These are among the key questions scientists are asking as they strive to control the health hazards – and exploit opportunities – of the millions of bacterial cells we end up drinking every day.

Ever since humans identified the critical link between disease and the presence of bacterial contamination of drinking water, we have toiled to remove microbes by filtering them from water supplies.

However, many bacteria slip through and evade filtration processes. Researchers are increasingly interested in the biofilms that call the inside of drinking-water distribution pipes their home. Some findings even suggest that bacteria in tap water can be traced to filters used in the water treatment process, rather than to the aquifers or rivers where it originated.

Dr Cindy Smith, a lecturer in microbiology based at NUI Galway’s Ryan Institute for environmental, marine and energy research, leads an international group investigating the diversity of microbes in drinking water pipes. “There can be hundreds, sometimes thousands, of bacteria in every drop of drinking water, even in bottled water,” says Smith.

Microbes are present inside water distribution pipes as biofilms – slimy layers of bacterial colonies that exude “sugary, glue-like substances that allow them to stick to the pipe surfaces”, Smith explains.

The biofilm slime layers are similar to the plaque that forms on teeth after each meal; the gunk on the hulls of ships; or the nuisance biofilms on medical implants, such as catheters, which can provide cover for such “superbugs” as methicillin- resistantStaphylococcus aureus (MRSA). “Biofilms in drinking water pipes are important because they may be a source of pathogens or even places that promote antibiotic resistance,” says Smith.
Could microbes be good for you?
But what if microbes in your water could be good for your health? Dr Ameet Pinto at the University of Glasgow explains that his team focuses not just on the water pipe network, but on the filters used for treatment.

Pinto and his colleagues analysed water at every stage of the treatment process, including before and after filtration; on the filter biofilm; after chlorination; and at the tap. They found that it was never possible to fully eliminate bacteria from the water supply.

Pinto groups the bacteria into three types. “Pass-through” bacteria never stuck to the filters, whereas “strict colonisers” were found both before filtration and on the filters. The third group was found before, on and after the filters.

“We called this group the ‘leaky colonisers’. These bacteria attach to the filter systems, forming biofilms, and clumps can break off and reach the drinking water distribution network,” Pinto explains.

Further investigations by Pinto and his colleagues found that the pH of the water influenced which species made it to the pipes and to consumers’ taps.

“It might be that simple changes to the filtration system, rather than the distribution system, could influence which microbes emerge from biofilms and reach our taps,” he says.

“We need a paradigm shift to where we are not only concerned with producing ‘safe’ water but also ‘healthy’ water.” He concedes that the delivery of ‘probiotic’ drinking water to household taps may be some way off, however.