Characterization of a mouse model of osteoarticular infection using clinical isolates of <i>Staphylococcus aureus</i>

Osteoarticular infection, most commonly due to <i>Staphylococcus aureus</i>, can become chronic, leading to many complications including irreversible joint damage and osteomyelitis. Here, we established a hindlimb model of infection in young (6-wk-old) mice by introducing <i>S. aureus</i> into the tibia through the knee joint, using methicillin-resistant <i>S. aureus</i> clinical isolates (TI1-5) derived from children with osteomyelitis, and a skin isolate (NRS384), all of which were previously transduced with a modified <i>lux</i> operon to induce bioluminescence. Weekly in vivo bioluminescent imaging (BLI) revealed that all strains could establish a local infection in >90% of recipients at 1 wk post-inoculation. Most infections persisted throughout 4 wk, and those that cleared did so by week 2, although 2 strains, TI2 and TI5, had lower incidence at 2 wk and significantly lower progression. Histologic examination of BLI-positive limbs revealed primarily septic arthritis, with variable and less frequent osteomyelitis, with abscesses, bacterial microcolonies, and neutrophil infiltration in affected tissues. We also found osteoclasts and marrow fibrosis in many infected bones. Immunofluorescence demonstrated less intense neutrophil and T cell infiltration around infection sites for TI2, compared to the more aggressive TI1 and TI3 strains. Micro-CT revealed significantly reduced tibial trabecular bone volumes (BV/TV) from infected, BLI-positive legs compared to uninfected, BLI-negative legs, although the latter were lower than those from noninfected mice. There was significant negative correlation between BV/TV and BLI for all strains except TI5. Lastly, we tested pain behavioral responses of mice to the aggressive TI3 strain. Electronic von Frey and hot plate assessments showed altered nociception suggestive of pain over 4 wk. In sum, intratibial injection of <i>S. aureus</i> causes persistent septic arthritis in young mice, with consistent differences between bacterial strains, allowing study of bacterial virulence and pathogenic host factors.