Why should ampicillin be added to extract plasmid dna?

Khan, I have never heard that ampicillin can destroy cell walls. In this case, alkali cracking is not needed. When expanding bacteria, ampicillin must be added to the culture medium, which will not affect the extraction itself, but it can prevent the growth of miscellaneous bacteria and increase the copy number of the target plasmid (provided that the plasmid is ampicillin-resistant, and it should not be added wrongly).