Microorganisms are now being used to make a huge range of food products and medicines, enzymes and even fuels. Many of these reactions take place in large vessels called fermenters. These are huge containers that hold up to 200 000 dm3 of liquid. They make it possible to control the environmental conditions such as temperature, oxygen and carbon dioxide concentrations, pH and nutrient levels, so that the microorganisms can grow and respire without being limited and can work as effi ciently as possible. It is very important that everything in the fermenter is sterile, so that only the microorganisms that are wanted grow in the culture. Penicillin, a widely used antibiotic, is made in an industrial fermenter using Penicillium mould.