A bioreactor is a large vessel in which a biological reaction or transformation occurs. Bioreactors are used in bioprocessing technology to carry out large-scale mammalian cell culture and microbial fermentation. For example, penicillin (an antibiotic) is made from the fungus Penicillium chrysogenum in a bioreactor; the vitamin riboflavin is made from the bacteria Eremothecium ashbyii; and the hormone insulin is made from the bacteria recombinant Escherichia.