Bacteria on Petri Dish

[deleted-782044]

I am doing a project testing on how well do disinfectants work. I used Petri Dishes. I am trying to find out how to calculate the amount of bacteria on the petri dish. I am having lots of trouble and I would very much appreciate any help.

I do not know how to calculate and find out the amount of bacteria on the dish.

Thank you

[deleted-674917]

Hello!
What do you see on your petri dish? Do you see dots? If yes, they are colonies of bacteria. Each individual dot starts from a single bacterium and you can report a colony count as a representative for bacterial numbers.

If you don't see dots, what do you see? Is the surface of your petri plate completely clean as if it were unused? If yes, the disinfectant concentration you used killed all bacteria and nothing grew on the plate. In this case you may want to dilute your disinfectant.
If the surface is covered in something slimy or glistening, then you have too much bacteria on there and you need to dilute what you are applying. The dilution should be done using sterile water so that you don't add external bacteria to your mix. Also, you need to remember how much you have diluted (e.g. 5 times, 10 times, 20 times) and multiply the final colony number with the dilution factor.

Hope this is helpful! Let me know if you have more questions.
MS

[deleted-674917]

Also, to make it easy to count colonies when you have many, use a marker and mark each colony that you have already counted, on the bottom side of the plate. That way you will not end up counting the same colony twice.