Bacteria are simple, single-celled organisms and are the most abundant type of life on Earth. A typical bacterial cell consists of a cell envelope, internal structures and external appendages. Unlike mammals and other eukaryotes, bacteria do not possess a nucleus; instead, the chromosomal DNA is found in a dense region of cytoplasm known as the nucleoid. Extra ring-shaped DNA is also found in some bacteria and these are known as plasmids (Ref 1,2).
A plasmid is a ring-shaped piece of DNA that is found within bacterial cells. Plasmids replicate independently of the chromosomal DNA found in the nucleoid but are always copied into next generation cells. Plasmids often contain genes that give bacteria genetic advantages such as antibiotic resistance. The genes within plasmids can be shared between bacterial cells in a process known as conjugation. It is this process that is partially responsible for the spread of antibiotic-resistant bacteria.