Introduction

Abstract

Concurrent with the development of bacterial genetics in the early 1950s was the discovery in Escherichia coli of genetic factors not localized routinely on the bacterial chromosome. These included:

• 1. The fertility (F⁺) factors responsible for bacterial conjugation;

• 2. Factors responsible for the production of the bacterial toxins of the colicin type;

• 3. Factors responsible for bacterial resistance to antibiotics. It became evident in time that these factors, termed plasmids by Lederberg (1952), consisted of double-stranded DNA (dsDNA) and were able to propagate in either of two alternative modes: either autonomously in the bacterial cytoplasm(replicative plasmids) or as an integral part of the bacterial chromosome(integrative plasmids)