Selective medium - how does it work? (Apr/11/2010 )
Hallo all,
I have a question regarding selective media.
I have been reading on a medium that they use to identify bacteria.
One of the statements they make is that the specific bacterium that you look for can be seen as a yellow bacterium since they can use the sucrose as a c-source and they will turn yellow because of this.
But why would they change from the original color of the medium into yellow?
I suppose they add some pH indicator in the medium too that will change color when the sucrose will be fermented and the pH drops?
or ?
lyok on Apr 11 2010, 09:10 AM said:
I have a question regarding selective media.
I have been reading on a medium that they use to identify bacteria.
One of the statements they make is that the specific bacterium that you look for can be seen as a yellow bacterium since they can use the sucrose as a c-source and they will turn yellow because of this.
But why would they change from the original color of the medium into yellow?
I suppose they add some pH indicator in the medium too that will change color when the sucrose will be fermented and the pH drops?
or ?
Ya thats right. Its a pH indicator that has one color in acidic medium and another in a basic medium.
Eum intersting topic, I have however seen the following question somewhere else and wonder what you guys think about it.
Will the pH drop then or can it go up too?
I can imagine it will only go down since you want to select by fermentation of a typical sugar or substance.
or?
You can have both: pH drop or a rise.
It depends on the medium and what you look for.
The use of peptone is an example. This can make the pH go up.