Sunday, 27 November 2011

Human-associated species dominate restroom bacterial communities

The results of a recent study characterizing microbial communities in public restrooms may be enough to change the ways of those who don't already wash their hands after using the toilet. The study found that restrooms are dominated by human-associated bacteria--particularly those from the skin, gut, and urinary tract. Although this is not particularly surprising, the "biogeography" of these species--or which surfaces they were most commonly found inhabiting--was a bit unexpected. While there were three distinct clusters of species (bacteria on or near toilets, those on the floor, and those on surfaces mostly touched by hands), there were also several groups of bacteria that were present on all surfaces. This includes gut and urine bacteria, which were found not only on the seats and handles of toilets, but also on sink faucet handles, soap dispensers, and even on the handles of doors leading out of the restroom--in other words, the areas touched after hand-washing has already been completed.

(Corynebacterium diphtheriae, an Actinobacteria)

The study, conducted by researchers from a variety of departments at the University of Colorado--Boulder, was not designed to produce results to frighten the squeamish, but to better understand the bacterial communities living inside human establishments. This has also been the goal of previous investigations, but those studies have generally relied on cultivation-based techniques that may fail to document the many bacterial species that cannot be grown in the lab. The current effort used a cultivation-independent technique known as barcoded pyrosequencing. This method was focused on 16 S rRNA, a highly conserved bacterial gene. Similar gene sequences from across the sampled communities can be clustered into "operational taxonomic units" (or OTUs, a bacterial equivalent of "species" that takes into account the unique way in which bacteria can share genes with each other) that can then be used to assign taxonomy and generate a phylogenetic tree.

In this case, the bacteria in question were sampled from 10 surfaces in each of 12 public restrooms (6 male and 6 female): door handles into and out of the restroom and restroom stall, faucet handles, soap dispensers, toilet seats, toilet flush handles, the floor around the toilet, and the floor around the sink. Bacteria in restroom communities were compared to those found in potential source locations, including the tap water flowing into the restrooms' sinks, soil that could be tracked in on shoes, and humans (from their skin, mouth, gut, and urine). The study also explored the similarity of bacterial communities on different surfaces within bathrooms in order to see whether certain types of bacteria typically cluster in particular areas.

(Streptococcus pneumoniae, a member of the phylum Firmicutes)

Although a total of 19 phyla were observed across all restroom surfaces, most samples were representatives from one of four phyla: Actinobacteria, Bacterioidetes, Firmicutes, and Proteobacteria. Interestingly, these were the same phyla identified by previous studies utilizing less powerful censusing techniques. Certain taxa were found on all surfaces. The most common of these were associated with human skin (Propionibacteriaceae, Corynebacteriaceae, Staphylococcaceae, and Streptococcaceae), which is not surprising given that they are known to be fairly hearty species that can survive for extended periods away from their preferred environment. More disturbing was the finding that some gut, mouth, and urine bacteria were also found on all surfaces--even on door handles leading out of the room.

The community-level analysis revealed that bacteria could be separated into three broad groups: those on the floor, those on toilet surfaces, and those on surfaces touched by hands. Floor surfaces had many low-abundance taxa and were the most diverse; these environments harbored an average of 229 OTUs, in comparison with 150 found in other areas. Occasionally, "floor species"--such as those typically found in soil--were also observed on toilet handles, indicating that some people were probably flushing with their feet in order to avoid touching germy surfaces. The most troubling observation was the "enrichment" of toilet seats with bacteria associated with the human gut; a high prevalence of these species could encourage outbreaks of intestinal illnesses.

Mouths (thankfully) were not major contributors to restroom bacteria. Neither was tap water running from the faucet. Ventilation systems and cleaning equipment were not sampled in the current study, and the authors admit that these could be important contributors to investigate in the future. Overwhelmingly, though, these results support previous findings that humans (in particular, our skin, gut, and urine) are the main source of bacteria in our environments. The patterns reported here also emphasize the importance of good hygiene in minimizing transmission of human pathogens: Even the cleanest person can pick up unfriendly bacteria by merely touching surfaces exposed to other individuals. Any readers feeling a bit disgusted about our species' cleanliness should take heart. While some of the bacteria may have been transferred directly to bathroom surfaces via contact with our skin, water splashing and aerosolization of particles during flushes are also likely culprits.

Flores, G.E., Bates, S.T., Knights, D., Lauber, C.L., Stombaugh, J., Knight, R., and Fierer, N. 2011. Microbial biogeography of public restroom surfaces. PLoS ONE: 6(11):e28132.

Thanks to the following websites for providing the images used in this post:

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