Lessons learned when looking for non-neutral ecological processes in the built environment: the bacterial and fungal microbiota of shower tiles

With periodic pulses of water, bathroom showers represent a habitat in the built environment with a high potential for microbial growth. We set out to apply a neutral model of microbial community assembly and to identify deviations from the model that would indicate non-neutral dynamics, such as selective pressures for individual taxa, in this particular indoor habitat. Following a cleaning event, the bacterial and fungal microbiota of the shower stalls in two residences in the San Francisco Bay Area were observed over a four-week period. We observed strong differences in composition between houses, preventing us from combining samples and thus limiting our statistical power. We also identified different aspects of the sampling scheme that could be improved, including increasing the sampling area (to ensure sufficient biomass) and increasing the number of replicates within an individual shower. The data from this pilot study indicate that immigrants to the built environment arising from human shedding dominate the shower ecosystem and that growth conditions are relatively unfavorable despite the water availability. We offer suggestions on how to improve the studying and sampling of microbes in indoor environments.