Establishment and multidimensional evaluation of a Staphylococcus aureus-induced empyema model in SD rats

Empyema is a severe pleural infection whose complex pathophysiology remains incompletely understood. Standardized animal models that reproduce its principal pathological features are needed to investigate disease mechanisms and evaluate new therapies. This study aimed to establish and multidimensionally characterize a Staphylococcus aureus -induced empyema model in Sprague-Dawley (SD) rats. Thirty-two female SD rats were used. A 12-rat concentration-gradient pilot study identified 0.2 mL of 5 × 10 8 CFU/mL S. aureus as the optimal inoculum. In the formal experiment, 20 rats were randomly assigned to receive a right intrapleural injection of live S. aureus ( n = 10) or an equal volume of heat-killed bacteria ( n = 10). Five rats per group were scheduled for euthanasia on days 3 and 5. Selected laboratory and image-based assessments were performed on coded samples. Assessments included gross pathology, hematology, pleural-fluid biochemistry and culture, scanning electron microscopy (SEM), histopathology, RT-qPCR, and ELISA. Groups were compared using the Mann-Whitney U test or Fisher’s exact test. Empyema developed in 9 of 10 experimental rats (90%); one rat died of sepsis on day 4 (10% mortality), and one failed to develop empyema. No control rat developed empyema or died. Experimental rats showed systemic illness, abundant purulent pleural effusion, pleural thickening, and fibrin deposition. Peripheral white blood cell counts and neutrophil percentages were higher than in controls. Pleural fluid had typical exudative characteristics, and S. aureus was recovered from all evaluable experimental samples (8/8). SEM demonstrated mature pleural biofilms, while histopathology showed severe inflammatory-cell infiltration and collagen deposition. Serum and pleural-fluid TNF-α, IL-1β, and IL-6 concentrations were higher than in controls, with pleural-fluid concentrations approximately 2.5-fold higher than paired serum concentrations. This S. aureus -induced rat model reproduced key features of human empyema, including purulent effusion, biofilm formation, intense local inflammation, and pleural fibrosis. Combined with the multidimensional evaluation system, it may provide a useful preclinical platform. The model achieved 90% success with 10% mortality in a single cohort, although reproducibility across independent cohorts requires confirmation.