UC San Diego

Most of the bacteria we encounter everyday do us no harm and many are even beneficial. Those that do cause human disease must be able to get past our initial immune defenses, establish an infection site and find proper nutrition to continue to proliferate. These disease- causing bacteria produce gene products, known as virulence factors, to aid in these pathogenic processes. Pore- forming toxins (PFTs) are among the most common and important virulence factors that bacteria can make. These toxins work by inserting into the membrane of a host cell to form a pore. Pneumolysin from Streptococcus pneumonia and streptolysin S made by Streptococcus pyogenes are examples of PFTs that are important for virulence. Despite their prevalence, it is often unclear how these toxins affect the host and contribute to virulence. The agriculturally important bacterium Bacillus thuringiensis produces a family of PFTs, called Cry toxins. Using a nematicidal Cry toxin, Cry5B, and the nematode Caenorhabditis elegans, I characterized the host response to a PFT at the genomic level. These studies indicated that C. elegans responds quickly to Cry5B exposure in a manner that is qualitatively distinct from other forms of stress response. The host response data also lead to the identification of two signal transduction pathways, the p38 MAP kinase cascade and a JNK-1ike MAP kinase, that orchestrate toxin defenses. I then carried out a study to identify the transcriptional targets of both the p38 and JNK pathways in C. elegans. The p38 targets were screened genetically for those that play a role in Cry5B defense. This investigation lead to the discovery of the ttm gene class, toxin-regulated targets of MAP kinase. I went on to find that toxin exposure also induces activation of the p38 pathway. To find some of the genes involved in this Cry5B-regulated activation, I performed a genetic screen in the C. elegans host. This screen identified a ceramide glucosyltransferase that is important for the toxin- regulated activation of p38 and is also mediates host protection to this toxin. Therefore, with respect to Cry5B response and defense, I identified both upstream and downstream components of the p38 MAP kinase pathway in C. elegans