Indoor gardening beneficially boosts our microbial exposure

Researchers at the University of Helsinki, Natural Resources Institute Finland and Tampere University have demonstrated that indoor gardening can increase the bacterial diversity of the skin and boost levels of anti-inflammatory molecules in the blood. The results of their research have been published in the journal Environment International.

As part of his doctoral thesis, Helsinki’s Mika Saarenpää investigated how microbial exposure that promotes the health of urban residents, particularly enhancing their immune regulation, could be increased easily through meaningful activities integrated into everyday life. According to Saarenpää, urbanisation has led to a considerable increase in immune-mediated diseases such as allergies, asthma and autoimmune diseases, as we live too ‘cleanly’ in cities.

Knowing from previous studies that contact with nature-derived, microbially rich material can alter the human microbiota, Saarenpää instructed his research subjects to commit to urban gardening for one month. The experimental group used a growing medium with high microbial diversity emulating forest soil, while the control group used a microbially poor peat-based medium.

“One month of urban indoor gardening boosted the diversity of bacteria on the skin of the [experimental] subjects and was associated with higher levels of anti-inflammatory cytokines in the blood,” Saarenpää said. In contrast, no changes in the blood or the skin microbiota were seen in the control group. This suggests that peat — the most widely used growing medium in the world and a large contributor to greenhouse gas emissions — does not bring any health benefits similar to a medium mimicking diverse forest soil.

“We know that urbanisation leads to reduction of microbial exposure, changes in the human microbiota and an increase in the risk of immune-mediated diseases,” Saarenpää said. “This is the first time we can demonstrate that meaningful and natural human activity can increase the diversity of the microbiota of healthy adults and, at the same time, contribute to the regulation of the immune system.”

Saarenpää’s study shows that microbial exposure can be increased easily and safely in the home, with only a small amount of space and financial investment required. The gardening took place in regular flower boxes, while the plants cultivated, such as peas, beans, mustards and salads, came from the shop shelf. Changes were observed in just one month, but as the research subjects enjoyed the gardening, many of them said they would continue the activity and switch to outdoor gardening in the summer.

“We don’t yet know how long the changes observed in the skin microbiota and anti-inflammatory cytokines persist, but if gardening turns into a hobby, it can be assumed that the regulation of the immune system becomes increasingly continuous,” Saarenpää said.

Saarenpää stated that microbe-mediated immunoregulation can reduce the risk of immune-mediated diseases or even their symptoms — so if health-promoting microbial exposure could be increased at the population level, the healthcare costs associated with these diseases could be reduced and quality of life increased. He said it is particularly important to invest in children’s exposure to nature and microbes — as the development of the immune system is at its most active in childhood — and so suggested introducing planter boxes filled with microbially rich soil in kindergartens, schools and hospitals, especially in densely built urban areas.

“My research emphasises the dependence of our health on the diversity of nature and that of soil in particular,” he said. “We are one species among others, and our health depends on the range of other species. Ideally, urban areas would also have such a diverse natural environment that microbial exposure beneficial to health would not have to be sought from specifically designed products.”

Image credit: Mika Saarenpää.