Detecting water-borne diseases

Jin Dayong

PhD student Jin Dayong is using cutting-edge techniques to detect potentially deadly diseases, such as Giardia and Cryptosporidium, in our drinking water.

After completing undergraduate and Masters degrees in China, Jin began his PhD under the supervision of Professor Jim Piper within the Centre for Lasers and Applications, Department of Physics at Macquarie University in 2003. The aim of his project is to develop a new device that can quickly spot these waterborne protozoan pathogens, the cause of much disease and even death for those with compromised immune systems.

About fluorescence

In recent years, our ability to detect microorganisms – both good and bad – in our environment and also within our bodies has increased dramatically due to the use of fluorescent molecules and chemicals, which naturally ‘glow’ when struck by light of a certain colour. These fluorophores can be attached as ‘labels’ to specific organisms, and when viewed under a powerful microscope and excited by a laser they help us to identify the target microorganisms, track their movements and observe their changes and processes.

The difficulty in detecting pathogens like Giardia and Cryptosporidium, however, is that they are very uncommon, yet just a few cysts can cause serious illness. And because the water in which they live is full of algae and mineral particles which also naturally fluoresce, it can be like looking for a needle in a haystack.

“Because of this problem, we use a technique called time-resolved fluorescence,” says Jin. “That means the fluorophores we label the specific targets with have a much longer fluorescence decaying lifetime than the naturally occurring autofluorescence from backgrounds, permitting time-gated detection of desired signals.”

Time, cost and size

The next obstacle Jin encountered was the enormous time it took to count the cells in any sample.

“We eliminated 100-fold background fluorescence by building a time-resolved fluorescent microscope, but it still took much too long to count each cell on the slide – you can count for a number of hours to find the number of cells in just one drop,” he says. “So I decided to introduce the sample as a continuously flowing sample. The sample will be sucked up through a micro capillary, be excited by the laser, and then pass through the microscope. Theoretically, this technique – called flow cytometry – can analyse 10,000 cells per second.”

The third, and most crippling dilemma, however, was the bulky size and high cost of the laser needed for the device. Because Jin wanted a compact, cheap instrument, he began to investigate other light sources.

“Instead of a laser, I decided to use an ultraviolet light emitting diode, which only became available last year,” says Jin. “In tests I found that with just a two milliwatt light emitting diode – costing just $5 – I can detect the time-resolved fluoropheres at cellular level. I will now move on to a more powerful diode, which can yield around one hundred milliwatts of power, equally sufficient to laser sources. These cost $100, whereas a laser costs at least $30,000.”

By the end of the year, Jin hopes to have produced a small, inexpensive device that can continuously monitor pathogens like Giardia and Cryptosporidium cysts in real-time, and which may lead to the detection of HIV in humans.

“It’s currently quite hard for medical researchers to find the exact number of contaminated blast cells,” he says. “Because we can look at 10,000 cells per second, and find Giardia or Cryptosporidium, we can perhaps use it in other areas like clinical diagnosis, the dairy industry and food industry.”

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