The production of a purified extracellular bacterial signal called autoinducer-2 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. Autoinducer-2 stimulates LuxQ luminescence genes, and is believed also to stimulate a variety of pathogenesis related genes in the bacterial species that produce it. A new class of bacterial genes is involved in the biosynthesis of autoinducer-2.