This video article describes the high throughput pipeline that has been successfully established to infect and analyze large numbers of zebrafish embryos providing a new powerful tool for compound testing and drug discovery using a whole animal vertebrate organism.
Zebrafish are becoming a valuable tool in the preclinical phase of drug discovery screenings as a whole animal model with high throughput screening possibilities. They can be used to bridge the gap between cell based assays at earlier stages and in vivo validation in mammalian models, reducing, in this way, the number of compounds passing through to testing on the much more expensive rodent models. In this light, in the present manuscript is described a new high throughput pipeline using zebrafish as in vivo model system for the study of Staphylococcus epidermidis and Mycobacterium marinum infection. This setup allows the generation and analysis of large number of synchronous embryos homogenously infected. Moreover the flexibility of the pipeline allows the user to easily implement other platforms to improve the resolution of the analysis when needed. The combination of the zebrafish together with innovative high throughput technologies opens the field of drug testing and discovery to new possibilities not only because of the strength of using a whole animal model but also because of the large number of transgenic lines available that can be used to decipher the mode of action of new compounds.
To date the zebrafish (Danio rerio) has been successfully established as an efficient model to study a variety of infectious diseases1. The zebrafish embryo offers unique in vivo imaging possibilities due to their transparency and the large number of existing transgenic reporter lines expressing fluorescent proteins. This powerful combination makes it possible to track different immune cell types in time while interacting with pathogens such as Mycobacterium marinum, the closest relative of M. tuberculosis2, or Staphylococcus epidermidis, the main causative of biomaterial associated infection3-5. Different routes of infection can be used in zebrafish embryos depending on the purposes of the study6.
One of these infection routes is yolk injection of the bacteria. The main advantage of this method compared to the others is that yolk infection can be performed automatically via robotic injection, significantly reducing the injection time and allowing high reproducibility of the infection7,8.
Previous work, using the zebrafish as a high throughput in vivo model system for the study of S. epidermidis and M. marinum infection showed to be successful7,8. This system is able to screen for disease progression via robotic yolk injection of early embryos and using fluorescence readout as a measure for the bacterial load. In agreement with this notion, this setup has been optimized and established a highly efficient high throughput pipeline with the potential to generate large numbers of homogenously infected embryos and track the progression of the infection during the time after treatment with a number of compounds. With the established setup it is possible to generate up to 8,000 synchronous embryos to screen for disease progression, processing in this way up to 2,500 embryos per hour. Embryos are sorted based on their bacterial load using an automated system, ensuring homogenous groups of infected larvae. Furthermore, to validate the setup, effects of reference known to prevent tuberculosis progression in mammals have been tested on embryos infected with M. marinum E11 strain or the more virulent M strain9.
This study describes in detail the high throughput pipeline that has been established to be able to generate large numbers of infected embryos and the subsequent analysis of the bacterial progression during development and after compound treatment.
1. Bacterial Strains and Growth Conditions
2. Prepare Zebrafish Eggs
3. Injection Needles
4. Experimental Outline of Injection
5. Flow-cytometer Analysis
6. Drug Treatment
7. High Resolution Imaging
The present results show that the high throughput pipeline to study S. epidermidis and M. marinum infection has been successfully established and that may be extended to other infection models. Firstly, the use of the large breeding vessel (Figure 1A), based on the published system by Adatto et al. (2011)11, enables to generate large numbers of synchronous eggs in single events affording a high control of the spawning process. Next to be able to inject large number of embryos in a short period of time, an improved version of the previously developed automated micro-injection system7 was used (Figure 1A). To asses which is the best developmental stage for yolk infection, injections with S. epidermidis and M. marinum were performed at all the different stages between 1 and 512 cell stage, according to the description made by Kimmel et al. (1995)12.
Injections with 100 cfu S. epidermidis between the 16 and 128 cell stage provided the best infection pattern (Figure 2). The bacteria proliferated inside the yolk for 3 days and spread into the body from 3 dpi onwards. Performing injections before the 16 cell stage led to high mortality from 4 dpi, and injection after the 256 cell stage showed mainly bacterial growth inside the yolk with hardly any bacteria spreading inside the body of the embryo. Quantification of bacterial burden was performed by fluorescence intensity analysis using the large particle flow cytometer as described by Veneman et al. (2013)8 (Figure 3).
Observations showed that the optimal developmental stage for injection of 30 cfu M. marinum injection is between 16 to 128 cell stage for the E11 strain (Figure 4A) and between 16-64 cell stage with the more virulent M strain (Figure 4B). Embryos injected at these stages showed bacterial growth inside the yolk and spreading of the bacteria through the embryo (Figure 7). Infection with both strains at earlier stages presented nonspecific generalized bacterial growth leading the embryos to die after 4 dpi. On the other hand, in embryos injected at later stages bacterial burden was restricted to the yolk.
