Deep-Tissue In Vivo Imaging Using Bioluminescence in a Mouse Infection Model and the Path to High Sensitivity With Near-Infrared Luminescence

The analysis of bacterial infections using animal models has primarily relied on the average evaluation of many individuals at specific time points. Consequently, tracking temporal changes in an infection within the same individual is challenging. InVivo imaging techniques enable the longitudinal assessment of infection in the same individual while reducing the number of animals required. Understanding the dynamics of bacterial infections over time is crucial for elucidating disease mechanisms and developing effective treatment strategies. In this review, we summarize the In Vivo imaging techniques used to detect bacterial colonization in deep tissues in animal models of bacterial infection, along with efforts to enhance their sensitivity. In particular, we introduce a recently developed In Vivo imaging system that employs near-infrared luminescence to achieve high sensitivity and versatility. Furthermore, we discuss strategies for further improving its sensitivity.

Usage of a Bioluminescence Reporter System to Image Promoter Activity During Host Infection

Bioluminescence is the process of production and emission of light by a living organism, usually as the by-product of the oxidative enzyme, luciferase. Currently available technology allows for the exploitation of a bioluminescent reporter system to study bacterial gene regulation during rodent infection, in real time, over a large dynamic range. Here we show how this imaging system can be used to study virulence gene regulation during Salmonella enterica infection in the mouse model. To demonstrate this technique we show the ex vivo expression pattern of the gene dksA, encoding a conserved and pleotropic regulator, which plays a key role in Salmonella pathogenicity [1].

In vivo bioluminescence imaging of the spatial and temporal colonization of lactobacillus plantarum 423 and enterococcus mundtii ST4SA in the intestinal tract of mice

Background

Lactic acid bacteria (LAB) are major inhabitants and part of the normal microflora of the gastrointestinal tract (GIT) of humans and animals. Despite substantial evidence supporting the beneficial properties of LAB, only a few studies have addressed the migration and colonization of probiotic bacteria in the GIT. The reason for this is mostly due to the limitations, or lack of, efficient reporter systems. Here we describe the development and application of a non-invasive in vivo bioluminescence reporter system to study, in real-time, the spatial and temporal persistence of Lactobacillus plantarum 423 and Enterococcus mundtii ST4SA in the intestinal tract of mice.

Results

This study reports on the application of the firefly luciferase gene (ffluc) from Photinus pyralis to develop luciferase-expressing L. plantarum 423 and E. mundtii ST4SA, using a Lactococcus lactis NICE system on a high copy number plasmid (pNZ8048) and strong constitutive lactate dehydrogenase gene promoters (Pldh and STldh). The reporter system was used for in vivo and ex vivo monitoring of both probiotic LAB strains in the GIT of mice after single and multiple oral administrations. Enterococcus mundtii ST4SA reached the large intestine 45 min after gavage, while L. plantarum 423 reached the cecum/colon after 90 min. Both strains predominantly colonized the cecum and colon after five consecutive daily administrations. Enterococcus mundtii ST4SA persisted in faeces at higher numbers and for more days compared to L. plantarum 423.

Conclusions

Our findings demonstrate the efficiency of a high-copy number vector, constitutive promoters and bioluminescence imaging to study the colonization and persistence of L. plantarum 423 and E. mundtii ST4SA in the murine GIT. The system allowed us to differentiate between intestinal transit times of the two strains in the digestive tract. This is the first report of bioluminescence imaging of a luciferase-expressing E. mundtii strain to study colonization dynamics in the murine model. The bioluminescence system developed in this study may be used to study the in vivo colonization dynamics of other probiotic LAB.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1315-4) contains supplementary material, which is available to authorized users.

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.

Prevalence of Yersinia enterocolitica Bioserotype 3/O:3 among Children with Diarrhea, China, 2010-2015

Yersinia enterocolitica is thought to not significantly contribute to diarrheal disease in China, but evidence substantiating this claim is limited. We determined the prevalence of Y. enterocolitica infection and strain types present among children <5 years of age with diarrhea in China. The overall prevalence of pathogenic isolates was 0.59%. Prevalence of pathogenic bioserotype 3/O:3 varied geographically. In this population, the presence of fecal leukocytes was a characteristic of Y. enterocolitica infection and should be used as an indication for microbiological diagnostic testing, rather than for the diagnosis of bacillary dysentery. In contrast with Y. enterocolitica isolates from adults, which were primarily biotype 1A, isolates from children were primarily bioserotype 3/O:3. Most pathogenic isolates from children shared pulsed-field gel electrophoresis patterns with isolates from pigs and dogs, suggesting a possible link between isolates from animals and infections in children. Our findings underscore the need for improved diagnostics for this underestimated pathogen.

