$$\rightleftharpoonup{xx}$$
$$\longleftharp{xx}$$,
$$\longrightharp{xx}$$,
A variety of medical devices (referred to as "biomaterials") are increasingly used in modern medicine to restore or replace human body parts1. However, the implantation of biomaterials predisposes a patient to infection, called a biomaterial-associated infection (BAI), which is a major complication of implants in surgery. Staphylococcus aureus and Staphylococcus epidermidis are two most prevalent bacterial species responsible for BAI2,3,4,5,6. Implanted biomaterials form a surface susceptible to bacterial biofilm formation. Moreover, local immune response may be deranged by the implanted biomaterials, causing reduced effectiveness of bacterial clearance. The initial clearance of infecting bacteria is performed mainly by infiltrating neutrophils, which have strongly reduced bactericidal capacity in the presence of an inserted or implanted biomaterial7. Moreover, macrophages infiltrating the tissue after the initial influx of neutrophils will phagocytose the remaining bacteria but cannot effectively kill them intracellularly, due to deranged immune signaling that is a consequence of the combined presence of the biomaterial and bacteria8. Thus, the presence of biomaterials can facilitate intracellular survival of bacteria9,10,11,12,13 and biofilm formation on the implanted biomaterials4,14. Consequently, BAI may lead to the failure and need for replacement of implanted biomaterials, causing increased morbidity and mortality and prolonged hospitalization with additional costs2,15.
An increasing number of anti-BAI strategies are being developed2,16,17. In vivo evaluation of the efficacy of these strategies in relevant animal models is essential. However, traditional experimental BAI animal models (e.g., mouse models) are usually costly, time-consuming, and therefore not suitable for high throughput testing of multiple strategies18. Recent development of bio-optical imaging techniques based on bioluminescent/fluorescent labeling of host cells and bacteria may allow for the continuous monitoring of BAI progression and host-pathogen/host-material interactions in single small animals such as mice18,19,20,21. However, this technique is relatively complex and still in its infancy, and several issues must be addressed for quantitative analysis of BAI18. For instance, a high challenge dose is required to visualize bacterial colonization. In addition, light scattering and adsorption of bioluminescence/fluorescence signals in tissues of mammalian test animals must also be addressed18,19,21. Therefore, novel, cost-effective animal models allowing for intravital visualization and quantitative analysis over time are valuable complementary systems for studying BAI in vivo.
Zebrafish (embryos) have been used as a versatile in vivo tool for dissecting host-pathogen interactions and infection pathogenesis of several bacterial species such as mycobacteria22, Pseudomonas aeruginosa23, Escherichia coli24, Enterococcus faecalis25, and staphylococci26,27. Zebrafish embryos have many advantages such as optical transparency, a relatively low maintenance cost, and possession of an immune system highly similar to that in mammals28,29. This makes zebrafish embryos a highly economic, living model organism for intravital visualization and analysis of infection progression and associated host responses28,29. To allow visualization of cell behavior in vivo, transgenic zebrafish lines with different types of immune cells (e.g., macrophages and neutrophils) and even with fluorescently tagged subcellular structures have been developed28,29. In addition, the high reproduction rate of zebrafish provides the possibility of developing high throughput test systems featuring automated robotic injection, automated fluorescence quantification, and RNA sequence analysis27,30.
In the present study, we aimed to develop a zebrafish embryo model for biomaterial-associated infection using fluorescence imaging techniques. To this end, we developed a procedure to inject bacteria (S. aureus) in the presence of biomaterial microspheres into the muscle tissue of zebrafish embryos. We used S. aureus RN4220 expressing mCherry fluorescent protein (S. aureus-mCherry), which was constructed as described elsewhere for another S. aureus strain10,31. The transgenic zebrafish line (mpeg1: UAS/Kaede) expressing Kaede green fluorescent protein in the macrophages32 and blue fluorescent polystyrene microspheres were used. In a previous study, we have shown that intramuscular injection of microspheres into zebrafish embryos to mimic biomaterial implantation is feasible33. To quantitatively analyze the progression of BAI and associated cell infiltration in single embryos over time, we used the "Zebrafish-Immunotest" project file which is operated within "ObjectJ" (a plug-in for ImageJ) to quantify the fluorescence intensity of bacteria residing and macrophages infiltrating in the vicinity of the injection site of microspheres33. In addition, we determined the numbers of colony-forming units (CFU) of bacteria in the presence and absence of microspheres in the embryos to study potential effects of biomaterials on infection. Our present study demonstrates that with the methods developed here, the zebrafish embryo is a promising, novel vertebrate animal model for studying biomaterial-associated infections in vivo.