Science Buddies: "Ask an Expert"

I want do do the Project - Death Rays: What Duration of Ultraviolet Exposure Kills Bacteria, and in step 6 it states, "Count colonies in both halves of each plate”. I am not sure how to count the bacteria. Could someone tell me the best way to accurately count bacteria. Also, how can you determine how many bacteria are alive and how many are dead? I will attach the link to the science project’s procedure page below.

https://www.sciencebuddies.org/science- ... #procedure

Hi there,

In this project, "counting colonies" means you should have clear-cut "dots" of bacteria, and you should count how many there are. The plate should look something like this: (https://en.wikipedia.org/wiki/Colony-fo ... unting.jpg)

The image shows someone marking the dots off with a marker and using a clicker in the other hand so you don't have to keep track, just click for every dot. You probably don't have a clicker; what I did once for colony counting was have my calculator out, hit +1, and then hit enter for every dot I counted.

In terms of alive vs dead bacteria, for the purposes of this project, you can assume the colonies you see are alive.

Let me know if that was confusing or if you have additional questions, such as if your plate doesn't look like this with clear colony dots.

–Eugene

Thank you so much Eugene. Also, what is the difference between a Petri dish and a nutrient agar plate?

Well, you might not really need to count bacteria. If you have a series of nutrient agar plates and they are uniformly covered by bacteria, you could expose them to different times and see which time simply stops bacteria from growing, and compare it to a non-irradiated plate. You can also grade the growth after each minute of irrdiation if you notice that after x minutes, the bacterial growth isn't as robust, but the bacteria aren't dead yet. A Petri dish is the plastic dish that holds the nutrient agar or other growth medium. A nutrient agar plate is a petri dish containing nutrient agar.