Science Buddies: "Ask an Expert"

I'm doing an experiment about preparing culture media and I've been curious about these following questions.

1. Why should I use agar plate after drying it for 23 days at room temperature?

2. Why is streaking and spreading needed during microbiological experiments?

3. Why is it necessary to cultivate the agar plate upside down after plating?

Thanks for your attention and I really look forward to get some good answers.

Hello and welcome to Science Buddies! Let's take your questions one at a time:

1. First off, I assume you're drying for 2-3 days, not 3 whole weeks! At any rate, a dry agar plate is ideal because moisture will interfere with your culture in a number of ways. It could invite mold to grow and contaminate your sample (as I've seen happen numerous times, even with sterile technique and a clean incubator). It could drown your samples if there's too much moisture, inhibiting proper growth. It could also interfere with the microbe's ability to draw up nutrients from the agar, depending on what type of organism you're working with. Essentially, a wet agar plate creates a lot of issues for your microbes that you don't want to have to deal with.

2. Streaking and spreading both let you seed the microbes evenly throughout the plate. This avoids having pockets of super-dense microbial growth at any one place. If you're culturing these microbes to pick off isolated colonies, you want to streak properly to let them grow into well-formed colonies. If you're doing a disc-diffusion test or something similar, having a well-formed lawn is crucial.

3. Storing the plates upside down prevents the condensation that forms on the lid cover from dripping onto the agar - which then brings us back to all the issues with moisture from your first question.

Hope that helps!

Thanks for your reply Eugene! I really appreciate your help!!
I just want to ask one more question about number 2, it is bad to have super-dense microbial growth at one place?

I'm sorry it took me so long to get back to you on this!

It depends on what you're planning to do with the culture. Are you culturing to use the microbes for something else, like putting it into a liquid culture? Or are you going to do something directly on the plate itself?

If it's the first, I don't think it's terrible, because you just want some spots of microbe colonies to be able to pick off from. If it's the second, it's more important to get an even distribution without heavy pockets of growth.

If you could let me know what your experimental design is, maybe I could help you further!

By first step of glycolysis, I mean about converting glucose --> fructose-1,6-bisphosphate.
2 ATPs are required during this process and I just wonder where these ATPs come from.
Are they already present in living cells or environment? If so, how are they generated and what kinds of pathways are there??
Thank you!

Moderator note: Please keep all of your posts on this topic in the same thread so the expert who is helping you will see that you have a follow-up post. Thank you!

Hi there,

So I'm not sure how this comment came to this thread but I'd be happy to help nonetheless.

ATP is a nigh-universal "energy" molecule that catalyzes a whole bunch of biochemical reactions. Because it's so important, your cells are pretty much constantly making ATP. The ATP molecules are generally then made available to these processes by being present in the cell.

The main pathway for ATP synthesis is through a molecule called ATP synthase, which works in the mitochondria (the powerhouse of the cell!) during cellular respiration. Glycolysis does produce a net gain of 2 ATP, but this is definitely not the most efficient way to make ATP.

As for specifically how cellular respiration works: that's an incredibly complex pathway that I'm not sure how far you want to get into. You can always do your own research if you are interested, but I'm going to avoid going into the biochemistry for now. Maybe a more basic concept is that ATP is an adenosine with three phosphates (as the name suggests), so your body will try to put a phosphate onto an ADP molecule, adenosine diphosphate.

Hope that helps!