We all want to know whether water is safe to swim in, and even more importantly, safe to drink. Unfortunately, standard tests for bacterial contamination can take days. Luckily, those long waits may be a thing of the past. Researchers from McMaster University have developed a quick and easy test for E. coli.
The scientists used a new version of bioactive paper, the little strips that, among other things, are used to detect glucose in urine. While there are paper strips that detect bacteria, they aren’t nearly sensitive enough. Bacterial counts are measured in colony-forming units per milliliter of water (cfu/mL). Currently available paper assays can only detect from 104 to 107 cfu/mL. The safe limit for recreational water (water used for swimming or bathing) is less than five cfu/mL. For drinking water, the limit is zero cfu/mL. In other words, to be useful, such a test would need to be about a million times more sensitive.
In order to approach this level of sensitivity, the researchers coated their bioactive papers with substrates that change color in the presence of enzymes produced by E. coli. These chemicals can be layered onto the paper using an ordinary inkjet printer. You then add a lysing agent (a chemical which not only kills cells but effectively blows them up so that their enzymes are exposed to the outside) to your water sample, and apply the result to the paper.
With this new method, it took only a few minutes to detect five cfu/mL of E. coli. By placing the water sample in growth media for a few hours, and thus allowing any bacteria present to replicate, the authors were able to tell whether a single bacterial cell had been in the original sample.
The same bioactive papers successfully calibrated bacterial counts in milk, orange juice and on heads of lettuce. As an added bonus, the reagents embedded in the papers remained stable at room temperature for at least two months. In short, these new bioactive papers are cheap, easy to use and easy to store.
Sample test strip, McMaster University.