Applications of bioluminescence imaging to the study of infectious diseases

Research Note: Evaluating the vertical transmission potential of Salmonella Reading in broiler breeders

Salmonella Reading (S. Reading) recently emerged as a foodborne pathogen causing extensive human outbreaks in North America from consuming contaminated poultry products, mostly from turkeys. Understanding the transmission dynamics of this pathogen is crucial for preventing future outbreaks. This study investigated the ability of S. Reading to colonize the tissues and contaminate eggs of broiler breeders. We utilized 2 S. Reading strains, marked with bioluminescence gene: the outbreak strain RS330 and a reference strain RS326. We used 32 commercially sourced broiler breeder hens, 34 wk of age, randomly assigned to the 2 treatments (16 hens per strain). Each hen was intravaginally inoculated with 108 CFU of the respective strain on d 1 and was rechallenged on d 4. Eggs were collected daily postchallenge to recover bioluminescent S. Reading strains from the external eggshell surface and internal egg contents. On d 7 postchallenge, 10 hens from each treatment group were euthanized. Ovaries, oviducts, and ceca were aseptically collected to detect S. Reading colonization. Results showed that 70.5% (36 of 51) and 34.5% (19 of 55) of external eggshell surfaces, and 4.0% (2 of 50) and 1.8% (1 of 54) of the internal egg contents tested positive for the outbreak and nonoutbreak strains. Additionally, 40.0% of ovaries, 70.0% of oviduct, and 70.0% of ceca samples from the outbreak strain group, and 20.0% of ovaries, 70.0% of oviduct, and 80.0% of ceca samples from nonoutbreak strain group were positive. No significant difference (P = 0.05) was observed in all the findings among the strains except for the eggshell surface contamination. These findings suggest that S. Reading can effectively colonize reproductive tissues, translocate to the ceca, and contaminate the eggs of hens. Future research is needed to determine whether S. Reading can remain viable within the eggs throughout incubation and until hatching.

Near-infrared in vivo imaging system for dynamic visualization of lung-colonizing bacteria in mouse pneumonia

In vivo imaging of bacterial infection models enables noninvasive and temporal analysis of individuals, enhancing our understanding of infectious disease pathogenesis. Conventional in vivo imaging methods for bacterial infection models involve the insertion of the bacterial luciferase LuxCDABE into the bacterial genome, followed by imaging using an expensive ultrasensitive charge-coupled device (CCD) camera. However, issues such as limited light penetration into the body and lack of versatility have been encountered. We focused on near-infrared (NIR) light, which penetrates the body effectively, and attempted to establish an in vivo imaging method to evaluate the number of lung-colonizing bacteria during the course of bacterial pneumonia. This was achieved by employing a novel versatile system that combines plasmid-expressing firefly luciferase bacteria, NIR substrate, and an inexpensive, scientific complementary metal-oxide semiconductor (sCMOS) camera. The D-luciferin derivative "TokeOni," capable of emitting NIR bioluminescence, was utilized in a mouse lung infection model of Acinetobacter baumannii, an opportunistic pathogen that causes pneumonia and is a concern due to drug resistance. TokeOni exhibited the highest sensitivity in detecting bacteria colonizing the mouse lungs compared with other detection systems such as LuxCDABE, enabling the monitoring of changes in bacterial numbers over time and the assessment of antimicrobial agent efficacy. Additionally, it was effective in detecting A. baumannii clinical isolates and Klebsiella pneumoniae. The results of this study are expected to be used in the analysis of animal models of infectious diseases for assessing the efficacy of therapeutic agents and understanding disease pathogenesis.

IMPORTANCE

Conventional animal models of infectious diseases have traditionally relied upon average assessments involving numerous individuals, meaning they do not directly reflect changes in the pathology of an individual. Moreover, in recent years, ethical concerns have resulted in the demand to reduce the number of animals used in such models. Although in vivo imaging offers an effective approach for longitudinally evaluating the pathogenesis of infectious diseases in individual animals, a standardized method has not yet been established. To our knowledge, this study is the first to develop a highly versatile in vivo pulmonary bacterial quantification system utilizing near-infrared luminescence, plasmid-mediated expression of firefly luciferase in bacteria, and a scientific complementary metal-oxide semiconductor camera. Our research holds promise as a useful tool for assessing the efficacy of therapeutic drugs and pathogenesis of infectious diseases.

Advances in the Development of Bacterial Bioluminescence Imaging

Bioluminescence imaging (BLI) is a powerful method for visualizing biological processes and tracking cells. Engineered bioluminescent bacteria that utilize luciferase-catalyzed biochemical reactions to generate luminescence have become useful analytical tools for in vitro and in vivo bacterial imaging. Accordingly, this review initially introduces the development of engineered bioluminescent bacteria that use different luciferase-luciferin pairs as analytical tools and their applications for in vivo BLI, including real-time bacterial tracking of infection, probiotic investigation, tumor-targeted therapy, and drug screening. Applications of engineered bioluminescent bacteria as whole-cell biosensors for sensing biological changes in vitro and in vivo are then discussed. Finally, we review the optimizations and future directions of bioluminescent bacteria for imaging. This review aims to provide fundamental insights into bacterial BLI and highlight the potential development of this technique in the future.

Akaluc bioluminescence offers superior sensitivity to track in vivo dynamics of SARS-CoV-2 infection

Monitoring in vivo viral dynamics can improve our understanding of pathogenicity and tissue tropism. Because the gene size of RNA viruses is typically small, NanoLuc is the primary choice for accommodation within viral genome. However, NanoLuc/Furimazine and also the conventional firefly luciferase/D-luciferin are known to exhibit relatively low tissue permeability and thus less sensitivity for visualization of deep tissue including lungs. Here, we demonstrated in vivo sufficient visualization of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection using the pair of a codon-optimized Akaluc and AkaLumine. We engineered the codon-optimized Akaluc gene possessing the similar GC ratio of SARS-CoV-2. Using the SARS-CoV-2 recombinants carrying the codon-optimized Akaluc, we visualized in vivo infection of respiratory organs, including the tissue-specific differences associated with particular variants. Additionally, we could evaluate the efficacy of antivirals by monitoring changes in Akaluc signals. Overall, we offer an effective technology for monitoring viral dynamics in live animals.

A rapid and versatile reverse genetics approach for generating recombinant positive-strand RNA viruses that use IRES-mediated translation

Reverse genetics systems have played a central role in developing recombinant viruses for a wide spectrum of virus research. The circular polymerase extension reaction (CPER) method has been applied to studying positive-strand RNA viruses, allowing researchers to bypass molecular cloning of viral cDNA clones and thus leading to the rapid generation of recombinant viruses. However, thus far, the CPER protocol has only been established using cap-dependent RNA viruses. Here, we demonstrate that a modified version of the CPER method can be successfully applied to positive-strand RNA viruses that use cap-independent, internal ribosomal entry site (IRES)-mediated translation. As a proof-of-concept, we employed mammalian viruses with different types (classes I, II, and III) of IRES to optimize the CPER method. Using the hepatitis C virus (HCV, class III), we found that inclusion in the CPER assembly of an RNA polymerase I promoter and terminator, instead of those from polymerase II, allowed greater viral production. This approach was also successful in generating recombinant bovine viral diarrhea virus (class III) following transfection of MDBK/293T co-cultures to overcome low transfection efficiency. In addition, we successfully generated the recombinant viruses from clinical specimens. Our modified CPER could be used for producing hepatitis A virus (HAV, type I) as well as de novo generation of encephalomyocarditis virus (type II). Finally, we generated recombinant HCV and HAV reporter viruses that exhibited replication comparable to that of the wild-type parental viruses. The recombinant HAV reporter virus helped evaluate antivirals. Taking the findings together, this study offers methodological advances in virology.

