Microorganisms populating the water-related indoor biome.

Modernisation of our households created novel opportunities for microbial growth and thus changed the array of microorganisms we come in contact with. While many studies have investigated microorganisms in the air and dust, tap water, another major input of microbial propagules, has received far less attention. The quality of drinking water in developed world is strictly regulated to prevent immediate danger to human health. However, fungi, algae, protists and bacteria of less immediate concern are usually not screened for. These organisms can thus use water as a vector of transmission into the households, especially if they are resistant to various water treatment procedures. Good tolerance of unfavourable abiotic conditions is also important for survival once microbes enter the household. Limitation of water availability, high or low temperatures, application of antimicrobial chemicals and other measures are taken to prevent indoor microbial overgrowth. These conditions, together with a large number of novel chemicals in our homes, shape the diversity and abundance of indoor microbiota through constant selection of the most resilient species, resulting in a substantial overlap in diversity of indoor and natural extreme environments. At least in fungi, extremotolerance has been linked to human pathogenicity, explaining why many species found in novel indoor habitats (such as dishwasher) are notable opportunistic pathogens. As a result, microorganisms that often enter our households with water and are then enriched in novel indoor habitats might have a hitherto underestimated impact on the well-being of the increasingly indoor-bound human population. KEY POINTS: Domestic environment harbours a large diversity of microorganisms. Microbiota of water-related indoor habitats mainly originates from tap water. Bathrooms, kitchens and household appliances select for polyextremotolerant species. Many household-related microorganisms are human opportunistic pathogens.