 Like all lifeforms , new strains of E. coli evolve through the natural biological processes of mutation , gene duplication , and horizontal gene transfer ; in particular , 18 % of the genome of the laboratory strain MG1655 was horizontally acquired since the divergence from Salmonella . E. coli K @-@ 12 and E. coli B strains are the most frequently used varieties for laboratory purposes . Some strains develop traits that can be harmful to a host animal . These virulent strains typically cause a bout of diarrhea that is unpleasant in healthy adults and is often lethal to children in the developing world . More virulent strains , such as O157 : H7 , cause serious illness or death in the elderly , the very young , or the immunocompromised . 
