Salt and yeasts,
a deadly story ?
What is the influence of salt on yeasts growth rate ?
INTRODUCTION
Yeasts are eukaryotic, single-celled microorganisms which possess a semi-permeable membrane that expose them to the phenomenon of osmosis.
Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane in the direction that tends to equalize the solute concentrations on the two sides.
Consequently, if yeasts are introduced in a solution with a higher salt concentration than their own, they tend to give water to the solution and shrink.
HYPOTHESIS
In an hypertonic solution, yeasts would lose too much water to grow easily and eventually die. I wanted to show that the increasing of the salt concentration of the media would decreases the growth rate of the population of yeasts.
EXPERIMENTS
To test this hypothesis, I prepared 3 YPD medias with different NaCl concentrations (0%, 1% and 2% of NaCl) with 3 replicate for each concentration in order to compare how the populations of yeasts would grow in each one by measuring the optical density evolution of these population after letting them grow for 23h at 30°C.
RESULTS
Evolution of the optical density of the populations versus time
Three absorbance measurement were taken after letting the populations grow for 0h, 9h and 23h and plotted into this graph. We can clearly distinguish the growing (encircled area) and the stationary phase of each populations and it seems that their differences are slightly influenced by the amount of salt in the media. To see it more clearly, the growth rate of these populations were calculated and plotted into the following graph :
Growth rate of each populations versus the NaCl concentration of their media
We can observe that the populations of yeasts tend to grow faster in the media with a concentration of 0% NaCl than in the media with 2% NaCl, which agrees with the hypothesis. However, the populations of yeasts in the media with a concentration of 1% NaCl grow slower.CONCLUSION & PERSPECTIVES
Even if at first glance the results tend to agree with my hypothesis, they are not very relevant because I don’t have enough data to conclude. To further improve this experiment, much higher salt concentrations would be tested in order to reach the point where the the yeast cells would die by osmosis and more measurements could be taken to be able to calculate more strictly the growth rate of the populations in the differents media. Finally, it would be interesting to observe the changes in the size and shape of the yeast cells to see if they shrink as expected.
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