Microbiota and health are closely connected. Allergies occur more frequently in people living in cities than in those living in rural areas. The main cause is considered to be the low microbial diversity combined with a higher exposure to pathogenic bacteria. This is partly due to stricter hygiene measures, increased use of antibiotics, and dietary patterns. A diet high in sugar and fat as well as highly processed foods is characteristic of the so-called Western diet and is associated with an increase in harmful bacteria along with a simultaneously lower bacterial diversity. All these factors promote dysbiosis - a microbial imbalance.
Especially in the early stages of life, microbial colonisation is important for healthy development. This colonisation is primarily influenced by the mode of birth, genetics, diet, and other environmental factors, including contact with nature. Interestingly, numerous similarities can be found between the bacterial communities of the skin and of the soil and immediate environment with which one comes into contact. Although the bacteria of the gut and the soil differ significantly, findings suggest that the type of soil cover and garden vegetation can influence gut microbiota and thus the immune system.
As allergies are occurring more frequently, particularly at an early age, the question of preventive measures is becoming increasingly urgent. Roslund et al. (2020) therefore examined the extent to which the ecological biodiversity of microorganisms in urban daycare centres affects the gut and skin microbiota in children and thus impacts their health.
Study design
The study was conducted at ten daycare centres in the Finnish cities of Lahti and Tampere. A total of 75 children aged three to five years were examined. The daycare centres were categorised as follows:
- Daycare centres with standard playgrounds (standard daycare centres): approximately 500 m² of courtyard with little or no green space as a control group with 16 children.
- Daycare centres with intervention playgrounds (intervention daycare centres): courtyards with 100 m² of forest soil and 200 m² of grass. Additionally, one-year-old plants were placed in pots, and peat blocks were set up for climbing and digging. 36 children spent time in these centres.
- Nature-oriented daycare centers, where children visited nearby forests daily (nature-oriented daycare centres): in this positive control group, 23 children were examined.
All children played twice daily for approximately half an hour to two hours in the courtyard or forest. They received three standardised meals daily from two central kitchens, prepared according to the guidelines of the Finnish Food Authority, the National Nutrition Council. Private consumption of fresh vegetables, contact with animals, time spent outdoors, and the number of siblings were comparable across all daycare groups.
Before and after the 28-day intervention period (May to June), the researchers determined the children’s skin and gut microbiota using skin swabs and stool samples. Children who received antibiotics or other medications during the intervention were excluded from the study. Immunological parameters were also measured. Furthermore, the researchers compared the environmental microbiota in the standard and intervention daycare centres before and after the intervention. For this, they analysed the relative abundance and diversity of bacteria in soil samples from playgrounds and sandpits.
Results
Soil microbiota
After the intervention, bacterial strains and entire bacterial communities had changed between the intervention and standard daycare centres. For example, Gammaproteobacteria accumulated in the intervention daycare centres, reflected in an eightfold higher relative abundance compared to the standard daycare centres. Overall, the intervention led to an increase in average diversity (alpha diversity) as well as greater variability between the microbial communities of the daycare centres (beta diversity).
Skin microbiota
Before the intervention, children in the nature-oriented daycare centres had a more diverse skin bacterial community compared to children in standard or intervention daycare centres, particularly with regard to Alphaproteobacteria. There were no significant differences between children in standard and intervention daycare centres before the intervention. After the intervention, children in the intervention daycare centres showed a more diverse skin microbiota, particularly regarding Proteo- and Gammaproteobacteria, compared to children in standard daycare centres. A particularly notable difference was observed in Alphaproteobacteria, whose diversity significantly increased in children in the intervention daycare centres (p<0.001). By the end of the study period, the composition of the skin microbiota in children in the intervention daycare centres resembled that of children in the nature-oriented daycare centres.
Gut microbiota
Similar to the skin microbiota, the composition of the gut microbiota in children in standard and intervention daycare centres was largely comparable before the intervention. This was particularly evident in the similar presence of Ruminococcaceae, a health-promoting bacterial family that occurred more frequently in children in the nature-oriented daycare centres. After the intervention, both the diversity and relative abundance of Ruminococcaceae in children in the intervention daycare centres increased significantly (p=0.01 and p=0.03, respectively) and were thus similar to those in children in the nature-oriented daycare centres. At the same time, the relative abundance of Clostridiales, a group of bacteria with rather pathogenic properties, decreased in children in intervention daycare centres (p=0.003).
Immunology
The intervention also affected immunology in that the blood profile showed improved regulation of inflammation.
Conclusion
The results show that exposure to microbial diversity in the environment can change the microbiome and influence the immune system in children. By redesigning the courtyard – introducing forest soil and grass as well as plants and peat blocks for play – the diversity of Gammaproteobacteria on the skin and the proportion of Treg cells in the blood increased. Gammaproteobacteria can support the immune system by promoting the maturation of Treg cells and thereby providing anti-inflammatory signals. Together, Gammaproteobacteria and Treg cells help prevent allergies. At the same time, a positive change in the gut microbiota was observed, reflected in the increase of Ruminococcaceae, a bacterial family associated with maintaining gut health. Additionally, the intervention improved cytokine ratios in favour of anti-inflammatory cytokines.
Discussion
Although the study is already from 2020, it serves as a good example of how significant the influence of the environment – particularly soil as a habitat for countless microorganisms – is on the human body: it reaches down to the smallest cells of the immune system. What is fascinating about the study is how quickly and easily the children’s microbiota changed – simply by being outdoors and coming into contact with soil while playing. A health resource that is easily accessible. Playing outdoors, or even digging in the dirt, is no longer a given today, when young children spend much of their time with tablets and similar devices. If, however, they play outside and can move freely, they are not only well-adjusted and happy but also strengthen their immune system in the long term and prevent allergies. For this health-promoting effect to occur, in addition to contact with it, soil that is healthy and has high microbial diversity is required. This is achieved when soil management is sustainable, i.e., without chemical and synthetic fertilisers and pesticides. This is precisely the aim of biodynamic agriculture: to manage soil so that humus is built up and high biodiversity is maintained. Such soil contains health-promoting bacteria that can positively influence the human microbiota and immune system – when contact is made.
References
[1] Roslund MI, Puhakka R, Grönroos M, Nurminen N, Oikarinen S, Gazali AM, Cinek O, Kramná L, Siter N, Vari HK, Soininen L, Parajuli A, Rajaniemi J, Kinnunen T, Laitinen OH, Hyöty H, Sinkkonen A, ADELE research group (2020): "Biodiversity intervention enhances immune regulation and health-associated commensal microbiota among daycare children" Sci. Adv. 6, eaba2578.
