Comparison of rapid and simple colorimetric microplate assays as an index of bacterial count

We investigated rapid and simple colorimetric microplate assays with tryptone–yeast–glucose broth (TYG) containing oxidation–reduction (redox) indicators (alamarBlue, MTS and WST-8), AquaTest (selective medium for coliforms) and PYP broth containing pH indicator (phenol-red) to assess cell counts using Escherichia coli, Staphylococcus aureus and Bacillus cereus strains as test organisms. In all redox systems, the estimation of aerobic cell concentration was possible by incubating at 35 °C for several hours. Although optical densities for MTS or WST were higher than that of alamarBlue, the time to obtain results (which may be correlated with doubling time) was longer. Color changes in AquaTest were 200–300 min later than for TYG containing alamarBlue and PYP.