Rapid in Situ Chondrocyte Death Induced by Staphylococcus Aureus Toxins in a Bovine Cartilage Explant Model of Septic Arthritis

Objective: To assess in situ chondrocyte viability following exposure to a laboratory strain and clinical isolates of Staphylococcus aureus.

Methods: Bovine cartilage explants were cultured in the presence of S. aureus 8325-4 (laboratory strain), clinical S. aureus isolates or non-infected culture medium of pH values 7.4, 6.4 and 5.4. All clinical isolates were isolated from the joint aspirates of patients presenting with S. aureus-induced septic arthritis (SA). At designated time points, in situ chondrocyte viability was assessed within defined regions-of-interest in the axial and coronal plane following live- and dead-cell image acquisition using the fluorescent probes 5-chloromethylfluorescein diacetate (CMFDA) and propidium iodide (PI), respectively, and confocal laser-scanning microscopy (CLSM). Cartilage water content, following S. aureus 8325-4 exposure, was obtained by measuring cartilage wet and dry weights.

Results: S. aureus 8325-4 and clinical S. aureus isolates rapidly reduced in situ chondrocyte viability (>45% chondrocyte death at 40 h). The increased acidity, observed during bacterial culture, had a minimal effect on chondrocyte viability. Chondrocyte death commenced within the superficial zone (SZ) and rapidly progressed to the deep zone (DZ). Simultaneous exposure of SZ and DZ chondrocytes to S. aureus 8325-4 toxins found SZ chondrocytes to be more susceptible to the toxins than DZ chondrocytes. Cartilage water content was not significantly altered compared to non-infected controls.

Conclusions: Toxins released by S. aureus have a rapid and fatal action on in situ chondrocytes in this experimental model of SA. These data advocate the prompt and thorough removal of bacteria and their toxins during the treatment of SA.