Mixed Bacterium and Yeast Interaction

Yeast is one of the most important organisms for industrial use in alcoholic fermentation. Bacterial contamination may occasionally occur during yeast fermentation, which may pose serious problems for the fermentation industry. This section deals with the interaction of Saccharomyces cerevisiae with Acetobacter suboxydans or E. coli in mixed-culture growth. S. cerevisiae was grown in mixed cultures with either A. suboxydans or E. coli in a culture medium and glucose at 28 °C. Growth was monitored by viable cell counts of yeast and bacteria using selective agars, direct count by microscopic reading of cells in a Petroff-Hauser counting chamber, and electronic counts by the Coulter counter. By selecting the appropriate threshold windows, the electronic instrument can differentiate yeast count and bacterial count in the same liquid. During the growth period, pH values were monitored by a conventional pH meter and glucose was determined by the Glucostat, a commercial kit designed to detect glucose in solution. A vinometer was used to measure alcohol level of the solution.

Figure 4 illustrates the interaction between S. cerevi-siae and A. suboxydans as reported by Fung et al. (39). For the yeast, both direct count and electronic count increased with time. The direct count registered about 10 times more cells than the electronic count. Because A. suboxydans does not grow well in solid agar medium, viable cell count data were not obtained. For A. suboxydans, the direct count increased with time, but the electronic count increased to about log 7.8 mL-1 and then started to decrease. The decrease in number is due to the adhesion of many of the bacterial cells to yeast cells. The electronic count cannot differentiate these two populations in this condition. However, microscopic observations can ascertain the different counts. The concentration of glucose completely disappeared in the first 10 h of yeast and bacterial interactions. The pH of the medium first decreased and then stabilized at pH 3. This is because of oxidation of alcohol (produced by S. cerevisiae) by A. suboxydans to acetic acid, making the reaction mixture acidic.

The interactions of yeast and E. coli (39) in Figure 5 show interesting contrasts compared with the yeast-Acetobacter interaction. Growth of yeast as monitored by viable cell count, direct count, and electronic counts all increased with time. After reaching a stationary phase, viable yeast count decreased, but direct count and electronic count registered no reduction in numbers. A likely explanation is that the former two biomass measurements counted both live and dead cells, but the viable cell count only registered living cells. After the stationary phase, some of the yeast cells went through the death cycle. The growth curves of E. coli showed an increase in the viable

Figure 4. Interaction of mixed cultures of S. cerevisiae and A. suboxydans in terms of cell numbers monitored by direct count and electronic count as well as product formation (alcohol and acid production. Source: Reprinted with permission from reference 39, page 527, by courtesy of Marcel Dekker, Inc.

Figure 4. Interaction of mixed cultures of S. cerevisiae and A. suboxydans in terms of cell numbers monitored by direct count and electronic count as well as product formation (alcohol and acid production. Source: Reprinted with permission from reference 39, page 527, by courtesy of Marcel Dekker, Inc.

cell count and direct cell count. The electronic count reached log 8 mL_1 and then declined, exhibiting a trend similar to that observed in the yeast-Acetobacter interactions.

Alcohol contents increased to about 2%, and the pH first dropped to around 4 and then rose to 6. This pattern differs from that obtained from the interaction for yeast and Ace-tobacter. E. coli cannot oxidize alcohol and thus did not create large amounts of acid to counteract the basic metabolites in the reaction vessel. This resulted in a medium that reverted to a more alkaline state.

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