Science Buddies: "Ask an Expert"

Hi, for my project I'm seeing the effect of probiotics on E. coli. I plan to plate E. coli on agar, and then add a disk of probiotics in the center of the petri dish. From there, I will see how probiotics inhibit E. coli by counting the colonies of each. The only problem is I don't know how I will be able to tell the difference between E. coli colonies and probiotic colonies. They are both bacteria and therefore will have similar looking colonies. Is there a way to accurately tell which colony is E. coli and which is probiotics?

Hello,

This is a great question, and perhaps another expert can also weigh in if there is a better suggestion. Some ways to tell differences between the two different types of bacteria on an agar plate could be by looking at differences in colony color or in colony morphology. See the link below, it has further information on this:

https://www.sciencebuddies.org/science- ... ates.shtml

Do you know which strains of probiotics you are using? Do you know how colonies of this type of bacteria compare with E. coli? Let us know and we can try to help you further.

Best,
Pharma

I'll be using yogurt as the probiotic and the bacteria that is found in the yogurt is lactobacillus acidophilus. My instructor said that both of the bacteria would look very similar and it would be difficult to tell which colony belongs to which bacteria. Would gram staining help? If we were to plate the E. coli, add the disk of L. acidophilus, let the bacteria grow and then use gram staining, would that help me tell how well probiotics inhibit the growth of E. coli? Oh and E. coli is gram negative while L. acidophilus is gram positive so the staining part would not be a problem

I apologize for the delay in getting back to you. Yes, if the colonies are not going to be distinguishable based on color/morphology, then Gram staining should help. A quick count of total colony numbers for each bacteria type in your experimental plates should give you your final data for the experiment.

Do you know how you want to set up the final experiment? How are you planning to plate the probiotics?

-Pharma

Yes, I plan to plate the E. coli on a petri dish. Then, I will add a disk of the probiotics in the center on the petri dish to see how it will grow and how E. coli will grow around it. After a day, I will use gram staining to determine the if the probiotic colonies inhibit the grow of E. coli colonies starting from the center of the petri dish.

Ok great! My question is are you using any type of isolation technique on the yogurt to separate out the bacteria before you plate them?

Best of luck with the experiment! Let us know if you have further questions.

-Pharma

I don't think I will be isolating the bacteria, I will ask my instructor about that. Thank you for the help Pharma!

You're welcome, I'm glad to have helped out. Best of luck!