The circular DNA molecule from a bacterium is called a plasmid.
Plasmids are small, circular pieces of DNA that can exist independently of the chromosomal DNA in bacteria. They often carry genes that can provide bacteria with advantageous traits, such as antibiotic resistance or the ability to metabolize certain substances. Unlike chromosomal DNA, which is essential for basic cellular functions, plasmids can be transferred between bacteria, allowing for genetic variation and adaptation. This property makes plasmids important tools in genetic engineering and biotechnology.