Ampicillin and LB broth

When you use Amp in LB broth, usually you are selecting the cells that had been genetically modified, i.e. they are resistant to Amp. This is a basic technique that been used in molecular biology in picking out the successfully modified cells during cloning. The other use of Amp is to kill off other cells like bacteris and fungi because LB is a medium that rich in nutrient, if you don't put an antibiotic, your solution will be very easily be containminated by other organisms that you don't want it to be in your medium. Also, according to the antibiotic genes in your cell, you can also put different antibiotic such as neromycin, tetracycline, and etc. But just Amp is the most usually and common as the restriction of using this is far less than using another antibiotic.

Usually when we prepare culture medium we will add some preservative to prevent other things, such as mold, bacteria, from growing in it.
So your point: ampicillin is used to screen off amp-sensitive cells is correct. Ampicillin is also used to keep away mold, and bacteria that are amp-sensitive is also correct. At the end, the mechanism is essentially the same as [selection] but we usually refer it to [preservation] rather than [selection].
What do you think?