Cellular nicotinamide adenine dinucleotide (NADH) as an indicator of bacterial metabolic activity dynamics in activated sludge

In this study, native fluorescent nicotinamide adenine dinucleotide (NADH) was used as a direct indicator of bacterial metabolic activity in activated sludge. Specific NADH concentration was dynamic and varied between 10(6)-10(8) molecules per bacterial cell. Low concentrations (10(6)-10(7) NADH molecules cell(-1)) indicate efficient bacterial metabolic activity while high concentrations (10(7)-10(8) NADH molecules cell(-1)) indicate inefficient bacterial metabolic activity. Specific [NADH] did not correlate to changes in dissolved organic carbon, but increases correlated to decreases in oxygen uptake rates. Perhaps a lack of oxygen as the terminal electron acceptor prevented efficient reoxidization of NADH to NAD(+), which resulted in an accumulation of NADH within the cells. Also, significant amounts of NADH were released and accumulated into the extracellular medium of metabolically active E. coli cells in log phase. Such overflow metabolism may be the product of favourable conditions. Thus, the flux of both specific intracellular and extracellular [NADH] indicates the dynamics of bacterial metabolic activity in activated sludge.