Development of a multiplexed microsphere PCR for rapid, culture-free detection and Gram-typing of bacteria in human blood samples.

Blood stream infection is a significant clinical problem, particularly in vulnerable patient groups such as those undergoing chemotherapy and bone marrow transplantation. Clinical diagnostics for suspected blood stream infection remain centered around blood culture (highly variable timing, hours to days), and empiric use of broad-spectrum antibiotics is often employed for patients presenting with febrile neutropenia. Gram-typing provides the first opportunity to target therapy (e.g. combinations containing vancomycin or teicoplanin for Gram-positives; piperacillin-tazobactam or a carbapenem for Gram-negatives), however current approaches require blood culture. In this study, we describe a multiplexed microsphere-PCR assay with flow cytometry readout, which can distinguish Gram-positive from Gram-negative bacterial DNA in a 3.5-hour time period. The combination of a simple assay design (amplicon-dependent release of Gram-type specific Cy3-labelled oligonucleotides) and the Luminex-based readout (for quantifying each specific Cy3-labelled sequence) opens opportunities for further multiplexing. We demonstrate the feasibility of detecting common Gram-positive and Gram-negative organisms after spiking whole bacteria into healthy human blood prior to DNA extraction. Further development of DNA extraction methods is required to reach detection limits comparable to blood culture.