Shigellosis is a diarrheal disease caused by the gram-negative bacterium Shigella flexneri. Following ingestion of the bacterium, S. flexneri interferes with innate immunity, establishes an infection within the human colon, and initiates an inflammatory response that results in destruction of the tissue lining the gut. Examination of host cell factors required for S. flexneri pathogenesis in vivo has proven difficult due to limited host susceptibility. Here we report the development of a pathogenesis system that involves the use of Caenorhabditis elegans as a model organism to study S. flexneri virulence determinants and host molecules required for pathogenesis. We show that S. flexneri-mediated killing of C. elegans correlates with bacterial accumulation in the intestinal tract of the animal. The S. flexneri virulence plasmid, which encodes a type III secretory system as well as various virulence determinants crucial for pathogenesis in mammalian systems, was found to be required for maximal C. elegans killing. Additionally, we demonstrate that ABL-1, the C. elegans homolog of the mammalian c-Abl nonreceptor tyrosine kinase ABL1, is required for S. flexneri pathogenesis in nematodes. These data demonstrate the feasibility of using C. elegans to study S. flexneri pathogenesis in vivo and provide insight into host factors that contribute to S. flexneri pathogenesis.The gram-negative bacterium Shigella flexneri is the causative agent of shigellosis, a diarrheal disease which affects up to 150 million people annually (22). The study of shigellosis in vivo has been hampered by the lack of a suitable model system. Unlike humans, mice do not develop intestinal disease upon ingestion of S. flexneri. While recent developments of mouse models using newborn mice (17) and infection paired with intraluminal injection of interleukin-8 (34) are promising, their utility in the study of host cell factors involved in shigellosis remains to be determined.Although Shigella is believed to specifically infect primates, several reports indicate that invertebrates, such as flies and nematodes, may serve as vectors of the bacterium (5,12,16,25,28,41,42). These results indicate that the host range of the pathogen may be broader than is suspected and open the possibility of using invertebrates to study conserved hostpathogen interactions that can be translated to mammalian systems. Caenorhabditis elegans has become a well-established model invertebrate for the study of bacterial pathogenesis and innate immunity. As in mammals, peristalsis, low pH, lytic enzymes, and antimicrobial substances prevent microbial colonization of the C. elegans intestine. However, pathogenic bacteria are capable of proliferating and killing C. elegans using different mechanisms. The lists of bacterial pathogens that induce nematode killing include both gramnegative pathogens such as Salmonella enterica, Pseudomonas aeruginosa, and Serratia marcescens and gram-positive bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, and Enterococcus faecalis (...