Antibiotic resistance is the resistance conferred to the host the ability to survive a given antibiotic by the plasmids containing resistance genes. If such kind of plasmid is present in a host, that host won't be killed by (moderate levels of) ampicillin or tetracycline. By transforming plasmids containing the antibiotic resistance genes, one can get rid of all contaminating bacteria which do not have such type of plasmid, thus ensuring that the plasmid will be propagated as surviving cells divide.