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Joined: Mon Jan 20, 2014 12:42 pm
Occupation: Student 7th grade

Advice needed

Postby Project101 » Mon Jan 20, 2014 1:02 pm

Hi Melissa,
I am currently working on my project, "is that really bacteria growing in my Yogurt?" I completed the experiment and want to know if my observations are correct. I used 4 agar plates, 3 of them had yogurt stains and the one had distiller water only. After 6 days, all plates showed bacterial growth, including the distiller water, although it was significantly less growth than the yogurt plates. does this mean the distiller water plate was contaminated, or did bacteria grow as a result of moisture.? All plates grew in an anaerobic environment. please help! Thank you

Posts: 1953
Joined: Fri Feb 01, 2013 7:00 am
Occupation: Retired molecular biologist, university researcher and teacher

Re: Advice needed

Postby SciB » Wed Jan 22, 2014 5:57 pm


My name is not Melissa, but I can try and answer your questions.

If you have growth on the plate with just distilled water (DW) it means that something was contaminated that should not have been. Did you use an unopened bottle of DW for the test? Are you absolutely positive that you used the CLEAN dropper to suck up the DW? If you used the one that had yogurt in it by mistake, then that would explain why you got growth on the DW plate.

When you spread the drops around on the plates with the ‘spreader’ did you do the DW FIRST? If you did the yogurt first then you would have transferred some of the culture to plate #4. The only other way I can think of that plate 4 got contaminated is if you accidentally touched the dropper containing the DW to one of the yogurt cultures and then used it to put drops on the control plate.

Let us know if you find out the reason for the growth on the DW plate, OK?

Best wishes,


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