Burkholderia LPS O-antigen-deficient bacteria had a reduced colonization rate in the host midgut. In vitro survival rates of three LPS O-antigen-deficient mutants were examined. A specific midgut contains substance killing LPS O-antigen-deficient mutants. 17 kDa trypsin inhibitor has antibacterial activity against the LPS O-antigen-deficient mutant.