IMPORTANCE

The lack of versatility of reverse genetics systems remains a bottleneck in viral research. Especially when (re-)emerging viruses reach pandemic levels, rapid characterization and establishment of effective countermeasures using recombinant viruses are beneficial in disease control. Indeed, numerous studies have attempted to establish and improve the methods. The circular polymerase extension reaction (CPER) method has overcome major obstacles in generating recombinant viruses. However, this method has not yet been examined for positive-strand RNA viruses that use cap-independent, internal ribosome entry site-mediated translation. Here, we engineered a suitable gene cassette to expand the CPER method for all positive-strand RNA viruses. Furthermore, we overcame the difficulty of generating recombinant viruses because of low transfection efficiency. Using this modified method, we also successfully generated reporter viruses and recombinant viruses from a field sample without virus isolation. Taking these findings together, our adapted methodology is an innovative technology that could help advance virologic research.

Diagnosis of Chronic Pulmonary Aspergillosis: Clinical, Radiological or Laboratory?

Chronic pulmonary aspergillosis (CPA) is a chronic progressive lung disease associated with a poor prognosis and a 5-year mortality rate of approximately 40-50%. The disease is characterized by slowly progressive destruction of the lung parenchyma, in the form of multiple cavities, nodules, infiltrates or fibrosis. CPA can be challenging to diagnose due to its non-specific symptoms and similarities with other respiratory conditions combined with the poor awareness of the medical community about the disease. This can result in delayed treatment even for years and worsening of the patient's condition. Serological tests certainly play a significant role in diagnosing CPA but cannot be interpreted without radiological confirmation of CPA. Although many data are published on this hot topic, there is yet no single definitive test for diagnosing CPA, and a multidisciplinary approach which involves a combination of clinical picture, radiological findings, microbiological results and exclusion of other mimicking diseases, is essential for the accurate diagnosis of CPA.

Virulence of Shigatoxigenic and Enteropathogenic Escherichia coli O80:H2 in Galleria mellonella Larvae: Comparison of the Roles of the pS88 Plasmids and STX2d Phage

The invasiveness properties of Shigatoxigenic and enteropathogenic Escherichia coli (STEC and EPEC) O80:H2 in humans and calves are encoded by genes located on a pS88-like ColV conjugative plasmid. The main objectives of this study in larvae of the Galleria mellonella moth were therefore to compare the virulence of eight bovine STEC and EPEC O80:H2, of two E. coli pS88 plasmid transconjugant and STX2d phage transductant K12 DH10B, of four E. coli O80:non-H2, and of the laboratory E. coli K12 DH10B strains. Thirty larvae per strain were inoculated in the last proleg with 10 μL of tenfold dilutions of each bacterial culture corresponding to 10 to 10⁶ colony-forming units (CFUs). The larvae were kept at 37 °C and their mortality rate was followed daily for four days. The main results were that: (i) not only the STEC and EPEC O80:H2, but also different E. coli O80:non-H2 were lethal for the larvae at high concentrations (from 10⁴ to 10⁶ CFU) with some variation according to the strain; (ii) the Stx2d toxin and partially the pS88 plasmid were responsible for the lethality caused by the E. coli O80:H2; (iii) the virulence factors of E. coli O80:non-H2 were not identified. The general conclusions are that, although the Galleria mellonella larvae represent a useful first-line model to study the virulence of bacterial pathogens, they are more limited in identifying their actual virulence properties.

Practical Guidance for Developing Small-Molecule Optical Probes for In Vivo Imaging

The WMIS Education Committee (2019-2022) reached a consensus that white papers on molecular imaging could be beneficial for practitioners of molecular imaging at their early career stages and other scientists who are interested in molecular imaging. With this consensus, the committee plans to publish a series of white papers on topics related to the daily practice of molecular imaging. In this white paper, we aim to provide practical guidance that could be helpful for optical molecular imaging, particularly for small molecule probe development and validation in vitro and in vivo. The focus of this paper is preclinical animal studies with small-molecule optical probes. Near-infrared fluorescence imaging, bioluminescence imaging, chemiluminescence imaging, image-guided surgery, and Cerenkov luminescence imaging are discussed in this white paper.

In Vivo Imaging of Nipah Virus Infection in Small Animal Rodent Models

In vivo imaging system (IVIS) is a powerful tool for the study of infectious diseases, providing the ability to non-invasively follow viral infection in an individual animal over time. Recombinant henipaviruses expressing bioluminescent or fluorescent reporter proteins can be used both to monitor the spatial and temporal progression of Nipah virus (NiV) infection in vivo as well as in ex vivo tissues. Virally produced luciferases react with systemically administered substrate to produce bioluminescence that can then be detected via IVIS imaging, while fluorescent reporters inherently generate detectable fluorescence without a substrate. Here we describe protocols applying bioluminescent or fluorescent reporter expressing recombinant viruses to in vivo or ex vivo imaging of NiV infection.

Paving the Way: Contributions of Big Data to Apicomplexan and Kinetoplastid Research

In the age of big data an important question is how to ensure we make the most out of the resources we generate. In this review, we discuss the major methods used in Apicomplexan and Kinetoplastid research to produce big datasets and advance our understanding of Plasmodium, Toxoplasma, Cryptosporidium, Trypanosoma and Leishmania biology. We debate the benefits and limitations of the current technologies, and propose future advancements that may be key to improving our use of these techniques. Finally, we consider the difficulties the field faces when trying to make the most of the abundance of data that has already been, and will continue to be, generated.

Novel Insights into Aspergillus fumigatus Pathogenesis and Host Response from State-of-the-Art Imaging of Host-Pathogen Interactions during Infection

Aspergillus fumigatus spores initiate more than 3,000,000 chronic and 300,000 invasive diseases annually, worldwide. Depending on the immune status of the host, inhalation of these spores can lead to a broad spectrum of disease, including invasive aspergillosis, which carries a 50% mortality rate overall; however, this mortality rate increases substantially if the infection is caused by azole-resistant strains or diagnosis is delayed or missed. Increasing resistance to existing antifungal treatments is becoming a major concern; for example, resistance to azoles (the first-line available oral drug against Aspergillus species) has risen by 40% since 2006. Despite high morbidity and mortality, the lack of an in-depth understanding of A. fumigatus pathogenesis and host response has hampered the development of novel therapeutic strategies for the clinical management of fungal infections. Recent advances in sample preparation, infection models and imaging techniques applied in vivo have addressed important gaps in fungal research, whilst questioning existing paradigms. This review highlights the successes and further potential of these recent technologies in understanding the host-pathogen interactions that lead to aspergillosis.

A Novel In Vitro Method to Assess the Microbial Barrier Function of Tissue Adhesives Using Bioluminescence Imaging Technique

Background. Tissue glues can minimize treatment invasiveness, mitigate the risk of infection, and reduce surgery time; ergo, they have been developed and used in surgical procedures as wound closure devices beside sutures, staples, and metallic grafts. Regardless of their structure or function, tissue glues should show an acceptable microbial barrier function before being used in humans. This study proposes a novel in vitro method using Escherichia coli Lux and bioluminescence imaging technique to assess the microbial barrier function of tissue glues. Different volumes and concentrations of E. coli Lux were applied to precured or cured polyurethane-based tissue glue placed on agar plates. Plates were cultured for 1 h, 24 h, 48 h, and 72 h with bioluminescence signal measurement subsequently. Herein, protocol established a volume of 5 μL of a 1 : 100 dilution of E. coli Lux containing around 2 × 10⁷ CFU/mL as optimal for testing polyurethane-based tissue glue. Measurement of OD_600nm, determination of CFU/mL, and correlation with the bioluminescence measurement in p/s unit resulted in a good correlation between CFU/mL and p/s and demonstrated good reproducibility of our method. In addition, this in vitro method could show that the tested polyurethane-based tissue glue can provide a reasonable barrier against the microbial penetration and act as a bacterial barrier for up to 48 h with no penetration and up to 72 h with a low level of penetration through the material. Overall, we have established a novel, sensitive, and reproducible in vitro method using the bioluminescence imaging technique for testing the microbial barrier function of new tissue glues.

