1. Inoculate S. meliloti 1021 cultures in 15-ml tubes with 2.0 ml TYR medium, incubate for 24 h at 30 °C on a rotary shaker (200 rpm). 2. Transfer 0.15 ml cultures to 50-ml tubes with 15 ml fresh TYR medium, measure OD600 at 600 nm. 3. When cultures reach OD600 = 0.7, split into 10 ml for biomass evaluation and 1.5 ml for TTC reduction measurement. 4. Centrifuge 10 ml cultures at 5,000 x g for 20 min, discard supernatant, dry cells at 65 °C for 4 h, weigh cells. 5. Centrifuge 1.5 ml cultures at 8,000 x g for 5 min, discard supernatant, wash bacteria with 1 ml 50 mM sodium phosphate buffer (pH 7.5), centrifuge, discard supernatant. 6. Resuspend cells in 1 ml 50 mM sodium phosphate buffer with 24 mM TTC, incubate at 30 °C for 1 h on a rotary shaker (200 rpm). 7. Centrifuge at 8,000 x g for 5 min, discard supernatant, resuspend cells in 1 ml 99.5% DMSO to dissolve formazan. 8. Centrifuge for 1 min at 13,000 x g, collect supernatants. 9. Prepare formazan standard solutions in 99.5% DMSO, read absorbance at 510 nm, use 99.5% DMSO as blank. 10. Plot calibration curve of formazan standards, determine formazan amount from reactions, normalize data to cell biomass, determine formazan amount per g cells.