As a model host, the nematode Caenorhabditis elegans has been used for studying unknown pathogen-host interactions and identifying novel virulence factors in bacterial pathogens. Among the bacterial pathogens that can induce death of C. elegans is enterohemorrhagic Escherichia call (EHEC) O157:H7, a major serotype of EHEC that causes hemorrhagic colitis and hemolytic uremic syndrome in humans and animals. However, it is unknown which EHEC O157:H7 factors are required for nematode death. In this study, bacterial ability to kill C elegans was tested for several EHEC O157:H7 wild-type and mutant strains missing one virulence-associated factor, including Shiga toxins, enterohemolysin, pO157 (a large virulence plasmid in EHEC O157:H7), Type 3 secretion system, LuxS, and lipopolysaccharide (LPS) O-side chains. Our results demonstrate that only mutants lacking either pO157 or LPS O-side chains cause full attenuation in killing C elegans. The LPS O-side chain-defective Delta perA mutant strain was not able to colonize in the intestine even at 24 h post-feeding with C elegans, while the wild-type strain began to accumulate and colonize in the intestine as early as 3 h post-feeding. A simple complementation of the mutant strain with the plasmid carrying the intact perA gene in trans completely restored the production of LPS O-side chains, as well as the ability to kill C elegans. Our results show that pO157 and PerA are required for EHEC O157:H7 to kill C elegans. (C) 2013 Elsevier Inc. All rights reserved.