My Scientific Question: What is the effect of pH levels on bacteria growth?
What I did:
- The independent variables in this science fair project are the concentration of bleach (alkaline) and vinegar (acid) – 20%, 40%, 60% and 80%. The dependent variable is the size of inhibition zone (area free of bacteria), which is determined by measuring the diameter of the area using a ruler. The constants (control variables) are the temperature of the room, the percentage of each solution, and the ingredients used to make the agar in the Petri dishes. My control in this project is water (negative control).
- Use the marker pen to label eight Petri dishes that were prepared with agar as acid 20%, acid 40%, acid 60%, acid 80%, alkaline 20%, alkaline 40%, alkaline 60% and alkaline 80%. I also divided the petri dishes into 4 quadrants to make more than one trial.
- Use eight test tubes to prepare different concentrations of alkaline and acidic solutions, and label them accordingly as stated below. Note that gloves should be worn when handling the acid and alkali.
- Use the pH meter to measure the different pH levels of the solutions.
- Use the hole puncher to cut out 72 circular discs out of the filter paper. There is be two discs for each trial.
- I took a cotton swab and I picked up some E. coli. Then, I swiped the E. coli on each of the 9 petri dishes.
- Next, use the forceps to pick up and immerse each punched filter paper into the test tube that contains the corresponding concentration of bleach or vinegar according to the labels made earlier. For each of the four sets of labeled filter paper, five circular discs will be immersed in the alkaline solution and placed in the Petri dishes labeled as alkali, while the remaining five circular discs will be immersed in the acidic solution and placed in the Petri dishes labeled as acid. The placement of each set of filter paper onto its corresponding Petri dish is done according to the labels made earlier.
- Place the petri dishes inside the incubator overnight. Which allows the bacteria to grow.
- Take out the incubator and use a ruler to measure the diameter of the inhibition zone in each of the petri dish. Then, calculate the average value.
What I learned:
My hypothesis was right for the acid part of this project. The 80% solution had the largest inhibition zone while the 20% solution had a very slight inhibition zone.
For the alkaline part of this project, the 80% and 60% solutions I was not able to measure since the inhibition zones crossed each other and the measurements would be inaccurate. But, I estimated how large the inhibition zones were.
There were a lot of things that went wrong in experiment. I actually had to redo it more 5 times. The first thing I did wrong was that I didn’t use actual filter paper because my mentor did not think that it was going to make a difference, but it obviously did. The second thing I did wrong was that my solutions were really low. The percentage of my solutions were 5%, 10%, 15%, and 20%. The only percentage that had a slight inhibition zone was the 20%. The third time I did this experiment, I changed the solutions to 20%, 40%, 60%, and 80%. I also changed the thickness of the filter paper. This time, it got really interesting because the acidic solutions grew bacteria but the basic solutions didn’t. So the fourth time I did it, I changed my alkaline from ammonia to bleach. Since bleach is a higher pH than ammonia.
The next time I do this project, I would changed the different types of acid/alkaline that I used in this project and see how that would affect the bacteria inhibition zone. I would change the amount of trials I do on each of the petri dish, like instead of doing four trials on one petri dish, I would do one trial of a solution on four petri dishes.
One new question I have is that are there different things in the products I used that helped killed the E. Coli? Another new question I have is what other factors could have caused the inhibition the grow or to not grow as large as the other inhibition zones.