Next, presorting with large particle flow cytometer (Figure 1B) generated large homogenous groups of infected fish excluding non- or highly infected embryos (Figures 5A and 6A). After presorting, M. marinum infected embryos were treated with Rifampicin, a first line antituberculosis drug. Previous studies demonstrated that treatment with Rifampicin at a dose of 200 μM efficiently reduces M. marinum infection in zebrafish7,13. Taking advantage of the large number of homogenously infected embryos generated with the high throughput setup, treatment with different doses was performed. Embryos infected with M. marinum M strain and treated for 48 hr with 12, 24, and 200 μM Rifampicin showed to reduce efficiently mycobacterial infection in a dose dependent manner (Figure 5B). In view of the efficient reduction of the infection using Rifampicin at a dose of 200 μM this concentration was used for the future experiments. In line with the previous result, studying bacterial burden progression using M. marinum E11 strain a significant reduction 24 hr and onwards after treatment with 200 μM Rifampicin was observed (Figure 6B).
Furthermore, if high magnification imaging is required of these embryos, they can be displayed automatically in 96-well plates (Figure 1C), from where the samples can be analyzed using the Vertebrate Automated Screening Technology system with the Large Particle Sampler mounted onto a CLSM.
The Vertebrate Automated Screening Technology system with the Large Particle Sampler is a system that can either be mounted onto a CLSM or stereo microscope. This device allows the loading of live or fixed embryos from a 96-well plate or bulk container automatically through a glass capillary, and orientates it in front of the camera at the desired angle (e.g., dorsal or lateral). Images of the embryo in all orientations can be made with the on board camera or with an external CLSM (Figure 7). Embryos will subsequently be transferred in the collection or waste container.
Figure 1. Mainstream experimental outline. A) Adult fish are put together to mate, eggs are collected, aligned into an agarose plate and injected. B) The injected eggs are incubated at 28 °C and will be pre-sorted for possible drug treatment. C) Subsequent analysis by large particle flow cytometer and/or Large Particle Sampler/Vertebrate Automated Screening Technology with CLSM. Please click here to view a larger version of this figure.
Figure 2. Establishment of best cell stage for S. epidermidis yolk injection. Zebrafish embryos were injected in the yolk at different developmental stages from 1 to 512 cell stage with 100 cfu of S. epidermidis. Embryos injected between 1 and 8 cell stage showed bacterial growth in the yolk and high mortality from 4 dpi. Embryos injected between 16 and 128 cell stage showed bacterial growth in the yolk and inside the body starting at 3 dpi. Embryos injected between 256 and 512 cell stage showed many bacterial growth inside the yolk. Please click here to view a larger version of this figure.
Figure 3. Quantification of bacterial burden using large particle flow cytometer. 100 cfu of S. epidermidis were injected into the yolk of zebrafish embryos. A) Up to 5 dpi, each day, groups of 10 embryos were homogenized and plated directly, showing the average exponential growth based on two biological replicas (error bars = SEM). B) Large particle flow cytometer analysis shows the average fluorescence signal from non-injected and S. epidermidis injected embryos. 30-160 embryos per condition were analyzed (error bars = SEM), different letters indicate statistical significant differences by one way ANOVA followed by Tukey's post-hoc test (P < 0.001), ns: not significant differences. C) Correlation between cfu and average fluorescence signal of groups of 10 S. epidermidis infected embryos (error bars = SEM). This figure has been modified from Veneman et al. (2013)8. Please click here to view a larger version of this figure.
Figure 4. Establishment of the best cell stage for M. marinum yolk injection. Zebrafish embryos were injected at all the different developmental stages from 1 to 512 cell stage with 30 cfu of M. marinum E11 and M strain. A, B) Embryos injected from 1-8 cell stage showed similar spreading and mortality with both strains. A) Embryos injected between 16-128 cell stage with E11 strain showed formation of granulomas and systemic infection while those injected from 256 to 512 cell stage kept bacterial burden into the yolk. B) Embryos injected between 16-64 cell stage with M strain showed formation of granuloma like structures and systemic infection while those injected from 128 to 512 cell stage kept bacterial burden into the yolk. Please click here to view a larger version of this figure.
