In Vivo Mouse Model of Spinal Implant Infection

Postoperative infection is a devastating complication of spinal surgery. This mouse model of spinal implant infection allows researchers to study the effect of host, implant, and therapeutic factors on infection. This technique quantifies real-time longitudinal bacterial burden, humanely and efficiently, in-vivo.

Using a frozen stock of S.aureus zen 36, streak the bacteria onto Luria broth plates containing 200 micrograms per milliliter of kanamycin and incubate for 12 to 16 hours at 37 degrees Celsius. At the end of the incubation, individually culture single colonies from the S.aureus zen 36 culture in tryptic soy broth containing 200 micrograms per milliliter kanamycin for 12 to 16 hours at 37 degrees Celsius and 200 revolutions per minute. The next day, dilute the culture in fresh broth at a one to 50 ratio and culture the bacteria for an additional two hours to allow the isolation of mid-logarithmic phase bacteria.

At the end of the incubation, pellet the bacterial cells by centrifugation and wash the cells three times in one milliliter of fresh PBS per wash. After the last wash, measure the absorbance at 600 nanometers. The ideal optical density is 0.750.

Then, re-suspend the bacteria at a one times 10 to the third colony forming units per two microliters of PBS concentration. Use clippers to remove hair from the sacrum and upper thoracic spine of the 12 week old male C-57 black six mouse. Deliver pain medicine via subcutaneous injection immediately before skin preparation and repeat in three days.

Sterilize the surgical site using three alternating Betadine and alcohol wipes. After the third wipe, place the mouse in the prone position on a sterile surgical bed and maximally flex the hips. Before making an incision, confirm a lack of response to toe pinch.

Palpate for the iliac crest to approximately the lumbar four vertebrae body and use a number 15 surgical scalpel blade to make a longitudinal, two centimeter skin incision. Palpate the spinous process to confirm the midline and continue the incision down to the bone. Dissect sub-periosteally on the right side of the L-4 spinous process, extending laterally to the transverse process, and pass an open absorbable braided suture size 5.0 cephalad and caudad to the L-4 body through the fascia.

Using a 25-gauge spinal needle, ream the spinous process of L-4 and insert a 0.1 millimeter diameter, one centimeter long, L-shaped surgical grade stainless steel implant along the lamina, with the long arm laying cephalad. Inoculate the implant with one times 10 to the third colony forming units per two microliters of bioluminescent S.aureus zen 36, taking care to ensure that all of the solution contacts the implant. Immediately tie the suture to ensure containment of the inoculum to the implant, and use a second absorbable suture to close the skin in a running fashion.

Place the mouse on a heating pad with monitoring until full recovery. Obtain postoperative radiographs to confirm the appropriate placement of the implant. To measure the bacterial burden, on the appropriate day post-infection, use clippers to remove any regrown hair from the sacrum to the upper thoracic spine and place the mouse onto a bioluminescent imaging platform.

Using large bending settings with a 13 centimeter field of view, capture the bioluminescent signal for five minutes. Then, use bioluminescent imaging software to isolate a standard ovoid region of interest to quantify the mean and maximum total flux of the bioluminescence. At the appropriate experimental endpoint, after preparing the mouse as demonstrated for the surgical procedure, use a number 15 scalpel blade to sharply incise the old incision and use sterile scissors to bluntly dissect to the L-4 spinous process.

When the implant can be identified, use a needle driver to gently twist and remove the implant from its position, and use sterile forceps and scissors to harvest approximately one gram of spinous process bone and soft tissue immediately surrounding the surgical implant. Place the tissue in one milliliter of PBS in a pre-weighed small conical rhino tube containing four sharp homogenizing beads and place the implant in 250 microliters of 0.3%TWEEN 80 in triptych soy broth. Weigh the tube of tissue and use a homogenizer to homogenize the sample.

Vortex the implants at medium strength for two minutes, then sonicate them for 10 minutes. Dilute the implant in tissue samples one to 10, one to 101, one to 1000, and plate them on triptych soy broth plates containing 1%agar. Place the plates in an incubator at 37 degrees Celsius overnight.

The following day, count the colony forming units from the implant and surrounding tissue cultures. Infected control mice demonstrate BLI signals that peak on post-infection day 10 and remain above one times 10 to the fifth photons per second per square centimeter per teridian until sacrifice, successfully modeling a chronic spinal implant infection. Mice treated with vancomycin monotherapy exhibit a significantly lower BLI signal compared to infected controls, with a two-fold reduction from post infections day 10 through 21.

After post-infection day 21, there is no significant difference in BLI between the monotherapy and infected control groups. Mice treated with vancomycin-rifampin combination therapy have an even lower BLI signal that is 20 fold lower than infected controls on post-infection day 10. This significant reduction persists until post infection day 28, after which no significant difference in BLI is observed between any of the groups.

Implants and surrounding tissue harvested and processed on day 35 revealed no significant differences in colony-forming units between infected control, monotherapy, and combination therapy groups. Following this procedure, the host immune response may also be studied such as by utilizing mice with fluorescently-labeled neutrophils. The development of this efficient mouse model has allowed us to identify effective novel treatments for spinal implant infection.