Imaging of burn induced sepsis in mice produced by gram negative bacteria thigh abscesses: Relationship to burn depth

Objectives Sepsis remains a major cause of death in patients with burn injury. In this investigation we studied the growth of the gram negative bacteria Proteus mirabilis, genetically engineered to be bioluminescent. The organisms were injected into the thigh of mice with burn injury and bioluminescence was studied in relation to size of injury.

Methods Male CD-1 mice (Charles River, 27-28 grams) were anesthetized and subjected to 20% TBSA burn by dipping the dorsum in a 90^oC water bath for 1,3,6 or 9 sec. After injury, the animals were resuscitated with 2 ml of saline given IP. Sham animals, received the same treatment without burn. Bioluminescent Proteus mirabilis (5x10 ⁶ CFU) were injected into the unburned thigh 2 hr after injury. The mice were studied 24 hrs later by monitoring the bioluminescence. Subsequently, the burn wound was excised and processed for histological examination.

Results Proteus injection in the thigh resulted in bacterial colonization of the burn eschar 24 hrs later (9 second burn). The 1 and 3 second burn resulted in minimal bioluminescence in the burn eschar, however, the 6 second burn did. No such bioluminescence was observed in the dorsum of sham animals. Histologically, all the wounds were full thickness. However, only the 6 and 9 second burns extended into the deep skeletal muscle.

Conclusions The current data suggest that burn induces colonization of burn eschar from gram negative Proteus mirabilis injected into a site distal to the burn injury. This higher bacterial load in the burn eschar is associated with depth of the burn. The present model may allow a better understanding of the development of burn induced sepsis at early times after burn injury, and could provide insights for the development of better treatments for gram negative sepsis after burn injury.

Research Support Shriners Hospitals for Childre