A detailed protocol of a mouse model for enterohemorrhagic E. coli (EHEC) colonization by using bioluminescence-labeled bacteria is presented. The detection of these bioluminescent bacteria by a non-invasive in vivo imaging system in live animals can advance our current understanding of EHEC colonization.
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.
EHEC O157:H7 is a pathogen that causes diarrhea1, HS2, HUS3, and even acute renal failure4 through contaminated water or food. EHEC is a pathogenic enterobacterium and colonizes to the gastrointestinal tract of humans1. When EHEC first adhere to host intestinal epithelium, they inject the colonization factors into host cells through the type III secretion system (T3SS) that functions as a molecular syringe inducing an attaching and effacing (A/E) lesion subsequently to enforce adhesion (colonization)5. These genes involved in A/E lesion formation are encoded by the locus of enterocyte effacement (LEE) pathogenicity island5.
Bioluminescence is a light-producing chemical reaction, in which luciferase catalyzes its substrate luciferin to generate visible light6. This enzymatic process often requires the presence of oxygen or adenosine triphosphate (ATP)6. Bioluminescence imaging (BLI) allows researchers the visualization and quantization of host-pathogen interactions in live animals7. BLI can characterize the bacterial infection cycle in live animals by following the bioluminescent bacteria as they migrate to and invade different tissues7; this reveals a dynamic progression of infection. Moreover, the bacterial load in animals is related to the bioluminescent signal8; thus, it is a convenient indicator to estimate the pathological conditions of experimental animals in a simple and direct way.
The plasmid used here contained the luciferase operon, luxCDABE, which is from the bacterium Photorhabdus luminescens that encodes its own luciferase substrate7,9. By transforming this luciferase-expressing plasmid into bacteria, the colonization and infection processes can be monitored by observing these bioluminescent bacteria in live animals. Overall, BLI and bioluminescence-labeled bacteria allow researchers to monitor the bacterial numbers and location, bacterial viability with antibiotics/therapy treatment, and bacterial gene expression in infection/colonization6,7. Numerous pathogenic bacteria have been reported that express the luxCDABE operon to examine their infection cycle and/or gene expression in infection. These bacteria, including uropathogenic E. coli10, EHEC8,11,12,13, enteropathogenic E. coli (EPEC)8, Citrobacter rodentium14,15, Salmonella typhimurium16, Listeria monocytogenes17, Yersinia enterocolitica18,19, and Vibrio cholerae20, have been documented.
Several experimental models have been developed to facilitate the study of EHEC colonization in vitro and in vivo21,22,23. However, there is a lack of suitable animal models to study the EHEC colonization in vivo, and thus a resulting paucity of details. To facilitate the study of the EHEC colonization mechanism in vivo, it is valuable to build animal models to observe and quantify EHEC colonization in live animals in a non-invasive method.
This manuscript describes a mouse-EHEC colonization model that uses a bioluminescent expressing system to monitor EHEC colonization over time in living hosts. Mice are intragastrically inoculated with bioluminescence-labeled EHEC and the bioluminescent signal is detected in mice with a non-invasive in vivo imaging system13. Mice infected with bioluminescence-labeled EHEC showed significant bioluminescent signals in their intestine after 2 days post infection, which suggested that those bacteria colonized in the host intestine after 2 days post infection. Ex vivo image data showed that this colonization is specifically in the cecum and colon of mice. By using this mouse-EHEC model, the bioluminescent EHEC colonization can be detected in the living host by an in vivo imaging system, to study the detailed mechanisms of enteric bacteria colonization, which may promote further understanding in EHEC-induced physiological and pathological changes.
Caution: EHEC O157:H7 is a biosafety level 2 (BSL-2) pathogen according to the Centers for Disease Control and Prevention (CDC) biosafety instruction (https://www.cdc.gov/). Therefore, all experimental procedures involving EHEC must be performed in a BSL-2 facility. Wear lab coats and gloves while performing the experiment. Work in a certified biosafety cabinet (BSC). Disinfect the experimental bench before and after the experimental procedure with 70% ethanol. All instruments or equipment that contact (or potentially contact) EHEC should be disinfected with 70% ethanol or bleach. Contaminated (or potentially contaminated) wastes should be sealed and autoclaved carefully. Wear a mask, eye protection, double gloves or a jumpsuit, if necessary. The 6-week-old C57BL/6 female mice were purchased and maintained at the Laboratory Animal Center of National Cheng Kung University (NCKU). The animal experiments were approved by the Institutional Animal Care and Use Committee of NCKU (Approval number 104-039).
