Plasmids carry certain genes that give the bacterial host a selective advantage. Plasmids contain antibiotic resistant genes which confer resistance to a number of antibiotics. Such plasmids are known as R Plasmids. Some bacteria contain plasmids that determine the formation of Bacteriocins which are proteins that kill same or other closely related species of bacteria.
Plasmids also give the bacteria new metabolic abilities, make them pathogenic or confer them with numerous other properties.
