New, quick method for identifying food-borne diseases

Published 16 July 2008

European researchers have developed a system which prepares samples and performs DNA tests on the salmonella and campylobacter bacteria in a portable and cost-effective chip

As the U.S. authorities try to identify the source of the recent salmonella attack which has already claimed more than 1,000 victims — the main suspects are red tomatoes and jalapeno peppers — it is good to read that food-borne diseases could take only minutes to detect with a new lab-on-a-chip device that uses a material more commonly found in the semiconductor industry. European researchers have developed a system which prepares samples and performs DNA tests on the salmonella and campylobacter bacteria in a portable and cost-effective chip.
They claim that it can provide results in twenty-five minutes, in comparison with existing methods that can take hours or days. Dr. Jesus M Ruano-Lopez is co-ordinator of the EU-funded OptoLabCard project based at Spanish research centre Ikerlan. He said: “The idea was to explore the possibilities of using SU-8, a structural material that has been used for microelectronics, for lab-on-a-chip applications. In order to validate that idea, we proposed two applications - one is the detection of salmonella for clinical purposes and the other the detection of campylobacter for chicken farms.” Other research partners include food safety and animal research centre Dianova in Denmark, the Institute of Electron Technology in Poland, and Germany’s Micro Resist Technology.

The researchers say the benefits of a SU-8 photoresist are that it is a relatively inexpensive material compared with silicon and glass-based materials, fairly chemically resistant and its surface properties can be easily controlled or altered by plasma or chemical treatment. One of the main challenges of the project was ensuring that a sample would contain a sufficient concentration of bacteria for a high level of accuracy in the detection of a disease. The team has used real-time polymerase chain reaction (PCR) techniques to overcome this in the analysis of the sample, in addition to using magnets in the sample preparation stage. PCR is a well-established method of replicating DNA to create higher concentrations in a sample. Ruano-Lopez said: “If you have a one milliliter sample, why waste it? If you took two microlitres, you might not catch the bacteria, so you need to use the whole one milliliter sample. So we have developed a technique based on magnetic bits implemented on a chip. The magnetic bits bond with the bacteria or DNA in the sample, and using magnets we can capture the magnetic bits as well as the DNA.” Hence, in order to carry out an analysis a syringe full of the sample containing magnetic bits is shaken, to enable the bonding between the magnetic bits and DNA, and then injected on to a disposable chip that is placed into the portable device. The injection is made with an external magnet close to the chip to allow it to separate and purify the sample. “When this is done, you add the PCR reagents contained in a second syringe, then you close the lid of the portable device and start doing the PCR. Following the PCR, using fluorescence probes we are able to measure the progress of this amplification and that’s it. After 25 minutes, you open the lid, take out the chip and put a new one in,” said Ruano-Lopez.

The results of the analyses are now being displayed on a laptop, but the scientists hope to design the system so that it can be connected to a PDA.
While the existing prototype requires a use of the second syringe, Ruano-Lopez said the consortium is also working on a way to avoid using a second syringe, perhaps by storing reagents within the chip. The researchers have also been careful to reduce any risks of contamination with a special “packaging” design. “We need three things to connect this chip [into the system] — the fluid, the electricity, which is the power, and the light for the fluorescence detection. So we have developed an interfacing system that does not use glue, wire bonding or any fixed connection, which allows us to remove anything that has been contaminated and replace it with a new one,” said Ruano-Lopez.