For the Thermal Wars final project, our group was interested by pigments, temperature and yeast. Yeast are unicellular microorganisms of the fungus kingdom. They are used in baking or in alcohol beverages. If the conditions allow it (temperature, pH…) and if they have nutrients, cells grow and divide over time. They can be different colors according to their pigments. Different colors absorb and reflect light in different ways, and this can be measured thanks to a spectrophotometer.
There are three main phases in cells’ growth (lag phase, exponential growth phase, stagnation phase), and we had already studied the exponential growth phase during the week. After stagnation, the cells start to die. Eventually, they multiply again, and the cycle starts over (if the conditions are favorable to their division).
Our teacher Tamara had different colored strains of yeast, and we knew that yeast are particularly resistant during stagnation. Therefore, we combined all these elements to come up with an experiment that interested us all : How does temperature and pigmentation of yeast influence the cell concentration at stagnation phase?
Indeed, we were wondering if pigments or temperature have an influence on cell concentration at stagnation phase, and how these factors were interwoven. This research could help if you need to pick yeast strains (for a study at stagnation phase) in function of their color.
To answer this question, we used two methods : microscopy and spectrophotometry. We used two methods because it is better to base your study on several and different manipulations in case the tendencies vary between each method. (Also, Tamara said we were smart enough to do both).
So we got to work: we made our 11 different colored yeast cultures multiply at three different temperatures, 25°C, 30°C and 35°C. We waited overnight so they would be all grown up and then got to measuring. For both methods, we needed the cells to be a little less concentrated to actually be accurate so we made dilutions. We then counted the cells under a microscope and used the spectrophotometer to measure the optical density (which is supposed to be closely related to cell concentration).
Our results
Using those two methods, we analysed the data, and plotted those two graphs:
These were some of our results: the graph to the left represents the microscopy method, where we simply counted cells in a small sample of the culture. In that graph, we’re representing the cell concentration in function of temperature and we can see that as the temperature increases between 25 and 25°C, the concentration of the yeast increases as well.
To the right, we represented optical density (the measures taken with the spectrophotometer) in function of temperature and here we see a difference compared to the actual cell concentration: for 30°C, the optical density is much lower! We weren’t expecting those results since optical density is supposed to be a good indicator for cell concentration and thought that both the left and right graph would look almost identical.
Conclusion
From our results, we could conclude that proceeding through several methods is always safer. Our results did seem to indicate that for lower temperatures, the cell concentration at stagnation phase were lower than at higher temperatures, however there isn’t much else we could conclude. To make sure we had accurate results, we wanted to simply reproduce the experiment as much as we could. We think that optical density might also be influenced by the color of the strains, this would make its relation to cell concentration flawed!
With that in mind, we thought we would reproduce the same experiment but with a different goal: to evaluate the efficiency of spectrophotometry for different colored cells!
If you want to know more:
A nice article summing up the growth phases of yeast: http://capricorn.bc.edu/wp/pathways/biology-bootcamp/yeast-techniques/#growth
A brief course on cells and the dynamics of growth: http://www.columbia.edu/cu/biology/courses/c2005/lectures/lec1_11.html
Another course on bacterial growth & generation time: http://study.com/academy/lesson/what-is-bacterial-growth-generation-time-curves-phases-stages.html
A scientific article on the stationnary phase of the bacterial life cycle: https://www.ncbi.nlm.nih.gov/pubmed/8257118
A wikipedia article on Bacterial Growth : https://en.wikipedia.org/wiki/Bacterial_growth
Another scientific article on the dynamics of Saccharomyces cerevisiae’s growth: https://www.sciencedirect.com/science/article/pii/0168165692901508
To go further into the study of the factors influencing cell concentration at stationnary phase: http://schaechter.asmblog.org/schaechter/2013/02/bacterial-antidepressants-avoiding-stationary-phase-stress.html
Our presentation of the project: https://docs.google.com/presentation/d/1lwf07pkZp9DUC3q728xI5gc8gLztR-amWk84gNtI9nU/edit#slide=id.gc6f90357f_0_0
June VK, Selim BS & Jacques CK
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