LuxCDE-luxAB-based promoter reporter system to monitor the Yersinia enterocolitica O:3 gene expression in vivo

It is crucial to understand the in vitro and in vivo regulation of the virulence factor genes of bacterial pathogens. In this study, we describe the construction of a versatile reporter system for Yersinia enterocolitica serotype O:3 (YeO3) based on the luxCDABE operon. In strain YeO3-luxCDE we integrated the luciferase substrate biosynthetic genes, luxCDE, into the genome of the bacterium so that the substrate is constitutively produced. The luxAB genes that encode the luciferase enzyme were cloned into a suicide vector to allow cloning of any promoter-containing fragment upstream the genes. When the obtained suicide-construct is mobilized into YeO3-luxCDE bacteria, it integrates into the recipient genome via homologous recombination between the cloned promoter fragment and the genomic promoter sequence and thereby generates a single-copy and stable promoter reporter. Lipopolysaccharide (LPS) O-antigen (O-ag) and outer core hexasaccharide (OC) of YeO3 are virulence factors necessary to colonization of the intestine and establishment of infection. To monitor the activities of the OC and O-ag gene cluster promoters we constructed the reporter strains YeO3-Poc::luxAB and YeO3-Pop1::luxAB, respectively. In vitro, at 37°C both promoter activities were highest during logarithmic growth and decreased when the bacteria entered stationary growth phase. At 22°C the OC gene cluster promoter activity increased during the late logarithmic phase. Both promoters were more active in late stationary phase. To monitor the promoter activities in vivo, mice were infected intragastrically and the reporter activities monitored by the IVIS technology. The mouse experiments revealed that both LPS promoters were well expressed in vivo and could be detected by IVIS, mainly from the intestinal region of orally infected mice.

Yersinia adhesins: An arsenal for infection

The Yersiniae are a group of Gram-negative coccobacilli inhabiting a wide range of habitats. The genus harbors three recognized human pathogens: Y. enterocolitica and Y. pseudotuberculosis, which both cause gastrointestinal disease, and Y. pestis, the causative agent of plague. These three organisms have served as models for a number of aspects of infection biology, including adhesion, immune evasion, evolution of pathogenic traits, and retracing the course of ancient pandemics. The virulence of the pathogenic Yersiniae is heavily dependent on a number of adhesin molecules. Some of these, such as the Yersinia adhesin A and invasin of the enteropathogenic species, and the pH 6 antigen of Y. pestis, have been extensively studied. However, genomic sequencing has uncovered a host of other adhesins present in these organisms, the functions of which are only starting to be investigated. Here, we review the current state of knowledge on the adhesin molecules present in the Yersiniae, and their functions and putative roles in the infection process.

Development of Bioluminescent Cronobacter sakazakii ATCC 29544 in a Mouse Model

Cronobacter sakazakii is an emerging pathogen that causes severe and life-threatening conditions including meningitis, bacteremia, and necrotizing enterocolitis. An animal model study for extrapolation of C. sakazakii infection can provide a better understanding of pathogenesis. However, methods for real-time monitoring of the course of C. sakazakii infection in living animals have been lacking. We developed a bioluminescent C. sakazakii strain (ATCC 29544) that can be used for real-time monitoring of C. sakazakii infection in BALB/c mice. C. sakazakii ATCC 29544 mainly colonized brain, liver, spleen, kidney, and gastrointestinal tract, as indicated by bioluminescence imaging. This work provides a novel approach for studying the progression of C. sakazakii infection and evaluating therapeutics in a living mouse model.

Propagation capacity of bacterial contaminants in platelet concentrates using a luciferase reporter system

INTRODUCTION

Currently the use of molecular tools and techniques of Genetic Engineering in the study of microbial behavior in blood components has replaced the employment of classical methods of microbiology. This work focuses on the use of a novel lux reporter system for monitoring the contaminating propagation capacity of bacteria present in platelet concentrates under standard storage conditions in the blood bank.

METHODS

A miniTn5 promotor probe carrying the lux operon from Photorhabdus luminiscens (pUTminiTn5luxCDABEKm2) was used to construct four bacterial bioluminescent mutants: Escherichia coli, Salmonella typhi, Proteus mirabilis and Pseudomonas aeruginosa. Luminescent mutants were used for contamination tests with 20 CFU in platelet concentrates bags and were stored under standard storage conditions in the blood bank (100 rpm at 22 °C). The measurements of luminous activity and optical density were used to monitor bacterial proliferation during 7 days (168 h).

RESULTS

During the exponential growth phase (log) of bacterial strains, a lineal correlation between luminous activity vs biomass was observed (R(2) = 0.985, 0.976, 0.981) for E. coli::Tn5luxCDABEKm2, P. mirabilis::Tn5luxCDABEKm2 and P. auriginosa::Tn5luxCDABEKm2, respectively. The above indicates that metabolic activity (production of ATP) is directly related to biomass in this phase of microbial growth. While conducting experiments, the inability to propagate S. typhi::Tn5luxCDABEKm2 was detected. We can speculate that platelet concentrates contain specific components that prevent the propagation of S. typhi.

