Bacteria Test

[daphne101996]

Ok so here is my question Which acidic substance kills the most bacteria, lime juice, lemon juice, grapefruit juice, or vinegar?


Well here are my procudures

1) Put ready Petri dishes, with the medium in it in the refrigerator
2) Prepare sterilized water by boiling water and letting cool to room temperature.
3) When ready take out the Petri dishes out of the refrigerator and let cool down for about a hour
4) Take the circles of filtered paper and drip the substance on the paper till all of it has been covered, and place a side till step 9.
5) Collect bacteria from the telephone with a sterilized cotton swab.
6) Fill a small cup half way full of sterilized water.
7) Dip the sterilized cotton swab with the bacteria into the sterilized water. Make sure that there isn’t so much water so it stays concentrated. This will transfer the bacteria you collected into the water.
Now, pour on the water with the bacteria onto the Petri dish till all of it is covered. Pour out excess water so there is a film of water.
9) Place the filtered paper with the substance onto the Petri dish.
10) Replace cover on dish, store upside down, and label each Petri dish with the name of the substance.
11) Place wooden pieces on the heating pad and put a tin box on top so it will stay insulted .Place the Petri dishes on top of wooden pieces .Then put the thermometer on the top of the tin can and put the tin box on top to keep insulated. Put the heating pad on low and make sure you keep the temperature around 100 degrees Fahrenheit.
12) You should see growth within a couple of days. And then after 3 day take out the Petri dishes. Don’t take off the filtered pieces of paper because the bacteria will grow on top of it, because bacteria need air to grow.
13) Take a picture of each experiment so you can compare later.
14) Repeat steps 1 - 13 for each substance and do two Petri dishes for each substance.


I got that the vinegar and grapefruit did the best at killing bacteria.But i wonder why did the things that had the most acid (lemon and lime) did the worst?It turns out that grape fruit has the most sugar out of the iteams.Does sugar have a cause in killing bacteria .???why did the things wiht the most acid kill the least?

[staryl13]

Hi!
You are correct, usually we think of acids as being the best bacterial killers. However, each bacteria grows at a different pH, some are actually well-suited to acidic pHs. It is a bit odd that something with sugar would inhibit bacterial growth but I would suggest you check the label, perhaps there is something else in the grapefruit juice that might have caused your result. Here's an extra link about factors affecting bacterial growth in general:
http://www.foodsafetysite.com/educators ... /bac2.html
Good luck!