Clinical significance and outcome of polymicrobial Staphylococcus aureus bacteremia

Objectives: The clinical significance of polymicrobial Staphylococcus aureus bacteremia (SAB) remains unclear. We therefore compared the clinical features and outcomes of polymicrobial and monomicrobial SAB. Methods: A prospective cohort study of patients with SAB was performed during a 20-months. Polymicrobial SAB was defined as the simultaneous isolation of S. aureus and other microorganisms from blood cultures. However, Corynebacterium spp., Bacillus spp., and coagulase-negative staphylococci were considered contaminants unless they were related to device infection and grew in two or more blood cultures. Results: During the study period, 44 (10%) patients had polymicrobial and 412 (90%) had monomicrobial SAB. A total of 54 microorganisms were isolated from the former, with Enterococcus spp. (22%) being the most common. Independent risk factors for polymicrobial SAB included neutropenia (odds ratio [OR] 3.5, p = 0.02), biliary tract catheters (OR 5.0, p = 0.001), and intra-abdominal infection (OR 10.3, p < 0.001). Clinical outcomes were significantly worse among patients with polymicrobial than monomicrobial SAB, including bacteremia-related and 7-day mortality rates. Independent predictors of bacteremia-related mortality were solid tumors (HR 2.0, p = 0.03) and polymicrobial SAB (HR 2.8, p = 0.007). Conclusions: Polymicrobial SAB is associated with more severe illness than monomicrobial SAB, with neutropenia, biliary tract catheters and intra-abdominal infection being significant risk factors for polymicrobial SAB.