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Bacteria Escape Water Treatment

By Stephen Luntz

Dangerous bacteria may be evading water treatment plants and getting into our drinking water by colonising amoebae, according to research at the University of NSW Water Research Centre.

The chlorine used during water treatment would normally kill microorganisms, but amoeba can form cysts that are resistant to chlorine at ten times the concentration used in treatment plants, says PhD student Jacquie Thomas. Once they are in more favourable conditions, the cysts become free-living amoebae again, capable of feeding and multiplying.

Although species of amoeba cause diseases that can blind those who wear contact lenses, or even kill people, these cases are exceptionally rare. On the other hand, dangerous bacteria such as Legionella and Mycobacterium can colonise amoeba. Once safely inside the cysts, they can escape treatments that would otherwise kill them.

Some bacterial species form a symbiotic relationship with their hosts, while others are parasitic, even killing the amoeba when their concentrations get too high.

Thomas conducted a review of 26 studies and was shocked to find that 45% of post-treatment water supplies had detectable levels of amoeba in them. “These amoebae are found in treated drinking water systems around the world and present an emerging health risk, although it is one that has not yet been quantified,” Thomas says.

“We don’t know how they are evading the treatment systems, but theoretically they must either be cysts going into the treatment or have a chance to change, possibly during the coagulation stage,” Thomas says.

Smaller amoebas can emerge from the cyst in a few hours, although larger species take longer. This gives those that survive a chance to multiply in the pipes to and within the house.

Thomas says work has begun to assess which species are a concern and what concentrations are dangerous. “Until we’ve done this we can’t provide advice on solutions,” she says, noting that most of the problem may occur within the home so treatment may need to move to the tap.

The problem has been overlooked until recently because the amoeba themselves pose such a low risk, but Thomas thinks that increased detection has occurred because more studies have looked for amoeba in water supplies rather than because the problem is becoming worse.