Intracellular redox equilibrium and growth phase affect the performance of luciferase-based biosensors

Light emission from the bacterial luciferase operon has been variously exploited during last two decades. The use of convenient inducible promoters has granted significant degrees of specificity to whole cell-based assays for high-throughput screening and environmental monitoring. Nevertheless, unexplained unspecific responses have been repeatedly reported. Here, we show that the impairment of the intracellular biochemical equilibrium interferes with the luminescence produced by Escherichia coli and Staphylococcus aureus strains carrying the lux operon under constitutive or inducible control. Compounds as trimethoprim and methotrexate, by indirectly inducing NADPH accumulation, enhance light emission. Conversely, molecules driving the cell toward an oxidized state, as dimethyl sulfoxide, inhibit luminescence. These findings fit into the accepted biochemical pathway for bioluminescence, where NADPH and reducing equivalents are necessary for the production of luciferase substrates, although they do not directly take part into the light-emitting reaction. Moreover, we investigated the influence of induction timing upon the bioluminescence response from inducible reporter systems and demonstrated a correlation between the emitted light and the growth phase at which induction is performed. Our results provide explanations for some unspecific responses recorded so far in whole cell-based luminescent biosensors and emphasize the intrinsic limitations of this kind of reporting system.