In everyday life, we use ethanol (a common
alcohol) to disinfect tools, sterilize our hands and even our wounds. However,
have you ever thought about what is the real effect of ethanol on the bacteria present
on your skin? For instance, we use hydro alcoholic solutions before going to
eat to avoid being contaminated by the germs we might have on our skin. I got
interested in Salmonella typhimurium
because it’s a microbe that causes several diseases like salmonellosis, gastro
enteritis or food poisoning. Then, I wanted to quantify the effect of ethanol
(and then of hydro alcoholic solutions) on the growth of a population of Salmonella typhimurium. The hypothesis
we can make at this time is that more we put ethanol in a bacteria solution;
more the speed of growth will decrease. We call this speed the “growth rate”.
In order to answer this question, I chose to
observe 5 different solutions along time of bacteria containing different
amounts of ethanol. When a colony grows in a liquid media, it makes the
solution blurrier because the density of cells increases when they divide.
Then, we can measure this density of cells thanks to a spectrophotometer which is a tool that sends a light ray through a
solution and measures how much of this light ray comes out of it. Thanks to it,
we can have the absorbance or optical density of the solution. Therefore, I
chose to measure the optical density of the 5 solutions (in which I added a 0%
ethanol solution, a 15% one, a 30% one, a 45% one and a 60% one) during 11h and
40min, allowing me to calculate the growth rate of each population. To improve
the accuracy of my results, I repeated the experiment 4 times (4 replicates).
This graph is representing my results. Each
line of dot represents the growth rate of 4 replicates for a given
concentration (when I put the 0%, 15%, 30%, 45% and 60% solutions in the cells
it diluted it, giving a lower final concentration in the whole solution). As we
predicted it, the more ethanol there is in the solution; the lower is the
growth rate. Indeed, we can see that the addition of a 45% ethanol solution is
less efficient to prevent bacteria from dividing that the addition of a 60%
ethanol solution. However, even if the growth rate of the solution in which we
added a 60% ethanol solution is lower than the others, it is still positive,
meaning that cells continue to grow, but slower.
Figure 1 : Graph of the growth rate as a function of
the final ethanol concentration for 4 replicates
We can conclude from this experiment that
ethanol has indeed an effect on the growth rate of a population of Salmonella typhimurium, so our initial
hypothesis is confirmed. However, we saw that the addition of a 60% ethanol
concentrated solution is not enough to stop all the bacteria divisions. This
concentration is in the range (60%-70%) usually used in industrial hydro
alcoholic solutions that we might use every day. Then, it would be interesting
to see if a higher concentration (70% for instance) can still more prevent the
bacteria populations from growing. We could also test different amount of these
solutions to estimate the good posology. Nevertheless, we know from previous
researches that a 95% concentrated ethanol solution is less efficient for
killing bacteria that a 70% one. It would be interesting to investigate why!
If you want to know
more:
aphp-hygiène.org “Les solutions hydro alcooliques en
43 questions”
Goldstein, D. B. (1986). Effect of alcohol on
cellular membranes. Annals of emergency medicine, 15(9), 1013-1018.
sdhe.fr « fiche technique de la solution instantanée pour les
mains »
https://www.youtube.com/watch?v=9RmeDPqYUMI
« Ethanol interacting with a membrane »
Acknowledgement : Petnica Center, Centre de Recherche Interdisciplinaire, Licence Frontière du Vivant, Fondation Bettencourt Schueller, Valentine, the teaching team, Bore promotion…
Clément Conil
