Easy nanosensor test for E. coli contamination

US researchers have unveiled a detector that can pick up diarrhoea-causing bacteria in milk and water in less than an hour. The nanosensor, reported in ACS Infectious Diseases, combines two lab technologies to quantify bacterial contamination. It could be used in developing countries as well as in food manufacturing.

Bacterial contamination is one of the world’s leading causes of illness and death, particularly in areas without, for instance, sanitised drinking water.

Escherichia coli is one species particularly linked to water and food contamination. Most strains are harmless but others, such as one known as O157:H7, can cause severe diarrhoea, vomiting and cramps.

In children, older people and those with immune disorders, it can be fatal. Part of its success is only a tiny amount of the bacterium is needed to make someone sick.

So testing food and water is imperative, especially in places far from adequate medical care. The simplest method is smearing a sample on an agar plate and analysing what grows.

But this takes time.

A number of lab tests can also find out if bacteria are present in a sample relatively quickly, but these complex tests are laborious and tend to need special expensive equipment.

An extra complication is they often require what’s called “sample amplification”, extra steps that boost the amount of a substance to a point where it becomes measurable.

Tuhina Banerjee and colleagues from Pittsburg State University combined two methods which don’t need sample amplification into a nanosensor test.

Why two methods? One technique, magnetic resonance, can detect bacteria in small quantities but not large. The other, fluorescence, is the opposite.

And together, they have all bases covered.

The nanosensors comprise special iron oxide particles combined with an optical dye, along with antibodies that specifically latch onto E. coli O157:H7 cells.

When mixed into a solution with bacterial colonies, the nanosensors swarm around their target’s outer membrane and cling to them but ignore other cells – even other strains of E. coli – and dead O157:H7.

This clustering is detectable using magnetic resonance. Just like MRI machines in hospitals, the bacteria detection test sends a magnetic field through the sample. But instead of measuring water molecules, as medical MRIs do, the detector picks up iron-rich nanosensor clumps.

The dye glows when the nanosensors cling to bacteria. If there are only a couple of colonies in a sample, the light won’t be bright enough to see. But if there’re loads, the sample will light up. The brighter the glow, the more contaminated the sample.

By tweaking antibodies, the technique could be used for a wide range of pathogens

The researchers tried their nanosensors on lake water as well as milk – common E. coli breeding grounds.

Sure enough, for small E. coli numbers the magnetic resonance detected the contamination. And when the solution contained lots of the bacterium, it glowed brightly.

And it’s not just O157:H7 the nanosensors can target. By tweaking antibodies, the technique could be used for a wide range of pathogens, the researchers write, to “solve old problems in new ways”.

The whole process takes less than an hour. 

Enzo Palombo, a microbiologist at Swinburne University in Melbourne, Australia, says he’s impressed at the technology’s ability to distinguish live O157:H7 from dead cells. He’d also like to see it tested in minced meat, which is where the O157:H7 strain tends to do its most damage.

But while it’s a quick method, the equipment needed to read the samples is still lab-based. The technology would certainly benefit from miniaturisation, Palombo says, so it could be used in the field.

So until that happens, current screening methods which give a simple yes-or-no answer without quantifying it, even if they take a few hours to amplify the bacteria numbers first, will prevail.