Stored water can develop slimy bacterial films: nanotechnology may be a solution

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When drinking water is stored in containers, a thin layer of microorganisms can grow at the interface of the container and water. This thin layer is called a biofilm. It's made up of bacteria that make the water unsafe to drink. Slimy biofilms are harder to kill with ordinary disinfectants like chlorine. So scientists are always looking for new ways to clean water in household storage systems. Nanotechnologists Lijo Mona and Muthumuni Managa are exploring innovative ways to eradicate pathogens in water. They've reviewed the ways that photosensitiser agents (molecules that absorb light and transfer energy to other molecules) can trigger the chemical and biological changes that can make water safer to drink.

What water problems does South Africa face?

Access to water and sanitation is a basic human right provided for in the South African constitution. However, only 77.1% of households have access to reliable water sources.

Many rural areas and villages experience severe water shortages because of minimal municipal infrastructure or water scarcity. Often, rural families get their water from polluted rivers.

In some areas, municipalities deliver water in tanks that people collect and store at home over a long time in buckets. But this poses an additional health risk as microorganisms may form bacteria-filled biofilms inside the buckets.

Using contaminated water has led to the loss of lives from waterborne diseases such as cholera, typhoid fever and diarrhoea. Most at risk are children aged five years and below and elderly people who have compromised immune systems. Water containing pathogens such as bacteria, viruses, fungi and protozoa causes diseases that can cause rapid dehydration, perforation of intestines, persistent digestive issues and kidney failure.

How can nanoparticles help?

We carried out a review of how different types of light-activated chemicals are used to kill microorganisms in water. Antimicrobial photodynamic inactivation is a light-based method that destroys disease-causing microorganisms in water. It involves the use of tiny nanoparticles – as small as a billionth of a metre.

These nanoparticles are usually made up of metals or their compounds containing other elements such as oxygen and sulphur as well as some non-metals. The compounds can be used as they are or combined with organic dyes that are activated by light.

When these materials interact with sunlight, they create several highly reactive oxygen-based molecules, including hydrogen peroxide and oxygen radicals. These molecules attack and damage key parts of bacterial cells, such as proteins and cell membranes, causing the bacteria to die.

Our review found that using molecules that can be activated by light to clean water prevents formation of biofilms. This means that water can be stored in buckets and other containers without people running the risk of getting waterborne diseases. We found that if the containers are exposed to sunlight occasionally, this works best.

Can we look forward to a breakthrough any time soon?

This method works against many different kinds of microorganisms, but its effectiveness can be improved depending on the properties of both the light-activated compounds and the microbes themselves. Different large ring-shaped molecules can be activated by light to help destroy pathogens, but how well they work depends on their structure.

For example, some bacteria attract positively charged molecules of photosensitisers to their surface. When exposed to oxygen and light, these molecules produce reactive compounds that can attack and kill the bacteria more effectively.

The more of these reactive species that are produced, the more effective they are at killing harmful microbes.

Scientists have found several ways to make antimicrobial photodynamic inactivation more effective. By slightly changing the structure of the light-sensitive molecules or adding certain metals and other chemical elements, they can increase the production of reactive oxygen compounds that kill microbes.

Attaching these molecules to nanoparticles can boost their performance even further. Because nanoparticles have unique properties, they can help generate more of the compounds that destroy harmful bacteria and other pathogens.

What needs to happen next?

Early research into using light-activated chemicals to stop slimy bacterial build-up (biofilms) in household water tanks is promising. Certain types of these chemicals seem good at killing many kinds of pathogens, including bacteria, viruses and fungi. But they still have challenges that make them difficult to use in real life, such as not working well against some tougher bacteria.

Scientists need to improve these chemicals so they produce more pathogen-killing molecules and work better overall. At the same time, we don't want them to introduce toxic metals into the water.

Studies in laboratories need to be scaled up, so that antimicrobial photodynamic inactivation can be tested in the field, especially in real life water storage systems.

Muthumuni Managa receives funding from The Royal Society (ISPF - International Collaboration Awards) and NRF.

Lijo Mona receives funding from The Royal Society through the ISPF - International Collaboration Awards.

By Muthumuni Elizabeth Managa, Associate professor, University of South Africa And

Lijo Mona, Postdoctoral fellow, University of South Africa