CONCLUSION

The use of luxCDABE system for the quantification of luminous activity is a rapid and sensitive alternative to study the propagation and auto-sterilization of bacterial contaminants in platelet concentrates.

Tracking bacterial pathogens with genetically-encoded reporters

During the infectious process, bacterial pathogens are subject to changes in environmental conditions such as nutrient availability, immune response challenges, bacterial density and physical contacts with targeted host cells. These conditions occur in the colonized organs, in diverse regions within infected tissues or even at the subcellular level for intracellular pathogens. Integration of environmental cues leads to measurable biological responses in the bacterium required for adaptation. Recent progress in technology enabled the study of bacterial adaptation in situ using genetically encoded reporters that allow single cell analysis or whole body imaging based on fluorescent proteins, alternative fluorescent assays or luciferases. This review presents a historical perspective and technical details on the methods used to develop transcriptional reporters, protein-protein interaction assays and secretion detection assays to study pathogenic bacteria adaptation in situ. Finally, studies published in the last 5 years on gram positive and gram negative bacterial adaptation to the host during infection are discussed. However, the methods described here could easily be extended to study complex microbial communities within host tissue and in the environment.

Human and animal isolates of Yersinia enterocolitica show significant serotype-specific colonization and host-specific immune defense properties

Yersinia enterocolitica is a human pathogen that is ubiquitous in livestock, especially pigs. The bacteria are able to colonize the intestinal tract of a variety of mammalian hosts, but the severity of induced gut-associated diseases (yersiniosis) differs significantly between hosts. To gain more information about the individual virulence determinants that contribute to colonization and induction of immune responses in different hosts, we analyzed and compared the interactions of different human- and animal-derived isolates of serotypes O:3, O:5,27, O:8, and O:9 with murine, porcine, and human intestinal cells and macrophages. The examined strains exhibited significant serotype-specific cell binding and entry characteristics, but adhesion and uptake into different host cells were not host specific and were independent of the source of the isolate. In contrast, survival and replication within macrophages and the induced proinflammatory response differed between murine, porcine, and human macrophages, suggesting a host-specific immune response. In fact, similar levels of the proinflammatory cytokine macrophage inflammatory protein 2 (MIP-2) were secreted by murine bone marrow-derived macrophages with all tested isolates, but the equivalent interleukin-8 (IL-8) response of porcine bone marrow-derived macrophages was strongly serotype specific and considerably lower in O:3 than in O:8 strains. In addition, all tested Y. enterocolitica strains caused a considerably higher level of secretion of the anti-inflammatory cytokine IL-10 by porcine than by murine macrophages. This could contribute to limiting the severity of the infection (in particular of serotype O:3 strains) in pigs, which are the primary reservoir of Y. enterocolitica strains pathogenic to humans.

OmpR, a response regulator of the two-component signal transduction pathway, influences inv gene expression in Yersinia enterocolitica O9

The environmental control of invasin (inv) expression in Yersinia enterocolitica is mediated by a regulatory network composed of negative and positive regulators of inv gene transcription. Previously, we demonstrated that OmpR, a response regulator of the two-component signal transduction pathway EnvZ/OmpR, negatively regulates inv gene expression in Y. enterocolitica O9 by direct interaction with the inv promoter region. This study was undertaken to clarify the role of OmpR in the inv regulatory circuit in which RovA protein has been shown to positively regulate inv transcription. Using ompR, rovA, and ompR rovA Y. enterocolitica mutant backgrounds we showed that the inhibitory effect of OmpR on inv transcription may be observed only when RovA is present/active in Y. enterocolitica cells. To extend our research on inv regulation we examined the effect of OmpR on rovA gene expression. Analysis of rovA-lacZ transcriptional fusion in Y. enterocolitica wild-type and ompR background indicated that OmpR does not influence rovA expression. Thus, our results indicate that OmpR influences inv expression directly via binding to the inv promoter, but not through modulation of rovA expression.

Bioluminescence imaging study of spatial and temporal persistence of Lactobacillus plantarum and Lactococcus lactis in living mice

Lactic acid bacteria, especially lactobacilli, are common inhabitants of the gastrointestinal tract of mammals, for which they have received considerable attention due to their putative health-promoting properties. In this study, we describe the development and application of luciferase-expressing Lactobacillus plantarum and Lactococcus lactis strains for noninvasive in vivo monitoring in the digestive tract of mice. We report for the first time the functional in vitro expression in Lactobacillus plantarum NCIMB8826 and in Lactococcus lactis MG1363 of the click beetle luciferase (CBluc), as well as Gaussia and bacterial luciferases, using a combination of vectors, promoters, and codon-optimized genes. We demonstrate that a CBluc construction is the best-performing luciferase system for the noninvasive in vivo detection of lactic acid bacteria after oral administration. The persistence and viability of both strains was studied by bioluminescence imaging in anesthetized mice and in mouse feces. In vivo bioluminescence imaging confirmed that after a single or multiple oral administrations, L. lactis has shorter survival times in the mouse gastrointestinal tract than L. plantarum, and it also revealed the precise gut compartments where both strains persisted. The application of luciferase-labeled bacteria has significant potential to allow the in vivo and ex vivo study of the interactions of lactic acid bacteria with their mammalian host.

