Escherichia coli strains, such as enterotoxigenic E. coli strains, genetically engineered to express from recombinant plasmids one or more colonization factors (CFs) associated with enterotoxigenic Escherichia coli bacteria (ETEC) in an increased amount compared to said CFs expressed by ETEC wild-type reference strains, as well as a method of producing such strains, and vaccine compositions against diarrhea comprising such strains, are described. Further, E. coli strains expressing unnatural combination of at least two different CFs, e.g., CFA/I+CS2, CFA/I+CS6, or CS2+CS4 are disclosed.
