Monday, November 22, 2010

Monitoring Yeast Cell Growth in the Presence of Ethanol

The yeast Saccharomyces cerevisiae is a single celled organism used in numerous tasks. The fast growing and unicellular nature of the yeast along with its asexual reproduction capabilities make it an ideal model to study the biology and genetics of eukaryotic cells. Besides its importance in scientific research yeast play an essential role in the baking of bread, the fermentation of alcohol for consumption and for biofuels.

Many of the assays employed with yeast involve variant or mutant strains the exhibit differences in growth under selective conditions. For example mutations that cannot grow without certain nutrients can be identified. Ethanol resistance to growth inhibition is another important parameter for the production of alcohol biofuel or the fermentation of beer and wine. Because individual yeast clones can be grown, one can identify unique strains that exhibit the desired growth characteristics in microplate.

Cell suspensions (bacterial or eukaryotic) result in light scatter when a light beam is passed through them. Using a spectrophotometer or a microplate reader this light scatter is detected as an increase in absorbance. While not strictly obeying Beer’s law, this relationship can still be exploited to monitor cell growth using absorbance. We have used the Synergy H1 Multi-Mode Microplate reader to monitor the growth of beer strains of yeast under different concentrations of ethanol.

Effect of Ethanol Concentration on the Growth of Strain 2000 Budvar lager beer yeast.
Figure 1. Effect of Ethanol Concentration on the Growth of Strain 2000 Budvar lager beer yeast.
By following the growth of yeast cells in suspension, the effect of environmental conditions such as ethanol concentration can be monitored (Figure 1). The log-phase of yeast cell growth is linear with respect to the change of absorbance (cell number) per unit of time for a given ethanol concentration. As such, changes in the slope as a result of ethanol concentration can be plotted for individual strains and compared (Figure 2).

Figure 2. Effect of ethanol on log-phase growth rate.
Figure 2. Effect of ethanol on log-phase growth rate.

These data demonstrate the utility of the Synergy H1 to monitor kinetically yeast cell growth in the presence of ethanol. Yeast cultures require constant temperature along with adequate agitation in order for consistent repeatable results. Besides providing adequate mixing of the cellular nutrient suspension for cellular growth, maintenance of a uniform suspension allows for accurate light scatter measurements. The Synergy H1 has a number of different shaking features including linear, orbital and double or figure eight shaking modes. Each mode can also be adjusted for speed and amplitude, providing a number of different mixing options for different materials.

Do you measure yeast cells in suspension? Do you have a need for a microplate reader?

By, BioTek Instruments, Paul Held,Ph.D, Principal Scientist

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