Petri Dishes

[Athienah]

Dear Expert

My daughter is studying about Bacteria and running a science fair project on Mold as well.

While doing so, we came across the following questions for which we would apprecite an answer or guidance:

1) Why is it necessary for the agar to be sterile before we start an experiment?

We believe the answer would be; "because we do not want to allow the growth of the Bacteria which might already be in the agar" (do you think this is correct? smart?)

2) Why do we have to use seperate Petri dishes for each sample?
We are not sure but maybe the answer here should be;" because different samples might contain different Bacteria and we would not want the different types to conjugate and develop new colonies which might not allow us to determine the original type"

We are not sure of our replies and we would be so thankful if you could provide us with the correct answers

Many thanks

Athienah

[MelissaB]

Hi,

Your first answer sounds perfect; if you already have bacteria in your agar and you get a colony growing, you will not know whether it was on there before or was put on there by your swab.

I don't really think that conjugation is an issue with the second question--I think there's a more straightforward answer, which is that if you put several samples on the same petri dish, you will not know which colony belongs to which sample unless you carefully mark exactly where you swabbed on the petri dish. However, I do know some people, to save money and to provide an additional control (within a petri dish) will, say, divide the dish into quarters and swab each sample only in a quarter of the plate, then compare the growth in the different quarter-plates. So I think this is a tricky question, because technically you don't have to put the samples in different dishes.

Hope this helps!

[Athienah]

Ms Melissa B

Thank you for your assistance .

Regards