Our laboratory is using whole genome approaches to investigate the molecular basis of microbial pathogenesis. We are particularly interested in elucidating those aspects of pathogenesis that are similar irrespective of the pathogen and the host.
The first project involves multi-host pathogen models that utilize human bacterial pathogens that are not only infectious in mammals, but also in plants, nematodes, or insects. The overall goal of this project is to identify previously unknown virulence-related genes in bacterial pathogens and to identify host genes that are involved in the defense response against pathogenic microorganisms. For example, we have identified a Pseudomonas aeruginosa isolate that is not only infectious in several mouse models, but also causes disease in the model plant Arabidopsis thaliana and in the insects Drosophila melanogaster and Galleria mellonella (wax moth caterpillar). PA14 also kills the nematode Caenorhabditis elegans. Clinical isolates of Salmonella typhimurium, Staphylococcus aureus, and Enterococcus faecalis also kill C. elegans. From the perspective of the pathogen, the advantage of using model non-vertebrate genetic hosts is that thousands of bacterial clones from a mutagenized library can be individually screened for avirulent mutants in separate host organisms. From the perspective of the host, the advantage of using model genetic organisms is that host genetic analysis can be used to identify host genes involved in pathogen defense by screening for more susceptible or more resistant mutants. To date we have identified a total of 32 P. aeruginosa genes, mutations in which result in a decrease in virulence in at least one of the non-vertebrate hosts and most of these mutants are also less pathogenic in mice. Remarkably, 16 of these 32 P. aeruginosa genes correspond to previously unknown proteins. From the host perspective, we have isolated a set of A. thaliana and C. elegans mutants that exhibit enhanced susceptibility or enhanced resistance to various pathogens. Specifically, we have shown that highly conserved MAPK signaling cascades are involved in the innate immune responses in both A. thaliana and C. elegans as they are in mammals. We have also studied a set of existing C. elegans mutants for susceptibility to PA14 killing and using these mutants we have demonstrated that a small molecule (a phenazine) excreted by P. aeruginosa plays a key role in C. elegans killing, most likely by exposing C. elegans to high levels of oxidative stress.
The second project concerns the identification of genes in the model plant Arabidopsis thaliana that are involved in the host response to the bacterial pathogen Pseudomonas syringae or to the fungal pathogens Erysiphe orontii, Botrytis cinerea or Fusarium oxysporum. The long-term goal of this project is to elucidate the mechanisms involved in pathogen recognition and how the recognition signals are transduced causing the elicitation of programmed cell death and the transcriptional activation of a battery of defense-related genes. Arabidopsis has several advantages as a model system, the most important of which is that a complete genome sequence is available, greatly facilitating the cloning of genes that have been identified by mutation. We have adopted both genetic and genomic strategies to dissect the mechanism of the Arabidopsis defense response to pathogen attack. Specifically, we have identified and are now characterizing a large set of Arabidopsis mutants that either exhibit enhanced disease susceptibility (eds mutants) or enhanced disease resistance (edr mutants) to P. syringae or E. orontii. This has resulted in the identification of at least 25 EDS and EDR genes, and many more remain to be discovered. In parallel, we are using DNA microarray technology to identify Arabidopsis genes that are activated in response to pathogen challenge and that
