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The pathogen enterotoxigenic E. coli (ETEC) was first described less than 50 years ago and is now recognized as a major global cause of morbidity and mortality due to diarrhea. Molecular studies of pathogenicity of this pathogen have however lagged behind compared to other enterobacterial pathogens such as EHEC, UPEC, Shigella spp, Salmonella spp and Yersinia spp. We have previously described a novel bile induced virulence regulon in certain ETEC subtypes and performed whole genome sequencing of the major clonal virotypes/lineages in ETEC. During studies of large virulence plasmids in ETEC lineages we found a virulence operon previously described in Shigella spp, EIEC, and EHEC that has not been described before in ETEC. Interestingly, this operon is linked to outer membrane modification and envelope stability and proposed to aid in invasiveness and intracellular survival, but ETEC is not considered to be invasive. Instead, we hypothezize that the ability to withstand envelope stress is important for ETEC to survive immune responses and environments in the host, as well as to persist in environmental conditions such as during water transmission. We will characterize the operon and several other newly discovered plasmid-borne putative virulence factors by use of microbiology studies, genetic modification, transcriptomics and proteomics with the hypothesis that presence of the new virulence traits determines why certain ETEC clones are so successful and able to spread globally.