Bacterial plasmids are small, circular pieces of DNA that are separate from the bacterial chromosome. Plasmids commonly contain genes that provide useful traits to bacteria like antibiotic resistance, but are not essential for survival. Plasmids can be transferred between bacteria through conjugation. There are different types of plasmids classified by their functions, such as fertility plasmids involved in conjugation, resistance plasmids containing antibiotic resistance genes, and virulence plasmids that can make bacteria pathogenic. Plasmids are useful in genetic engineering and cloning because they can be easily manipulated in the laboratory by inserting foreign DNA fragments using restriction enzymes and ligase. This allows bacteria to produce proteins like human insulin.