1. Bioluciferase-labeled EHEC Bacteria Generation
2. Bioluminescent EHEC Bacteria Preparation for Oral Inoculation
NOTE: The timeline flowchart of the experimental procedures for EHEC preparation and mouse oral gavage is presented in Figure 1 to aid in experimental preparation.
3. Mouse Oral Gavage of EHEC
4. Visualization
5. Data Acquisition
NOTE: The software used for data acquisition is listed in the Table of Materials.
We administered bioluminescence-labeled EHEC (~ 109 bacterial cells) to 6-week old female C57BL/6 mice by oral gavage. After oral inoculation of EHEC to mice within 1 h, the animals were examined for bioluminescent signal by the in vivo imaging system as shown in Figure 7. The results showed a strong bioluminescent signal in gavage mice with bioluminescence-labeled EHEC. We examined the signals on 2 days post infection. As shown in Figure 8A, the mice inoculated with bioluminescence-labeled wild-type EHEC EDL933 showed intense bioluminescent signals even after 2 days post infection, which suggested EHEC colonized in hosts by 2 days. We also intragastrically infected bioluminescence-labeled EDL933ΔrfaD (ΔrfaD) to mice (Figure 8A). This mutant, defected in lipopolysaccharide (LPS), has been shown to reduce colonization in the host in our previous study. As shown in Figure 8A, there is no bioluminescent signal detected in ΔrfaD-infected mice, which suggests that there are no or less bacteria cells colonized in the mice. Quantification of the fluorescent signal is shown in Figure 8B. Next, the location of these bioluminescence-labeled bacteria was determined. The infected mice were sacrificed humanely and their whole intestine removed. The intestines of mice 2 days post infection were positioned on 9 cm Petri dishes and imaged ex vivo (Figure 9A). The intestinal tissues of bioluminescence-labeled EDL933 infected mice revealed a significant increase in bioluminescent signals in the cecum and colon, which suggest that these bioluminescent EHEC colonized in the cecum and colon of infected mice for 2 days at least. In contrast, mice infected with bioluminescence-labeled ΔrfaD (Figure 9A), revealed decreased bioluminescent signal in their intestinal tissue, which is consistent with the in vivo image (Figure 8A). Quantification of the fluorescent signal is shown in Figure 9B.
Figure 1: Timeline of the experimental preparation flow chart.
Overview of the timing needed to prepare bioluminescent EHEC bacteria and pretreat mice with streptomycin. (A) EHEC preparation. (B) Mice preparation. Please click here to view a larger version of this figure.
Figure 2: In vivo imaging system acquisition control panel.
Before imaging samples, open IVIS Acquisition Control Panel. Select "Luminescent," "Photograph," and "Overlay." Set Exposure Time as "Auto." Set Binning as "Medium." Set ƒ/stop as 1 for luminescent and 8 for photograph. ƒ/stop controls the amount of light received by the CCD detector. Once samples are ready for imaging, click "Acquire" to acquire images. Please click here to view a larger version of this figure.
Figure 3: Tool Palette panel.
After image acquiring, use the Tool Palette panel for quantifying bioluminescent intensity. Open the Tool Palette panel and image the data. Choose one of the ROI Tools to range the bioluminescent signals on images. Please click here to view a larger version of this figure.
Figure 4: Bioluminescent signal from sample for quantification.
Bioluminescent signal area on images encircled by ROI Tools. All bioluminescent signals shown here are in the red circle. Please click here to view a larger version of this figure.
Figure 5: ROI measurements.
After circling bioluminescent signals and clicking "Measure ROIs" on the Tool Palette panel, values are presented as shown. The values of the column Total Flux (p/s) are used for the bioluminescent intensity quantification. Please click here to view a larger version of this figure.
Figure 6: Add different quantification information.
