Researchers from the Georgia Institute of Technology and the Centers for Disease Control and Prevention (CDC) have developed a new laboratory test that can rapidly identify Staphylococcus aureus in blood cultures. This new test takes advantage of unique isotopic labeling combined with specific bacteriophage amplification.
Before bacteria can be identified in a blood sample, they have to be multiplied enough times to reach detection thresholds and be incubated at least 18 to 24 hours. The new method, published in Molecular and Cellular Proteomics this month, reduces valuable culturing time to only two hours. The identification method takes advantage of the faster replication rate of viruses that can infect bacteria. A nitrogen-15 labeled bacteriophage is added to the test which specifically targets living Staphylococcus aureus cells. The phage replicates inside the bacteria and reaches the detection threshold much quicker. The phage proteins that were introduced with nitrogen-15 labels can be distinguished from the freshly replicated virus particles created in the S. aureus host cells. With mass spectrometry, a protein identification technique based on the mass and charge ratio of peptide fragments, the phage proteins can easily be detected. By comparing the labeled with the new replicated phage particles it is possible to calculate the number of original S. aureus host cells in the test sample.
This new technique makes it possible to quickly screen a sample for Staphylococcus aureus infection and allows an early start with empirical antibiotic therapy. The test does not provide any information on antibiotic susceptibility and cannot discriminate between MRSAs or other specific types of S. aureus strains. This prototype mass spectrometry-based technique has been optimized to detect low concentrations of bacteria that should allow clinicians to diagnose staph infections without the need for a significant culture period.
Carrie Pierce, one of the authors on the paper and research chemist at the CDC commented in the press release:
“The simplicity of sample preparation, the low cost of required reagents and the increased availability of mass spectrometers in clinical laboratories make this new method a cost-effective way to rapidly and effectively detect staph infections, which must be treated quickly to prevent spread of the disease.”
Abstract in Molecular & Cellular Proteomics: Viable Staphylococcus aureus Quantitation using 15N Metabolically Labeled Bacteriophage Amplification Coupled with a Multiple Reaction Monitoring Proteomic Workflow