Don't be fooled: Yeast have two growth spurts!

by Cameron Davidson-Pilon

Did you know that the traditional S-shaped growth curve in microbiology is terribly inaccurate? It's inaccurate for many reasons, but the reason I want to discuss today is that some microbes have multiple growth spurts - like two "S"s ontop of each other. For example, when there are two sources of sugar in the media, the microbes will first consume the more accessible sugar, and then after a pause to reconfigure their metabolism, and then start to consume the second source of sugar. This leads to two growth spurts, and hence two "S"s. 

Interestingly, yeast have another trick up their sleeves (cell membranes?). The consumption of sugar, like glucose, gives yeast a large initial spurt of growth, as you can see in the below figure (the output of our Pioreactor). As a byproduct, the yeast expel alcohol, specifically ethanol. 

After this consumption of sugar, the yeast are not done. After all the sugars are consumed, they take some time, reconfigure their internal metabolism, and start to consume the ethanol they just expelled! 
Let's zoom out further:
In the above, we can see this new "double-S" growth of the yeast culture. But this new growth is a lot less pronounced in the implied growth rate chart. Why is this? When the yeast consume the available sugar, the culture 5x its initial size (starts at 1, and goes to ~5 after 3 days). That implies a very high growth rate. However, when the yeast consume the ethanol, which has less available energy in it, the yeast are only able to 2x their population size (from ~5 to ~10). Hence the implied growth rate is much smaller and flatter. 

This phenomenon is called the diauxic shift, and only some yeast and bacteria can achieve it. The ethanol is oxidized, which means that oxygen is needed for this second-stage of growth to occur. 

The above experiment was performed using our Pioreactor. The Pioreactor can help you study the impact of media, temperature, and other variables on the diauxic shift. Have fun! 

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