Did you know that our stomach is an acid environment ? The pH (which is the unit that measures the acidity) is low. Very low. Its pH is equal at 2 on a scale that goes from 0 to 14. 0 being the most acid value. If you put your hand in this kind of environment, your cells would probably die quickly. However, some bacteria survive in our stomach (if you want to know more : click here) Furthermore, they help the stomach to digest some nutriments. Without lactobacillus bacteria, we can’t digest sugar milk for instance. Lactobacillus changes the sugar from the milk in lactic acids which is used to form lactate. Lactobacillus acidophilus is very helpful for humans in different ways, as explained in this video (Xin Yun channel). Most interesting fact about this bacteria is that they are particularly adapted to survive in low pH. Why ? Because, they have a sensor which is activate in low pH. This sensor, situated in the membrane of the bacteria, active a system which adapt the internal pH of the bacteria according to the external pH. This point avoid cell damage. Moreover, this system ask energy. Thus, the production of lactic acid increase because the bacteria produce energy. So, we decided to observe the amount of lactic acid produced by Lactobacillus acidophilus according to the external pH. 1
How can we evaluate the amount of lactic acid ?
First, we prepare an environment where our bacteria can live. As we said before, this bacteria need sugar milk to form lactic acid. But they also live in an environment without oxygen. They practice fermentation. Thus, we prepare a medium with tomato juice (which have all the nutriment and allow fermentation) and milk.
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Next, we fill three different Erlenmeyer with the same quantity of tomato juice.
We change the pH of each Erlenmeyer in order to get 3 different pH. We chose 5.2, 4.3, 3.1. Why ? 5.2 is the optimum pH for the bacteria. 4.3 is a pH tested in a previous research and 3 is a random pH.
After that, we added lactobacillus acidophilus in the three Erlenmeyer and after one night, we measured the pH. If, we observe a pH variation that would means that our bacteria produce lactic acids. We expect that lower is the pH higher is the production of lactic acid or the pH variation.
However, we could not assume that the change of pH is due to the production of lactic acid by the bacteria and not because of natural phenomenons or other reasons. Then, we needed to correlate the evolution of the amount of bacteria in each solution to the pH observed. This would allow us to make a link between the amount of bacteria and the pH of the solution, and then begin to confirm our basic hypothesis : the pH change (or not) is due to the production of lactic acid by Lactobacillus acidophilus.
Then, we took 3 others recipients in which we regulated the pH as the 3 others but we did not put any bacteria in it. We measured their pH at the same time than the 3 others.
In order to measure the pH, we used a pH-meter which is a device that you put inside a solution and it gives on a screen the pH measured. The disadvantage of these devices is their precision and accuracy. To minimize the errors of measurement, we repeated the measure of pH 3 times for each measure.
On the other hand, to estimate the amount of bacteria in each recipient, we took some samples that we put in a spectrophotometer (a device that gives the opacity of a solution by measuring the light that can goes through it) during a whole night until the measures of pH. This value of opacity (called optical density) along time gives us informations about the speed of the bacteria divisions and then the amount of them.
For the first solution at pH=5.2, we observed a decrease of the pH of 0.2 unit. For the other solutions there is no significant pH variation between the solutions with and without cell which make impossible to conclude on any implication of L. acidophilus in pH change. Eventually, all the measurements taken are not precise enough and the incubation time is not long enough to let L. acidophilus produce enough acid that would have a significant impact.
Here is our github gathering all the files you will need to do this experiment !
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