In the technique developed by Walsh et al., a sample of positive blood culture is treated with lysis buffer to "pop" the blood cells, then transferred to a specialized optical tube. The tube is centrifuged, which drives bacteria or fungi, which are denser than the solution, down through a liquid density cushion to form a pellet at the bottom of the tube.
Then comes the intrinsic fluorescence spectroscopy (IFS): the microbial pellet is irradiated with light ranging from the deep ultraviolet to infrared, which excites certain organic molecules in the microorganisms, including tryptophan, NADH, FAD, porphyrins, and others, and causes them to fluoresce in a characteristic way depending on the identity of the microbe. The exact pattern of fluorescence is compared with a database of 37 of the most common known pathogens to identify the organism present.
"We're using intrinsic fluorescence to identify the microorganisms. It's an innate property of most living organisms. Because it's intrinsic, no reagents are needed for the identification step," which removes many of the opportunities for mistakes and lowers test costs, says Walsh.
Testing in a controlled laboratory study shows the method can correctly identify the species in 96.5% of all test samples, and in the 2.7% of samples for which no species identity was provided, the system was able to correctly identify 67% to the family level, which is often enough information to select an effective therapy. Among over a thousand samples tested, the method never gave an incorrect result as to the family level or the Gram type.
Walsh says the research and development team in Durham is actively working on automating the system with robotics to make it a fully hands-off process. Blood cultures grow in their own time, often producing a positive result at an
|Contact: Garth Hogan|
American Society for Microbiology