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The number of colonies on an Agar Plate

Postby Jdastewart » Sun Apr 07, 2019 8:20 pm


My 5th header hypothesized that surfaces that come into contact with more people will have more bacteria.

So we bought a science kit and tested 5 surfaces.

However now that we’ve got the results, we need help qualifying them.

The cell phone for example produced over 300 distinct small colonies.

However a dollar bill produced what appears to be far fewer colonies. But the colonies themselves are far larger.

One would assume that a dollar bill would have more bacteria.

Is it correct to say that the more colonies produced means, the more bacteria present on that surface.

Could it be in fact, that the larger colonies (on the dollar bill) are in fact smaller colonies that have grown so rapidly they have joined together to form what appears to be a larger colony?

What if we have 300 colonies covering 2/8 of an agar plate vs 5 colonies covering 7/8 of a plate.

Which one would have the most bacteria?

Which one would be the dirtiest?

We honestly thought this would be an easy experiment. But after getting the results - we have more questions than we started with. Lol.


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Re: The number of colonies on an Agar Plate

Postby MS15 » Thu Apr 11, 2019 2:53 pm

The hypothesis should be easily testable and I'm sorry you've had more confusion than you anticipated.
First - if the nature of your colonies are different, then it is most likely because they are different kinds of bacteria and grow differently.

Although it's true that one would have expected the dollar bill to be more dirty, it is possible that you used a rather fresh dollar bill that has undergone limited handling and change of hands. On the other hand, we handle our cell phone screens a lot more than we handle dollar bills nowadays (and not always with washed hands :lol:) so it is not impossible that the cell phone has a higher bacterial load.
One option is to compare two different dollar bills- one that appears fresh and one that appears more used (and old and dirty) - these two cases should work exactly as you predicted.

Regarding the actual experiment-
1) Try to collect bacteria from each surface under identical conditions (for example, if you are using a damp swab then rub each swab with identical number of strokes and for identical times on the individual surfaces)
2) Be careful while transferring from the swab to the agar plates: do it vey gently and also identically for each plate and surface for suitable comparisons.
3) Use a 'control' experiment to make sure your plates are fine and nothing grows if you don't add contents from a swab. For this, I would just use a clean swab and perform a blank transfer. Everything should be similar but without having touched a test surface. This will help you determine if you are getting colonies from the air or from the plates themselves.
4) Also after performing the experiment, try to keep the plates together in the same place for same amount of time. This will ensure all plates get identical temperature conditions and time for growth for observing comparable nature of colonies. This is a good practice but keep in mind that different types of bacteria will grow differently.

You can find more information here: https://sites.google.com/a/crestmem.edu/science-fair/past-projects/how-much-bacteria-is-on-a-dollar-bill

Hope this is helpful and I'm happy to answer any more questions you may have,

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