Chondrocyte Invasion May Be a Mechanism for Persistent Staphylococcus Aureus Infection In Vitro

Background: Recurrent bone and joint infection with Staphylococcus aureus is common. S. aureus can invade and persist in osteoblasts and fibroblasts, but little is known about this mechanism in chondrocytes. If S. aureus were able to invade and persist within chondrocytes, this could be a difficult compartment to treat.

Question/purpose: Can S. aureus infiltrate and persist intracellularly within chondrocytes in vitro?

Methods: Cell lines were cultured in vitro and infected with S. aureus. Human chondrocytes (C20A4) were compared with positive controls of human osteoblasts (MG63) and mouse fibroblasts (NIH3T3), which have previously demonstrated S. aureus invasion and persistence (human fibroblasts were not available to us). Six replicates per cell type were followed for 6 days after infection. Cells were treated daily with antibiotic media for extracellular killing. To determine whether S. aureus can infiltrate chondrocytes, fluorescence microscopy was performed to qualitatively assess the presence of intracellular bacteria, and intracellular colony-forming units (CFU) were enumerated 2 hours after infection. To determine whether S. aureus can persist within chondrocytes, intracellular CFUs were enumerated from infected host cells each day postinfection.

Results: S. aureus invaded human chondrocytes (C20A4) at a level (2.8 x 10 5 ± 5.5 x 10 4 CFUs/mL) greater than positive controls of human osteoblasts (MG63) (9.5 x 10 2 ± 2.5 x 10 2 CFUs/mL; p = 0.01) and mouse fibroblasts (NIH3T3) (9.1 x 10 4 ± 2.5 x 10 4 CFUs/mL; p = 0.02). S. aureus also persisted within human chondrocytes (C20A4) for 6 days at a level (1.4 x 10 3 ± 5.3 x 10 2 CFUs/mL) greater than that of human osteoblasts (MG63) (4.3 x 10 2 ± 3.5 x 10 1 CFUs/mL; p = 0.02) and mouse fibroblasts (NIH3T3) (0 CFUs/mL; p < 0.01). S. aureus was undetectable within mouse fibroblasts (NIH3T3) after 4 days. There were 0 CFUs yielded from cell media, confirming extracellular antibiotic treatment was effective.

Conclusion: S. aureus readily invaded human chondrocytes (C20A4) in vitro and persisted viably for 6 days after infection, evading extracellular antibiotics. Chondrocytes demonstrated a greater level of intracellular invasion and persistence by S. aureus than positive control human osteoblast (MG63) and mouse fibroblast (NIH3T3) cell lines.

Clinical relevance: Chondrocyte invasion and persistence may contribute to recurrent bone and joint infections. Additional research should assess longer periods of persistence and whether this mechanism is present in vivo.