Introduction

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)