Plasma DNA Many naturally occurring plasmids contain genes that provide some benefit to the host cell, fulfilling the plasmid’s portion of the symbiotic relationship. For example, some bacterial plasmids encode enzymes that inactivate antibiotics. Such drug-resistance plasmids have become a major problem in the treatment of a number of common bacterial pathogens. As antibiotic use became widespread, plasmids containing several drug-resistance genes evolved, making their host cells resistant to a variety of different antibiotics simultaneously. Many of these plasmids also contain “transfer genes” encoding proteins that can form a macromolecular tube, or pilus, through which a copy of the plasmid can be transferred to other host cells of the same or related bacterial species. Such transfer can result in the rapid spread of drug-resistance plasmids, expanding the number of antibiotic-resistant bacteria in an environment such as a hospital. Coping with the spread of drug-resistance plasmids is an important challenge for modern medicine