Staphylococcal Internalization into Osteoblasts: A Partially Conserved Mechanism Across the Genus

Abstract

Staphylococcus aureus, considered 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, such as biofilm formation and invasion/internalization of/into host cells, support the pathogenicity of S. aureus as they enable it to evade from the immune system and most antibiotic treatments. S. aureus can be internalized into non-professional phagocytic cells like fibroblasts, epithelial cells, endothelial cells and osteoblasts. The main pathway of internalization of S. aureus is FnBP-fibronectin-5{beta}1 integrin dependent. Interestingly, S. pseudintermedius, S. delphini and S. argenteus are able to invade osteoblasts, depending on the presence of FnBP-like proteins such as Staphylococcus pseudintermedius surface proteins D and L (SpsD/L) or Staphylococcus delphini surface protein Y (SdsY). However, the internalization capacity and mechanism have been poorly investigated in other staphylococci species. Here, we investigated the internalization capacity of staphylococci into osteoblasts at the genus level and attempted to correlate it with the presence of FnBP-like proteins by combining fibronectin adhesion assays, infection of osteoblasts and genome analysis. Over the 53 Staphylococcus species tested, half of them exhibited high internalization into osteoblasts. We highlighted that the "FnBP-fibronectin-5{beta}1 integrin" dependent internalization pathway of S. aureus, is well-conserved in 27 Staphylococcus species. In silico analysis identified multiple FnBP-like proteins correlating with the highly internalized species and showing diversity in their sequence organization, likely due to multiple acquisitions of such encoding genes throughout Staphylococcus evolution. AUTHOR SUMMARYStaphylococcus aureus is a pathogenic bacterium that causes severe infections including bone and joint infections. It invades bone cells, such as osteoblasts, using the bacterial surface protein FnBP, which binds to fibronectin, an extracellular compound, which subsequently binds to 5{beta}1 integrin on the surface of osteoblasts. This cross-linking enables active internalization of S. aureus by the cells and potential intracellular persistence, which are responsible for the ability to induce staphylococcal chronic infections. While four species have been studied for their internalization into osteoblasts, a comprehensive genus-level investigation remains unexplored. Therefore, we investigated the conservation of this internalization capacity among the genus Staphylococcus, including a significant number of species of animal origin. Approximately half of the genus is capable of invading osteoblasts at varying levels via 5{beta}1 integrin. Additionally, homologous proteins to FnBP were identified in most highly internalized species, suggesting a similar pathway of cell internalization to that of S. aureus. Genomic analysis reveals these proteins were acquired multiple times during evolution, suggesting they provide an advantage for host infection. In the context of One Health approach and the increasing number of animal pathogens causing human infections, understanding staphylococcal pathogenicity will help anticipate the emergence of new infectious diseases.