Clinical significance and outcome of polymicrobial Staphylococcus aureus bacteremia

The clinical significance of polymicrobial Staphylococcus aureus bacteremia (SAB) remains unclear. We therefore compared the clinical features and outcomes of polymicrobial and monomicrobial SAB.

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.

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).

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.