Powerful screens for bacterial virulence proteins

Abstract

Many mammalian and plant pathogenic bacteria inject virulence effector proteins into host cells by means of the type III secretion system (TTSS) (1). Effector proteins attack the host innate immune system, modify cytoskeleton and membranes, or alter vesicle trafficking (2, 3). The collective action of these proteins promotes bacterial entry into, growth and movement within, and dissemination from infected host cells/tissues. The full complement of TTSS effectors is not known for most bacterial pathogens; their identification remains a crucial step toward a comprehensive understanding of bacterial pathogenesis, host range, and pathogen evolution. In a recent issue of PNAS, Chang et al. (4) describe a powerful method for discovering TTSS effectors. Using this method, those authors identified two nearly complete repertoires of TTSS effectors from two pathovars (specific to different plant species) of the plant pathogen Pseudomonas syringae . Here, we discuss this article in the context of earlier efforts to discover TTSS effectors in plant pathogens. In both mammalian and plant pathogenic bacteria, the initial method for identification of proteins secreted through the TTSS was to monitor extracellular proteins secreted in bacterial cultures in a TTSS-dependent manner (5–7). Unfortunately, the amounts of TTSS effector proteins secreted by plant pathogenic bacteria, although detectable by antibodies, are too low to be useful for systematic identification of effectors. A breakthrough came in 1996 with the discovery that bacterial “avirulence” ( avr ) genes encode TTSS effector proteins that function inside the plant cell (reviewed in ref. 8). These avr genes had previously been cloned based on their ability to convert virulent strains into avirulent ones in specific plant genotypes (9). This breakthrough, besides significantly expanding the number of known TTSS effectors in several bacteria, provided an important clue that would prove …