Optimization of redox dye, buffer washing volume, and IEC for NESPA screening. (A) Down-selection results for 24 redox dyes based on reduction potential, O2 tolerance, molar absorptivity, and IEC selectivity. (B) Chemical structures of selected tetrazolium dyes and their colorimetric changes for mNADH detection. (C, D) Optimization of the redox dye for NESPA. Heat-treated colonies were overlaid by melted agarose solution containing all reagents for color development (NMN+ + 6PG). The percentages of NMNH signal, NAD(P) background, and dye background of the total signal are presented in the stacked bar chart. (E, F) Optimization of the buffer washing volume for NESPA. (G, H) Optimization of the IEC for NESPA. Heat-treated colonies were washed and overlaid by melted agarose solution containing enzymatic IEC (i.e., GsDI, PfuNROR, or TmDI) or PMS to identify the most selective IEC.