding tissues prior to implantation of a secondary implant.

A mouse model of post-arthroplasty Staphylococcus aureus joint infection to evaluate in vivo the efficacy of antimicrobial implant coatings

BACKGROUND

Post-arthroplasty infections represent a devastating complication of total joint replacement surgery, resulting in multiple reoperations, prolonged antibiotic use, extended disability and worse clinical outcomes. As the number of arthroplasties in the U.S. will exceed 3.8 million surgeries per year by 2030, the number of post-arthroplasty infections is projected to increase to over 266,000 infections annually. The treatment of these infections will exhaust healthcare resources and dramatically increase medical costs.

METHODOLOGY/PRINCIPAL FINDINGS

To evaluate novel preventative therapeutic strategies against post-arthroplasty infections, a mouse model was developed in which a bioluminescent Staphylococcus aureus strain was inoculated into a knee joint containing an orthopaedic implant and advanced in vivo imaging was used to measure the bacterial burden in real-time. Mice inoculated with 5x10(3) and 5x10(4) CFUs developed increased bacterial counts with marked swelling of the affected leg, consistent with an acute joint infection. In contrast, mice inoculated with 5x10(2) CFUs developed a low-grade infection, resembling a more chronic infection. Ex vivo bacterial counts highly correlated with in vivo bioluminescence signals and EGFP-neutrophil fluorescence of LysEGFP mice was used to measure the infection-induced inflammation. Furthermore, biofilm formation on the implants was visualized at 7 and 14 postoperative days by variable-pressure scanning electron microscopy (VP-SEM). Using this model, a minocycline/rifampin-impregnated bioresorbable polymer implant coating was effective in reducing the infection, decreasing inflammation and preventing biofilm formation.

CONCLUSIONS/SIGNIFICANCE

Taken together, this mouse model may represent an alternative pre-clinical screening tool to evaluate novel in vivo therapeutic strategies before studies in larger animals and in human subjects. Furthermore, the antibiotic-polymer implant coating evaluated in this study was clinically effective, suggesting the potential for this strategy as a therapeutic intervention to combat post-arthroplasty infections.

In vitro and in vivo comparisons of staphylococcal biofilm formation on a cross-linked poly(ethylene glycol)-based polymer coating

Poly(ethylene glycol) (PEG) coatings are known to reduce microbial adhesion in terms of numbers and binding strength. However, bacterial adhesion remains of the order of 10(4)cm(-2). It is unknown whether this density of bacteria will eventually grow into a biofilm. This study investigates the kinetics of staphylococcal biofilm formation on a commercially produced, robust, cross-linked PEG-based polymer coating (OptiChem) in vitro and in vivo. OptiChem inhibits biofilm formation in vitro, and although adsorption of plasma proteins encourages biofilm formation, microbial growth kinetics are still strongly delayed compared to uncoated glass. In vivo, OptiChem-coated and bare silicone rubber samples were inserted into an infected murine subcutaneous pocket model. In contrast to bare silicone rubber, OptiChem samples did not become colonized upon reimplantation despite the fact that surrounding tissues were always culture-positive. We conclude that the commercial OptiChem coating considerably slows down bacterial biofilm formation both in vitro and in vivo, making it an attractive candidate for biomaterials implant coating.

Microscopic detection of viable Staphylococcus epidermidis in peri-implant tissue in experimental biomaterial-associated infection, identified by bromodeoxyuridine incorporation

Infection of biomedical devices is characterized by biofilm formation and colonization of surrounding tissue by the causative pathogens. To investigate whether bacteria detected microscopically in tissue surrounding infected devices were viable, we used bromodeoxyuridine (BrdU), a nucleotide analogue that is incorporated into bacterial DNA and can be detected with antibodies. Infected human tissue was obtained postmortem from patients with intravascular devices, and mouse biopsy specimens were obtained from mice with experimental biomaterial infection. In vitro experiments showed that Staphylococcus epidermidis incorporated BrdU, as judged from staining of the bacteria with anti-BrdU antibodies. After incubation of bacteria with BrdU and subsequent staining of microscopic sections with anti-BrdU antibodies, bacteria could be clearly visualized in the tissue surrounding intravascular devices of deceased patients. With this staining technique, relapse of infection could be visualized in mice challenged with a low dose of S. epidermidis and treated with dexamethasone between 14 and 21 days after challenge to suppress immunity. This confirms and extends our previous findings that pericatheter tissue is a reservoir for bacteria in biomaterial-associated infection. The pathogenesis of the infection and temporo-spatial distribution of viable, dividing bacteria can now be studied at the microscopic level by immunolabeling with BrdU and BrdU antibodies.

