271Vol. 2, No. 4—October-December 1996 Emerging Infectious Diseases Synopses Discovery of Two Traits Used by a Broad

In the past decade, there has been an explosion of new information about bacterial pathogens as researchers have begun to examine the molecular and genetic bases of microbial pathogenicity. Because microbes invade many niches in humans and cause a wide variety of syndromes, it initially appeared that each disease might be created by a distinct molecular mechanism. However, the spectrum of methods is not as broad as first imagined; rather, bacteria exploit a number of common molecular tools to achieve a range of goals (1). Among these tools are pathogenicity islands, which enable bacteria to gain complex virulence traits in one step, and type III secretion systems, which provide a means for bacteria to target virulence factors directly at host cells. These factors then tamper with host cell functions to the pathogens ’ benefit. Early in the search for virulence genes, researchers discovered that many of these genes resided on plasmids or phages; however, it was also clear that these genes did not produce all of the physiologic changes induced in host cells by various pathogens (2). Thus, researchers searched the chromosome. Surprisingly, as when found on plasmids, virulence genes often clustered in functionally related groups. Furthermore, these groups often appeared to have been acquired from another organism, as features of their DNA sequence differed from the bulk of the genome. These observations gave rise to the concept of pathogenicity islands— discrete segments of DNA that encode virulence traits and often appear to have a foreign origin (3,4). Researchers found that a particular set of virulence genes appeared several times on both plasmids and pathogenicity islands (5-8). These genes were discovered in both plant and animal pathogens and were homologou