Look and see if it is time to induce protein expression in Escherichia coli cultures.

It is shown that Methyl Red can be used as an indicator dye that changes color in Escherichia coli culture as a result of time- and cell density-dependent bleaching by azoreductase produced by the bacteria. For cell cultures that are being used to express a recombinant protein, this phenomenon can be exploited to provide a simple visual cue that cell cultures have reached an appropriate growth phase for addition of an agent to induce protein expression, such as isopropyl thiogalactoside.