Molecular Probes for Autofluorescence-Free Optical Imaging

Optical imaging is an indispensable tool in clinical diagnostics and fundamental biomedical research. Autofluorescence-free optical imaging, which eliminates real-time optical excitation to minimize background noise, enables clear visualization of biological architecture and physiopathological events deep within living subjects. Molecular probes especially developed for autofluorescence-free optical imaging have been proven to remarkably improve the imaging sensitivity, penetration depth, target specificity, and multiplexing capability. In this Review, we focus on the advancements of autofluorescence-free molecular probes through the lens of particular molecular or photophysical mechanisms that produce long-lasting luminescence after the cessation of light excitation. The versatile design strategies of these molecular probes are discussed along with a broad range of biological applications. Finally, challenges and perspectives are discussed to further advance the next-generation autofluorescence-free molecular probes for in vivo imaging and in vitro biosensors.

Construction and applications of SARS-CoV-2 pseudoviruses: a mini review

The ongoing coronavirus disease 2019 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has posed a serious threat to global public health and social stability. There is an urgent need for understanding the nature and infection mechanism of the virus. Owing to its high infectivity and pathogenicity and lack of effective treatments, live SARS-CoV-2 has to be handled in biosafety level 3 laboratories, which has impeded research into SARS-CoV-2 and the development of vaccines and therapeutics. Pseudotyped viruses that lack certain gene sequences of the virulent virus are safer and can be investigated in biosafety level 2 laboratories, providing a useful virological tool for the study of SARS-CoV-2. In this review, we will discuss the construction of SARS-CoV-2 pseudoviruses based on different packaging systems, current applications, limitations, and further explorations.

Tracing Lactobacillus plantarum within the intestinal tract of mice: green fluorescent protein-based fluorescent tagging

BACKGROUND

Lactobacillus plantarum is an important probiotic with a variety of physiologic functions. Studies have focused on the effects of L. plantarum on host physiology and microbiota, but studies of the fate of strains after they enter the intestine are lacking. In this study, L. plantarum ST-III was genetically engineered to express green fluorescent protein (GFP). Mice were administered ST-III-GFP, and fluorescence imaging was used to study the distribution, location and quantity of strains within 8 h after entry into the intestine.

RESULTS

The results indicated that genetic modification did not affect the growth of ST-III, tolerance to simulated gastric juice and intestinal fluid or tolerance to antibiotics (with the exception of chloramphenicol). Fluorescence imaging and colony counting indicated that ST-III-GFP can be detected in the small intestine 5 min after oral gavage. After 30 min, nearly all ST-III-GFP was located in the small intestine. After 1.5 h, ST-III-GFP was detected in both the cecum and large intestine. After 4 and 8 h, ST-III-GFP was mainly concentrated in the cecum and large intestine. Compared to the initial amount ingested, the survival rate of ST-III-GFP within the intestine of mice was 10% after 8 h. In addition, a strong linear relationship was found between the fluorescence intensity and the viable count of ST-III-GFP.

CONCLUSIONS

The obtained data indicate that the amount of ST-III-GFP can be estimated by measuring the fluorescence intensity of this novel strain within the intestinal tract. © 2020 Society of Chemical Industry.

Rapid visualized assessment of drug efficacy in live mice with a selectable marker-free autoluminescent Klebsiella pneumoniae

Klebsiella pneumoniae is an opportunistic pathogen that is responsible for community acquired infections and nosocomial infections. Antibiotic-resistant K. pneumoniae and/or hypervirulent K. pneumoniae are emerging as a serious threat to public health. For the sake of alleviating and conquering current dilemma, discovery of effective new drugs against K. pneumoniae is a tough challenge. However, traditional anti-K. pneumoniae drug discovery methods cost considerable amount of time, animals, labor and so on. So an efficient technique for in vitro and in vivo drug screening with the least time duration, animals and labor cost is highly needed for the discovery of new effective compounds. Hence, in this study we constructed a selectable marker-free autoluminescent K. pneumoniae (SfAlKp) harboring luxCDABE by combining Tn7 transposon and Xer-dif system. SfAlKp can be used for discovery of new drugs via detecting luminescence intensity as a surrogate marker. The energy-consuming autoluminescent reaction catalyzed by the LuxAB enzymes which use the substrates produced by LuxCDE using the metabolites of the bacteria. Tn7 can insert exogenous genes into the bacterial genome and the DNA fragment in between dif sequences can be recognized and removed by endogenous XerCD recombinases of K. pneumoniae. The drug susceptibility and growth rate of SfAlKp are identical to its parent strain, meanwhile the luminescence intensity and stability are also significant characteristics of SfAlKp. Compared to conventional techniques, the autoluminescence-based measurement is more applicable to high throughput screening for compounds both in vitro as well as in vivo in animal model.

Applications of bioluminescence in biotechnology and beyond

Bioluminescent probes have hugely benefited from the input of synthetic chemistry and protein engineering. Here we review the latest applications of these probes in biotechnology and beyond, with an eye on current limitations and future directions.

Live Imaging

Being able to vizualize a pathogen at a site of interaction with a host is an aesthetically appealing idea and the resulting images can be both informative as well as enjoyable to view. Moreover, the approaches used to derive these images can be powerful in terms of offering data unobtainable by other methods. In this article, we review three primary modalities for live imaging Borrelia spirochetes: whole animal imaging, intravital microscopy and live cell imaging. Each method has strengths and weaknesses, which we review, as well as specific purposes for which they are optimally utilized. Live imaging borriliae is a relatively recent development and there was a need of a review to cover the area. Here, in addition to the methods themselves, we also review areas of spirochete biology that have been significantly impacted by live imaging and present a collection of images associated with the forward motion in the field driven by imaging studies.

Advances in the In Vivo Molecular Imaging of Invasive Aspergillosis

Invasive pulmonary aspergillosis (IPA) is a life-threatening infection of immunocompromised patients with Aspergillus fumigatus, a ubiquitous environmental mould. While there are numerous functioning antifungal therapies, their high cost, substantial side effects and fear of overt resistance development preclude permanent prophylactic medication of risk-patients. Hence, a fast and definitive diagnosis of IPA is desirable, to quickly identify those patients that really require aggressive antimycotic treatment and to follow the course of the therapeutic intervention. However, despite decades of research into this issue, such a diagnostic procedure is still not available. Here, we discuss the array of currently available methods for IPA detection and their limits. We then show that molecular imaging using positron emission tomography (PET) combined with morphological computed tomography or magnetic imaging is highly promising to become a future non-invasive approach for IPA diagnosis and therapy monitoring, albeit still requiring thorough validation and relying on further acceptance and dissemination of the approach. Thereby, our approach using the A. fumigatus-specific humanized monoclonal antibody hJF5 labelled with ⁶⁴Cu as PET-tracer has proven highly effective in pre-clinical models and hence bears high potential for human application.

