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Some patient populations and types of orthopaedic surgery could be at particular risk for anaerobic infections. In this retrospective cohort study of operated adult patients with infections from 2004 to 2014, we assessed obligate anaerobes and considered first clinical infection episodes. Anaerobes, isolated from intra-operative samples, were identified in 2.4% of 2740 surgical procedures, of which half (33/65; 51%) were anaerobic monomicrobial infections. Propionibacterium acnes, a penicillin and vancomycin susceptible pathogen, was the predominantly isolated anaerobe. By multivariate analysis, the presence of fracture fixation plates was the variable most strongly associated with anaerobic infection (odds ratio: 2.1, 95% CI: 1.3-3.5). Anaerobes were also associated with spondylodesis and polymicrobial infections. In contrast, it revealed less likely in native bone or prosthetic joint infections and was not related to prior antibiotic use. In conclusion, obligate anaerobes in our case series of orthopaedic infections were rare, and mostly encountered in infections related to trauma with open-fracture fixation devices rather than clean surgical site infection. Anaerobes were often co-pathogens, and cultures most frequently recovered P. acnes. These observations thus do not support changes in current practices such as broader anaerobe coverage for perioperative prophylaxis.

Summary Background Staphylococcus aureus is the most common cause of soft tissue infections. It is unknown, however, if a patient who has had such an infection is at greater risk for future soft tissue infections with S. aureus . Methods We conducted an epidemiological survey of adult patients hospitalized in the only public hospital in Geneva for treatment (usually combined surgical and medical) of a soft tissue infection caused by S. aureus . By reviewing nursing and medical records from the emergency department and hospital wards, we assessed whether or not they developed any other soft tissue infections (excluding a recurrence) after or before the index one. Results Among 1023 index episodes of soft tissue infections, 670 (65%) were caused by S. aureus, of which 47 were caused by methicillin-resistant strains (30 healthcare-associated and 17 community-acquired). The patients' median age was 51 years and 334 (34%) were immune-compromised. The median time span between the patient's first and last consultation (for any reason) in our hospital was 21.4 years (interquartile range, 10-30 years). In addition to their index infection, 124 patients (12%) developed a new nosocomial or community-acquired soft tissue infection. Among the index cases with an S. aureus infection, 92 (14%) had another soft tissue infection, compared to 32 (9%) who had a non-staphylococcal index infection (Pearson-χ 2 -test; p =0.03). Similarly, patients with an index S. aureus infection, compared to those with a non- S. aureus infection, had a higher rate of another soft tissue infection caused by S. aureus (χ 2 -test; p 0.01). In multivariate analysis, an index infection due to S. aureus shows a high association to further S. aureus soft tissue infections (logistic regression; odds ratio 2.5, 95% confidence interval 1.4-4.6). Conclusion Among adult patients hospitalised for a soft tissue infection, those infected with S. aureus (compared with other pathogens) may be at higher risk of a subsequent soft tissue infection, particularly with S. aureus .