Science Buddies: "Ask an Expert"

I am making a broth of bacteria for my science project. I used a sponge to wipe off a toilet seat. I am now soaking the sponge in warm water for 24 hours. I have inserted a thermometer into the water, and intend to keep the temperature at 95ºF. If the water cools down, should I add more hot water to increase the temperature? Or will this cause problems with the bacteria sample? Thank you for your help.

As long as the water is not -too- hot, it should be okay. If it's boiling, for example, you might accidentally kill the bacteria. So, I would suggest keeping the temperature of the water fairly similar to the temperature you're using to incubate the sponge.

Good luck!

Thank you very much. When the broth is done, I will transport a millimeter of the broth, along with Micrology Labs Easygel into a petri dish. Every 24 hours, I will record the number of colonies in each bacterial species, as well as the diameter of five selected colonies. I was wondering whether you had an idea of how to successfully record the diameter. My thoughts were to place a grid with 1-cubic-centimeter squares on the underside of the petri dish. Using this grid, I will estimate the diameter of the colonies. Please advise as to whether you think this will work. Thank you, again.

Hi,

I think you mean a mililiter, not a milimeter .

Two things: First, consider that you may get too many colonies to count. In this case, you might select a random part of the plate (say, 1/4 or 1/8) and count that instead, then extrapolate your results.

Second, your idea is fine and should work, but it might be easier to just put a ruler on the back of a plate and use it to measure the diameter directly. I'm assuming you're using agar that's translucent, and in my (admittedly limited) experience it's usually pretty easy to see through it and use a ruler to measure colony properties. It might help if the ruler itself is see-through, though.

Thank you. You are correct, I mean milliliter. Sorry for my mistake. I would just like to clarify on your idea of using the ruler. Could I just put the ruler down on the table, and set the petri dish on top of it to measure.
I also have a few questions:
What do you estimate might be the average diameter of the colonies? I understand if this is a hard question to answer, and if you cannot give me a definitive idea.
What types of bacteria do you expect to turn up in my broths: one is from a toilet seat, and the other is from a pan used to cook hamburger meat?
Thank you for all your help.

Hi,

Yes, I think you can just put the ruler down and measure the colonies.

As for diameter, it's going to depend on what temperature you keep the plates at. Unfortunately, I'm not a microbiologist, so I don't even have a well-informed guess.

Here's a Science Buddies site that should answer your final question: http://sciencebuddies.com/science-fair- ... ates.shtml . I suggest also looking through the microbiology guide, here: http://sciencebuddies.com/science-fair- ... ndex.shtml .

Hope this helps!

Thank you. I will be keeping some bacteria at 100º, some at about 90º, and some at about 80º. Do you think these temperature variances of 10º will exhibit enough difference to draw conclusions on the effect of temperature on bacterial growth patterns? I am not just trying to decide which temperature provides the bacteria with the necessary nutrients to reproduce the fastest. I am also trying to discover how does the temperature effect the pattern. For example, will the higher temperature-bacteria stay in the exponential phase for longer or will there be a longer stationary phase.
Also, if my project is about determining the effect of temperature on growth patterns of bacteria, should I observe and record the form, elevation, and margin? Is this important to my project? Or is it unnecessary and distracting information?
Thank you for all your help.

Hi there,

You have a very interesting project! I want to help you with a few of your questions.

First, keep in mind that you may get so many bacteria that you produce a "lawn." In other words, you might not be able to distinguish the different colonies; instead, you might see your entire plate covered in bacteria by the end of the first 24 hours. Because of this, you might want to check your plates at shorter intervals at first (say, every 6 hours for the first 24 hours), to make sure that you can see the different colonies as they develop.

Another thing to keep in mind is that higher temperatures increase the rate of all chemical reactions, and so increased temperatures will likely make your bacteria grow faster. Also, some bacteria may not grow at higher or lower temperatures, while others will. As Melissa pointed out, your 100 degree (boiling) temperature may kill many of your bacteria.

As a scientist, it is always useful to record as much data as possible. If you don't use some of it, that's okay; but it's better to record as much as you can so that you don't look back and think, "Hey, if only I had recorded those observations!" So go ahead and record the form, elevation, and margin. The more you observe, the more informed your conclusions will be!

I hope that helps.

Heather