Saccharomyces cerevisiae and Candida albicans Yeast Cells Labeled with Fe(III) Complexes as MRI Probes

The development of MRI probes is of interest for labeling antibiotic-resistant fungal infections based on yeast. Our work showed that yeast cells can be labeled with high-spin Fe(III) complexes to produce enhanced T2 water proton relaxation. These Fe(III)-based macrocyclic complexes contained a 1,4,7-triazacyclononane framework, two pendant alcohol groups, and either a non-coordinating ancillary group and a bound water molecule or a third coordinating pendant. The Fe(III) complexes that had an open coordination site associated strongly with Saccharomyces cerevisiae upon incubation, as shown by screening using Z-spectra analysis. The incubation of one Fe(III) complex with either Saccharomyces cerevisiae or Candida albicans yeast led to an interaction with the β-glucan-based cell wall, as shown by the ready retrieval of the complex by the bidentate chelator called maltol. Other conditions, such as a heat shock treatment of the complexes, produced Fe(III) complex uptake that could not be reversed by the addition of maltol. Appending a fluorescence dye to Fe(TOB) led to uptake through secretory pathways, as shown by confocal fluorescence microscopy and by the incomplete retrieval of the Fe(III) complex by the maltol treatment. Yeast cells that were labeled with these Fe(III) complexes displayed enhanced water proton T2 relaxation, both for S. cerevisiae and for yeast and hyphal forms of C. albicans.

Hydrogen Peroxide-Activatable Nanoparticles for Luminescence Imaging and In Situ Triggerable Photodynamic Therapy of Cancer

Abnormally increased reactive oxygen species (ROS) are intimately related to the development and metastasis of cancer. Since hydrogen peroxide (H₂O₂) is a major component of ROS, molecular imaging and selective treatment in response to high H₂O₂ are intriguing for the management of cancers. Herein, we report novel self-assembly luminescent nanoparticles, which can be activated by H₂O₂, thereby serving as an effective nanotheranostics for luminescence imaging and in situ photodynamic therapy (PDT) of tumors with high H₂O₂. This functional nanomedicine was assembled from an amphiphilic conjugate (defined as CLP) based on chlorin e6 (Ce6) simultaneously conjugated with luminol and poly(ethylene glycol), exhibiting a well-defined core-shell nanostructure. Upon triggering by pathologically relevant levels of H₂O₂, CLP nanoparticles produced luminescence due to the luminol unit and simultaneous excitation of Ce6 by chemiluminescence resonance energy transfer, enabling in vitro and in vivo imaging of tumors with highly expressed H₂O₂. In addition, excited Ce6 can produce singlet oxygen (¹O₂) for in situ PDT of H₂O₂-high tumors and inhibiting lung metastasis, which was demonstrated by in vitro and in vivo experiments. Furthermore, preliminary studies revealed the biosafety of CLP nanoparticles. Consequently, the self-illuminating nanoparticles are promising for noninvasive imaging and therapy of tumors with high expression of H₂O₂.

Recent advances in functional research in Giardia intestinalis

This review considers current advances in tools to investigate the functional biology of Giardia, it's coding and non-coding genes, features and cellular and molecular biology. We consider major gaps in current knowledge of the parasite and discuss the present state-of-the-art in its in vivo and in vitro cultivation. Advances in in silico tools, including for the modelling non-coding RNAs and genomic elements, as well as detailed exploration of coding genes through inferred homology to model organisms, have provided significant, primary level insight. Improved methods to model the three-dimensional structure of proteins offer new insights into their function, and binding interactions with ligands, other proteins or precursor drugs, and offer substantial opportunities to prioritise proteins for further study and experimentation. These approaches can be supplemented by the growing and highly accessible arsenal of systems-based methods now being applied to Giardia, led by genomic, transcriptomic and proteomic methods, but rapidly incorporating advanced tools for detection of real-time transcription, evaluation of chromatin states and direct measurement of macromolecular complexes. Methods to directly interrogate and perturb gene function have made major leaps in recent years, with CRISPr-interference now available. These approaches, coupled with protein over-expression, fluorescent labelling and in vitro and in vivo imaging, are set to revolutionize the field and herald an exciting time during which the field may finally realise Giardia's long proposed potential as a model parasite and eukaryote.

Preclinical Models and Methodologies for Monitoring Staphylococcus aureus Infections Using Noninvasive Optical Imaging

In vivo whole-animal optical (bioluminescence and fluorescence) imaging of Staphylococcus aureus infections has provided the opportunity to noninvasively and longitudinally monitor the dynamics of the bacterial burden and ensuing host immune responses in live anesthetized animals. Herein, we describe several different mouse models of S. aureus skin infection, skin inflammation, incisional/excisional wound infections, as well as mouse and rabbit models of orthopedic implant infection, which utilized this imaging technology. These animal models and imaging methodologies provide insights into the pathogenesis of these infections and innate and adaptive immune responses, as well as the preclinical evaluation of diagnostic and treatment modalities. Noninvasive approaches to investigate host-pathogen interactions are extremely important as virulent community-acquired methicillin-resistant S. aureus strains (CA-MRSA) are spreading through the normal human population, becoming more antibiotic resistant and creating a serious threat to public health.

Bioluminescence Imaging to Study Mature Biofilm Formation by Candida spp. and Antifungal Activity In Vitro and In Vivo

The widespread use of indwelling medical devices has increased the number of device-related infections in hospitalized patients. These infections are often associated with the formation of biofilms on the medical implants that are difficult to treat because of their resistance to the classical antifungal drugs. The most common fungi isolated from catheters and other medical devices are Candida species. The Candida genus contains multiple species of which C. albicans and C. glabrata are the two most common pathogenic yeasts in humans. A limited number of animal models is available for investigating host-pathogen interactions and testing novel antifungal drugs in vivo against these species. Fungal load in biofilms in these models is traditionally analyzed postmortem, requiring host sacrifice and enumeration of microorganisms from individual biofilms in order to evaluate the amount of colony forming units and the efficacy of antifungal treatment. Bioluminescence imaging (BLI) made compatible with small animal models for in vivo biofilm formation is a valuable tool to follow biofilm development and its treatment longitudinally. Due to the noninvasive nature of BLI, the imaging procedure can be repeated in the same animal, allowing for follow-up of the biofilm growth in vivo without removing the implanted device or detaching the biofilm from its substrate. Although detecting a quantifiable in vivo BLI signal from biofilms formed on the inside of implanted catheters is challenging, BLI proved to be a practical tool in the study of fungal biofilms. This method describes the use of BLI for in vitro and in vivo follow-up of device-related fungal biofilm formation in mice and rats and antifungal activity testing against both C. albicans and C. glabrata device-associated biofilms. It can further be applied for efficient in vivo screening for interesting genes of the pathogen and the host involved in biofilm formation.

Rapid Tuberculosis Diagnosis Using Reporter Enzyme Fluorescence

Tuberculosis is the most frequent cause of death in humans from a single infectious agent. Due to low numbers of bacteria present in sputum during early infection, diagnosis does not usually occur until >3 to 4 months after symptoms develop. We created a new more sensitive diagnostic that can be carried out in 10 min with no processing or technical expertise.

Tuberculosis is the most frequent cause of death in humans from a single infectious agent. Due to low numbers of bacteria present in sputum during early infection, diagnosis does not usually occur until >3 to 4 months after symptoms develop. We created a new more sensitive diagnostic that can be carried out in 10 min with no processing or technical expertise. This assay utilizes the Mycobacterium tuberculosis-specific biomarker BlaC in reporter enzyme fluorescence (REF) that has been optimized for clinical samples, designated REFtb, along with a more specific fluorogenic substrate, CDG-3. We report the first evaluation of clinical specimens with REFtb assays in comparison to the gold standards for tuberculosis diagnosis, culture and smear microscopy. REFtb assays allowed diagnosis of 160 patients from 16 different countries with a sensitivity of 89% for smear-positive, culture-positive samples and 88% for smear-negative, culture-positive samples with a specificity of 82%. The negative predictive value of REFtb for tuberculosis infection is 93%, and the positive predictive value is 79%. Overall, these data point toward the need for larger accuracy studies by third parties using a commercially available REFtb kit to determine whether incorporation of REFtb into the clinical toolbox for suspected tuberculosis patients would improve case identification. If results similar to our own can be obtained by all diagnostic laboratories, REFtb would allow proper treatment of more than 85% of patients that would be missed during their initial visit to a clinic using current diagnostic strategies, reducing the potential for further spread of disease.

