In genetic engineering, DNA molecules are placed within bacteria and/or virus cells and allowed to grow, producing new cells that can then be placed into humans or animals. Once there, it is believed that they can be used to replace diseased or abnormal genes.
As eNotes.com says . . .
"Bacteria are cheap, easy to grow, clonal, multiply quickly, relatively easy to transform and can be stored at -80°C almost indefinitely."
It also states that the new gene can be stored inside the bacterium indefinitely. But, genetic engineering doesn't work with bacteria, so scientists have started using yeast. And, a genetically modified virus has been used to develop a lithium ion battery that is easier on the environment.
Some of what is being done in this relatively new field is exciting, but a lot of it is frightening! Let's hope that we don't create a real live sci-fi horror sometime in our future!