A laser-tracking device has allowed researchers to follow bacterial movement with near-atomic resolution, which has led to a better understanding of how bacteria spread among cells. The group studied Listeria monocytogenes, which can cause a severe form of food poisoning and which usually evade immune response by hiding inside cells. The bacteria use proteins inside the host cell to form rocketlike tails. They then ride these tails to the cell wall, deform the wall and invade neighboring cells. Researchers at Johns Hopkins University created a device to track Listeria monocytogenes while inside a host cell. Studying their movement could help scientists understand how bacteria spread among cells. Scientists thought that these tails push bacteria in a continuous fashion, even on the molecular level. Testing this idea required tracking the bacteria's movement inside cells with near-atomic resolution. Biomedical engineering researchers at Johns Hopkins University developed a suitable technique by modifying the setup used for some optical tweezers. TInstead of using a medium-power laser to generate optical forces, they focused on the bacteria with a very weak index-guided diode laser from Toshiba in Irvine, Calif., and analyzed the far-field forward-scattered light with a quadrant photodiode detector. The device is on an inverted microscope manufactured by Carl Zeiss Inc. in Thornwood, N.Y., allowing two-dimensional tracking. The scientists found that Listeria do not move continuously on the molecular level, but instead move in a steplike fashion. Each step may correspond to the addition of individual protein building blocks to the tail. The researchers think the movement points to a molecular motor that may also push other infectious agents between cells. Their results are detailed in the Oct. 26, 2000, issue of Nature.