Development of a Staphylococcus aureus reporter strain with click beetle red luciferase for enhanced in vivo imaging of experimental bacteremia and mixed infections

In vivo bioluminescence imaging has been used to monitor Staphylococcus aureus infections in preclinical models by employing bacterial reporter strains possessing a modified lux operon from Photorhabdus luminescens. However, the relatively short emission wavelength of lux (peak 490 nm) has limited tissue penetration. To overcome this limitation, the gene for the click beetle (Pyrophorus plagiophtalamus) red luciferase (luc) (with a longer >600 emission wavelength), was introduced singly and in combination with the lux operon into a methicillin-resistant S. aureus strain. After administration of the substrate D-luciferin, the luc bioluminescent signal was substantially greater than the lux signal in vitro. The luc signal had enhanced tissue penetration and improved anatomical co-registration with infected internal organs compared with the lux signal in a mouse model of S. aureus bacteremia with a sensitivity of approximately 3 × 10⁴ CFU from the kidneys. Finally, in an in vivo mixed bacterial wound infection mouse model, S. aureus luc signals could be spectrally unmixed from Pseudomonas aeruginosa lux signals to noninvasively monitor the bacterial burden of both strains. Therefore, the S. aureus luc reporter may provide a technological advance for monitoring invasive organ dissemination during S. aureus bacteremia and for studying bacterial dynamics during mixed infections.

Non-invasive bioluminescence imaging of HCoV-OC43 infection and therapy in the central nervous system of live mice

Human coronaviruses (HCoVs) are important pathogens that cause upper respiratory tract infections and have neuroinvasive abilities; however, little is known about the dynamic infection process of CoVs in vivo, and there are currently no specific antiviral drugs to prevent or treat HCoV infection. Here, we verified the replication ability and pathogenicity of a reporter HCoV-OC43 strain expressing Renilla luciferase (Rluc; rOC43-ns2DelRluc) in mice with different genetic backgrounds (C57BL/6 and BALB/c). Additionally, we monitored the spatial and temporal progression of HCoV-OC43 through the central nervous system (CNS) of live BALB/c mice after intranasal or intracerebral inoculation with rOC43-ns2DelRluc. We found that rOC43-ns2DelRluc was fatal to suckling mice after intranasal inoculation, and that viral titers and Rluc expression were detected in the brains and spinal cords of mice infected with rOC43-ns2DelRluc. Moreover, viral replication was initially observed in the brain by non-invasive bioluminescence imaging before the infection spread to the spinal cord of BALB/c mice, consistent with its tropism in the CNS. Furthermore, the Rluc readout correlated with the HCoV replication ability and protein expression, which allowed quantification of antiviral activity in live mice. Additionally, we validated that chloroquine strongly inhibited rOC43-ns2DelRluc replication in vivo. These results provide new insights into the temporal and spatial dissemination of HCoV-OC43 in the CNS, and our methods provide an extremely sensitive platform for evaluating the efficacy of antiviral therapies to treat neuroinvasive HCoVs in live mice.

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We verified the pathogenicity of a reporter HCoV-OC43 strain expressing Renilla luciferase (rOC43-ns2DelRluc) in mice.

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HCoV-OC43 spatio-temporal progression in CNS of mice was monitored by non-invasive bioluminescence imaging (BLI).

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Chloroquine was validated strongly inhibited rOC43-ns2DelRluc replication in in live mice.

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rOC43-ns2DelRluc-based BLI was reported as a promising platform for non-invasively screening antiviral compounds in vivo.

Preclinical Optical Imaging to Study Pathogenesis, Novel Therapeutics and Diagnostics Against Orthopaedic Infection

Preclinical in vivo optical imaging includes bioluminescence imaging (BLI) and fluorescence imaging (FLI), which provide noninvasive and longitudinal monitoring of biological processes in an in vivo context. In vivo BLI involves the detection of photons of light from bioluminescent bacteria engineered to naturally emit light in preclinical animal models of infection. Meanwhile, in vivo FLI involves the detection of photons of a longer emission wavelength of light after exposure of a fluorophore to a shorter excitation wavelength of light. In vivo FLI has been used in preclinical animal models to detect fluorescent-labeled host proteins or cells (often in engineered fluorescent reporter mice) to understand host-related processes, or to detect injectable near-infrared fluorescent probes as a novel approach for diagnosing infection. This review describes the use of in vivo optical imaging in preclinical models of orthopaedic implant-associated infection (OIAI), including (i) pathogenesis of the infectious course, (ii) monitoring efficacy of antimicrobial prophylaxis and therapy and (iii) evaluating novel near-infrared fluorescent probes for diagnosing infection. Finally, we describe optoacoustic imaging and fluorescence image-guided surgery, which are recent technologies that have the potential to translate to diagnosing and treating OIAI in humans. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2269-2277, 2019.

Sensitive bioluminescence imaging of fungal dissemination to the brain in mouse models of cryptococcosis

Cryptococcus neoformans is a leading cause of fungal brain infection, but the mechanism of dissemination and dynamics of cerebral infection following pulmonary disease are poorly understood. To address these questions, non-invasive techniques that can study the dynamic processes of disease development and progression in living animal models or patients are required. As such, bioluminescence imaging (BLI) has emerged as a powerful tool to evaluate the spatial and temporal distribution of infection in living animals. We aimed to study the time profile of the dissemination of cryptococcosis from the lung to the brain in murine models by engineering the first bioluminescent C. neoformans KN99α strain, expressing a sequence-optimized red-shifted luciferase. The high pathogen specificity and sensitivity of BLI was complemented by the three-dimensional anatomical information from micro-computed tomography (μCT) of the lung and magnetic resonance imaging (MRI) of the brain. These non-invasive imaging techniques provided longitudinal readouts on the spatial and temporal distribution of infection following intravenous, intranasal or endotracheal routes of inoculation. Furthermore, the imaging results correlated strongly with the fungal load in the respective organs. By obtaining dynamic and quantitative information about the extent and timing of brain infections for individual animals, we found that dissemination to the brain after primary infection of the lung is likely a late-stage event with a timeframe that is variable between animals. This novel tool in Cryptococcus research can aid the identification of host and pathogen factors involved in this process, and supports development of novel preventive or therapeutic approaches.

Summary: A novel combination of bioluminescence and anatomical imaging non-invasively identified the timeframe and extent of Cryptococcus neoformans dissemination to the brain in animal models of systemic and pulmonary fungal infection.

Replacement, Refinement, and Reduction in Animal Studies With Biohazardous Agents

Animal models are critical to the advancement of our knowledge of infectious disease pathogenesis, diagnostics, therapeutics, and prevention strategies. The use of animal models requires thoughtful consideration for their well-being, as infections can significantly impact the general health of an animal and impair their welfare. Application of the 3Rs—replacement, refinement, and reduction—to animal models using biohazardous agents can improve the scientific merit and animal welfare. Replacement of animal models can use in vitro techniques such as cell culture systems, mathematical models, and engineered tissues or invertebrate animal hosts such as amoeba, worms, fruit flies, and cockroaches. Refinements can use a variety of techniques to more closely monitor the course of disease. These include the use of biomarkers, body temperature, behavioral observations, and clinical scoring systems. Reduction is possible using advanced technologies such as in vivo telemetry and imaging, allowing longitudinal assessment of animals during the course of disease. While there is no single method to universally replace, refine, or reduce animal models, the alternatives and techniques discussed are broadly applicable and they should be considered when infectious disease animal models are developed.

