Is that water drinkable? Fast test may hold the answer


Enlarge / Because not all water can come from the Britta or fridge filter, after all.

Wodicka/ullstein bild via Getty Images

Detecting the agents of disease is often really hard. Imagine that you live in a village in a developing country. You may not have electricity, and your water comes via a well of unknown quality. Is the lining in that well sufficient to keep shallow, polluted groundwater from seeping in?

No matter how good your well-building skills are, you still need to regularly test drinking water to ensure that it is safe. A new development in detecting bacterial nasties has scientists saying there’s a solution, one that looks like high-tech litmus paper. But I’m not so sure it’s all it’s cracked up to be.

Testing your water

I have a brother who runs a non-governmental organization devoted to water safety and sanitation. On his last visit, he entertained my kids by testing the water from the local canal. Essentially, you put a sample of water in a test tube and put another on a plate with some bacteria food. The plate and test tube are left in a nice warm place for 24 hours. In the absence of electricity, this involves taping the samples in your armpit.

After you’ve cuddled up with your test tube and slept the night away, you visually examine the plate for colonies of bacteria and shine a UV light on the test-tube. E. coli glow when lit by UV light. A glowing test tube is the harbinger of some very bad diarrhea with an added dice roll on more serious health problems. E. coli is mostly harmless, but it provides an indication that sewage has entered your drinking water.

The advantage of this testing kit is that it is very low tech and relatively cheap. The UN hands these things out with a combination flashlight/UV lamp. And that low tech, cheap combination is actually the key to success: with a little bit of training, anyone can use it pretty much anywhere.

The only real downside is that you have to wait 24 hours to know if the water is safe. (Well, and you must walk around with a test tube in your armpit.)

Hidden high tech

To eliminate the 24-hour incubation time, researchers have developed a test based on a molecular recognition scheme. A bit of paper is printed with an ink that has DNA in it. The DNA consists of one short strand that is interrupted with a molecule that can emit light when it is excited. It doesn’t emit light in this state because the DNA attached to it suppresses its inner glow. The second strand is much longer, with a sequence that only matches the short strand at the ends. When they are mixed, the two strands pair up, which leaves a large section in the center of the long strand unpaired. This tangles up with itself, and the molecule is nicely stable in its tangled up state. This is important, because it means that the paper with the DNA will still be useful after months in the back of the cupboard.

The fun happens when you kill some E. coli by splitting them open. The contents of the cell spills out into the mixture. The sequence of the unpaired strand just happens to match a portion of E. coli’s DNA. The E. coli sequence pairs up with the un-paired portion of DNA, which strains the hell out of the DNA molecule, snapping it off from the molecule that glows.

Afterward, if you shine a UV light on the paper, the now-freed molecule will glow, revealing the presence of deadly E. coli.

The problem is that living E. coli—the sort you’d find in water samples—tend to keep DNA inside their cells, where it will never react with the paper. So the researchers added an ingredient to their ink: lysozyme, an enzyme that specializes in opening up bacteria.

With everything in place, the test goes as follows: a sample of water is dropped onto the paper. The lysozyme kills the E. coli. The contents of the E. coli snap the DNA, releasing the glowing molecule. After a few minutes, the paper glows in the presence of UV, warning you not to drink the water.

To glow or not to glow?

The researchers tested their kit on water, as well as milk and some fruit juices. It works in all cases, though the juices and milk had to be watered down. But frankly, even for water (where it performed best), the test is not good enough. It’s sensitive to 10,000 E. coli per ml—recreational swimming water should have less than four E. coli per ml, and drinking water needs a level that should be below the limit of detection. So, even with this fancy detection technique, you would have to incubate bacteria to let their population expand before you could reach the desired level of sensitivity.

You might be wondering why I am telling you about a poorly performing paper protector. It’s because I see potential. Perhaps not where the researchers intend it, but I do see it. First of all, the detection technique can also identify—at least to some extent—any bacteria that turn up. E. coli is mostly used as a proxy for sewage in the water. But, if we can directly detect specific harmful bacteria in the water, then we might be able to use that to quickly identify sources of pollution, making it easier to maintain clean water.

Still, for that to work, the sensitivity needs to increase to the point where incubation is not required.

Only the rich need apply

I predict, however, that this paper test will not turn up in the developing world for a very long time. The paper and the printing process are all quite cheap and can be made even cheaper by moving to large-scale production. The molecule, though, is not. You pay for (or prepare) this stuff by the microliter. While each dot only uses a tiny amount of molecule, it is still going to be relatively expensive.

In Europe and the USA, I can imagine this seeing use. If the researchers can increase the sensitivity, then it could provide valuable early warning for water supply operators. Knowing that water tests only take 10 minutes would both increase the safety of the water supply but also possibly allow for more efficient water treatment. It also seems like a good approach to providing a first rough survey when tracing infections in hospitals. So this research may be going places, just not to developing countries.

Scientific Reports, 2017, DOI: 10.1038/s41598-017-12549-3

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