By clicking on Configure Measurement on the left corner of the ROI Measurements panel, you can select other desired quantification values/information. Please click here to view a larger version of this figure.
Figure 7: Representative image of mice after inoculated with bioluminescent EHEC.
Representative image of mice inoculated with bioluminescent EHEC by oral gavage within 1 h. The color scale represents the radiance (p/s/cm2/sr). Please click here to view a larger version of this figure.
Figure 8: Images of mice inoculated with bioluminescence-labeled EHEC after 2 days.
(A) Represent image of mice inoculated with bioluminescent wild-type EHEC EDL933 and EDL933:ΔrfaD by oral gavage after 2 days post infection. (B) Quantification of bioluminescence intensity of mice infected with EHEC. Error bars indicate the standard deviations. Representative images are shown.All experiments were conducted independently three times with 2 – 3 animals each time, and error bars indicate the standard deviations. P-values denote the results of statistical analysis by t-test. The color scale represents the radiance (p/s/cm2/sr). Please click here to view a larger version of this figure.
Figure 9: Images of intestinal tissues of infected mice with bioluminescence-labeled EHEC.
(A) 2 days after inoculation with bioluminescence-labeled EHEC, the mice were euthanized and whole intestinal tissues were removed and imaged ex vivo. Representative images are shown.(B) Quantification of bioluminescence intensity of intestinal tissues from mice infected with EHEC. All experiments were conducted independently three times with 2 - 3 animals each time, and error bars indicate the standard deviations. P-values denote the results of statistical analysis by t-test. The color scale represents the radiance (p/s/cm2/sr). Scale bar represents 1 cm. Please click here to view a larger version of this figure.
Steps | Temperature | Time | Number of cycles |
Initial denaturation | 95 °C | 10 min | 1 |
Denaturation | 95 °C | 30 sec | 35 |
Annealing | 58.4 °C | 30 sec | |
Extension | 72 °C | 1.5 min | |
Final extension | 72 °C | 10 min | 1 |
Hold | 4 °C | ∞ | 1 |
Table 1: Polymerase chain reaction (PCR) conditions
Primers name | Sequence |
nptII F | 5’CCTATGCATAATAATTCCGCTAGCTTCACG3’ |
nptII R | 5’GCTCCACCGATAATATTCCTGAGTCATACT3’ |
Table 2: Primer sequences used to amplify nptII
It has been reported that EHEC transformed with luciferase plasmid has been utilized to examine its localization in hosts or gene expression in vivo8,11,12. The murine model demonstrated here has also been reported to detect the EHEC colonized timing and localization in murine host8. Nevertheless, we provide the detail protocol of how to administer EHEC inoculation to mice intragastrically and how to carefully prepare the bioluminescent bacteria for oral gavage. Notably, for the mouse oral gavage of EHEC (step 3.7), the position of the mouse head is critical when the gavage needle is inserted. If the position is not vertical, it will be difficult to pass the needle, and it could possibly injure the mouse. In step 3.8, when the gavage needle is inside the mouth of the mouse, the tongue will lay outside of the mouth slightly. If resistance is encountered when passing the gavage needle to the esophagus, stop moving the needle forward and withdraw it immediately. Alter the needle position to make sure that the needle is entering the esophagus. The needle could be entering the trachea when resistance occurs, which would lead to injecting bacteria in the lungs instead of the stomach.
Application of green fluorescent protein (GFP) as a biosensor is common in biological experiments. However, using GFP as a reporter to observe the pathogen infection/colonization in live animals by in vivo imaging is not recommended, because the absorbance by hemoglobin, proteins and water are high between 200-650 nm26, which overlaps with GFP (excitation 480 nm, emission 510 nm)27. Therefore, using GFP signal as a reporter for in vivo imaging can be interrupted by hemoglobin, proteins, and water in animals26. The near-infrared (NIR) fluorescence is ideally suited for in vivo imaging because its absorbance window is around 650 – 900 nm28, which is in the region of lowest absorption coefficient of hemoglobin (<650 nm) and water (>900 nm)26,28 . Moreover, when tissue absorbs light, there is a chance to induce autofluorescence. When the wavelengths of excitation and emission range in the GFP window, it induces much more autofluorescence than NIR29. Use of NIR can improve the signal to background ratio compared to that of GFP by eliminating the autofluorescence background29. Bioluminescence does not require energy excitation to generate visible light. It depends on the reaction to catalyze substrate luciferin by its enzyme luciferase and generate light. Since bioluminescence does not require light directly on a sample, the background signal from a sample is very low. Therefore, use of bioluminescence as a reporter is more general and easier for in vivo imaging. In contrast, fluorescence requires light excitation to induce signal light. When tissues absorb light, there is a chance that the fluorescent light will be emitted and induce autofluorescence so that their signal-to-noise is higher compared to that of bioluminescence.
