Bacteria are one celled organisms, and so they do not reproduce sexually, like mammals. Instead, most bacterial reproduction comes in the form of simple cell division, where the parent bacteria basically makes a copy of itself. However, some bacteria have developed a way to increase the genetic diversity of their population. They do this using smaller, circular strands of DNA, that can be passed along from one bacterium to the next without the need for cellular division. These small DNA strands are known as what?