Figure 5. Treatment of M. marinum acute infection with a first-line anti-tuberculosis drug. Embryos injected between 16-64 cell stage with 30 cfu of M. marinum M strain were run through the large particle flow cytometer at 3 dpi to be sorted in two groups after discarding the non- and/or highly infected embryos. A) Fluorescence of individual embryos in both groups. B) Embryos treated with Rifampicin (RIF) for 48 hr at doses of 12, 24, and 200 μM were analyzed at 4 dpi; their bacterial load is significantly reduced. C) Representative COPAS profiles of embryos treated with DMSO and Rifampicin at doses of 12, 24, and 200 μM for 24 hr. Bacterial load and distribution is indicated by the red peaks. Blue line represents the profile of the element sorted (4 dpf zebrafish embryo) by the COPAS. 60-90 embryos per condition were analyzed. Each data point represents an individual embryo. Values are indicated as mean ± SEM. ns: not significant differences. Analysis of statistical significance of differences was performed by one way ANOVA followed by Tukey's posthoc test. Please click here to view a larger version of this figure.
Figure 6. Treatment of M. marinum chronic infection with a first line antituberculosis drug. Embryos injected between 16-64 cell stage with 30 cfu of M. marinum E11 strain were run through the large particle flow cytometer at 3 dpi to be sorted in two groups after discarding the non- and/or highly infected embryos. A) Fluorescence of individual embryos in both groups. B) Embryos treated with Rifampicin (RIF) at 200 μM during 4 days were analyzed showing a significant reduction of the bacterial load after 1 day of treatment. 90 embryos per condition were analyzed. Values are indicated as mean ± SEM. Different letters indicate significant differences between time points of the same treatment. * indicates significant differences with control group. ns: not significant differences. Analysis of statistical significance of differences was performed by one way ANOVA followed by Tukey's posthoc test. (P < 0.05). Figure B) has been modified from Spaink et al. (2013)13. Please click here to view a larger version of this figure.
Figure 7. Result of M. marinum E11 yolk injection imaged using Vertebrate Automated Screening Technology and CLSM. Confocal Z stack (stitched 3 images) of a 5 dpi fli1-egfp14 embryo. A) Live embryo showing proliferation of M. marinum E11 bacteria (red) throughout the body. B) Fixed 5 dpi fli-egfp embryo showing M. marinum E11 bacteria (red) throughout the body co-localizing with leucocytes (light blue) detected by L-plastin immunostaining15.
The high throughput methodology described in this paper provides a fast and cost effective pipeline to screen high number of fish embryos and larvae with different types of infections. Using the large breeding vessel instead of traditional single or family breeding tanks facilitated control of the spawning process and generation of larger number of synchronous eggs. With an improved version of the automated micro-injection system7, it is possible to inject up to 2,500 eggs almost all in the same cell stage within 1 hr. With these updates and improved software it is feasible to inject more eggs than previously was possible which can be used to perform large drug screens with bacterial proliferation as a read out. However this method is still limited to yolk injection, other injection routes for example described by Benard et al. (2012)6, will hopefully be incorporated in the automated micro-injection system in the near future.
Although these methods are benchmarked for screening zebrafish, it would be useful for applications with other fish species as well. For instance, the common carp has been indicated to have advantages for drug screens. Like zebrafish, eggs and early stage embryos from common carp are transparent but with the main advantage of its large spawn size of hundred thousands of eggs and the availability of inbred lines that offer a more constant genetic background16.
The analysis of large amounts of infected embryos is done with the high throughput large particle flow cytometer. This device can sort analyzed embryos into multi well plates or a Petri dish making it especially suitable for testing large numbers of compounds. If a higher imaging resolution is needed, than the setup is adapted in a way that the large particle flow cytometer technology can be used for pre-screening and subsequently analyze the samples at a medium throughput at a higher resolution. This can be done using the Vertebrate Automated Screening Technology17,18. This device can automatically collect live or fixed embryos between 2 and 7 days post fertilization from a multi well plate or bulk container, image 360° through a capillary using CLSM or stereo microscopy and dispose again in 2 bulk containers allowing manual sorting of the embryos based on the microscopic images. Future improvements will allow the sorting of the embryo after imaging into the multi well plate, therefore making it possible to screen automatically large number of individual embryos over time with CLSM. Assuming that in future applications the Vertebrate Automated Screening Technology system can also be connected to the large particle flow cytometer technology without the need of prior dispensing larvae into multi well plates, will lead to a more advanced sorting.
