Upon infection, pathogenic bacteria must evade the immune defence of their host in order to multiply. To this end, many bacteria secrete toxins as part of their virulence mechanism. In a classical view, toxins are molecules that cause intoxication upon their release by bacteria into the body fluids. However, in the last 10 years a different class of bacterial toxin has been recognised. These molecules are not simply secreted by the bacterium, but instead they are delivered directly from the bacterial cytoplasm into the cytoplasm of the eukaryotic cell by specialised secretion machines present exclusively in Gram-negative bacteria. These are the so-called type III or type IV secretion systems, depending on whether they use a structure resembling the flagella or conjugative pili, respectively. In this chapter, we will describe the mode of action of toxins delivered by type III secretion systems (TTSSs). These molecules, currently known as type III effectors, have been shown to act on different host signalling pathways controlling a number of responses, and in some cases interfere with cell growth.

TYPE III SECRETION SYSTEMS

TTSSs are present not only in bacteria that are pathogenic for animals but also in bacteria pathogenic for plants or even in symbionts for plants and insects (Cornelis and Van Gijsegem, 2000). We will restrict our analysis to the action of type III effectors of animal pathogens. Among these, type III effectors have been identified in Yersinia spp., in Salmonella spp., in Shigella spp., in enteropathogenic and enterohaemorrhagic Escherichia coli, in Pseudomonas aeruginosa, and more recently, in Burkholderia pseudomallei (Stevens et al., 2003).