Lipopolysaccharide O-antigen of enterohemorrhagic Escherichia coli O157:H7 is required for killing both insects and mammals.

Studies of enterohemorrhagic Escherichia coli (EHEC) infection mechanisms using mammals require large numbers of animals and are both costly and associated with ethical problems. Here, we evaluated the pathogenic mechanisms of EHEC in the silkworm model. Injection of a clinically isolated EHEC O157:H7 Sakai into either the silkworm hemolymph or intraperitoneal fluid of mice killed the host animals. EHEC O157:H7 Sakai deletion mutants of the rfbE gene, which encodes perosamine synthetase, a monosaccharide component synthetase of the O-antigen, or deletion mutants of the waaL gene, which encodes O-antigen ligase against the lipid A-core region of lipopolysaccharide (LPS), had attenuated killing ability in both silkworms and mice. Introduction of the rfbE gene or the waaL gene into the respective mutants restored the killing ability in silkworms. Growth of both mutants was inhibited by a major antimicrobial peptide in the silkworm hemolymph, moricin. The viability of both mutants was decreased in swine serum. The bactericidal effect of swine serum against both mutants was inactivated by heat treatment. These findings suggest that the LPS O-antigen of EHEC O157:H7 plays an important defensive role against antimicrobial factors in the host body fluid and is thus essential to the lethal effects of EHEC in animals.