Considering EHEC is naturally less colonized in mice by oral infection2,22, a natural mucosal pathogen of mice, called Citrobacter rodentium, has been utilized to study the colonization mechanism to murine host as a surrogate bacterium22,30. Both of EHEC and C. rodentium colonize the intestinal mucosa and induce the formation of A/E lesions in host22,30. They also contain the LEE pathogenicity island, which encodes a T3SS and several effector proteins that induced A/E lesion22,30. Therefore, the use of luciferase expressing plasmid as a reporter in C. rodentium to detect the colonization pathology and study the colonization mechanism via an in vivo imaging system has also been reported14,15. Nevertheless, while C. rodentium infection of mice is a useful model to investigate the function of T3SS and the mechanism of A/E lesion, C. rodentium does not contain Shiga toxin (Stx)30, which is a dominant virulence factor that causes kidney failure in EHEC, particularly serotype O157:H73. Although a Stx-expressing C. rodentium strain has been constructed recently31, which is more realistic to EHEC infection, it does not include other potential EHEC virulence factors that are crucial for colonization and/or infection. Furthermore, C. rodentium shares 67% of its genes with EHEC32, which suggests that EHEC may use a virulence distinct from C. rodentium during colonization and/or infection.
The luciferase expressing plasmid used in here, pWF27913, was modified from pAKlux29 whose backbone is pBBR1MCS433. Although pBBR1MCS4 have been tested and replicated in various bacteria33, it is crucial to ensure the origin of replication (ORI) of this plasmid is suitable for the bacterial host before using this plasmid-based luciferase system for the experiment and thus confirming that this luciferase expressing plasmid can replicate in the bacterial host. We use antibiotic stress to maintain the stability of plasmid in the bacteria. When bacteria enter animals in the absence of antibiotics, the bioluminescent signal of wild-type EHEC has been detected for at least 2 days. However, we didn't following infection for longer than 2 days because we had already seen a significant difference in luminescent intensity between EHEC WT and EHEC rfaD (that encodes a gene required for EHEC synthesizing intact LPS) mutant at 2 days. To maintain the plasmid stably in bacteria under the absence of antibiotics, a plasmid pCM1710,34 can be used for this purpose. pCM17 encodes a two-plasmid partitioning system and a post-segregational killing mechanism to ensure the maintenance of the plasmid in bacteria in the absence of antibiotics10,34. The plasmid pCM17 containing the luxCDABE operon driven by the OmpC promoter can be detected by bioluminescent signal for at least 7 days8. An alternative method to obtain a continuous bioluminescent expression bacteria in the absence of antibiotics is to insert luxABCDE gene into the bacterial chromosome35. Francis et al. used transposon inserted the luxABCDE operon and antibiotics cassette randomly inserted into the chromosome of Streptococcus pneumoniae to obtain the bioluminescence stable strain35.
In our previous study13, we utilized this model system to examine EHEC colonization in a host and compare the difference of colonization ability between the EHEC wild type (WT) and mutant13. When mice were administered the bioluminescent EHEC rfaD mutant, the bioluminescent signals diminished dramatically compared to that of WT EHEC after 2 days post infection. It provides evidence that this murine model can analyze the mutation effect of EHEC colonization in the host. Furthermore, therapeutic treatments for reducing the colonization of EHEC is a considered, potential solution to EHEC infection since the use of antibiotics is contraindicated5,36. Therefore, it is worth testing whether this model system can be used to examine the efficacy of anti-colonization drugs/treatments against enterobacteria colonization in the host. We believe that by using this model system, it is possible to examine the timing and location of not merely EHEC, but also other enterobacteria colonization in vivo. By using this animal model, the process of EHEC colonization in mice can be monitored and the colonization burden in the host can be quantified to determine spatial and temporal colonization of EHEC in live animals. The visualization and quantification of enterobacteria colonization by using this model makes it a great tool to investigate and analyze the fine mechanisms of enterobacterial colonization, and thus compensate for the deficiency of colonization research and improve current knowledge.
