The genomic constitution of several bacteria may consist of two components, viz., chromosome that carries genes for all essential functions and their regulation, and an extra-chromosomal but autonomous unit, plasmid that was initially thought to carry functions required for its own replication, maintenance, and distribution. However, plasmids should not be considered selfish or promiscuous molecules as many of them have been described to carry genes providing functions of selective advantage. For example, they may code for resistance to several antimicrobial agents, such as antibiotics, heavy metals, or toxins, or for production of antibiotics, pigments, toxins, H2S, and others, or may provide unusual catabolic capabilities, or endow fertility, etc. They may also induce plant tumors, and other symbiotic and pathogenic responses in plants and animals, to name a few. There may also exist another category, however, where a plasmid may carry some essential genes as well.