 Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors.For example, "E. coli" (when growing aerobically using glucose as an energy source) uses two different NADH dehydrogenases and two different quinol oxidases, for a total of four different electron transport chains operating simultaneously.A common feature of all electron transport chains is the presence of a proton pump to create an electrochemical gradient over a membrane.