Abstract: The effects of antibacterial compounds produced by Saccharomyces cerevisiae in Koumiss on pathogenic Escherichia coli O₈ and its cell surface characteristics were investigated.  S. cerevisiae isolated from Koumiss produced antibacterial compounds which were active against pathogenic E. coli O₈ as determined by the Oxford cup method.  The aqueous phases from S. cerevisiae at pH=2.0 (S2) and pH=8.0 (S8) were extracted and tested, respectively.  The organic acids of S2 and S8 were determined by high performance liquid chromatography (HPLC), and the concentrations of killer toxins were determined by enhanced bicinchoninic acid (BCA) Protein Assay Kit.  The minimum inhibition concentration (MIC) and the minimum bactericidal concentration (MBC) of S2 and S8 on E. coli O₈ were determined by the broth microdilution method.  The effects of S2 and S8 on the growth curve of E. coli O₈ were determined by turbidimetry, and the hydrophobicities of E. coli O₈ cell surface were determined using the microbial adhesion to solvents method, the permeation of E. coli O₈ cell membrane were determined by the o-nitrophenyl-β-D-galactoside (ONPG) method.  Aqueous phases at pH 2.0 and 8.0 had larger inhibition zones and then S2 and S8 were obtained by freeze-drying.  The main component in S2 was citric acid and it was propanoic acid in S8.  Other organic acids and killer toxins were also present.  Both the MICs of S2 and S8 on E. coli O₈ were 0.025 g mL^–1, the MBCs were 0.100 and 0.200 g mL^–1, respectively.  The normal growth curve of E. coli O₈ was S-shaped, however, it changed after addition of S2 and S8.  E. coli O₈ was the basic character, and had a relatively hydrophilic surface.  The hydrophobicity of E. coli O₈ cell surface and the permeation of E. coli O₈ cell membrane were increased after adding S2 and S8.  The present study showed that S2 and S8 inhibit the growth of pathogenic E. coli O₈ and influence its cell surface characteristics.

Key words: antibacterial compound, Saccharomyces cerevisiae, Escherichia coli, hydrophobicity, permeation