Sonic barrier

The detection of bioagents and bacteria could be greatly improved with a technology that uses ultrasound waves to purify and concentrate bacteria from substances like blood or soil samples.

By cleaning and concentrating the bacteria, ProKyma, a spin-out from the Defence Science and Technology Laboratory (DSTL), says its Sonic Separation platform would enable scientists to more easily and quickly identify low-levels of bacteria in highly contaminated samples, and increase the sensitivity of molecular testing methods such as polymerase chain reaction (PCR).

‘We are generating ultrasound waves by passing electricity through a piezoquartz transducer,’ said ProKyma chief executive Damian Bond.

‘We have a specially-designed chamber where the sound passes through a number of different layers at the centre of which is a liquid layer, which is the chamber.

‘On the far side of that we have a reflector layer where the sound goes through and is reflected back. All the dimensions are optimised so we create a standing wave inside the liquid chamber,’ he said.

Normally, two soundwaves moving in a sine wave cancel each other out when they cross one another, although this can depend on device design.

In ProKyma’s technology, the scientists have designed the chamber so that the emitted and reflected soundwaves pass each other out of phase, creating a standing wave with a pattern that resembles a figure eight with the ends chopped off. The point where they cross over in the middle is known as the node.

‘The chamber is designed in such a way that there are acoustic forces in there which push any particles [of bacteria] that are in the liquid into that node,’ said Bond.

‘It is either a force which is pulling the particles into the node, or it is the fact that at the node you have high pressure and low pressure at the antinodes, and therefore particles are pushed from low pressure to high pressure,’ he added.

By collecting particles at the node, the technology washes the particles of the substance in which they were originally found (from blood and soil, to water and faeces) and concentrates them into a single area so that there is a greater chance of detection in a small sample.

‘One of the things we are trying to do is concentrate all the bacteria that is in a 10ml sample down into 100 microlitres. If we can do that we have improved the sensitivity of detection 100-fold,’ said Bond. ‘If you can do that with tests such as for TB, you can improve the sensitivity of the screening, which people are starting to say is not good enough,’ he added.

This means that scientists would be able to analyse a more concentrated sample, rather than a dilute sample of between 20 and 100 microlitres, which is all that existing methods can handle.

Cell cleaning is required in a number of diagnostic systems. This includes methods such as using an antibody to ‘speak’ to a bacteria; using a DNA probe to stick to another piece of DNA; and in PCR systems, where enzymes in the body are mapped then purified and fused out of the body to create a magnified replica of a DNA strand.

‘To make it [a diagnostic system] work properly, you need very controlled washing steps that are very efficient and rigorous to make sure that you have removed everything that could give a non-specific response,’ said Bond.

‘PCR works by multiplying copies of DNA up 20, 30 or 50 times. The problem we are starting to find with PCR is if you cannot guarantee that that sample will reproduce precisely 20 or 30 times, you are not going to get the same results from test to test,’ he said.

To diagnose bacteria, scientists break it open to release and test DNA. At the point of breaking, there is usually still a lot of original sample present, and this is seen as the main inhibitor to the PCR reaction. According to Bond, all existing methods break open the cell while there is still some element of the sample remaining, whereas ProKyma’s technology removes the sample before the cell is broken.

‘In engineering terms, we are trying to provide a stable baseline so that the reaction will be reproducible and reliable every time it is performed,’ he said.

At present, the company has a prototype of its technology, which comprises an enclosed fluidics chamber housing a disposable flow cell device, which allows samples to be analysed without the risk of cross-contamination.

Anh Nguyen