Felipe Lombo and his BIONUC research team from the University of Oviedo in Spain exploited a naturally occurring bacteria-binding protein, colicin S4, fused to green fluorescent protein. This fluorescent sensor produces a light signal when exposed to UV, making it a great tool for detecting bacterial contamination in drinking water.

Colicin S4 is produced by some bacteria to kill rival bacteria and specifically detects Escherichia coli, a bacterium that is an indicator of faecal water contamination. While colicin S4 provides specificity, green fluorescent protein emits a light signal that allows detection. The fusion of the two parts results in a sensor that can bind E. coli in contaminated water and produce light signals. Using a filter that retains the bacteria, unbound sensor molecules are washed away. If bacteria were present in the water sample, a light signal will occur in response to UV light. If no bacteria were present, all sensor molecules are washed away and no light signal is observed.

The signal can then be detected by a portable device that weighs only half a kilogram. Not only can it determine whether water is contaminated, but it also gives information about how many E. coli bacteria are present in the water. The researchers showed that this device can accurately determine between 20 and 1000 bacteria.

The current bacteria detection method takes several days to provide a result, which leaves a long time span between sample collection and response to contamination, during which, a large population could be contaminated. In most countries, water samples are taken at some key distribution points only once a day. The faster detection method would therefore allow for more immediate responses and makes more frequent sampling possible. This way, outbreaks can be prevented before they happen.

Lombo's method is not the first that offers faster testing of drinking water. However, it is the first method that is capable of doing so without a lot of equipment and cost. While the authors say that their method is cheap, they do not mention exact numbers; how the price compares to the current wait-and-observe-growth method remains to be seen.

Dangers of contamination
In recent years, disease outbreaks occurred in Norway and the USA, originating from contaminated water distribution networks. One of the microorganisms causing trouble was Giardia, which causes severe diarrhoea. In Canada, another microorganism called Toxoplasma, spread through contaminated water, damaging the brain of babies. Tropical countries face frequent outbreaks of cholera, another disease causing diarrhoea. In total, the World Health Organisation estimates that 663 million people lack access to safe drinking water and that contaminated water is one of the leading cause of deaths worldwide, after lung infections and AIDS.

Testing drinking water for all possible disease-causing organisms is impossible; there are simply too many of them. E. coli, which originates from faeces, is considered to be a good indicator organism because it does not multiply outside animals and therefore allows estimation of how much contaminating material was not removed from the water. The absence of E. coli generally indicates that the water was cleaned well. When the team examined water containing other bacteria, tests using this fluorescent sensor protein remained negative, indicating its good sensitivity.

Not just drinking water?
If this method is implemented, it would allow quicker monitoring of water quality and overcome the delay between contamination, detection and response. The low price of this fluorescent analysis and the inexpensive equipment necessary to carry it out could allow placement of several systems along the drinking water distribution pipelines, facilitating water quality monitoring every few kilometres, from source to final consumer.

The output is an electrical signal that, when above a threshold level, indicates bacterial contamination. Thus the results along a whole water distribution pipeline could be sent from a mobile phone to the water control headquarters, allowing a faster response in case of any contamination and preventing outbreaks. Finally, if the sensor turns out to also work in less pure water, its use could be expanded to the other two types of water that need regular testing: recreational water in pools and water used in farming.