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

<i>Staphylococcus aureus</i>, 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 <i>S. aureus</i> pathogenicity by allowing it to evade the immune system and most antibiotic treatments. <i>S. aureus</i> 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, <i>S. pseudintermedius, S. delphini</i>, and <i>S. argenteus</i> can also invade osteoblasts, relying on FnBP-like proteins such as <i>S. pseudintermedius</i> surface proteins D and L (SpsD/L) or <i>S. delphini</i> surface protein Y (SdsY). However, the internalization capacity and mechanisms remain poorly explored in other <i>Staphylococcus</i> 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 <i>Staphylococcus</i> species tested, half exhibited high internalization into osteoblasts. We demonstrated that the FnBP-fibronectin-α5β1 integrin-dependent internalization pathway of <i>S. aureus</i> is well conserved in 27 <i>Staphylococcus</i> species. <i>In silico</i> analyses revealed multiple FnBP-like proteins associated with highly internalized species, showing sequence diversity likely resulting from multiple gene acquisitions throughout <i>Staphylococcus</i> evolution.IMPORTANCEThe internalization of <i>Staphylococcus aureus</i> 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 <i>Staphylococcus</i> species. In this study, we demonstrated that half of the species within the <i>Staphylococcus</i> genus are capable of being internalized by osteoblasts using a mechanism similar to that of <i>S. aureus</i>. Internalization into NPPCs may therefore represent a partially conserved process within the <i>Staphylococcus</i> genus, raising important questions about the evolution of pathogenicity in staphylococci.