Septic arthritis and prosthetic joint infections: microbial spectrum and evolving resistance patterns

Background: Septic arthritis (SA) and prosthetic joint infection (PJI) are severe musculoskeletal emergencies associated with rapid joint destruction, functional disability, and high mortality. Accurate microbiological diagnosis remains challenging, particularly in PJIs where biofilm formation reduces culture sensitivity. Local epidemiological data are essential to optimize empirical therapy and stewardship strategies. This study aimed to determine the distribution of microorganisms isolated from synovial fluid cultures and to evaluate antimicrobial resistance trends, with a direct comparison between native SA and PJI over a 5-year period. Methods: We retrospectively analyzed 3171 synovial fluid specimens collected between January 2020 and December 2024 at a tertiary referral hospital. Microorganisms were identified by MALDI-TOF MS, and antimicrobial susceptibility was tested with VITEK-2 according to EUCAST criteria. Resistance trends were assessed for major pathogens. Results: Overall, 651 samples (20.5 %) yielded growth, with significantly higher positivity in blood culture bottles than sterile containers (29.6 % vs. 16.1 %, $p<0.001$ ). PJIs accounted for 47.8 % of positive cultures. The most frequent pathogens were Staphylococcus aureus (33.6 %), coagulase-negative staphylococci (CoNS) (24.9 %), and Pseudomonas aeruginosa (8.6 %). CoNS ( $p=0.017$ ) and E. faecalis ( $p=0.009$ ) were significantly more common in PJIs. Methicillin resistance increased among S. aureus (20.0 % $\to$ 30.8 %) and remained high among CoNS (51 %-85 %). Extended-spectrum $\beta$ -lactamase (ESBL) prevalence rose in K. pneumoniae (14.3 % $\to$ 42.1 %) and remained high in E. coli (57 %-80 %). All staphylococcal isolates remained susceptible to glycopeptides, linezolid, and daptomycin. Conclusions: Staphylococci remain the dominant pathogens in joint infections, with CoNS and E. faecalis strongly associated with PJIs. Rising methicillin resistance and ESBL-producing Enterobacterales highlight the need for careful empirical coverage, while preserved activity of last-line agents is reassuring. Routine inoculation into blood culture bottles significantly improves diagnostic yield.