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.
The bacteria detection process takes about 30 minutes and is as sensitive as processes that use culture samples grown in the lab. The results are described in the journal Angewandte Chemie.
“Trying to diagnose very low levels of bacteria in a sample while maintaining high quality is not an easy thing to do,” says pathologist Miller. The Harvard test is very sensitive–that is, it can detect low levels of bacteria–but until the device undergoes more tests, it’s impossible to say how specific it is. If it proves to have high rates of false positives, says Miller, it won’t be viable in places like the United States, where tuberculosis rates are low. However, says Miller, “there could be a lot of value for a cheap and easy test like this in areas with high tuberculosis prevalence.”
The researchers are collaborating with the Harvard School of Public Health to test the device on clinical samples from patients carrying tuberculosis.