Animal cells are much more fragile. They are often much larger than any microorganism and evolved to live in a collective
as part of organs or tissues within complex anatomical systems. They are held together only by a delicate membrane, they are
more difficult to grow in suspension, and they often have to grow attached to surfaces. These cells are complex, with systems
of cellular machinery inside of them. Animal cells replicate slowly, require complex nutrients, and do not grow as well at
high densities because of waste accumulation and oxygen stress. Animal cell culture is more complicated and thus more expensive
than traditional fermentation.
TABLE 1: Approximate times from introduced gene to protein production at usable levels.
But ease of growing cells is not the only issue. Yeasts and bacteria may be easy to grow, but there are limits on the size
of genes that can be planted in them. If it is necessary to clone a larger chunk of DNA, then it may be necessary to use animal
or plant cells.
In addition, though various kinds of cells can all be made to express the same protein, a protein produced by a bacterium
may have different effects than the same protein produced by animal cells. Why? 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.
Some cells express a protein of interest and then keep it within their cellular membranes. Others secrete the protein, transporting
it across those membranes into the liquid the cell is growing in. For biotechnology, secreted proteins are preferred because
they are easier and more cost-effective to collect and purify.
What follows is a brief introduction to the types of cells currently used in biopharmaceutical production. A more detailed
discussion of the pluses and minuses of particular cells and processes can be found.
But first, a note on terminology: In discussions of biotechnology, you will hear the process of growing cells referred to
as both fermentation and cell culture. The terms are close in meaning, but the biopharmaceutical industry tends to distinguish
between them, using fermentation to refer to the cultivation of single-celled organisms such as bacteria and yeast, and cell
culture to describe a specific kind of fermentation used to grow cells that come from multicelluar organisms such as animals
Bacteria. Many biopharmaceuticals are produced by bacteria, especially the species Escherichia coli and Bacillus subtilus.
The DNA in bacteria occurs on a single circular chromosome and in small ring-like plasmids. These circular shapes keep the
DNA safe from fraying at the ends, and they can be taken up and used by many kinds of cells, which makes them convenient for
use as cloning vectors.