This paper describes the establishment and optimization of a high throughput setup to study S. epidermidis and M. marinum infection as a model for drug discovery. It demonstrates that the outcome of these bacteria injected into the yolk depends on the developmental stage of the eggs at the time of injection. Injecting M. marinum E11 at 16-128 cell stage or the M strain at 16-64 cell stage leads to the same infection pattern as caudal vein injection2,6. However this setup is not limited to the proliferation of bacterial pathogens only. It was shown before that it is possible to robotically inject solutions containing DNA, RNA or morpholinos for transgenesis, over-expression and gene knock-down studies, respectively13. Furthermore, it was shown that this setup is also useful for the study of cancer cell proliferation and migration. Therefore this pipeline presents a versatile method for high throughput screens of a variety of signal mechanisms in the context of innate immunity, applied to infectious disease and the development of cancer. These screens can be combined with others for medicine discovery but also with analysis of possible toxic effects of identified applicable drugs.
The authors have nothing to disclose.
We are grateful to Leonie de Boer and Bas Zaat (Academic Medical Centre) for providing us with the S. epidermidis O-47 strain. We thank Rico Bongaarts, Francis Smet and Angela Comas (Union Biometrica) for help and advice with the COPAS XL and VAST BioImager analysis. We thank Davy de Witt, Ulrike Nehrdich, and Laura van Hulst for fish caretaking, and other colleagues from Leiden University for helpful discussions. This research forms part of the Project P5.03 IBIZA of the research program of the BioMedical Materials institute, co-funded by the Dutch Ministry of Economic Affairs, and of the Smart Mix Program (NWOA_6QY9BM) of The Netherlands Ministry of Economic Affairs and of The Netherlands Ministry of Education, Culture and Science. Additional support was obtained from the EU project ZF-Health (FP7-Health-2009-242048), and RMJ was supported by Marie Curie fellowship as Experienced Researcher in the EU Initial Training Network FishForPharma (PITN-GA-2011-289209). SJR received funding from the Innovative Medicines Initiative Joint Undertaking under grant agreement n°115337, resources of which are composed of financial contribution from the European Union’s Seventh Framework Programme (FP7/2007-2013) and EFPIA companies in kind contribution.The authors further acknowledge financial support from the Leiden University Fund (LUF) for robotics and from Cyttron, in the Besluit Subsidies Investeringen Kennisinfrastructuur program, which in turn is financially supported by the Netherlands Organization for Scientific Research for imaging facilities. The COPAS system acquisition was partially supported by the Division for Earth and Life Sciences (ALW) with financial aid from the Netherlands Organization for Scientific Research (NWO, 834.10.004).
Middlebrook 7H10 agar | BD Bioscience, Franklin Lakes, New Jersey, USA | 262710 | |
Middlebrook 7H9 broth | BD Bioscience, Franklin Lakes, New Jersey, USA | 271310 | |
BBL Middlebrook oleic acid albumin dextrose catalase (OADC) enrichment | BD Bioscience, Franklin Lakes, New Jersey, USA | 211886 | |
BBL Middlebrook albumin dextrose catalase (ADC) enrichment | BD Bioscience, Franklin Lakes, New Jersey, USA | 211887 | |
LB agar | Sigma-Aldrich, Saint Louis, Missouri, USA | L5542 | Multiple suppliers |
LB broth | Sigma-Aldrich, Saint Louis, Missouri, USA | L3022 | Multiple suppliers |
Chloramphenicol | Bio-connect, Huissen, the Netherlands | 16785.03 | Multiple suppliers |
Hygromycin | Bio-connect, Huissen, the Netherlands | 25966.01 | Multiple suppliers |
Tween-80 | Sigma-Aldrich, Saint Louis, Missouri, USA | P1754 | Multiple suppliers |
Polyvinylpyrrolidone40 | Calbiochem, San Diego, California, USA | 529504 | Multiple suppliers |
Ethyl 3-aminobenzoate methanesulfonate salt (Tricaine) | Sigma-Aldrich, Saint Louis, Missouri, USA | A5040 | |
Agarose | Sphaero-Q, Gorinchem, the Netherlands | S103 | Multiple suppliers |
Instant ocean sea salt | Sera Marin, Heinsberg, Germany | 5460 | |
iSPAWN | Techniplast, Buguggiate, Italy | iSPAWN | |
Automated microinjection system | Life Science Methods BV, Leiden, the Netherlands | Automated microinjection system | |
Complex Object Particle Analyzer and Sorter XL (COPAS XL) | Union BioMetrica Inc., Holliston, Massachusetts, USA | COPAS XL | |
Vertebrate Automated Screening Technology BioImager (VAST BioImager) | Union BioMetrica Inc., Holliston, Massachusetts, USA | VAST BioImager | |
LP Sampler | Union BioMetrica Inc., Holliston, Massachusetts, USA | LP Sampler | |
Confocal laser scanning microscope | Leica Microsystems, Wetzlar, Germany | TCS SL | |
Injection needle 10 µm inner diameter | Qvotek, Mississauga, Canada |