A Dual-Color Bioluminescence Reporter Mouse for Simultaneous in vivo Imaging of T Cell Localization and Function

Non-invasive imaging technologies to visualize the location and functionality of T cells are of great value in immunology. Here, we describe the design and generation of a transgenic mouse in which all T cells constitutively express green-emitting click-beetle luciferase (CBG99) while expression of the red-emitting firefly luciferase (PpyRE9) is induced by Nuclear Factor of Activated T cells (NFAT) such as during T cell activation, which allows multicolor bioluminescence imaging of T cell location and function. This dual-luciferase mouse, which we named TbiLuc, showed high constitutive luciferase expression in lymphoid organs such as lymph nodes and the spleen. Ex vivo purified CD8+ and CD4+ T cells both constitutively expressed luciferase, whereas B cells showed no detectable signal. We cross-bred TbiLuc mice to T cell receptor-transgenic OT-I mice to obtain luciferase-expressing naïve CD8+ T cells with defined antigen-specificity. TbiLuc^*OT-I T cells showed a fully antigen-specific induction of the T cell activation-dependent luciferase. In vaccinated mice, we visualized T cell localization and activation in vaccine-draining lymph nodes with high sensitivity using two distinct luciferase substrates, D-luciferin and CycLuc1, of which the latter specifically reacts with the PpyRE9 enzyme. This dual-luciferase T cell reporter mouse can be applied in many experimental models studying the location and functional state of T cells.

Bioluminescence Imaging as a Tool for Poxvirus Biology

Bioluminescence imaging, with luciferase as a reporter-encoding gene, has been successfully and widely used for studies to follow viral infection in an organism and to measure therapeutic efficacy of antiviral agents in small animal models. Bioluminescence is produced by the reaction of a luciferase enzyme stably inserted into the viral genome with a defined substrate systemically delivered into the animal. The light emitted is captured allowing the detection of viral infection sites and the quantification of viral replication in the context of tissues of a living animal. The goal of this chapter is to provide a technical background for the evaluation of poxvirus infection in cells and animals through bioluminescence imaging technology using luciferase-expressing recombinant poxviruses.

Automated quantification of bioluminescence images

We developed a computer-aided analysis tool for quantitatively determining bioluminescent reporter distributions inside small animals. The core innovations are a body-fitting animal shuttle and a statistical mouse atlas, both of which are spatially aligned and scaled according to the animal’s weight, and hence provide data congruency across animals of varying size and pose. In conjunction with a multispectral bioluminescence tomography technique capitalizing on the spatial framework of the shuttle, the in vivo biodistribution of luminescent reporters can rapidly be calculated and, thus, enables operator-independent and computer-driven data analysis. We demonstrate its functionality by quantitatively monitoring a bacterial infection, where the bacterial organ burden was determined and validated with the established serial-plating method. In addition, the statistical mouse atlas was validated and compared to existing techniques providing an anatomical reference. The proposed data analysis tool promises to increase data throughput and data reproducibility and accelerate human disease modeling in mice.

Analysis of bioluminescence images of bacterial distributions in living animals is mostly manual and semiquantitative. Here, the authors present an analysis platform featuring an animal mold, a probabilistic organ atlas, and a mirror gantry to perform automatic in vivo bioluminescence quantification.

Directed Evolution of an Influenza Reporter Virus To Restore Replication and Virulence and Enhance Noninvasive Bioluminescence Imaging in Mice

Reporter viruses provide a powerful tool to study infection, yet incorporating a nonessential gene often results in virus attenuation and genetic instability. Here, we used directed evolution of a luciferase-expressing pandemic H1N1 (pH1N1) 2009 influenza A virus in mice to restore replication kinetics and virulence, increase the bioluminescence signal, and maintain reporter gene expression. An unadapted pH1N1 virus with NanoLuc luciferase inserted into the 5' end of the PA gene segment grew to titers 10-fold less than those of the wild type in MDCK cells and in DBA/2 mice and was less virulent. For 12 rounds, we propagated DBA/2 lung samples with the highest bioluminescence-to-titer ratios. Every three rounds, we compared in vivo replication, weight loss, mortality, and bioluminescence. Mouse-adapted virus after 9 rounds (MA-9) had the highest relative bioluminescence signal and had wild-type-like fitness and virulence in DBA/2 mice. Using reverse genetics, we discovered fitness was restored in virus rPB2-MA9/PA-D479N by a combination of PA-D479N and PB2-E158G amino acid mutations and PB2 noncoding mutations C1161T and C1977T. rPB2-MA9/PA-D479N has increased mRNA transcription, which helps restore wild-type-like phenotypes in DBA/2 and BALB/c mice. Overall, the results demonstrate that directed evolution that maximizes foreign-gene expression while maintaining genetic stability is an effective method to restore wild-type-like in vivo fitness of a reporter virus. Virus rPB2-MA9/PA-D479N is expected to be a useful tool for noninvasive imaging of pH1N1 influenza virus infection and clearance while analyzing virus-host interactions and developing new therapeutics and vaccines.IMPORTANCE Influenza viruses contribute to 290,000 to 650,000 deaths globally each year. Infection is studied in mice to learn how the virus causes sickness and to develop new drugs and vaccines. During experiments, scientists have needed to euthanize groups of mice at different times to measure the amount of infectious virus in mouse tissues. By inserting a foreign gene that causes infected cells to light up, scientists could see infection spread in living mice. Unfortunately, adding an extra gene not needed by the virus slowed it down and made it weaker. Here, we used a new strategy to restore the fitness and lethality of an influenza reporter virus; we adapted it to mouse lungs and selected for variants that had the greatest light signal. The adapted virus can be used to study influenza virus infection, immunology, and disease in living mice. The strategy can also be used to adapt other viruses.

Detection of Enterohemorrhagic Escherichia Coli Colonization in Murine Host by Non-invasive In Vivo Bioluminescence System

Enterohemorrhagic E. coli (EHEC) O157:H7, which is a foodborne pathogen that causesdiarrhea, hemorrhagic colitis (HS), and hemolytic uremic syndrome (HUS), colonize to the intestinal tract of humans. To study the detailed mechanism of EHEC colonization in vivo, it is essential to have animal models to monitor and quantify EHEC colonization. We demonstrate here a mouse-EHEC colonization model by transforming the bioluminescent expressing plasmid to EHEC to monitor and quantify EHEC colonization in living hosts. Animals inoculated with bioluminescence-labeled EHEC show intense bioluminescent signals in mice by detection with a non-invasive in vivo imaging system. After 1 and 2 days post infection, bioluminescent signals could still be detected in infected animals, which suggests that EHEC colonize in hosts for at least 2 days. We also demonstrate that these bioluminescent EHEC locate to mouse intestine, specifically in the cecum and colon, from ex vivo images. This mouse-EHEC colonization model may serve as a tool to advance the current knowledge of the EHEC colonization mechanism.

Endocrine influence on neuroinflammation: the use of reporter systems

Most of the ageing-associated pathologies are coupled with a strong inflammatory component that accelerates the progress of the physiopathological functional decline related to ageing. The currently available pharmacological tools for the control of neuroinflammation present several side effects that restrict their application, particularly in chronic disorders. The discovery of the potential anti-inflammatory action exerted by endogenous oestrogens, as well as the finding that activation of oestrogen receptor α results in a significant decrease of inflammation at the cellular level and in models of inflammatory diseases, prompted us to embark in a series of studies aimed at the generation of reporter systems, allowing us to (i) understand the anti-inflammatory action of oestrogens at molecular level; (ii) evaluate the extent to which the action of this steroid hormone was relevant in models of pathologies characterised by a strong inflammatory component; and (iii) investigate the efficacy of novel, synthetic oestrogens endowed with anti-inflammatory activity. Accordingly, we conceived the NFκB-luc2 reporter mouse, a model characterised by dual reporter genes for fluorescence and bioluminescence imaging under the control of a synthetic DNA able to bind the transcription factor nuclear factor kappa B, the master regulator of the expression of most of the cytokines responsible for the initial phase of acute inflammation. Here, we summarise the philosophy that has driven our research in the past years, as well as some of the results obtained so far.

