MadSci Network: Microbiology

Re: What are filter discs and should I use them for this project?

Date: Wed Sep 24 21:03:37 2008
Posted By: Steve Mack, Assistant Staff Scientist, Molecular and Cell Biology
Area of science: Microbiology
ID: 1222273917.Mi

I'm doing an experiment pertaining to finding out which toothpaste combats bacteria best. I am planning on swiping a Q-tip in the inside of my mouth and putting it on an agar plate. How should I apply the toothpaste? I read that I can use sterile filter discs, but I tried to find out what those were and haven't been able to find out what they are. Is there a better way that you know of?
Hi Emily,

Filter disks are small circles of filter paper that have been soaked with whatever substance you are testing in your bacterial growth project (more about that below). These are sold by biological supply companies like VWR; you can find a description of them here. You probably don't need 1000 filter disks, so if you wanted to purchase some, you could purchase 50 disks from Carolina Biological Supply here.

However, I don't think that you need to purchase filter disks to do your project. You can make your own filter disks from a coffee filter using a hole punch. You'll have to sterilize them after you make them, which you should be able to do by wrapping your disks in aluminum foil and baking them at 300 degrees F, or by steaming them (in a bamboo steamer, or a fish steamer). If you have access to an autoclave then you should autoclave them, but most students don't have an autoclave at their disposal.

With respect to the specifics of your project, we have some answers in our archives that you should review. First, you should look at this answer (932075989.Mi) to a question about bacterial inhibition by toothpaste, and this answer (971383197.Bt) about bacterial inhibition by eucalyptus oil. Those will give you the idea of what you should be doing in your experiment.

Then, you should take a look at our bacterial culture FAQ page, and in particular answer 879518241.Mi discussing the use of filter disks to test the effect of peppermint oil on bacterial growth. That answer also describes a different way to approach the question of bacterial inhibition, using serial dilutions. That approach is also described in this answer (986096141.Mi). You should also look at this page.

In order to do this project properly, I think that you'll have to do a little more work than simply streaking a cotton swab across a plate and adding toothpaste-soaked disks. You will have to streak your swabs across plates, and you can read about how to do that properly in this answer (916783675.Mi) and also here.

The purpose of this initial streaking is to isolate individual colonies, which represent a single bacterial cell that was on the surface of your swab. Then, you can culture that colony, and make plates that are covered with many identical bacterial cells at a uniform density (a lawn). That is when you place one of your treated paper disks in the center of the plate. Then, you incubate the plate so that the bacterial cells will grow into colonies. This is described in greater detail in this answer (862241783.Mi). If your toothpaste has an inhibitory effect on bacterial growth, then the bacterial cells closest to the disk will grow poorly in comparison to those further away. You can compare the extent to which different toothpastes can inhibit the growth of bacteria from the same culture.

To make your disks, you will first have to make sterile solutions of your toothpastes. You'll need to make sure that you add the same volume of each solution to each disk, so that they are all the same. You will also have to make some disks that have only the sterile water that you used to make your toothpaste solutions on them, and you will have to use some disks with nothing on them. These later two types of disks are the negative controls that you will use to determine the inhibitory effect of the dry disks and the water that you used on bacterial growth. If your toothpaste-soaked disks inhibit more bacterial growth than the dry disks or the water-soaked disks, then you can argue for an inhibitory effect for that toothpaste.

To really do this project properly, you will want to vary the concentrations of different toothpastes, and you will want to test them on different bacterial cultures (different colonies that you have picked and cultured). That will be a lot of work, so you should start small, with one culture and one toothpaste at one concentration, to make sure you are doing each step correctly.

Finally, I need to remind you that working with bacteria can be dangerous if you do not take the proper precautions. You should pursue this project with the assistance and oversight of your teachers.

Good luck!

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