Matt First, a postdoctoral scientist at Woods Hole Oceanographic Institution, collects water samples from Wood Neck Beach in Falmouth, Mass., searching for microbial "Trojan horses" in the ocean. These are bacteria that can resist being digested by the single-celled organisms that eat them. Instead, the bacteria persist within the organisms, biding their time until they are released back into the environment, much as the ancient Greeks did during the Trojan War. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)
Biologist Matt First uses fluorescent green stain (in sample tubes at right) to "tag" bacteria ingested by single-celled organisms. The green color allows him to easily track how they are digested—or in some cases, not digested. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)
Under a microscope, green-stained bacteria are quite visible within a live single-celled ciliate that has ingested the bacteria.
This electron micrograph shows an amoeba, Hartmannella vermiformis (orange) using an extended pseudopod ("false foot") to entrap a Legionella pneumophila bacterium (green). After it is ingested, the Legionella pneumophila bacterium, which causes Legionnaires' disease, can survive within the amoeba, which then becomes a "Trojan horse" that harbors the pathogen. (Image courtesy of the Centers for Disease Control and Prevention and Dr. Barry S. Fields)