Detecting Contaminated Water With A Simple Color Change

A canny choice of a color-changing molecule—and a simple way to make it—has helped researchers develop an easy and potentially inexpensive test for Escherichia coli in drinking water for use in developing countries (Environ. Sci. Technol. 2014, DOI: 10.1021/es502319n).

Anthony P. Davis of the University of Bristol, in England, and his team based the new test on existing ones that use glucuronides as indicators. An enzyme that’s common in E. coli but not other bacteria, β-glucuronidase, hydrolyzes these compounds to release glucuronic acid and a dye molecule. When released, these dyes either change colors or develop fluorescence, which serves as the signal that E. coli is present. But commercially available glucuronides are expensive or require extra equipment, like a fluorescent lamp, to detect.

To lower costs, Davis looked for a glucuronide with a dye that produced an intense color change so that researchers could use less indicator for each test and save money. He and his team picked resorufin β-D-glucuronide (REG), which previously had been used for histochemical analysis. REG is orange, but when hydrolyzed it releases resorufin, which turns pink at a normal biological pH.

In their test, the scientists used just 0.1 mg of REG per 100 mL of sample solution and got a distinct orange-to-pink change after 20 hours’ incubation. With the indicator, the researchers could detect as little as 10 E. coli bacteria in a sample.

Davis’s team tried to lower costs further by designing a new synthesis for REG. They removed expensive steps, including the need to purify the final product using reversed-phase HPLC, presumably used in the commercial manufacture of REG, Davis says.