Dense, insoluble (i.e., not easily dissolved) protein bodies (i.e., clumps) that are produced within the cells of certain microorganisms. The refractile bodies function as a sort of natural storage device for the microorganism). They are called refractile bodies because their greater density (than the rest of the microorganism’s body mass) causes light to be refracted (bent) when it is passed through them. This bending of light causes the appearance of very bright and dark areas around the refractile body and makes them visible under a microscope.
Relatively rare in natural occurrence, refractile bodies can be induced (i.e., caused to occur) in procaryotes (e.g.. bacteria) when the procaryotes are genetically engineered to produce eucaryotic (e.g., mammal) proteins. The proteins are stored in refractile bodies. For example, the Escherichia coli bacterium can be genetically engineered to produce bovine somatotropin (BST, a cow hormone) which is stored within refractile bodies in the bacterium. After some time of growth when a significant amount of BST has been synthesized the Escherichia coli cells are disrupted (i.e., broken open), and the refractile bodies are removed by centrifugation and washed. They are then dissolved in appropriate solutions to release the protein molecules. This step denatures (unfolds, inactivates) the BST molecules and they are refolded to their native conformation (i.e., restored to the natural conformation found within the cow) in order to regain their natural activity. The protein is then formulated in such a way as to be commercially viable as a biopharmaceutical.
Refractile bodies are also known as inclusion bodies, protein inclusion bodies, and refractile inclusions. One point of interest is that the prerequisite for the generation of a mammalian protein by (in) a living foreign system such as E. coli is that the system used to generate the protein (1) must not have an immune system capable of destroying the foreign protein it is making or (2) the foreign protein made must be camouflaged or protected from any defense mechanisms possessed by the synthesizing organism.