Prosthetic joint infection (PJI) is a devastating complication following arthroplasty. An utmost effort has been done to clarify risk factors and microbiology of PJI. We now know the most frequent infecting microorganisms are members of the skin microbiome. These microorganisms are thought to get into the joint and infect the prosthesis during the peri-operative and/or post-operative period. Nevertheless, recent studies have suggested, bacteria may be present in the joint even before the first incision, suggesting the existence of a joint microbiome. Therefore, we aimed to determine the bacterial composition from different knee conditions.

Inclusion Criteria:

• All patients 18 years or older who give verbal and written informed consent for the retrieval of experimental samples and data collection.
• Group A: Subjects with lack of Osteoarthritis (OA) evidence, invasive examinations, minor procedures, cartilage damage, and/or synovitis in joints.
• Group B: Patients undergoing primary knee arthroplasty (partial or total).
• Group C: Contralateral non-affected (native) joint without knee arthroplasty, history of septic arthritis (SA), invasive examinations and/or minor procedures.
• Group D: Patients undergoing aseptic knee revision surgery.
• Group E: Patients undergoing septic knee revision surgery

Exclusion Criteria:

• Withdrawal of informed consent, suspicion of PJI, surgical site infection (SSI) or septic arthritis (SA) before samples retrieval, use of antibiotics within 4 weeks before collecting the samples (not including perioperative antimicrobial prophylaxis), inflammatory arthritis, systemic rheumatic disorder, high risk of infection.
• Group A: History and/or suspicion before/at samples retrieval of SA in native joint.
• Group B: History and/or suspicion before/at samples retrieval of SSI or SA in the affected joint.
• Group C: No additional criterion.
• Group D: History and/or suspicion before/at samples retrieval of PJI, SSI, SA in the affected joint
• Group E: No additional criterion.

Participants will be recruited preoperatively during their elective procedure scheduling visit (diagnostic/treatment algorithm for group A or surgery for groups B-E, accordingly). Patients presenting for Orthopedic Sports Medicine consultation (group A), with end-stage knee osteoarthritis who are candidates for TKA (group B/C), patients with a TKA who require an aseptic knee revision surgery (group C/D), or those patients complicated by PJI (group E), and who agree to participate in the study will undergo the informed consent process