The major components of a bacterial cell are the plasma membrane, cell wall, and a nuclear region containing a single, circular DNA molecule. Plasmids—small circular pieces of DNA that exist independently of the bacterial chromosome—are also present in a bacterial cell. In addition, some bacteria may have flagella, which aids in movement; pili or fimbriae, which are short, hairlike appendages that help bacteria adhere to various surfaces, including the cells that they infect; or a capsule of slime around the cell wall that protects it from other microorganisms.