Swabbing, growing and counting bacteria

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Swabbing, growing and counting bacteria

Postby barefootmom » Wed Jan 18, 2012 5:28 pm

My apologies for a new thread but I'm trying to get all this information together as quickly as possible as I believe we have already made mistakes with our first sample collection and I believe that we will need to re-do this part of the project in order to obtain accurate data. I have another set of petri-dishes arriving in two days and I really hope to have a clearer idea of exactly what we need to do to assure we have the most accurate data possible.

Our question is "Which surface at school has the most bacteria?" Research shows that is should be the cafeteria tables and wrote the hypothesis based on that. We are collecting samples from various surfaces around the school (ie. lunch tables, desks, computer mouse, etc.). My research as to the actual collection and growing procedures has led me in several different directions so I am hoping to get a clearer picture of the best way to go about this.

All my research shows that without diluting the sample most of the bacteria will be to numerous to count but I haven't been able to locate any specific information on how to dilute a surface sample. Could you explain the procedure for diluting the particular samples we will be working with.

?#1: Would it be best to use a dry (sterile) swab or a wet (sterile) swab to collect the samples? and if we should use a wet swab what is the best method to assure that it remains sterile? (and why is one better than the other?)

?#2: What is the best method to apply these samples to the dish for this project? Some say to "swab using a zig-zag pattern" others say "cover as much of the dish as possible" neither of which seem very "controlled" so how do I assure that my counts are accurate?

?#3: Do the sample petri-dishes need to be kept in an incubator? (other students have said they do but I have not yet found anything that says this) (again why?)

I greatly appreciate your input.
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Re: Swabbing, growing and counting bacteria

Postby ivyh » Thu Jan 19, 2012 8:52 am


Here is a link detailing the procedure for diluting a surface sample:
http://www.waksmanfoundation.org/labs/r ... lution.htm
(If any of the steps are unclear, feel free to ask)
You can purchase the "sterile broth" online - just look up Luria Broth and you should be able to find a link to it.

as for the other questions:

1. Assuming you're going to follow the procedure for dilution, use a sterile saline solution to wet the swab. As long as you don't touch the swab on any other surface before collecting the samples or leave the saline solution or swab out in the open for too long, it should be fine. Just make sure you open the swab, wet the swab, and take your sample quickly.

2. Both methods can be used. As long as you keep it consistent throughout all samples, either should be fine.

3. The samples should be incubated. Otherwise, you will be waiting a very, very long time for your bacteria to grow.

I hope this was helpful. Do you have any more questions?
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Re: Swabbing, growing and counting bacteria

Postby microkts » Tue Jan 24, 2012 9:10 pm

Hi barefootmom,

I love this project idea. I am a microbiologist and we do this kind of experiment regularly to make sure our labs aren't contaminated. ivyh gave you some really great answers to your questions. I just wanted to add a couple things. For your diluting you can also use sterile saline instead of broth. Here is a previous post on diluting bacteria in your home.

http://www.sciencebuddies.org/mentoring ... ?f=3&t=456

I would use the teaspoon method they talk about. You can use sterile saline purchased from the drug store, just make sure it doesn't have any extra ingredients. You can also sterilize some glass jars or cups covered with foil in your oven (225 F for at least 10 minutes). Then you can use the sterile jars to do your dilutions in. The foil is nice because you can take it off and put it back on as needed.

Let us know if you have any more questions.

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