Provided are: a method for blocking the biosynthesis of an outer membrane protein (OMP), which is a protein required for the survival of bacteria, by inhibiting the formation of a YaeT complex in an outer membrane of a gram-negative bacterium and a substance for use in the method, for the purpose of drastically solving the problem of the acquisition of multi-drug resistance of a gram-negative bacterum. Specifically disclosed is an anti-gram-negative bacteria agent which is characterized by being capable of inhibiting the formation of a YaeT complex to thereby exhibit a bactericidal activity, a proliferation-inhibiting activity and/or a drug excretion-inhibiting activity on gram-negative bacteria. The agent is preferably a peptide molecule comprising an amino acid sequence composed of at least LTLR or a peptide molecule comprising an amino acid sequence composed of at least FIRL.
