More than Just Contaminants: Frequency and Characterization of Polymicrobial Blood Cultures from a Central Clinical Microbiology Laboratory Serving a Large Healthcare System

Background: Polymicrobial blood stream infection is often considered uncommon, and corresponding cultures may be assumed to represent contamination. Here we characterized the prevalence and epidemiology of these cultures submitted to a central clinical microbiology laboratory. Methods: Blood cultures from 2017 to 2018 (n = 104 547) were evaluated. Polymicrobial blood cultures were defined by the presence of more than one organism in a blood culture set (set = one aerobic and one anaerobic bottle). Data were stratified by patient location and characteristics of the microbiota detected. Results: Of all blood culture sets, 14 600 (14.0%) were positive. Among these, 1651 sets (11.3% of positive cultures; 1.6% of total cultures) were polymicrobial. Most cultures (85.2%) grew two microorganisms; the greatest number of microorganisms in a culture was 6. The most common microorganism pairs were (a) two coagulase-negative staphylococci (CoNS), (b) Corynebacterium and CoNS, and (c) S. aureus and CoNS. Microorganisms in polymicrobial cultures represented microbiota from skin (46.1%), the gastrointestinal (GI) tract (33.9%), strict anaerobes (1.4%), and "other"microorganisms (18.6%). Most cultures with GI microbiota originated from an adult academic medical center compared to community or pediatric settings (40.5% of polymicrobial cultures vs 27.2% and 25.8%, P < 0.0001). Within the academic medical center, patients in an intensive care unit (ICU) or who had bone marrow transplants (BMT) had more cultures suggestive of GI microbiota compared to those from the emergency department (ED) (50.8% and 52.8% vs 31.2%, P < 0.0001). Conclusions: Polymicrobial blood cultures are common in a variety of healthcare settings and the recovered microorganisms can be clinically relevant.