A Murine Skin Wound Infection Model to Study the Immune Response against a Bacterial Pathogen

Overview

This video demonstrates a technique to generate a murine model of skin wound infection. Upon infecting mice — engineered to express a fluorescent protein in neutrophils — with a genetically modified bioluminescent Staphylococcus aureus — a pathogenic bacteria. The bacterial burden at the infection site and the neutrophil recruitment dynamics can be studied using whole animal imaging.

Protocol

All procedures involving animal models have been reviewed by the local institutional animal care committee and the JoVE veterinary review board.

1. Mouse Source and Housing

1. Derive LysM-eGFP (enhanced green fluorescent protein (eGFP) inserted in the Lysozyme M (LysM) promoter region) mice on a C57BL/6J genetic background as described previously. Derive LysM-EGFP×MyD88^-/- mice by crossing LysM-EGFP mice with MyD88^-/- (Myeloid differentiation primary response 88 protein) mice on a C57BL/6J background.
2. House mice in a vivarium. For these studies, animals were housed at the University of California, Davis in groups prior to surgery and singly housed following surgery. Use mice between the ages of 10-16 weeks.

2. Bacterial Preparation and Quantification

1. Remove the bioluminescent Staphylococcus aureus strain of interest from -80 °C storage to thaw on ice. Streak on a 5% bovine blood agar plate. Incubate the streaked plate in a humidified incubator at 37 °C overnight (16 h).
   NOTE: In this protocol, the ALC2906 SH1000 strain was used. This strain contains the shuttle plasmid pSK236 with the penicillin-binding protein 2 promoters fused to the luxABCDE reporter cassette from Photohabdus luminescens.
2. Prepare tryptic soy broth (TSB) by mixing 0.03 g of TSB powder per mL of pure water, and autoclave TSB to sterilize. When cooled, add any necessary antibiotics using a sterile technique. In this protocol, add 10 µg/mL of chloramphenicol to the TSB to select for expression of the pSK236 shuttle plasmid, which contains the bioluminescence luxABCDE cassette.
3. Pick 3-4 separate colonies from the S. aureus plate into TSB with 10 µg/mL chloramphenicol to start an overnight culture. Incubate bacteria on an incubating shaker at 37 °C overnight (16 h).
4. Start a new bacterial culture from the overnight culture by diluting a sample 1:50 into TSB with 10 µg/mL chloramphenicol. Culture in an incubating shaker at 200 rpm and 37 °C.
5. Two hours after splitting the S. aureus, monitor the optical density at 600 nm (OD₆₀₀) on a spectrophotometer. Observe the OD₆₀₀ vs. time to find mid-logarithmic phase growth. For the ALC2906 SH1000 strain, an OD₆₀₀ of 0.5 is mid-logarithmic and corresponds to a concentration of 1 x 10⁸ CFU/mL (CFU = colony-forming unit) (Figure 1).
6. When OD₆₀₀ is 0.5, wash bacteria 1:1 with ice-cold Dulbecco's phosphate-buffered saline (DPBS). Centrifuge the bacteria for 10 min at 3,000 x g and 4 °C. Carefully decant the supernatant and add additional chilled DPBS and vortex thoroughly. Centrifuge once more for 10 min at 3,000 x g and 4 °C.
7. Carefully decant the supernatant. Resuspend the bacterial pellet at the desired concentration. For these studies, collect 3 mL of ALC2906 SH1000 and resuspend in 1.5 mL of phosphate-buffered saline (PBS), correlating to a bacteria concentration of about 2 x 10⁸ CFU/mL. Keep bacteria on ice until use.
8. To verify bacteria concentration, dilute 100 µL of the bacterial sample 1:10,000 and 1:100,000 in PBS. Plate 20 µL aliquots on an agar plate. Incubate at 37 °C in a humidified incubator for 16 h. Count CFUs by gross examination and calculate a bacterial concentration the following day.

3. Excisional Skin Wounding and Inoculation with S. aureus

1. Administer 100 µL of 0.03 mg/mL buprenorphine hydrochloride (~0.2 mg/kg) to each mouse via intraperitoneal injection.
2. Twenty minutes post-injection, place 2-4 mice in a chamber with 2-3 LPM oxygen with 2-4% isoflurane. Once mice are fully anesthetized, transfer the mice one at a time to a nose cone connected to 2-3 LPM oxygen with 2-4% isoflurane. Verify mice are fully anesthetized by firmly pinching each rear paw between a thumb and forefinger. Proceed to the next step if the animal does not respond to the pinch.
3. Shave a 1-inch by 2-inch section on the back of the mouse with electric clippers and clear the area of fur clippings using a clean wipe or gauze. Avoid using depilatory lotion because it may cause excess inflammation.
4. Clean the back of the mouse first with 10% povidone-iodine-soaked gauze and then with 70% ethanol-soaked gauze. Clean the area in a spiral pattern, moving outward from the center of the surgical area. Wait approximately 1 minute for the surgical area to dry prior to surgery.
5. Hold the shaved back of the mouse loosely between two fingers and firmly press a sterile 6 mm punch biopsy at the center of the prepared surgical area. Do not pull the skin taut.
   1. Twist the punch biopsy to create a circular outline on the skin that fully cuts through the skin in at least one section of the outline. Be careful not to cut into the underlying fascia or tissue.
   2. Use sterile scissors and forceps to cut through the epidermis and dermis following the circular pattern imprinted by the punch biopsy.

