Science Buddies: "Ask an Expert"

I am a high school student enrolled in a Scientific Research class and I am preparing a science fair experiment.
In my experiment I am comparing the bacteria levels on dishes after washing the plates with three different temperatures of water (15 C, 26 C, 38 C) and three different soaps (Dawn, Palm Olive, Life Tree Citrus). After each trial, I will swab the presently washed dish with a sterile swab, then swab onto an agar Petri dish. I will place the agar Petri dishes into an incubator, and after two days I will count the bacteria colonies. The issue I have run into is using the colonies to determine how many bacteria are actually in the Petri dish. In my research I found that Culture Counts (using the assumption that each colony started from one individual bacteria) might work, but I am skeptical of this process and would like to know if it is accurate. Another process I found is calculating fractal dimension. I comprehend that by using a grid to count the bacteria, you count the number of bacteria through the number of boxes filled, yet I don’t understand how to know how many bacteria are in one box. I would like to know your input, and any other processes that involve direct counts (no dilutions).
~Thank you for your assistance.

From my experience outside of a serial dilution which shows relative growth you could do two things. You could use a culture counter as you suggested. They are fairly accurate. However to be used, the bacteria must be suspended in a liquid media. Another way to measuring growth is a spectrophotometer. This will also measure particles in a liquid. However you must have some kind of conversion chart to what the spectrophotometer's reading equals in cells. Most labs that use a spectrophotometer for this purpose should have that.

Good Luck

Adam.

Hi,

Adam gave you some good advice on selecting the method of measuring your results. Plate counts are a standard method for measuring the number of bacteria, and this would be a good way to measure your dependent variable. In my experience it is possible to count colonies on a plate when the quantity is between 30 and 300 colonies. If there are too many colonies on the plate, results are reported at TNTC (too numerous to count). You might want to do a preliminary experiment to find out if the swab might have too many organisms for a plate. The swab could be transferred to a tube of sterile water (1 ml or 10 ml) and a dilution of the water plated to extend the range of your count and give you quantitative results.

The science buddies website has lots of information on microbiological techniques and safety information.

https://www.sciencebuddies.org/science- ... ndex.shtml


Let us know if you have any questions. This sounds like a great project.


Donna Hardy