The authors have nothing to disclose.
We acknowledge Chi-Chung Chen from the Department of Medical Research, Chi Mei Medical Center (Tainan, Taiwan) for the help in mouse infection, and the support from the laboratory animal center of National Cheng Kung University. This work is supported by the Minister of Science and Technology (MOST) grants (MOST 104-2321-B-006-019, 105-2321-B-006 -011, and106-2321-B-006 -005) to CC.
Shaker incubator | YIH DER | LM-570R | bacteria incubation |
Orbital shaking incubator | FIRSTEK | S300 | bacteria incubation |
pBSL180 | source of nptII gene | ||
pAKlux2 | source of luxCDABE operon | ||
T&A Cloning Kit | Yeastern Biotech | FYC001-20P | use for TA cloning |
Nsi I | NEB | R0127S | use for plasmid cloning |
Sca I | NEB | R0122S | use for plasmid cloning |
Spe I-HF | NEB | R0133S | use for plasmid cloning |
Sma I | NEB | R0141S | use for plasmid cloning |
T4 ligase | NEB | M0202S | use for plasmid cloning |
Ex Taq | TaKaRa | RR001A | use for PCR amplification |
10X Ex Taq Buffer | TaKaRa | RR001A | use for PCR amplification |
dNTP Mixture | TaKaRa | RR001A | use for PCR amplification |
PCR machine | applied Biosystem | 2720 thermal cycler | for PCR amplification |
Glycerol | SIGMA | G5516-1L | use for bacteria stocking solution |
NaCl | Sigma | 31434-5KG-R | chemical for making LB medium, 10 g/L |
Tryptone | CONDA pronadisa | Cat 1612.00 | chemical for making LB medium, 10 g/L |
Yeast Extract powder | Affymetrix | 23547-1 KG | chemical for making LB medium, 5 g/L |
Agar | CONDA pronadisa | Cat 1802.00 | chemical for making LB agar |
kanamycin | Sigma | K4000-5G | antibiotics, use for seleciton |
streptomycin | Sigma | S6501-100G | antibiotics, eliminate the microbiota in mice |
EDL933 competent cell | Homemade | method is on supplemental document | |
Electroporator | MicroPulser | for electroporation | |
Electroporation Cuvettes | Gene Pulser/MicroPulser | 1652086 | for electroporation |
High-speed centrifuge | Beckman Coulter | Avanti, J-26S XP | use for centrifuging bacteria |
Fixed-Angle Rotor | Beckman Coulter | JA25.5 | use for centrifuging bacteria |
Fixed-Angle Rotor | Beckman Coulter | JLA10.5 | use for centrifuging bacteria |
centrifuge bottles | Beckman Coulter | REF357003 | use for centrifuging bacteria |
centrifuge bottles | Thermo Fisher scientific | 3141-0500 | use for centrifuging bacteria |
eppendorf biophotometer plus | eppendorf | AG 22331 hamburg | for measuring the OD600 value of bacteria |
C57BL/6 mice | Laboratory Animal Center of NCKU | ||
lab coat, gloves | for personnel protection | ||
isoflurane | Panion & BF Biotech Inc. | G-8669 | for mice anesthesia, pharmaceutical grade |
1ml syringe | use for oral gavage of mice | ||
Reusable 22 G ball-tipped feeding needle | φ0.9 mm X L 50 mm | use for oral gavage of mice | |
surgical scissors | use for mice experiment | ||
Xenogen IVIS 200 imaging system | Perkin Elmer | IVIS spectrum | use for bioluminescent image capture |
Living Image Software | Perkin Elmer | version 4.1 | use for quantifying the image data |