The choice of cells to use in biopharmaceutical production depends on a number of factors, some technical and some economic. Bacteria and yeast, for instance, are relatively simple to grow. Each cell of a bacterium or yeast is an independent organism capable of its own metabolism. Yeasts and bacteria have fairly simple nutritional needs and grow well suspended in a liquid medium, as well as in large fermentors.
Various kinds of cells can all be made to express the same protein. However, a protein produced by a bacterium may have different effects than the same protein produced by animal cells. After a protein is expressed in the cell, it goes through a process called posttranslational modification. Molecules of sugars and carbohydrates attach themselves to the protein (a process called glycosylation). The protein may fold itself into a different configuration, changing the surface available to attach to other molecules in the body. Folding and glycosylation have a great effect on the ability of a protein to be used for a particular process, and different types of cells perform these modifications in different ways. The choice of the right cell to culture may result in a protein that is more appropriate for use—or in the elimination of extra steps in the manufacturing process.