Researchers from the University of Michigan have managed to test drug susceptibility in individual bacteria. Because there is no need for the bacteria to grow into a visible colony before testing, results can be obtained in minutes, rather than in days. The scientists, led by Raoul Kopelman, managed this feat by bypassing the need to see the bacteria with a microscope.
Currently, in order to tell what sort bacterial infection a person has, doctors have to allow the microorganisms time to grow into a visible culture. This can take hours or even days. Testing whether a culture is susceptible to a particular drug can take just as long as researchers wait to see whether bacteria have died or stopped dividing.
Rather than trying to observe bacteria through a microscope, Kopelman and his colleagues used a device called an asynchronous magnetic bead rotation (AMBR) sensor. Simply put, the scientists attach a single bacterium to a spinning magnetic bead. If the bacterium grows (divides in two), the increased drag slows the bead down. On the other hand, if the bacterium dies (after administration of a drug, for example), the bead speeds up. The changes in speed can be detected for size differences as small as 80 nanometers, one third the limit of the most powerful light microscope and easily small enough to register the addition or subtraction of a single bacterium.
You can watch the following animation to see how this works:
The researchers expect this technique to be useful not only for identifying and treating bacterial infections, but for treating cancer as well. After all, cancer cells can be attached to an AMBR sensor and subjected to a variety of drugs and treatments just as well as bacteria can.