Linking pathogens to pH: A novel way to detect E.coli?

By Joseph James Whitworth contact

- Last updated on GMT

Litmus test results using apple juice, milk and lake water, with clean water as a negative control (NC). Photos are when litmus test just begins (0 min) and proceeds for 15, 30 and 60 minutes. NC sample does not change colour over time
Litmus test results using apple juice, milk and lake water, with clean water as a negative control (NC). Photos are when litmus test just begins (0 min) and proceeds for 15, 30 and 60 minutes. NC sample does not change colour over time

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A method to detect E.coli has been developed by linking pH to the pathogen.

Dr Yingfu Li, professor at McMaster University, and his team created a test for E. coli based on the Litmus test for pH. The test is a colorimetric sensor where existing Litmus dyes and pH papers respond to pH changes by producing a color signal.

Li, president and chief scientific officer of start-up company Innovogene, said the new colour of the Litmus paper represents the presence of bacteria.

The firm develops paper-based systems which use pH and colour scaling to illustrate the concentration of pathogens or carcinogens.

Simple test

Li said the team worked fast and had a proof of concept in a few weeks, a workable test a few months later and then they took a year to validate the results. The paper can be viewed here​.

“The whole idea is a simple test that everyone can perform in an industrial setting or as a home-based test,” ​he told FoodQualityNews.com.

“After sample preparation the method takes about eight hours. Six or seven hours to detect single cells so if there was more it would take less time.

“We are working on paper-based biosensors and trying to use biological molecules printed on and keeping them performing there as we would like them to do.”

The method uses a bacteria-specific RNA-cleaving DNAzyme probe as the molecular recognition element and protein enzyme urease to elevate the pH value of the test solution.

By coupling urease to the DNAzyme on magnetic beads, the detection of bacteria is translated into a pH increase, which can be detected using a litmus dye or pH paper.

Linking molecular recognition to pH change

Through devising a method that links molecular recognition to the pH change of the sensing solution, the researchers said they could take advantage of litmus dyes and pH papers.

“Our sensing system also takes advantage of magnetic separation, which is easy to implement, and pH sensitive dyes or pH paper strips, which are cheap and widely available,” ​said the researchers.

“To our knowledge, this is the first example where a molecular recognition event of an aptazyme (or any functional nucleic acid) is translated into a pH change.”

Although an E. coli-sensing aptazyme was used, the sensor design can be extended to any RNA-cleaving aptazyme.

Li said there was also opportunity for high throughput.

“We are hoping first to continue to expand the technology into other food pathogens and go into disease,” ​he said.

“We are looking at special needs, such as collaboration in Africa to test water to see if it needs to be treated and get the sensor to detect a single cell in eight hours which is one work shift.” ​  

Related topics: Food Safety & Quality

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