1001 lights: luciferins, luciferases, their mechanisms of action and applications in chemical analysis, biology and medicine

Bioluminescence (BL) is a spectacular phenomenon involving light emission by live organisms. It is caused by the oxidation of a small organic molecule, luciferin, with molecular oxygen, which is catalysed by the enzyme luciferase. In nature, there are approximately 30 different BL systems, of which only 9 have been studied to various degrees in terms of their reaction mechanisms. A vast range of in vitro and in vivo analytical techniques have been developed based on BL, including tests for different analytes, immunoassays, gene expression assays, drug screening, bioimaging of live organisms, cancer studies, the investigation of infectious diseases and environmental monitoring. This review aims to cover the major existing applications for bioluminescence in the context of the diversity of luciferases and their substrates, luciferins. Particularly, the properties and applications of d-luciferin, coelenterazine, bacterial, Cypridina and dinoflagellate luciferins and their analogues along with their corresponding luciferases are described. Finally, four other rarely studied bioluminescent systems (those of limpet Latia, earthworms Diplocardia and Fridericia and higher fungi), which are promising for future use, are also discussed.

Detection of Bioluminescent Borrelia burgdorferi from In Vitro Cultivation and During Murine Infection

Borrelia burgdorferi, etiologic agent of Lyme disease, is the leading tick-borne disease in the United States with approximately 300,000 cases diagnosed annually. Disease occurs in stages beginning localized infection at the site of a tick bite and progresses to disseminated infection when antibiotic treatment is not administered in a timely manner. A multi-systemic infection develops following dissemination to numerous immunoprotective tissues, such as the heart, bladder, and joints, resulting in late Lyme disease. B. burgdorferi undergoes dynamic genetic regulation throughout mammalian infection and defining the exact role of virulence genes at distinct stages of disease is challenging. The murine model allows for the characterization of the pathogenic function of genes in B. burgdorferi, but traditional end point studies limit the ability to gather data throughout an infection study and greatly increase the required number of mice. Molecular genetic techniques to evaluate and quantitate B. burgdorferi infection are laborious and costly. To partly circumvent these issues, a codon optimized firefly luciferase, under the control of a constitutive borrelial promoter, was introduced into B. burgdorferi enabling the characterization of mutant or modified strains under in vitro growth conditions and throughout murine infection. The detection of bioluminescent B. burgdorferi is highly sensitive and allows for the repeated real-time quantitative evaluation of borrelial load during murine infection. Furthermore, bioluminescence has also been utilized to evaluate alteration in tissue localization and tissue-specific gene expression of B. burgdorferi. In this chapter, we describe the generation of bioluminescent borrelial strains along with methods for in vitro, in vivo, and ex vivo B. burgdorferi studies.

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.

Current status on the development of pseudoviruses for enveloped viruses

Emerging and reemerging infectious diseases have a strong negative impact on public health. However, because many of these pathogens must be handled in biosafety level, 3 or 4 containment laboratories, research and development of antivirals or vaccines against these diseases are often impeded. Alternative approaches to address this issue have been vigorously pursued, particularly the use of pseudoviruses in place of wild‐type viruses. As pseudoviruses have been deprived of certain gene sequences of the virulent virus, they can be handled in biosafety level 2 laboratories. Importantly, the envelopes of these viral particles may have similar conformational structures to those of the wild‐type viruses, making it feasible to conduct mechanistic investigation on viral entry and to evaluate potential neutralizing antibodies. However, a variety of challenging issues remain, including the production of a sufficient pseudovirus yield and the inability to produce an appropriate pseudotype of certain viruses. This review discusses current progress in the development of pseudoviruses and dissects the factors that contribute to low viral yields.

Bioluminescent tracing of a Yersinia pestis pCD1+-mutant and Yersinia pseudotuberculosis in subcutaneously infected mice

Yersinia pestis has evolved from Yersinia pseudotuberculosis serotype O:1b. A typical Y. pestis contains three plasmids: pCD1, pMT1 and pPCP1. However, some isolates only harbor pCD1 (pCD1⁺-mutant). Y. pestis and Y. pseudotuberculosis share a common plasmid (pCD1 or pYV), but little is known about whether Y. pseudotuberculosis exhibited plague-inducing potential before it was evolved into Y. pestis. Here, the luxCDABE::Tn5::kan was integrated into the chromosome of the pCD1⁺-mutant, Y. pseudotuberculosis or Escherichia coli K12 to construct stable bioluminescent strains for investigation of their dissemination in mice by bioluminescence imaging technology. After subcutaneous infection, the pCD1⁺-mutant entered the lymph nodes, followed by the liver and spleen, and, subsequently, the lungs, causing pathological changes in these organs. Y. pseudotuberculosis entered the lymph nodes, but not the liver, spleen and lungs. It also resided in the lymph nodes for several days, but did not cause lymphadenitis or pathological lesions. By contrast, E. coli K12-lux was not isolatable from mouse lymph nodes, liver, spleen and lungs. These results indicate that the pCD1⁺-mutant can cause typical bubonic and pneumonic plague-like diseases, and Y. pestis has inherited lymphoid tissue tropism from its ancestor rather than acquiring these properties independently.

New insights into experimental visceral leishmaniasis: Real-time in vivo imaging of Leishmania donovani virulence

Visceral leishmaniasis is an insidious neglected disease with worldwide distribution. It is caused by parasites from the Leishmania donovani complex, which are able to be transmitted by different species of phlebotomine sand flies and to infect numerous mammal hosts. Despite the high number of people infected or at risk, and the remarkable quantity of studies focusing on this disease, a proper experimental model to efficiently decipher the infectious process of visceral leishmaniasis taking into account the nuances of parasite’s virulence and the duration of the infection is still lacking. Therefore, using golden Syrian hamsters and BALB/c mice, state-of-the-art genetic manipulation applied on a fully virulent L. donovani strain and in vivo imaging approaches, we describe herein three benefits for experimental visceral leishmaniasis: (i) the development of a double transfected bioluminescent (firefly luciferase) and fluorescent (E2-crimson) virulent strain of L. donovani (Ld1S_luci_E2-crimson), favoring a wide range of both in vivo and in vitro investigations, (ii) the establishment of a non-invasive mouse model to evaluate the infectious process during visceral leishmaniasis and the parasite’s virulence in real time, allowing longitudinal studies with the same animals, and (iii) the elaboration of a suitable method to reinstate (and verify anew) the virulence in a population of attenuated parasites, by recovering persistent parasites from chronic infected mice. Consequently, these results open up new perspectives on the study of visceral leishmaniasis, especially in the fields of therapeutics and vaccinology, since the model described herein renders now possible long-lasting follow up studies, with easy and accurate day-by-day verifications of the infection status along with a reduced number of laboratory animals.

TRIAL REGISTRATION

ClinicalTrials.gov 2013-0047.

Fluorescent Lactic Acid Bacteria and Bifidobacteria as Vehicles of DNA Microbial Biosensors

Control and quantification of effector molecules such as heavy metals, toxins or other target molecules is of great biotechnological, social and economic interest. Microorganisms have regulatory proteins that recognize and modify the gene expression in the presence or absence of these compounds (effector molecules) by means of binding to gene sequences. The association of these recognizing gene sequences to reporter genes will allow the detection of effector molecules of interest with high sensitivity. Once investigators have these two elements-recognizing gene sequences and reporter genes that emit signals-we need a suitable vehicle to introduce both elements. Here, we suggest lactic acid bacteria (LAB) and bifidobacteria as promising carrier microorganisms for these molecular biosensors. The use of fluorescent proteins as well as food-grade vectors and clustered regularly interspaced short palindromic repeats (CRISPR) are indispensable tools for introducing biosensors into these microorganisms. The use of these LAB and bifidobacteria would be of special interest for studying the intestinal environment or other complex ecosystems. The great variety of species adapted to many environments, as well as the possibility of applying several protocols for their transformation with recognizing gene sequences and reporter genes are considerable advantages. Finally, an effort must be made to find recognizable gene sequences.

