Portable Bacteria Counter Detects TB Using Miniature MR Unit

Modern methods of detecting tuberculosis are either very slow (growing Mycobacterium tuberculosis in blood agar) or are not particularly specific or sensitive. Researchers from Harvard Medical School and Massachusetts General Hospital are now working on a portable device that can count acid-fast staining bacteria in a sample with only about a dozen of bacterial organisms. The system uses magnetic nanoparticles to attach to the bacteria and a tiny MR sensor to then detect the coupled pairs.
MIT Technology Review, that first reported about the technology, explains:

The Harvard detector can find very small loads of bacteria. It’s a miniaturized version of a nuclear magnetic resonance imager, a very sensitive but typically large and expensive device used for clinical and chemical applications such as brain imaging and determining protein structures. The size and expense of typical nuclear magnetic resonance imagers is dictated by the need for a strong magnet. Weissleder’s group simplified the instrument into a portable, one-pound device with disposable parts by compromising on signal quality and by placing the sample chamber right inside the radio-frequency coils. “When you’re measuring bacteria, you don’t need high resolution–you just need to pick up one pattern,” says Lee [Hakho Lee, instructor at Mass General].
As proof of principle, Weissleder and Lee demonstrated they could detect a bacterium very similar to tuberculosis in sputum samples. First, the viscous sample must be liquefied. Then it’s mixed with a solution of cannonball-shaped iron nanoparticles coated in antibodies that stick to the bacteria. The sample is loaded onto the detector, which uses microfluidics to force the sample through a channel fitted with a screen that traps bacteria and washes free any nanoparticles that didn’t meet a target. This channel is surrounded by a metal coil that pulses the trapped bacteria with radio-frequency waves under the influence of a magnet. This causes the iron nanoparticles to emit a magnetic signal, in turn affecting the protons in the surrounding water molecules. The Harvard device picks up on these changes, whose magnitude and duration are directly proportional to the number of labeled bacteria in the sample.

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