On-line monitoring of NAD(P)H fluorescence and 2D fluorescence spectroscopy was performed with Enterobacter aerogenes, a bacterium sensitive to oxygen availability. The organism was grown in a reactor under low and high dissolved oxygen concentrations and circulated through a bypass attached to the reactor. Under low dissolved oxygen concentration in the reactor, NAD(P)H fluorescence in the reactor and the bypass showed a deviation, but not when the dissolved oxygen level in the reactor was high. The pattern of growth curves was identical under low and high oxygen levels. This indicates a difference in the metabolic activity of E. aerogenes in response to oxygen. The difference spectrum of the 2D fluorescence shows that growing E. aerogenes under high dissolved oxygen levels increases the NAD(P)H content of the cells.