Science Buddies: "Ask an Expert"

Hello
I am performing an experiment about the effect of headphone use on bacteria count in the ear.

I was wondering whether I need to try the experiment on a variety of people using a variety of headphones before being able to come to a conclusion.
I will be using a solution I found from the internet instead of agar, it consist of water, gelatine and sugar. Is this suitable?
I do not know where would be the best place to grow the bacteria
Would I need to identify the strain of bacteria and will counting colonies by eye be enough for the results?
Also if there is an increase in bacteria can I conclude that the regular use of headphones increases chances of ear infections if, ear infections are caused by accumulation of bacteria ?

Thank You

Hi Spongebob

There are quite a few variables you should consider. And because of this, in answer to your question whether to try the experiment on a variety of people, I'd say yes. What are some of the variables? People will vary in the amount of bacteria they naturally have. Furthermore, a person could vary from one day to the next. In addition, there can be variability in the swabbing and culturing techniques, to name a few. So, you might consider getting some number (maybe 10?) of people who don't use headphones (controls), and 10 who use ear buds on a regular basis. (You'll have to establish what regular is.) You might also find 10 who use earphones that go outside the ear. Besides asking people what type of headphone they use, you may want to ask your subjects about how much time they spend using headphones, and when they last used them. If time permits, you might consider taking samples from the same subjects 2+ times to moniter variability. (Please see the rules and / or talk with your teacher about the use of human subjects in your experiment!)

The solution of gelatin, water, and sugar should be ok but be sure it's sterile to begin with and that you put it into sterile dishes. The more consistent your culture media, the better! Though many bacteria like warmer temperatures, you can probably keep your plates at room temperature. Keep them out of direct sun, and put someplace where they won't be disturbed and the conditions are quite constant. (If you have access to an incubator, that's great!) You will probably want to set a time, say one week, to score your plates. (You might need to adjust this depending on how much grows.) I would recommend using a digital camera to record what each plate looks like, and to try to count the colonies. You might set up a scoring system with respect to colony size. I don't know what grade you're in, how much time you have, nor what lab facilities you have, but I don't think it is probably necessary to try to ID the bacteria. (There are keys if you wish to try. For example: Microbiology: A Photographic Atlas for the Laboratory by Steven K. Alexander or Bergey's Manual of Determinative Bacteriology by D. H. Bergey)

As for your last point - ear infections are caused by bacteria, but even if you find bacteria in the external ear region that may be attributed to headphone use, I don't think you can make the leap to say that this bacteria can cause ear infection.

In addition to what DrHamil said, you may need special permissions to culture bacteria. Many bacteria are harmful (as you note, some cause ear infections!) so growing large cultures can be dangerous. Check with your teacher about the proper procedures to use for this experiment.