This study is to obtain a comprehensive view of S. aureus adaptations in the infected human host and (i) to improve the understanding of the interface between antibiotic therapy, resistance development and virulence factor adaptation in S. aureus infected patients, and (ii) to adapt these findings into a more sustainable use of antimicrobials for therapy.

Staphylococcus (S.) aureus is notorious for its ability to develop resistance. This study is to obtain a comprehensive view of S. aureus adaptations in the infected human host and (i) to improve the understanding of the interface between antibiotic therapy, resistance development and virulence factor adaptation in S. aureus infected patients, and (ii) to adapt these findings into a more sustainable use of antimicrobials for therapy.

Patients with S. aureus infections who will routinely undergo diagnostic and surgical procedures will be identified. Intraoperative samples from the site of infection during surgery will be taken. Additionally, two blood drawing events (22.5ml in total) will be performed and nose swabs will be taken. The biological material will be analysed. This project will be the first to study different phenotypes of invasive S. aureus directly from its environment in vivo.

Inclusion criteria for patients:

• In collaboration with the orthopedic surgeons, patients with probable or proven S. aureus infections will be identified who will routinely undergo diagnostic and surgical procedures:

Inclusion criteria for healthy participants:

• Healthy control group should be generally well-being

Exclusion criteria for patients and for healthy participants:

• Pregnancy: Pregnant or lactating women will be excluded (urinary pregnancy test will be performed in women of childbearing age)
• General health condition: Current or recurrent disease that could cause complications that may affect the study or may set the patient at risk
• Diseases/ Illness: Psychiatric or other mental disorders which in the opinion of the investigator may probably affect the full understanding of the study (including signs of dementia)
• Language: Insufficient knowledge of german language to understand the procedure and rationale of the research question
• Vulnerability: Individuals whose willingness to volunteer in the research project may be unduly influenced by the expectation, whether justified or not, of benefits associated with participation, such as members of a group with a direct hierarchical structure (refer to ICH Guidelines E6 (R1) for GCP, 1.61)

Hospitalized patients with probable or proven S. aureus infection will be screened at the University Hospital of Basel. Patients with confirmed S. aureus infection who correspond to the eligibility criteria will be asked to participate in the research project.

Healthy volunteers who correspond to the eligibility criteria will be asked to participate in the research project and recruited by oral communication from the institutions Biozentrum, University Hospital Basel, and University of Basel.