Urbanization Reduces Transfer of Diverse Environmental Microbiota Indoors

Abstract

Expanding urbanization is a major factor behind rapidly declining biodiversity. It has been proposed that in urbanized societies, the rarity of contact with diverse environmental microbiota negatively impacts immune function and ultimately increases the risk for allergies and other immune-mediated disorders. Surprisingly, the basic assumption that urbanization reduces exposure to environmental microbiota and its transfer indoors has rarely been examined. We investigated if the land use type around Finnish homes affects the diversity, richness, and abundance of bacterial communities indoors. Debris deposited on standardized doormats was collected in 30 rural and 26 urban households in and near the city of Lahti, Finland, in August 2015. Debris was weighed, bacterial community composition determined by high throughput sequencing of bacterial 16S ribosomal RNA (rRNA) gene on the Illumina MiSeq platform, and the percentage of four different land use types (i.e., built area, forest, transitional, and open area) within 200 m and 2000 m radiuses from each household was characterized. The quantity of doormat debris was inversely correlated with coverage of built area. The diversity of total bacterial, Proteobacterial, Actinobacterial, Bacteroidetes, and Firmicutes communities decreased as the percentage of built area increased. Their richness followed the same pattern except for Firmicutes for which no association was observed. The relative abundance of Proteobacteria and particularly Gammaproteobacteria increased, whereas that of Actinobacteria decreased with increasing built area. Neither Phylum Firmicutes nor Bacteroidetes varied with coverage of built area. Additionally, the relative abundance of potentially pathogenic bacterial families and genera increased as the percentage of built area increased. Interestingly, having domestic animals (including pets) only altered the association between the richness of Gammaproteobacteria and diversity of Firmicutes with the built area coverage suggesting that animal ownership minimally affects transfer of environmental microbiota indoors from the living environment. These results support the hypothesis that people living in densely built areas are less exposed to diverse environmental microbiota than people living in more sparsely built areas.

Short‐term direct contact with soil and plant materials leads to an immediate increase in diversity of skin microbiota

“…Soils including composted gardening materials host especially diverse microbial communities (Yu et al, 2015) and thus increasing contact with such soils for urban citizens could provide a means for increasing the diversity of their microbiota and further decrease the prevalence of asthma and atopies (von Hertzen & Haahtela, 2006). This is especially significant in the light of our earlier findings, which suggest that urbanization and pollution could lead to changes in soil microbiota and urbanization reduces microbial transfer indoors (Parajuli et al, 2017(Parajuli et al, , 2018.…”

“…These changes may affect the normal development of immune system in early childhood (Stiemsma, Reynolds, Turvey, & Finlay, ). Most recently, the hygiene hypothesis has been extended to a biodiversity hypothesis (von Hertzen, Hanski, & Haahtela, ), which suggests that the rapid global biodiversity decline is related to the increase in immune‐mediated diseases: the decreased microbial diversity in the urban living environment contributes to the reduction in natural exposure to microorganisms (Parajuli et al., ) and prevent the natural development of immune system.…”

A biodiversity hypothesis

“…Are the pets running in and out, are the shoes taken off when stepping in? Parajuli et al studied debris deposited on standardized doormats and confirmed that urban built environment reduces transfer of diverse environmental microbiota indoors.…”

Rekindling old friendships in new landscapes: The environment–microbiome–health axis in the realms of landscape research

“…Liddicoat et al (2019) found that disturbed land may favour opportunistic bacteria (including pathogenic strains), albeit in a non‐urban setting, and Talamantes, Behseta, and Zender (2007) found anthropogenically disturbed land can release pathogenic fungal spores. Moreover, densely urbanized environments can prevent the transfer of diverse microbiota indoors (Parajuli et al, 2018), and indoor environments can harbour higher proportions of human associated pathogens (Kembel et al, 2012). As such, creating socially inclusive, high‐quality biodiverse greenspaces may also help to reduce contact with pathogens.…”