To guard against harmful waterborne pathogens, many consumers, including managers of health-care facilities, install antimicrobial silver-containing showerheads. But in ACS ES&T Water, researchers now report that these fixtures are no "silver bullet." In real-world showering conditions, most microbes aren't exposed to the silver long enough to be killed. However, the composition of rare microbes in water from these showerheads varied with each type of fixture tested.

The stream of droplets and fine mist that form during a shower could be inhaled or swallowed. Installing showerheads containing silver -- a naturally antimicrobial metal -- is a cost-effective way to potentially protect against drinking water-associated pathogens (DWPIs). DWPIs, such as the strains Pseudomonas and Legionella, can cause infections in people with compromised immune systems. However, researchers have previously reported that reducing DWPI by adding copper and silver to a building's water system to eliminate bacteria and viruses (i.e., copper-silver ionization) is a temporary or minor fix. The copper-silver ionization study results have raised concerns in the scientific community that short-term silver exposure in showerheads could fortify microbes instead and increase the risk of antimicrobial resistance, rather than curb it. So, Sarah-Jane Haig and colleagues wanted to see how showerheads containing silver affected the microbial composition of the water.

The team compared two conventional showerheads made of either plastic or metal with three fixtures that contained silver -- as a silver mesh within the showerhead, a silver-coated copper mesh in the showerhead and hose, or as a silver-embedded polymer composite. The showerheads were run in Haig's full-scale shower lab facility, which was developed to mimic real-world conditions.

Contrary to the antimicrobial claims of manufacturers, silver did not reduce overall DWPI concentrations or total bacteria in water samples in the researchers' shower lab. Haig and her team reasoned that this is a result of differences in their testing conditions versus those of the manufacturers'. More specifically, in this study the water came out of the showerhead in a quick rush, as would happen when someone takes a shower. However, the standard tests used by manufacturers expose microbe cultures to the silver material alone for 16 to 24 hours, which the team says is not an appropriate timeframe or a realistic scenario.

Although the overall pathogen concentration in the water samples didn't vary between standard and silver showerheads, the composition of the microbiome did. The populations were different for each of the silver-containing fixtures, which suggests that the way the silver was incorporated into the showerheads mattered, and rare microbes accounted for most of the differences. Further experiments by the team suggest that microbes in the silver-containing showerheads could be more prone to forming microbial communities called biofilms in response to the stressful environment. In addition, biofilms might explain the association between the number of different species and showerhead age, say the researchers, because these communities are reservoirs for DWPIs.

"These findings underscore the need for improved testing standards, real-world performance evaluation, and innovative solutions to mitigate microbial risks in water systems, benefiting consumers and vulnerable populations," says Haig.