Transgenic bacteria contain foreign gene that is intentionally introduced into its genome. They are manipulated to express the desirable gene for the production of various commercially important products.

An example of transgenic bacteria is $E$.coli. In the plasmid of $E$.coli, the two $DNA$ sequences corresponding to $A$ and $ B$ chain of human insulin are inserted, so as to produce the respective human insulin chains. Hence, after the insertion of insulin gene into the bacterium, it becomes transgenic and starts producing chains of human insulin.

Later on, these chains are extracted from $E$. coli and combined to form human insulin.