Standard curve relating Optical Density and the Slope of Methylene Blue (MB) reduction in the mixed culture of E. coli and B. subtilis . (a) Linear relationship between optical density and MBRT slope. Symbol open triangle represents experimental points. Solid line represents best fit with R2 = 0.987. Best-fit equation is given by Y = 0.0063 X. (b) Variation of OD at different time points during the growth of the mixed culture. Symbol open triangle represents equivalent OD at 600 nm as determined by MBRT; plus or positive symbol represents actual OD measured.