The accessory chromosomes known as plasmids have figured of bacterial genetics and of molecular biology as a whole. They include drug-resistance factors, which are accessory chromosome proper that have become of great practical concern, causing, as they do, a large proportion of the antibiotic resistance prevalent in medicine and veterinary practice. Two other well-known classes are the sex factors, which enable bacteria to transfer their genes to recipients by conjugation, and the bacterial phages, viruses that grow in bacteria. Despite the great properties, all plasmids exhibit strong fundamental similarities, and it is these general properties, rather than their individual peculiarities, which are emphasized here. The book first shows how different types of plasmid were noticed and then goes on to the physical structure of their chromosomes, their manner of replication, and how they act to bring about conjugation and other processes they determine. Although the treatment is primarily biological, the practical scope of the subject, regarding both laboratory techniques and the implications for human and animal medicine, has been kept in mind by including appropriate references.