Giardia Colonizes and Encysts in High-Density Foci in the Murine Small Intestine

Giardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Giardia lamblia is a highly prevalent yet understudied protistan parasite causing significant diarrheal disease worldwide. Hosts ingest Giardia cysts from contaminated sources. In the gastrointestinal tract, cysts excyst to become motile trophozoites, colonizing and attaching to the gut epithelium. Trophozoites later differentiate into infectious cysts that are excreted and contaminate the environment. Due to the limited accessibility of the gut, the temporospatial dynamics of giardiasis in the host are largely inferred from laboratory culture and thus may not mirror Giardia physiology in the host. Here, we have developed bioluminescent imaging (BLI) to directly interrogate and quantify the in vivo temporospatial dynamics of Giardia infection, thereby providing an improved murine model to evaluate anti-Giardia drugs. Using BLI, we determined that parasites primarily colonize the proximal small intestine nonuniformly in high-density foci. By imaging encystation-specific bioreporters, we show that encystation initiates shortly after inoculation and continues throughout the duration of infection. Encystation also initiates in high-density foci in the proximal small intestine, and high density contributes to the initiation of encystation in laboratory culture. We suggest that these high-density in vivo foci of colonizing and encysting Giardia likely result in localized disruption to the epithelium. This more accurate visualization of giardiasis redefines the dynamics of the in vivo Giardia life cycle, paving the way for future mechanistic studies of density-dependent parasitic processes in the host.

IMPORTANCEGiardia is a single-celled parasite causing significant diarrheal disease in several hundred million people worldwide. Due to limited access to the site of infection in the gastrointestinal tract, our understanding of the dynamics of Giardia infections in the host has remained limited and largely inferred from laboratory culture. To better understand Giardia physiology and colonization in the host, we developed imaging methods to quantify Giardia expressing bioluminescent physiological reporters in two relevant animal models. We discovered that parasites primarily colonize and encyst in the proximal small intestine in discrete, high-density foci. We also show that high parasite density contributes to encystation initiation.

Transcriptomic Profiling of High-Density Giardia Foci Encysting in the Murine Proximal Intestine

Giardia is a highly prevalent, understudied protistan parasite causing significant diarrheal disease worldwide. Its life cycle consists of two stages: infectious cysts ingested from contaminated food or water sources, and motile trophozoites that colonize and attach to the gut epithelium, later encysting to form new cysts that are excreted into the environment. Current understanding of parasite physiology in the host is largely inferred from transcriptomic studies using Giardia grown axenically or in co-culture with mammalian cell lines. The dearth of information about the diversity of host-parasite interactions occurring within distinct regions of the gastrointestinal tract has been exacerbated by a lack of methods to directly and non-invasively interrogate disease progression and parasite physiology in live animal hosts. By visualizing Giardia infections in the mouse gastrointestinal tract using bioluminescent imaging (BLI) of tagged parasites, we recently showed that parasites colonize the gut in high-density foci. Encystation is initiated in these foci throughout the entire course of infection, yet how the physiology of parasites within high-density foci in the host gut differs from that of cells in laboratory culture is unclear. Here we use BLI to precisely select parasite samples from high-density foci in the proximal intestine to interrogate in vivo Giardia gene expression in the host. Relative to axenic culture, we noted significantly higher expression (>10-fold) of oxidative stress, membrane transporter, and metabolic and structural genes associated with encystation in the high-density foci. These differences in gene expression within parasite foci in the host may reflect physiological changes associated with high-density growth in localized regions of the gut. We also identified and verified six novel cyst-specific proteins, including new components of the cyst wall that were highly expressed in these foci. Our in vivo transcriptome data support an emerging view that parasites encyst early in localized regions in the gut, possibly as a consequence of nutrient limitation, and also impact local metabolism and physiology.

A replication-competent foot-and-mouth disease virus expressing a luciferase reporter

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We have generated a replication-competent foot-and-mouth disease virus expressing Nanoluciferase, designated as Nano-FMDV.

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Nano-FMDV is genetically stable.

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The replication of Nano-FMDV can be monitored by bioluminescent methods.

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This reporter virus has potential applications in real-time monitoring of FMDV infection in vitro and in vivo, and in screening of antivirals and antibodies.

Bioluminescence is a powerful tool in the study of viral infection both in vivo and in vitro. Foot-and-mouth disease virus (FMDV) has a small RNA genome with a limited tolerance to foreign RNA entities. There has been no success in making a reporter FMDV expressing a luciferase in infected cell culture supernatants. We report here for the first time a replication-competent FMDV encoding Nanoluciferase, named as Nano-FMDV. Nano-FMDV is genetically stable during serial passages in cells and exhibits growth kinetics and plaque morphology similar to its parental virus. There are applications for the use of Nano-FMDV such as real-time monitoring of FMDV replication in vitro and in vivo.

Tracking Bioluminescent ETEC during In vivo BALB/c Mouse Colonization

Enterotoxigenic Escherichia coli (ETEC) is a leading cause of diarrhea worldwide. Adhesion to the human intestinal tract is crucial for colonization. ETEC adhesive structures have been extensively studied; however, colonization dynamics remain uncharacterized. The aim of this study was to track bioluminescent ETEC during in vivo infection. The promoter region of dnaK was fused with the luc gene, resulting in the pRMkluc vector. E. coli K-12 and ETEC FMU073332 strains were electroporated with pRMkluc. E. coli K-12 pRMkluc was bioluminescent; in contrast, the E. coli K-12 control strain did not emit bioluminescence. The highest light emission was measured at 1.9 OD₆₀₀ (9 h) and quantified over time. The signal was detected starting at time 0 and up to 12 h. Streptomycin-treated BALB/c mice were orogastrically inoculated with either ETEC FMU073332 pRMkluc or E. coli K-12 pRMkluc (control), and bacterial colonization was determined by measuring bacterial shedding in the feces. ETEC FMU073332 pRMkluc shedding started and stopped after inoculation of the control strain, indicating that mouse intestinal colonization by ETEC FMU073332 pRMkluc lasted longer than colonization by the control. The bioluminescence signal of ETEC FMU073332 pRMkluc was captured starting at the time of inoculation until 12 h after inoculation. The bioluminescent signal emitted by ETEC FMU073332 pRMkluc in the proximal mouse ileum was located, and the control signal was identified in the cecum. The detection of maximal light emission and bioluminescence duration allowed us to follow ETEC during in vivo infection. ETEC showed an enhanced colonization and tropism in the mouse intestine compared with those in the control strain. Here, we report the first study of ETEC colonization in the mouse intestine accompanied by in vivo imaging.

A bioluminescent imaging mouse model for Marburg virus based on a pseudovirus system

Marburg virus (MARV) can cause lethal hemorrhagic fever in humans. Handling of MARV is restricted to high-containment biosafety level 4 (BSL-4) facilities, which greatly impedes research into this virus. In this study, a high titer of MARV pseudovirus was generated through optimization of the HIV backbone vectors, the ratio of backbone vector to MARV glycoprotein expression vector, and the transfection reagents. An in vitro neutralization assay and an in vivo bioluminescent imaging mouse model for MARV were developed based on the pseudovirus. Protective serum against MARV was successfully induced in guinea pigs, which showed high neutralization activity in vitro and could also protect Balb/c mice from MARV pseudovirus infection in vivo. This system could be a convenient tool to enable the evaluation of vaccines and therapeutic drugs against MARV in non-BSL-4 laboratories.