This invention provides a purified extracellular bacterial autoinducer-2 signaling molecule, the production of which is regulated by changes in environmental conditions associated with a shift from a free-living existence to a colonizing or pathogenic existence in a host organism. The signaling molecule stimulates LuxQ luminescence genes, and is believed also to stimulate a variety of pathogenesis related genes in the bacterial species that produce it. This invention also provides is a new class of bacterial genes involved in the biosynthesis of the signaling molecule.