Comparison of animal models with soft tissue infection by different bacilli

In clinical medicine improved diagnostic methods for the detection of infection are needed. A good infectious animal model is very important for the development of a new diagnostic method or drug. The purpose of this study was to establish a good animal model with soft tissue infection. Twenty-four SD rats were divided into four groups (6 in each group). Various bacilli including Staphylococcus aureus (S. aureus), Streptococcus pneumoniae (S. pneumoniae), and Escherichia coli (E. coli) were injected intramuscularly into the left caudal thighs of three groups of rats to create soft tissue infection. In addition, normal saline was injected into the left caudal thighs of ten rats which were used as controls. Before and 48 hr after inoculation of the bacilli, a blood sample (0.5 ml) was taken from each rat and analyzed to determine the white blood cell count and differentiated cell count. In addition, 48 hr after the inoculation, 0.2 mCi of gallium-67 was injected via the tail vein. Gallium scan was performed at 24 hr and 48 hr after administration of the radiotracer. The dorsal view of both hind legs was imaged and analyzed by computers to calculate the lesion-to-normal (L/N) ratio. After imaging, all rats were sacrificed and specimens from portions of the infected thigh muscle were sent for histopathologic investigation to confirm the infection. The increase in both the WBC counts and the segmented polymorphonuclear leukocytes (PMNs) were most significant in the S. aureus group, followed by the S. pneumoniae group, E. coli group and normal control groups. The rats with S. aureus infection had significant gallium uptake at the site of infection and the highest L/N ratio of 2.14 on the 24-hr image and 2.0 on the 48-hr image. The rats with S. pneumoniae had the second highest L/N ratio (1.41 at 24 hr, and 1.48 at 48 hr). The L/N ratio for the E. coli group was 1.27 at 24 hr and 1.35 at 48 hr. No obviously abnormal gallium uptake was demonstrated in the normal controls. We conclude that all three bacilli induced a soft tissue infection in SD rats. S. aureus resulted in the most significant infectious signs.