# Staphylococcal internalization into osteoblasts: a partially conserved mechanism across the genus

## TL;DR

This study shows that half of Staphylococcus species can enter bone cells using a mechanism similar to the well-known S. aureus pathway, suggesting a partially conserved process across the genus.

## Contribution

The study reveals that internalization into osteoblasts is a partially conserved mechanism across the Staphylococcus genus, with insights into FnBP-like proteins and evolutionary gene acquisitions.

## Key findings

- Half of the 53 Staphylococcus species tested showed high internalization into osteoblasts.
- The FnBP-fibronectin-α5β1 integrin-dependent pathway is conserved in 27 species.
- Sequence diversity in FnBP-like proteins suggests multiple gene acquisitions during evolution.

## Abstract

Staphylococcus aureus, recognized as a major human pathogen, is associated with severe infections such as bacteremia, endocarditis, skin and soft tissue infections, and bone and joint infections. Virulence mechanisms, including biofilm formation and host cell invasion/internalization, contribute to S. aureus pathogenicity by allowing it to evade the immune system and most antibiotic treatments. S. aureus can be internalized by non-professional phagocytic cells such as fibroblasts, epithelial cells, endothelial cells, and osteoblasts. Its primary internalization pathway is FnBP-fibronectin-α5β1 integrin-dependent. Interestingly, S. pseudintermedius, S. delphini, and S. argenteus can also invade osteoblasts, relying on FnBP-like proteins such as S. pseudintermedius surface proteins D and L (SpsD/L) or S. delphini surface protein Y (SdsY). However, the internalization capacity and mechanisms remain poorly explored in other Staphylococcus species. Here, we investigated staphylococcal internalization into osteoblasts at the genus level and examined its correlation with FnBP-like proteins, integrating fibronectin adhesion assays, osteoblast infections, and genome analysis. Among the 53 Staphylococcus species tested, half exhibited high internalization into osteoblasts. We demonstrated that the FnBP-fibronectin-α5β1 integrin-dependent internalization pathway of S. aureus is well conserved in 27 Staphylococcus species. In silico analyses revealed multiple FnBP-like proteins associated with highly internalized species, showing sequence diversity likely resulting from multiple gene acquisitions throughout Staphylococcus evolution.

The internalization of Staphylococcus aureus into non-professional phagocytic cells (NPPCs) is considered a key mechanism in the development of persistent infections. The primary internalization pathway has been clearly identified. However, the capacity for internalization and its underlying mechanism remain poorly studied in other Staphylococcus species. In this study, we demonstrated that half of the species within the Staphylococcus genus are capable of being internalized by osteoblasts using a mechanism similar to that of S. aureus. Internalization into NPPCs may therefore represent a partially conserved process within the Staphylococcus genus, raising important questions about the evolution of pathogenicity in staphylococci.