When bacteria grow in aerobic environments, the terminal electron acceptor (O2) is reduced to water by an enzyme called an "oxidase". When bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase. In mitochondria the terminal membrane complex ("Complex IV") is cytochrome oxidase. Aerobic bacteria use a number of different terminal oxidases. For example, "E. coli" (a facultative anaerobe) does not have a cytochrome oxidase or a "bc1" complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.