The etiologic agent of the majority of adult joint infections is which organism?

• A. Abiotrophia sp.
• B. Leuconostoc sp.
• C. Staphylococcus aureus
• D. Streptococcus pneumoniae

Answer: (C)

Septic arthritis in adults is most often caused by Staphylococcus aureus, a skin colonizer that readily seeds joints through the bloodstream or after direct injury. Its virulence factors enable strong adherence to synovial tissue, rapid invasion, and, in the case of prosthetic joints, biofilm formation that protects it from antibiotics. While other organisms like Abiotrophia and Leuconostoc can cause joint infections, they are much rarer and typically linked to particular conditions such as dental disease or specific patient contexts; Streptococcus pneumoniae can cause septic arthritis but is less common than Staphylococcus aureus in adults. This overall prevalence makes Staphylococcus aureus the leading etiologic agent for adult joint infections.