Showerheads—A Microbial Haven

Showerheads, something we all rely on but frequently forget to clean. Their design makes showerheads a perfect mechanism for the aerosolization of germs and a harbor for microbial growth. Bacteria belonging to the Mycobacterium genus have been known to propagate within showerheads and infect people through inhalation of the aerosolized mycobacteria. In hospitals, they are sometimes a source for disease transmission of waterborne microbes, such as Legionella. As such, the authors of a recent study sought to assess the microbial diversity of showerheads and evaluate just what is going on within these devices.

Investigators collected and assessed showerhead biofilm from 656 households across the United States and Europe. Not surprisingly, most common bacteria were Mycobacterium, but there was also a considerable amount of diversity. A total of 13.5% of the samples were identified as Mycobacterium genus.

Interestingly, Mycobacteria tended to be more common—twice as common!—in showerheads belonging to households that received municipal water rather than those receiving well water. The investigators noted that this is likely because of varying levels of chlorine use. Homes on municipal water had measured chlorine concentrations 15 times higher than those with well water. Mycobacteria are more resistant to chlorine and chlorine by-products than other bacteria, and so the chlorine levels play into their prevalence. Water source, chemistry, and household location also impacted microbial diversity.

Furthermore, the investigators found hot spots where there was a high abundance of pathogenic mycobacteria in showerheads within the United States. These hotspots tended to overlap in areas where nontuberculous mycobacterial lung infections were more prevalent. These areas varied by species, but hot spots for M avium complex were found in Florida, the Northeast, and Washington state, while hotspots for M. abscessus complex were found in Florida. These areas had the highest reported incidence of patients diagnosed with nontuberculous mycobacterial lung infections.

Personally, I was surprised to learn that “the abundance of taxa assigned to the genus Mycobacterium in showerhead biofilms in the United States was, on average, 2.3 times higher than that measured for showerheads in Europe.” The investigators suggested this is likely a result of differences in water treatment, water heating systems, source water type, showerhead design, etc. The considerable mycobacterial diversity within showerheads was impressive—dozens of difference clades were found.

Overall, this study, coined the ‘Showerhead Microbiome Project’, shed considerable light onto not only the prevalence of mycobacteria in showerheads across the United States and Europe, but also the impact of varying chlorine levels and even water pH. For example, households in the United states that received municipal water not only had higher concentrations of chlorine and iron, but also lower pH and nitrate levels. Chlorine levels in shower water were, on average, 11 times higher in US households than those in Europe. The investigators found significant correlations between median mycobacterial abundance and prevalence of patients suffering from pulmonary nontuberculous mycobacterial lung infections.

The consistent prevalence of mycobacteria in showerheads sheds light into their ability to transmit disease. This, coupled with the hot spots of nontuberculous mycobacterial lung infections, reveals a desperate need to increase the cleaning and disinfection of these household devices.