Implant-Derived S. aureus Isolates Drive Strain-Specific Invasion Dynamics and Bioenergetic Alterations in Osteoblasts

<b>Background:</b> Implants are integral to modern orthopedic surgery. The outcomes are good, but infections remain a serious issue. <i>Staphylococcus aureus</i> (<i>S. aureus</i>), along with <i>Staphylococcus epidermidis</i>, are predominant pathogens responsible for implant-associated infections, as conventional antibiotic treatments often fail due to biofilm formation or the pathogens' ability to invade cells and to persist intracellularly. <b>Objectives:</b> This study therefore focused on interactions of <i>S. aureus</i> isolates from infected implants with MG63 and SaOS2 osteoblasts by investigating the adhesion, invasion, and the impact on the bioenergetics of osteoblasts. <b>Methods and Results:</b> We found that the ability of <i>S. aureus</i> to adhere to osteoblasts depends on the isolate and was not associated with a single gene or expression pattern of characteristic adhesion proteins, and further, was not correlated with invasion. However, analysis of invasion capabilities identified better invasion conditions for <i>S. aureus</i> isolates with the SaOS2 osteoblastic cells. Interestingly, metabolic activity of osteoblasts remained unaffected by <i>S. aureus</i> infection, indicating cell survival. In contrast, respiration assays revealed an altered mitochondrial bioenergetic turnover in infected cells. While basal as well as maximal respiration in MG63 osteoblasts were not influenced statistically by <i>S. aureus</i> infections, we found increased non-mitochondrial respiration and enhanced glycolytic activity in the osteoblasts, which was again, more pronounced in the SaOS2 osteoblastic cells. <b>Conclusions:</b> Our findings highlight the complexity of <i>S. aureus</i>-host interactions, where both the pathogen and the host cell contribute to intracellular persistence and survival, representing a major factor for therapeutic failures.