do disinfectants really work?

[gmickjord]

my daughter is doing this project with her friends for the science fair and
they have already swabbed the petri dishes and bacteria is growing now they are supposed to count the colonies
my question is how to do this
the dishes look different some have lots of dots but really small while others have only one or two but large spots
is it the size or the quantity of the spots that matter
if someone could respond quickly that would be good because the petri dishes only have a 2 week life span and we have already passed one thank you

[staryl13]

Hi!
The project (following sciencebuddies link)
https://www.sciencebuddies.org/science- ... p012.shtml
has a chart which can be filled in with the # of colonies and other observations. The size can be place under "other observations" and the # of colonies should be counted and put under its respective column. Usually the amount of colonies matter more than the size. Hope this helps, good luck!

[adance]

staryl13 is right--it's the number of colonies that counts. If there are only a few, they will grow really big, and if there are a lot they tend to stay small. When I used to count colonies, I would use a fine-tip Sharpie, look at the bottom of the plate, and mark each colony as I counted. Then you don't count the same one twice.
If you have tons of colonies on a plate to count, one way to make it easier is get a sheet of clear plastic, trace a petri dish on it, and use a ruler to divide it into quarters. Then, you can count how many colonies just in one quarter, and multiply by four, if the quarters look roughly the same.