Portrait of Tom Brock, US microbiologist and discoverer of the bacterium Thermus aquaticus (TaqI). The bacterium is found in hot springs, and so is a good source of heat stable DNA polymerase. This is used in the high-temperature polymerase chain reaction (PCR), a method for amplifying DNA samples in vitro. The TaqI PCR has many advantages over previous methods which used E.coli DNA polymerase. Photographed in Yellowstone Park, USA.