In this study, a reduced nicotinamide adenine dinucleotide hydride (NADH) biosensor (Frex) was introduced into lactic acid bacteria for the first time. By detecting the changes of NADH with environmental perturbations through NADH specificity, glucose metabolism, electron transfer chain inhibition, and pyruvate regulation, the feasible application of Frex in Lactobacillus paracasei was verified. Furthermore, the physiological and metabolic characteristics of the recombinant strain exhibited good performances on cell growth and metabolism, as well as the yield of L-lactic acid. Simultaneously, the level of oxidative and reductive potential (ORP) could be adopted to characterize the evolution of fermentation. Finally, from the viewpoint of process engineering with growing and resting cells, the in vivo measurement results using Frex revealed that intracellular redox conditions would be significantly influenced by extracellular ORP environments, as well as cell growth and metabolism were dependent on different optimal ORP levels in L. paracasei. (C) 2015 Published by Elsevier Ltd.