Science Buddies: "Ask an Expert"

I'm doing my year-long project on the bacteria from various water sources. These include the water fountain in the school, and the ice, tap water, and toilet water in 3 local restaurants. I was going to put a sample of each water source onto a separate agar plate and let them incubate for at least 48 hours, then count the bacteria colonies that grew. Is this an accurate way to run this experiment, and are there any suggestions to make it better or change how it's done?

That sounds like a great project!

Some suggestions:

Put only 1mL of water sample on each agar plate (let the ice melt). After you do that, spread the water over the surface of the agar plate with either a sterile inoculating loop or sterile swab. (If you are using disposable varieties of each, use a different loop or swab for each plate.) Remember to use the same volume of each water sample (don't use 1mL of one sample on one petri dish and then 2mL of another sample on another petri dish).

Let the plates air-dry by only partially covering the petri dish (not for long, otherwise, your plates will get contaminated by bacteria in the air.) To shorten the time you have to expose/dry your plates, you can put them in the incubator partially covered, then remove them after they are dry, put the lids back on, and then put them back into the incubator. If the agar is still wet when you incubate it, the colonies will not be circular. Instead, they will form into "splotches" or they will run together with other colonies. If that happens, you can't count the seperate colonies.

Make sure you run multiple trials (2 or 3) of each sample in seperate petri dishes. This will insure accuracy of your data. You could use either boiled water (100 degrees Celsius) or sterile deionized water as a "sterility control" (to insure that there are no bacteria in the agar).

You might want to check the bacterial content of bottled water (a couple brands) as a comparison. Some people drink bottled water because they say that it is "cleaner."

I hope this helps! Best of luck!

~Maryam M.

Hello, the bacteria content in ice, tap water and water fountain will be significantly lower than the toilet water, so you might consider using a smaller loop for toilet water (0.001 mL) and use 0.01 mL loop for the rest, and remember to factor in the difference in your final calculation, because the bacteria in the toilet water sample might grow all over the Petri dish making it difficult to do a accurate colonies count. (If the bacteria still grow all over with the dish with the smaller loop consider doing dilution either using sterile water or sterial saline before streaking.) I agree with Maryam that you should consider using boiling water as a control, and run multiple trails.

You can see a picture of the inoculum counter-streak (a technique use to quantify bacteria) in the follow website. (Figure 2 urine culture).

http://mmi.creighton.edu/Manuals/Lab%20Manual1_2.htm

Michael

It might be interesting to compare the inoculum counter-streak technique, with using 1mL of sample on petri dish, and see what happen.

Michael