To test the possible killer sensitivity of each strain, 100mL of a suspension containing 1×〖10〗^8 cells/mL were spread as a lawn on the surfaces of two plates containing YPD-MB agar (1% yeast extract, 2% peptone, 2% glucose, 2% agar, 0.003% methylene blue,buffered at pH 4.2 with 0.2 M citric acid and 0.2 M potassium biphosphate). The reference killer-positive strain ofS. cerevisiae CECT 1407 was seeded on the plate by the streak plate technique. To test the possible killer activity of each strain, 100mL of a suspension containing 1×〖10〗^8cells/mL of the reference killer-sensitive strain S. cerevisiae CECT 1893 were spread as a lawn on the surfaces of two plates containing YDP-MB agar. The tested strain was then seeded by the streak plate technique. Plates were incubated for 72 h at 28 C. A clear zone of growth inhibition and/or a region of bluish cells surrounding the streaked yeast culture indicated a killerpositive strain. Cross-inhibition was tested by a similar method. Possible killer activity and sensitivity of each strain compared to all other strains were tested. All assays were performed in duplicate.