According to infectious disease specialists at the Hospital de Sant Pau in Barcelona, Spain, Staphylococcus aureus is the most commonly cultured microorganism in surgical site infection (SSI) related to prosthetic joint replacement in their institution. The study was reported on at the 51st Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC), in Chicago, USA. Consecutive patients receiving primary total hip or knee prosthetic joint replacement between 2004 and 2009 in the Department of Orthopedic Surgery of their tertiary hospital were included. The surveillance of SSI was prospectively carried out with a follow-up period of one year. Infections were classified according to the American Centers for Disease Control and Prevention definitions. A total of 2,222 joint replacements were performed during the study period. There were 1,430 knee (64%) and 792 hip arthroplasties; SSI was identified in 104 (4.7%), incisional SSI in 58, and prosthetic-joint infection in 38. Staphylococci were the most common cause of infection (59 [63%]), followed by gram-negative bacilli (GNB) (30 [32%]). Staphylococci aureus was found in 30 patients (32%), followed by coagulase-negative staphylococci 29 (31%), Escherichia coli 11 (12%), and Pseudomonas aeruginosa 10 (11%). Among isolated S. aureus, 9 (30%) were resistant to methicillin (MRSA). In the first 3 years of the study (1st period, the rate of MRSA infections was 20%; it was 40% in the final 3 years (2nd period, p 0.43). The investigators also observed no differences in the percentages of GNB infections between the two study periods (35% vs 29%, p 0.53). The rate of polymicrobial infections was 7% during the 1st study period and 21% during the 2nd period (p 0.07). Following their analysis of the current etiology of SSI related to primary total knee and hip prosthetic joint replacement, the Spanish researchers concluded that the most commonly cultured microorganism in SSI related to prosthetic joint replacements in their setting is indeed S. aureus, followed closely by coagulase-negative staphylococci.