Science Buddies: "Ask an Expert"

For my science project I am comparing waters to see which one has the most bacteria in them. I’m of course using pertri dishes to growth them. But I was wondering, how should I measure the amount of bacteria in the dishes so I am able to compare them? Should I count the amount of colonies in each dish and then compare? and if so what happens if the growth becomes to rapid and I am unable to distinguish between the colonies? I want to be able to but all this data into a table and compare them.

Hi,

In order to accurately compare the different 'waters' you need to keep all your independent variables the same:

1. use the same amount of water on each plate (about 0.2 ml); measure the volume as accurately as possible
2. use fresh nutrient agar plates all from the same batch
2. spread the water evenly over the entire plate using a sterile spreader made from a large paper clip
3. incubate the plates upside down at a temperature of 18-26 C for 24 to 48 hours
4. count or estimate all the colonies on the plate (if there are too many, divide the plate into sectors and just count all the colonies in a couple of sectors, average them and multiply to get the total number of colonies on the plate)

What 'water' are you going to use? City water from the spigot has been treated with chlorine and won't contain very many microbes. pond water can contain a lot of micro-organisms, but they are not all bacteria. There are algae, fungi, and protozoans. Usually you can tell bacterial colonies from those of other organisms by their appearance. Here is some help for distinguishing colonies by type on agar:
https://www.sciencebuddies.org/science- ... gar-plates

Let us know if you need more help.

Sybee