That is the question I wanted to answer ! Not so easy … Because first of all, you have to know what an endospore is ! An endospore is a heat resistance structure that forms within certain cells. It allows some cells to survive under stress conditions !
Then, I had to choose only two bacteria, among all those that exist … Not an easy choice ! After some hard hours of thinking, I choose Bacillus subtilis, a beautiful little bacterium that has the superpower to grow spores and Escherichia coli, another wonderful bacterium not able to form spores.
Now that I have my two protagonists, I have to be sure that some endospores form on B. subtilis. For that, I took some of those bacteria and decided to stain them, so that they become pink and the endospores become green. With colors, it is easier to distinguish the endospores ! Once I did the staining, I observe it under the microscope. What did I see !? Nothing … There was no green little thing … only pink bacteria … no endospores !
I
was sad but I decided to move on : it’s not because there is no
endospores that I cannot do my experiment ! Therefore, I did some
overnight cultures : my bacteria could grow all night, and I will
observe them in the morning. Then, when I woke up, I took a few amount
of the overnight cultures to let them grow for another 6 hours in
erlenmeyers, at 37°C, their optimum temperature to live, and 41°C, a
higher temperature for them to stress and form endospores. After the 6
hours waiting, I measure the absorbance, with a spectrophotometer, to
observe their growth rate. With my data, I manage to plot this graph :
 |
| First graph is at 37°C and the graph below is at 41°C |
On this graph, nothing was happening like I thought it would … I saw that E. coli was growing at 41°C even though it should die and that B. subtilis was dying at 41°C even though it should be resistant. Everything was upside down ! Also, the growth rate was near to 0 while the bacteria were suppose to grow…
I still had to draw some conclusions. I think the bacteria are not growing because I had to let them grow for 6 hours in order to the endospores to form. Therefore, when I take my measures, they might already be in their stationary phase, which is why we cannot see any growth. I also had some material problems to put the bacteria at 41°C which could be the reason why E. coli are still growing at this temperature. Also I am not sure that the endospore form in B. subtilis, which could explain their die at 41°C. Maybe next time I should first be sure that endospores form, by stressing B. subtilis, by undernourishment.
If you want to know more, please take a look at the poster I did : https://docs.google.com/document/d/1Ju9UHWRhJi3lInW2Xy_V-iADRkeao3flViloPTj1NFU/edit
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