One third of the world's population is infected with tuberculosis. Detecting the bacteria is time-consuming and expensive, even in hospitals with sophisticated lab equipment. And in the poor countries where the infection is most prevalent, people often don't have access to this equipment. Researchers at Massachusetts General Hospital in Boston and Harvard University have now demonstrated that a handheld device can be used to count as few as 20 bacteria in a sputum sample in a half hour. They hope to develop the test into an inexpensive product that can be deployed for TB testing.

The bacteria counter is being developed by researchers led by Ralph Weissleder, director of the Center for Systems Biology and the Center for Molecular Imaging Research at Harvard Medical School, and Hakho Lee, an instructor at Mass General. The technology uses magnetic-nanoparticle labels and a detector that works on the same principles as magnetic resonance imaging. They're focused on tuberculosis, says Lee, because "even one bacterium can cause the disease, but at this point there is no easy way to detect the bacteria at high sensitivity."

The biggest problem with existing tests is that they are too slow, says Peter Katona, associate clinical professor of infectious diseases at the University of California, Los Angeles. The most accurate way to identify the infection is to grow a sample in the lab. But because TB grows slowly, that can take as long as six weeks. What's more, expensive culture equipment is typically not available in poor areas where the infection is prevalent.

The cheapest and fastest way to detect TB is a skin test that screens for an immune reaction. But such tests are not particularly accurate. "There are a number of conditions where there is no immune reaction" even if the patient carries the infection, says Steven Miller, director of the clinical laboratories at the University of California, San Francisco Medical Center. TB and HIV often go hand in hand, but in HIV-positive patients the skin test doesn't work. Another common test, staining a sputum sample with a dye that targets TB and examining it under the microscope, also has a high rate of false negatives. "Unless there is a very high load of bacteria, you can't pick it up," says Miller.

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.