4. S. aureus Inoculation

1. Fill a 28 G insulin syringe with the desired bioluminescent bacterial inoculant. In this study, administer a concentration of 1 x 10⁸ CFU/mL (50 µL). Do not administer more than 100 µL of volume.
2. Inject 50 µL of inoculant between the fascia and tissue in the center of the wound on the back of the mouse. Ensure that the inoculant forms a bubble at the center of the wound with minimal leakage or dispersion.
   1. Pull the dermis to the side, hold the syringe nearly parallel to the tissue, and slowly push the syringe into the tissue until a sudden decrease in resistance is felt, which indicates piercing of the fascia. Carefully lead the syringe into the center of the wound and dispense the inoculant slowly. Remove the syringe slowly from the animal.
3. Inject the same volume of sterile PBS into the wounds of uninfected animals as described above.
4. Return the animal to its cage. Place the cage under a heat lamp or on top of a heating pad, and monitor the animal until recovery from anesthesia.

5. In Vivo bioluminescence imaging (BLI) and fluorescence imaging (FLI)

1. Initialize the whole animal imager through the instrument software. Anesthetize mice in a chamber receiving 2-3 LPM oxygen with 2-4% isoflurane. Deliver anesthesia to the nosecones inside the imager.
2. Place the wounded and infected mouse into the imager. Position the mouse such that the wound is as flat as possible. Use the following sequence set-up to image the mice.
   1. Select Luminescence and Photograph as the imaging mode. The exposure time is 1 min at small binning and F/stop 1 (luminescence) and F/stop 8 (photograph). The emission filter is Open. Click the Acquire button to record the image.
   2. Select Fluorescence and Photograph as the imaging mode. The exposure time is 1 s at small binning and F/stop 1 (Fluorescence) and F/stop 8 (photograph) with an excitation wavelength of 465/30 nm and an emission wavelength of 520/20 nm with a high lamp intensity. Click the Acquire button to record the image.
3. Return the animal to its cage and monitor it until it recovers from anesthesia.
4. Image mice daily as described above.

Representative Results

Figure 1: Correlation between OD₆₀₀ and CFU counts for ALC2906. 3-4 ALC2906 SH1000 colonies were picked from an agar plate and transferred into TSB with 10 µg/mL chloramphenicol for overnight culture. The next day, the suspension was split 1:50 into TSB with 10 µg/mL chloramphenicol and cultured. Optical density at 600 nm (OD₆₀₀) was measured in regular intervals after 2 hours using a spectrophotometer. At each measurement, the bacteria were diluted 1:100,000 in ice-cold PBS and aliquoted onto an agar plate for overnight incubation. CFUs were counted the following day to calculate the initial concentration and correlated to OD₆₀₀. N = 3 with 4 different OD₆₀₀ measurements per experiment.

Materials

Name	Company	Catalog Number	Comments
14 mL Polypropylene Round-Bottom Tube	Falcon	352059
6mm Disposable Biopsy Punch	Integra Miltex	33-36
Bioluminescent S. aureus	Lloyd Miller, Johns Hopkins	ALC 2906 SH1000
Bovine Blood Agar, 5%, Hardy Diagnostics	VWR	10118-938
Buprenoprhine hydrochloride injectable	Western Medical Supply	7292	0.3 mg/mL
C57BL/6J Mice	Jackson Labratory	664
Chloramphenicol (crystalline powder)	Fisher BioReagents	BP904-100
DPBS (1X)	Gibco	14190-144
Insulin Syringes	Becton, Dickson and Company	329461	.35 mm (28 G) x 12.7 mm (1/2'')
IVIS Spectrum In Vivo Imaging System	Perkin Elmer	124262
Living Image Software – IVIS Spectrum Series	Perkin Elmer	128113
LysM-eGFP Mice	Thomas Graff Albert Einstein College of New York	NA
Microvolume Spectrophotometer	ThermoFisher Scientific	ND-2000
MyD88 KO Mice	Jackson Labratory	9088
Non-woven sponges	AMD- Ritmed Inc	A2101-CH	5 cm x 5 cm
Povidone Iodine 10% Solution	Aplicare	697731
Tryptic Soy Broth	Becton, Dickson and Company	211825