Correlation between bioluminescence and bacterial burden in passively protected mice challenged with a recombinant bioluminescent M49 group A streptococcus Strain

Streptococcus pyogenes, also known as group A streptococcus (GAS), is a human pathogen which infects people of all age groups. Previous work has reported that conformationally constrained chimeric peptide J8 protects against GAS challenge. In the current study, we demonstrate the utility of bioluminescent imaging as a rapid technique for monitoring bacterial dissemination following the passive transfer of rabbit anti-J8 antibodies to naïve mice and subsequent challenge with recombinant GAS strain M49, an emm type shown to be associated with poststreptococcal glomerular nephritis.

The noninvasive, quantitative, in vivo assessment of adenoviral-mediated gene delivery in skin wound biomaterials

Because there are few reports using gene delivery in clinically-approved synthetic matrices, we examined the feasibility of using a noninvasive imaging system to study the kinetics of luciferase gene expression when delivered in an adenoviral vector. Using a mouse model of full thickness injury, we quantified the kinetics of gene expression, determined the optimal dose of particle delivery, and established the temporal importance of drug delivery in obtaining optimal gene expression. Specifically, we found that the ideal time to deliver adenovirus to a graft is during the early phase of graft wound closure (days 0-3 post-operatively) for a peak of gene expression to occur 7 days after delivery. Under these conditions, there is a saturating dose of 6 x 10(8) adenoviral particles per graft. In light of these findings, we examined whether the efficacy of delivery could be increased by modulating the composition of the grafts. When a collagen gene-activated matrix (GAM) containing basic fibroblast growth factor (FGF2) was compared to matrix alone, a significant increase in gene expression is observed when identical amounts of vector are delivered (p<0.05). Taken together, these results show how a noninvasive and quantitative assessment of gene expression can be used to optimize gene delivery and that the composition of matrices can dramatically influence gene expression in the wound bed.

Spatiotemporal progression of localized bacterial peritonitis before and after open abdomen lavage monitored by in vivo bioluminescent imaging

BACKGROUND

Bacterial peritonitis is a life-threatening abdominal infection associated with high morbidity and mortality. The rat is a popular animal model for studying peritonitis and its treatment, but longitudinal monitoring of the progression of peritonitis in live animals has been impossible until now and thus required a large number of animals. Our objective was to develop a noninvasive in vivo imaging technique to monitor the spatiotemporal spread of bacterial peritonitis.

METHODS

Peritonitis was induced in 8 immunocompetent male Wistar rats by placing fibrin clots containing 5x10(8) cells of both Bacteroides fragilis (American Type Tissue Culture [ATCC)] 25,285 and bioluminescent Escherichia coli Xen14. After 1 or 2 days, infected clots were removed and open abdomen lavage was performed. In vivo bioluminescent imaging was used to monitor the spread of peritonitis.

RESULTS

Bioluminescent in vivo imaging showed an increase in the area of spread, and the number of E. coli tripled into the rat's abdominal cavity on day 1 after clot insertion; however, on day 2, encapsulation of the clot confined bacterial spread. Bioluminescent E. coli respread over the peritoneal cavity after lavage; within 10 days, however, in vivo imaging showed a decrease of 3-4 orders of magnitude in bacterial load.

CONCLUSION

Bioluminescent in vivo imaging can be effectively used to monitor the spatiotemporal behavior of the peritonitis during 3 different stages of the disease process: initiation, treatment, and follow-up. Imaging allows researchers to repeatedly image the same animal, thereby reducing variability and providing greater confidence in determining treatment efficacies for therapeutic interventions using a small number of animals.

Antimicrobial effects of an NO-releasing poly(ethylene vinylacetate) coating on soft-tissue implants in vitro and in a murine model

Infection of surgical meshes used in abdominal wall reconstructions often leads to removal of the implant and increases patient morbidity due to repetitive operations and hospital administrations. Treatment with antibiotics is ineffective due to the biofilm mode of growth of the infecting bacteria and bears the risk of inducing antibiotic resistance. Hence there is a need for alternative methods to prevent and treat mesh infection. Nitric oxide (NO)-releasing coatings have been demonstrated to possess bactericidal properties in vitro. It is the aim of this study to assess possible benefits of a low concentration NO-releasing carbon-based coating on monofilament polypropylene meshes with respect to infection control in vitro and in vivo. When applied on surgical meshes, NO-releasing coatings showed significant bactericidal effect on in vitro biofilms of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa and CNS. However, using bioluminescent in vivo imaging, no beneficial effects of this NO-releasing coating on subcutaneously implanted surgical meshes in mice could be observed.