 Many proteins previously thought difficult or impossible to be expressed in E. coli in folded form have been successfully expressed in E. coli . For example , proteins with multiple disulphide bonds may be produced in the periplasmic space or in the cytoplasm of mutants rendered sufficiently oxidizing to allow disulphide @-@ bonds to form , while proteins requiring post @-@ translational modification such as glycosylation for stability or function have been expressed using the N @-@ linked glycosylation system of Campylobacter jejuni engineered into E. coli . 
