Many water purification methods introduced into the developing world are quickly abandoned as people don't know how to use and maintain them, says Norma Alcantar at the University of South Florida in Tampa. So she and her colleagues decided to investigate the prickly pear cactus, Opuntia ficus-indica, which 19th-century Mexican communities used as a water purifier. The cactus is found around the world.

The team extracted the cactus's mucilage - the thick gum the plant uses to store water. They then mixed this with water to which they had added high levels of either sediment or the bacterium Bacillus cereus.

Alcantar found that the mucilage acted as a flocculant, causing the sediment particles to join together and settle to the bottom of the water samples.

The scientists see communities in developing countries using the cactus on daily basis. They could boil a slice of it to release the gum and then add it to water just like the scientists did. But there are hurdles to overcome. What resources would be necessary for widespread growth of the cactus for this purpose and how can people ensure the "treated" water is truly bacteria free? If these problems can be solved, cheap, clean water could be accessible for millions of people that currently don't have any.