Development of bioluminescent bioreporters for in vitro and in vivo tracking of Yersinia pestis

Yersinia pestis causes an acute infection known as the plague. Conventional techniques to enumerate Y. pestis can be labor intensive and do not lend themselves to high throughput assays. In contrast, bioluminescent bioreporters produce light that can be detected using plate readers or optical imaging platforms to monitor bacterial populations as a function of luminescence. Here, we describe the development of two Y. pestis chromosomal-based luxCDABE bioreporters, Lux(PtolC) and Lux(PcysZK). These bioreporters use constitutive promoters to drive expression of luxCDABE that allow for sensitive detection of bacteria via bioluminescence in vitro. Importantly, both bioreporters demonstrate a direct correlation between bacterial numbers and bioluminescence, which allows for bioluminescence to be used to compare bacterial numbers. We demonstrate the use of these bioreporters to test antimicrobial inhibitors (Lux(PtolC)) and monitor intracellular survival (Lux(PtolC) and Lux(PcysZK)) in vitro. Furthermore, we show that Y. pestis infection of the mouse model can be monitored using whole animal optical imaging in real time. Using optical imaging, we observed Y. pestis dissemination and differentiated between virulence phenotypes in live animals via bioluminescence. Finally, we demonstrate that whole animal optical imaging can identify unexpected colonization patterns in mutant-infected animals.

Unique cell adhesion and invasion properties of Yersinia enterocolitica O:3, the most frequent cause of human Yersiniosis

Many enteric pathogens are equipped with multiple cell adhesion factors which are important for host tissue colonization and virulence. Y. enterocolitica, a common food-borne pathogen with invasive properties, uses the surface proteins invasin and YadA for host cell binding and entry. In this study, we demonstrate unique cell adhesion and invasion properties of Y. enterocolitica serotype O:3 strains, the most frequent cause of human yersiniosis, and show that these differences are mainly attributable to variations affecting the function and expression of invasin in response to temperature. In contrast to other enteric Yersinia strains, invasin production in O:3 strains is constitutive and largely enhanced compared to other Y. enterocolitica serotypes, in which invA expression is temperature-regulated and significantly reduced at 37°C. Increase of invasin levels is caused by (i) an IS1667 insertion into the invA promoter region, which includes an additional promoter and RovA and H-NS binding sites, and (ii) a P98S substitution in the invA activator protein RovA rendering the regulator less susceptible to proteolysis. Both variations were shown to influence bacterial colonization in a murine infection model. Furthermore, we found that co-expression of YadA and down-regulation of the O-antigen at 37°C is required to allow efficient internalization by the InvA protein. We conclude that even small variations in the expression of virulence factors can provoke a major difference in the virulence properties of closely related pathogens which may confer better survival or a higher pathogenic potential in a certain host or host environment.

Determination of spatial and temporal colonization of enteropathogenic E. coli and enterohemorrhagic E. coli in mice using bioluminescent in vivo imaging

Infectious diarrhea is a major contributor of child morbidity and mortality in developing nations. Murine models to study the pathogenesis of infectious diarrhea caused by organisms such as enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) are not fully characterized. More emphasis has been placed on infection of mice with the murine specific pathogen Citrobacter rodentium. While these three organisms are genetically related they are not identical. Our goal was to better characterize the murine model of EPEC and EHEC infection by using bioluminescent bacteria to determine temporal and spatial colonization of these two human pathogens. EPEC and EHEC were transformed with a bacterial luciferase expression plasmid containing the constitutive OmpC promoter. C57BL/6 mice were orally inoculated with bioluminescent EPEC or EHEC and bacterial localization in the intestine was monitored ex vivo and in vivo by IVIS. At 3 days after infection, EPEC, EHEC and Citrobacter rodentium were all localized in the cecum and colon. EPEC colonization peaked at day 2-3 and was undetectable by day 7. The bioluminescent EPEC adheres to the cecum and colon of the mouse intestine. However, when EPEC infected mice were administered xylazine/ketamine for in vivo live imaging, the EPEC persisted at high densities for up to 31 days. This is the first report of a bioluminescent imaging of luciferase expressing EPEC in a mouse model.