A few years ago, Christopher H. Contag attended a meeting at which investigators presented data obtained with the latest techniques for detecting viruses. As he listened to each speaker, the Stanford University virologist realized how frustrated he had become with the tools available to his profession. Consider the way researchers usually establish the normal course of a bacterial or viral infection, remarks Contag. They infect dozens, if not hundreds, of animals, usually mice. As the infection proceeds, the investigators kill groups of the mice and, using a variety of means, painstakingly examine the animals' tissues to determine how far the microorganisms have spread. Such experiments are costly and laborious; they also raise the ire of animal rights activists. Contag felt there had to be a better way. If we can track the trajectory of a bullet in space, we must be able to track a pathogen in a living animal, he says. Contag began to take a look at bioluminescence, the natural emission of light by organisms such as fireflies and certain bacteria. These creatures all owe their glow to an enzyme called luciferase. For more than a decade, scientists have made use of luciferase or the genes that encode it. For example, with the aid of this light-producing enzyme, they've discovered and studied the genes that establish circadian rhythms (SN: 8/12/95, p. 108). Pamela Contag, also at Stanford, has popped luciferase genes into salmonella bacteria to better study how the microbes interact with animal cells. After reading an article about detecting light that passes through tissue, Christopher Contag wondered if bioluminescent microorganisms inside living animals might be observable from the outside. Fortunately, David Benaron, a medical imaging researcher at Stanford, was nearby. virologist strolled over to Benaron's office and outlined his proposition. thought it was an unusual mixture of ideas. But I have to tell you my first reaction was, that's impossible, says Benaron. Benaron's main concern was that the greenish yellow glow emitted by the standard luciferase-induced chemical reaction would be mostly absorbed and scattered by the animal tissues. That reservation didn't stop him from agreeing to give the idea a try, however. Benaron and the two Contags decided to test first whether bioluminescent microbes could shine through dead tissue. The easiest source of tissue was the grocery store, so we picked up chicken breast, beef liver, and lamb kidney, laughs Christopher Contag. In their initial experiment, the investigators placed a vial of Pamela Contag's glowing salmonella inside a thawed chicken breast. bacterial light had to penetrate about half a centimeter of meat to reach a highly sensitive photon detector. Sure enough, light came out, says Benaron. That glimmer has since led to what hntirrininaict (.nrr1nn qtiwnrt of the I Tniversity of Nottingham in England calls some of the most important work in microbiology in a decade.