Escherichia coli O157:H7 is an intestine-inhabiting bacterium associated with many severe disease outbreaks worldwide. It may enter the soil environment with the excreta of infected animals (including horses, cattle and chickens) and human. Earthworms are able to protect themselves against invading pathogens due to their efficient innate defense system. To better understand the mechanisms earthworms (Eisenia fetida) utilize to defend and eliminate Escherichia coli O157:H7, we examined the changes in the growth rate, as well as antioxidant and antimicrobial functions of earthworms in response to various concentrations of Escherichia coli O157:H7 in an artificial soil. Our results show that earthworms can inhibit the growth of Escherichia coli O157:H7. Upon exposure to Escherichia coli O157:H7 from 0 d to 24 d, coelomic cytolytic factor (CCF) in earthworms is induced, the reactive oxygen species (ROSs) and superoxide dismutase (SOD) are also elevated. Under lower bacterial concentrations (10(5)-10(6)CFU g(-1)), these ROSs can be rapidly scavenged by superoxide dismutase (SOD) to avoid peroxidation damage, but when the bacterial concentrations are high (10(7)-10(8)CFU g(-1)), excess amount of ROSs then cause accumulation of lipid peroxidation molecular malondialdehyde (MDA). In addition, exposure to Escherichia coli O157:H7 can induce the gene expression of antimicrobial peptide lumbricin I in the tissues of the earthworms. In conclusion, antioxidant systems and antimicrobial immune function may play important roles in the defense of the earthworms Eisenia fetida against Escherichia coli O157:H7.