Researchers at Nagasaki University have developed a new PCR-based method designed to improve detection of bacteremia by identifying bacterial DNA inside circulating neutrophils. Instead of looking only for bacteria freely circulating in blood, the assay detects bacteria that have already been engulfed by immune cells as part of the body’s response to infection.
Blood cultures remain the standard method for diagnosing bacteremia, but they can take more than a day to produce results and may become negative after antibiotics are started. Molecular tests that directly detect bacteria in blood have also faced technical challenges and limited clinical adoption.
The new method, called droplet digital cell PCR (ddcPCR), combines droplet digital PCR with single-cell analysis. Researchers isolated neutrophils from blood, placed individual cells into microscopic droplets, and amplified bacterial DNA within those cells. The assay was designed to detect both human DNA and bacterial DNA at single-cell resolution.
The team focused on Staphylococcus aureus, a major cause of bloodstream infection. In laboratory experiments, the assay successfully identified neutrophils that had engulfed S. aureus and showed good agreement with flow cytometry measurements. It was sensitive enough to detect very small numbers of infected cells, identifying phagocyte populations representing as little as 0.05 percent of neutrophils.
To evaluate clinical performance, the researchers tested 255 patient blood samples collected at the same time as routine blood cultures. Blood cultures identified S. aureus in three cases, while ddcPCR detected neutrophils containing S. aureus DNA in six cases.
Four patients were positive by ddcPCR despite negative blood cultures. According to the authors, all four patients had already received antibiotics before blood cultures were collected, which may have prevented bacterial growth in culture while bacterial DNA remained detectable inside neutrophils.
The study highlights a different diagnostic approach: measuring the host immune response alongside pathogen detection. Because neutrophils actively engulf bacteria during infection, detecting bacterial DNA inside these cells may help identify bloodstream infections even when free bacteria are difficult to culture.
The assay also required only around 1 mL of blood, substantially less than the volume typically needed for blood cultures. However, performance was reduced in some patients with severe neutropenia or hematologic malignancies because too few normal neutrophils were available for testing.
The authors stated that ddcPCR is not intended to replace blood cultures, but could serve as a complementary tool for faster assessment of suspected bacteremia and potentially support earlier treatment decisions.
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