 E. coli is a very versatile host for the production of heterologous proteins , and various protein expression systems have been developed which allow the production of recombinant proteins in E. coli . Researchers can introduce genes into the microbes using plasmids which permit high level expression of protein , and such protein may be mass @-@ produced in industrial fermentation processes . One of the first useful applications of recombinant DNA technology was the manipulation of E. coli to produce human insulin . 
