Roslund MI, Puhakka R, Grönroos M, et al. Biodiversity intervention enhances immune regulation and health-associated commensal microbiota among daycare children. Sci Adv. 2020;6(42):eaba2578.
To experimentally determine if naturally occurring soil bacteria in “green” vs “standard” landscaped daycare yards influence inflammatory immune system response among preschool-aged children
Seventy-five preschool-aged children (aged 3-5 years) in Finland. All children received the same 2 meals and 1 snack per day throughout the experiment, as well as the same amount of outside time per day (~1.5 hours). The researchers assessed activities occurring away from daycare (eg, meals at home, weekend outdoor/nature time, contact with animals) using surveys completed by parents and found minimal variation between groups.
Children were excluded from the study if they had any of the following: presence of an immunodeficiency, autoimmune, or immunoregulatory medical condition (eg, human immunodeficiency virus [HIV], Crohn disease, Down syndrome); current use of immunosuppressive medication (eg, corticosteroids); or current use of antibiotics or probiotics.
Researchers assessed children before (@Day 0) and after (@Day 28) a landscaping intervention or no-change control at the urban daycare centers they attended. They included 3 setting types:
- Standard daycare (no-change negative control setting): Gravel and concrete yard with built elements (ie, swing set, jungle gym, and sandbox)
- Nature daycare (no-change positive control): Yard and surroundings containing naturally occurring forest elements (eg, blueberry bushes, heather, mosses, meadow grass, peat)
- Greening daycare (variable setting): Standard daycare yard @Day 0, with forest elements brought and planted to facilitate similarity to Nature daycare by Day 28.
Researchers took pre-post measures of skin and stool bacteria, as well as blood samples, from all participants. They also measured soil/ground bacteria @Day 0 and @Day 28 across all 3 settings in a standardized manner (eg, adjacent to daycare front door, swing set, jungle gym, sandbox).
Researchers took pre-post intervention blood samples to assess children’s immune system status. Specifically, they measured the following inflammatory immune markers (cytokines):
- Interleukin 10 (IL-10): an important anti-inflammatory interleukin cytokine
- Transforming growth factor-beta 1 (TGF-β1): an important anti-inflammatory regulatory cytokine
- Interleukin 17A (IL-17A): an important pro-inflammatory regulatory cytokine.
Blood samples were also used to measure regulatory T cells (Treg), modulatory white blood cells affecting inflammatory immune response. Researchers also measured skin and stool bacteria, as well as environmental soil bacteria, to assess any pre-post between-group changes.
Soil samples from all 3 daycare settings @Day 0 revealed extensive presence of multiple classes of bacteria, as anticipated. Microbial diversity prior to intervention was vastly higher in the Nature daycare group than the Standard daycare group (which included the pre-intervention Greening daycare group). Children who attended the Nature daycare were found to have greatly elevated IL-10 levels (P=0.0000023). Conversely, children in the Day 0 Standard daycare group had lower levels of TGF-β1 (P=0.01).
"Greening” a daycare yard led to changes in the soil, skin, and stool bacteria that corresponded directly with alterations in measures of immune function.
At Day 28, significant increases were measured in bacterial abundance and diversity for the Greening daycare, up to 8 times what was measured in the Standard daycare group. This was mirrored by increased microbial diversity on the skin and in the stool samples of children in the Greening daycare group, resembling findings in the Nature daycare group.
Most importantly, changes in soil, skin, and stool bacteria for the Greening daycare group corresponded directly with alterations in measures of immune function. Increases in microbial diversity resulted in significant increases in anti-inflammatory TGF-β1 (P=0.01) and Treg (P=0.016) and decreases in pro-inflammatory IL-17A (P=0.002). In particular, a decrease in IL-17A was most associated with increases in the stool bacterium Faecalibacterium prausnitzii (P=0.045), which has been shown to be beneficially immune-modulatory for individuals with Crohn disease.1
This is the first field study to experimentally measure how environmental bacteria modulate the human immune system. It has been known for decades that environmental factors have an impact on immune system function, beginning with the development of the “Hygiene Hypothesis” in 1989.2 Since that time, multiple studies have demonstrated the importance of microbial biodiversity in influencing immune system function.3,4 This includes the corollary refined “Old Friends Hypothesis,” which suggests rising incidences of atopic disease (ie, allergy, asthma, eczema) and autoimmune disease (eg, multiple sclerosis, rheumatoid arthritis, Crohn disease) in the developed vs developing world are the result of microbial Th1/Th2/Th17/Treg immune modulation more representative of our traditional evolutionary past.5 A past in which our prehistoric ancestors had an intimate physical exposure to the rich microbial biodiversity of the natural world surrounding them.6,7
The evidence for the Old Friends Hypothesis is not just based on comparisons of developed vs developing countries. Even within developed countries there are many studies demonstrating the health benefits of people, especially children, being exposed to more biodiverse microbial environments. Writings about these benefits are even common in popular lay literature.8,9 One common method for attaining these benefits is spending time outside, where naturally occurring microbial diversity is high. Multiple studies have shown the benefits of such outdoor activities on rates of atopic and autoimmune disease,10,11 as well as mental health conditions such as depression and anxiety12,13 mediated by neuronal anti-inflammatory and neurotransmitter-supportive changes caused by microbial exposures.14–16 Of course, this is in addition to all of the other mental and physical benefits known to occur from spending time outside, including greater physical activity, social interaction, creative play, empathy development, academic performance, etc.17–19
Prior to this current study, all research on the Hygiene/Old Friends hypotheses was either observational (making associations across large population-based data sets) or laboratory-based in vivo inoculation trials to measure the effects of specific microbial species. This field study demonstrates for the first time that altering real-world settings can change environmental microbiomes in ways that have measurable, impactful effects on human inflammatory status and immune system function.
This was a well-designed study that established a direct connection between changes in external and internal environmental microbes and immune system function. A more definitive association could have been established by exposing participants directly to select microbial species, but that association has been established elsewhere.20 This study did not demonstrate how long after Day 28 participants’ microbial and immunological changes lasted. Future studies could include a crossover or washout period after exposure to determine longevity. In addition, a longitudinal study with clinical populations (eg, children with atopic or autoimmune diagnoses) would be useful in determining the therapeutic value of this type of exposure.
The world in which we live has intricate and intimate impacts on our health and well-being. As it is said in the holistic worldview, “everything is connected.” In the modern era, many people have attempted to remove themselves from the natural world around them, often to their detriment. Exploration of ways we can return to a more relational interaction with our surroundings makes biological sense and supports the physiological functioning of our bodies’ adaptations over millions of years. Research such as this daycare study shows that simple environmental adjustments can affect the health and lives of ourselves and our children in positive ways.