Low-dose infectivity of Staphylococcus aureus (SMH strain) in traumatized rat tibiae provides a model for studying early events in contaminated bone injuries

Animal models of post-traumatic acute osteomyelitis (OM) that closely mimic human scenarios, including infection prophylactic procedures such as debridement and lavage, may provide a better understanding of OM. We contaminated mechanically traumatized rat tibiae (n = 69) with various doses of a Staphylococcus aureus strain (SMH) known to cause human OM and then performed curettage and lavage. Tibiae were harvested 24 h after lavage for assessment of bacterial load and determination of minimal infective doses for 50% (ID ₅₀) and 95% (ID₉₅) of rats. Some experiments varied tibial harvest time after lavage (n = 10); for progressive infection, tibiae were evaluated at 7 and 15 days after contamination (n = 17 for each time point). At 24 h after contamination, the ID₅₀ was 1.8 × 10³ CFU, and the ID₉₅ was 9.2 × 10³ CFU. Tibial bacterial loads did not increase with inocula greater than the ID₉₅. Lavage removed many bacteria from bone, but it did not prevent subsequent infection or disease. At 15 days after contamination, most tibiae (14 of 17) were infected, with macroscopic and radiological signs of established OM. This newly described rat OM model, with a low ID₉₅ despite prophylactic curettage and lavage, closely mimics events in contaminated human bone injuries. This situation will allow study of early factors in contaminated bone injuries, including clinical interventions that may reduce infection and prevent disease.