Science Buddies: "Ask an Expert"

Hello. I am designing and experiment for my IB biology class. My plan is to test the effect coffee on yeast cells. I will be using saccharomyces cerevisiae, HAO Strain, a, wild type in my experiment. I will measure the yeast cell growth using a spectrophotometer. During my experiment, I plant to calculate the yeast cell growth at different intervals (i.e. 0,2,4,6,8 hours). In order to this as accurately as possible I want to use a fixative to inhibit cell growth.

What specific fixative should I add into my sample before using the spectrophotometer?

Thank you so much!

Hi hollymora314,

Thank you for your question! I like your ideas for your science project, it reminds me of a similar project I did with caffeine and bacteria

I don't think it's necessary to use fixative in your yeast samples at the different time points -- if the spectrophotometer readings are taken in a timely manner at each time point, the data should be accurate. Generally, fixatives are critical when doing staining/immunohistochemistry or if you plan to look at the yeast under a microscope (such as in electron microscopy or fluorescence microscopy). I would suggest using a fixative only if you plan to count the cells under the microscope (using a hemocytometer).

Also, I am not sure if this is in your experimental design already, but I suggest using different amounts of coffee and seeing if there is a dose-dependent effect of coffee on the growth of S. cerevisiae.

I hope I have been able to offer some help as you begin your science fair project. Please feel free to respond on this forum if you have any more questions or concerns.

-Brandi

Hello Brandi,

Thank you so much for your response! That helped me so much! I am planning on using different amounts of coffee and seeing the different effects. However, I still am trying to decide which brand of coffee and how much to put in each test tube.

Also, I have another quick question! I plan on growing my yeast in YPD media. When using the spectrophotometer, should the blank contain just YPD or should it also contain coffee? If it needs to contain coffee, will I have to change the blank depending on the concentration of coffee that are in the other test tubes?

Hopes this makes sense. Thank you so much!


Holly Mora

Hi Holly,

Do you plan to use instant coffee for your experiments? I suggest this as it will be easy to dilute in water, at different concentrations. Or, you could even add this straight to the cultivation media. Regarding the concentrations of coffee to put in your tubes, I would try 0 mg/ml, 1 mg/ml, 2 mg/ml, 5 mg/ml, 10 mg/ml, and 25 mg/ml of coffee per ml of media. I really have no idea about the effects of coffee on yeast -- I am finding several papers about caffeine, but not coffee.

For the blanks, you should use the exact media that your yeast are cultivated in. So, you should use YPD with varying concentration of coffee -- in other words, each concentration should have its own blank. You will also need a blank of YPD alone for the tubes that do not have coffee (controls). Remember to include both a negative control (YPD alone with no coffee or yeast) and a positive control (YPD with yeast, no coffee)!

Also, I want to offer a suggestion that I thought of as I was re-reading your original question. I don't think you should measure the effects of the coffee on yeast at certain time points (i.e. 2h, 4h, 6h, etc.). Instead, I think you should control the cultivation time; that way, you can more easily distinguish the effects of different coffee concentrations and determine if coffee has dose-dependent effects on S. cerevisiae. This paper talks about the growth curve for S. cerevisiae: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6235381/. If you wanted to and have the time, you could generate your own growth curve (in YPD media alone) by plotting time versus OD at 600 nm. That way, you can optimize growth time and control it throughout your experiments. Again, this is just a suggestion

I know this is a lot of information, but I hope it helps! Feel free to reach out if any more questions come up! Good luck!

--Brandi