Science Buddies: "Ask an Expert"

I just want clear steps on how to actually distribute the water on the petri dishes and make sure it's all correct and count the bacteria colonies.

The Procedure section of the project (https://www.sciencebuddies.org/science- ... #procedure) tells you exactly how to put the water onto the agar surface and how to spread it around:

Put on a pair of disposable gloves. Get a clean medicine dropper from the glass jar and attach a bulb to the top. Get out three tryptic soy agar plates and a glass stirring rod from your aluminum foil package. Suck some of the boiled (but now room-temperature) water into the medicine dropper.
Remove the cover of one of the plates. Apply three drops of the boiled water to the soy agar. Use the glass rod and smear the water drops in a zigzag pattern on the surface of the soy agar, starting in the center—smear the water sample from the center of the plate to the edge of the plate. Replace the cover.

If you are still unsure what to do check out videos on Youtube that show how to spread bacteria and count colonies. For example: https://www.youtube.com/watch?v=eXKqgxmQ5As
Don't worry about using a micropipettor as they do in the video. That's just how we would do it in a professional lab. A sterile dropper is not as accurate but it is fine for this project. Pay attention to all the steps as they are important for maintaining sterility and making sure you have the water spread evenly over the agar surface.

After spreading all the plates put them in a warm [24-30C] place in the house where they won't be disturbed. I tape the lids to the plates to make sure that they don't come off by accident. Also, using a Sharpie marker, write on the side of the plate [NOT the lid] what you spread on it and the date.

If you have any specific questions let us know

Good luck!

Sybee

I am assuming that you are talking about petri dishes with agar. In this case, a simple way to distribute water (including any other liquid) is to use a L-shaped glass rod. A picture of it is here:< http://www.thomassci.com/Supplies/Spatu ... 20Spreader>. Make sure to disinfect it which typically involves dipping it in 70% ethanol followed by flaming it on the bunsen burner and cooling it prior to using it to spread the water.

If you are talking about how to spread suspended microbial dilution on the agar, first do the dilution, dispense the dilution onto the agar surface using a sterile pipette, and then spread it using the L-shaped glass rod as described above.

Good luck!

-Sally Kuzniewski