Bacteraemia (bloodstream infection) can lead to sepsis, septic shock, and death if not promptly detected and treated. ICU patients are particularly vulnerable, with reported mortality rates as high as 36-42%. The current gold standard for bacteraemia detection relies on traditional culture methods and exhibits low sensitivity, which presents a significant challenge for detecting polymicrobial bacteraemia. Polymicrobial bacteraemia accounts for ~10% of cases and is associated with increased progression to sepsis and septic shock, longer hospital stays, and delays in adjustment of antimicrobial therapy. Mortality rates can be over double that of monomicrobial infections. In this study, we aimed to evaluate the efficacy of using a viability quantitative polymerase chain reaction (vPCR) approach for detecting polymicrobial infections that may be missed by traditional methods.
Four combinations of clinical bacterial strains (E. faecalis + E. cloacae, S. aureus + E. coli, S. pneumoniae + E. coli and B. fragilis + K. pneumoniae) that are commonly found in clinical settings were evaluated using 5 models: organism A monoculture, organism B monoculture, 1:1 combination, 1:10 combination, and 10:1 combination. Mock blood cultures were created by spiking BACTEC Peds Plus bottles with 500 µL heparinised donor blood and 100 µL bacterial suspension containing a total of 10 CFUs for each model. The bottles were incubated in a BACTEC device until they signalled positive. The samples were tested using both the routine culture method (Gram stain and sub-culture on non-selective agar) and our qPCR assay, which incorporates PMA (a viability dye) to ensure amplification of DNA from viable bacteria only. Bacterial loads were also determined by spiral plating.
Our results demonstrated that the vPCR assay could detect all organisms in all five models, whereas the routine method showed occasional failures when the bacterial ratio was uneven, or the combination included a significantly slower-growing organism. This suggests that the true incidence of polymicrobial bacteraemia could be underestimated and consequently, its clinical importance may not be fully recognised.