What is a plasmid and how is it passed between bacteria?

A plasmid is a small, circular DNA molecule that exists independently of the bacterial chromosome. It often carries genes that provide a selective advantage, such as antibiotic resistance genes. Plasmids can be transferred between bacteria via conjugation (direct transfer through a sex pilus), transformation (uptake of free plasmid DNA from the environment), or transduction (transfer via bacteriophages). This gene transfer allows for the rapid spread of traits like antibiotic resistance.