Abstract: The co-cultivation of a strain of lactic acid bacteria and a strain of yeast isolated from koumiss in Ximeng region of Inner Mongolia was studied. The best fermentation conditions of two-strain system were preliminarily explored. The optimal conditions for maximum viable cell count of lactic acid bacteria were shake culture at 30 ℃ for 12 h and then stationary culture at 37 ℃ for another 20 h in a medium based on skim milk with the addition of 1 g/100 mL peptone, 0.5 g/100 mL sucrose and 0.5 g/100 mL yeast extract powder, while maximum viable cell count of yeast was achieved after stationary culture at 37 ℃ for 8 h and then shake culture at 30 ℃ for another 32 h in skim milk with the addition of 0.5 g/100 mL peptone, 0.5 g/100 mL sucrose and 0.5 g/100 mL yeast extract powder. The inoculum was a mixture of lactic acid bacteria and yeast at a ratio of 1:1 (m/m). In addition, the effect of secondary fermentation on the symbiotic interaction between the two species of microorganisms was explored under the optimal growth conditions. Active substances promoting the growth of lactic acid bacteria were formed during the first 12 h of shake culture of yeast in medium No. 5 at 30 ℃, while those promoting the growth of yeast were formed during the first 16 h of stationary culture of lactic acid bacteria in medium No. 1 at 37 ℃. During the secondary fermentation, the viable cell counts of yeast and lactic acid bacteria in co-cultures were both significantly higher than those in pure cultures (P ＜ 0.01).

Key words: lactic acid bacteria, yeast, interaction