Bacterial and fungal DNA extraction from positive blood culture bottles: a manual and an automated protocol

Shotgun-Metagenomics on Positive Blood Culture Bottles Inoculated With Prosthetic Joint Tissue: A Proof of Concept Study

Clinical metagenomics is actively moving from research to clinical laboratories. It has the potential to change the microbial diagnosis of infectious diseases, especially when detection and identification of pathogens can be challenging, such as in prosthetic joint infection (PJI). The application of metagenomic sequencing to periprosthetic joint tissue (PJT) specimens is often challenged by low bacterial load in addition to high level of inhibitor and contaminant host DNA, limiting pathogen recovery. Shotgun-metagenomics (SMg) performed directly on positive blood culture bottles (BCBs) inoculated with PJT may be a convenient approach to overcome these obstacles. The aim was to test if it is possible to perform SMg on PJT inoculated into BCBs for pathogen identification in PJI diagnosis. Our study was conducted as a laboratory method development. For this purpose, spiked samples (positive controls), negative control and clinical tissue samples (positive BCBs) were included to get a comprehensive overview. We developed a method for preparation of bacterial DNA directly from PJT inoculated in BCBs. Samples were processed using MolYsis5 kit for removal of human DNA and DNA extracted with BiOstic kit. High DNA quantity/quality was obtained, and no inhibition was observed during the library preparation, allowing further sequencing process. DNA sequencing reads obtained from the BCBs, presented a low proportion of human reads (<1%) improving the sensitivity of bacterial detection. We detected a 19-fold increase in the number of reads mapping to human in a sample untreated with MolYsis5. Taxonomic classification of clinical samples identified a median of 96.08% (IQR, 93.85-97.07%; range 85.7-98.6%) bacterial reads. Shotgun-metagenomics results were consistent with the results from a conventional BCB culture method, validating our approach. Overall, we demonstrated a proof of concept that it is possible to perform SMg directly on BCBs inoculated with PJT, with potential of pathogen identification in PJI diagnosis. We consider this a first step in research efforts needed to face the challenges presented in PJI diagnoses.

A nanoparticle-based method for culture-free bacterial DNA enrichment from whole blood

Point-of-care (POC) diagnostics are one of the quick and sensitive detection approaches used in current clinical applications, but always face a performance tradeoff between time-to-result and assay sensitivity. One critical setting where these limitations are evident is the detection of sepsis, where 6-10mL of whole blood may contain as little as one bacterial colony forming unit (cfu). The large sample volume, complex nature of the sample and low analyte concentration necessitates signal enhancement using culture-based or molecular amplification techniques. In the time-critical diagnosis of sepsis, waiting for up to 24h to produce sufficient DNA for analysis is not possible. As a consequence, there is a need for integrated sample preparation methods that could enable shorter detection times, whilst maintaining high analytical performance. We report the development of a culture-free bacterial enrichment method to concentrate bacteria from whole blood in less than 3h. The method relies on triple-enrichment steps to magnetically concentrate bacterial cells and their DNA with a 500-fold reduction in sample volume (from 10 to 0.02mL). Using this sample preparation method, sensitive qPCR detection of the extracted S. aureus bacterial DNA was achieved with a detection limit of 5±0.58cfu/mL within a total elapsed time of 4h; much faster than conventional culture-based approaches. The method could be fully automated for integration into clinical practice for point-of-care or molecular detection of bacterial DNA from whole blood.