Discussion

Based on a systematic search, ours is the first systematic review and meta-analysis aiming to summarize current knowledge on the role of outer (environmental) and inner layer (human microbiota) biodiversity in the development of asthma, wheezing, and allergic sensitization. The systematic review and meta-analysis showed a protective trend of exposure to high environmental biodiversity on the development of asthma, wheezing, and allergic sensitization. Although the evidence on the effect of inner layer biodiversity suggested that bacterial diversity was slightly higher among individuals with asthma, there was no clear evidence of a significant association between inner layer biodiversity and the risk of asthma, wheezing or allergic sensitization.
Validity of results
In addition to the four databases, we also searched the reference lists of all the relevant articles identified. Our meta-analysis included also evidence from longitudinal studies on biodiversity and respiratory outcomes,17, 18, 20-26 which allowed assessment of the time-dependent effects related to outer and inner layer biodiversity on the development of asthma, wheezing, and allergic sensitization. Both funnel plot and Egger’s test showed an asymmetric pattern when addressing the effect of exposure to bacterial richness (outer layer biodiversity), but no significant publication bias was observed for the other exposure indicators.
The small number of studies included in the meta-analysis did not allow conduction of subgroup analyses. The different sampling methods applied and different types of samples (including stool and airway samples) may also complicate the comparison of different studies. We used NOS to evaluate the risk of bias.13 Overall, the rather low scores on the quality scale were achieved (Table S1). Study design-related limitations were largely due to potential unadjusted confounding, which varied from study to study, as well as due to self-reported outcomes, which may have increased heterogeneity in the summary estimates of the present study. In addition, heterogeneity and validity of self-reported outcomes may lead to information bias. The variation in the outcome assessment method may have had impact on the ability to reach consistent summary effect estimates, which on the other hand may have contributed to heterogeneity of the study results. However, as there were only a limited number of studies identified, we were not able to perform sensitivity analysis according to the outcome definitions.
Synthesis with previous knowledge
Although the evidence on the role of biodiversity in the development of respiratory outcomes was inconsistent, the findings of the present systematic search and meta-analysis suggested that the effects related to the inner and outer layer biodiversity on asthma, wheezing, and allergic sensitization were variable. However, caution is needed when interpreting the summary results, because of small number of studies included, as well as due to heterogeneity in definitions of exposure and outcomes, and confounders that were adjusted for.
Previous studies have reported an association between early life environmental exposures and airways inflammation.11, 27 These studies reported that those children who had contact with animals and allergens during early life were less likely to develop asthma.7, 28, 29 Furthermore, exposure to higher environmental biodiversity, assessed based on land use or vegetation types, has been associated with a lower risk of respiratory outcomes, such as asthma and allergic sensitization.30-32Moreover, the biodiversity hypothesis proposed that contact with natural environments including environmental microbiota enriches the human microbiome, promotes immune balance, and protects from developing allergies and/or inflammatory diseases 6. While exposure to beneficial microbiota seems to play an important role, the complexity of different routes of exposure to microbiota and their timing, duration, intensity and frequency make studying the role of outer and inner biodiversity on respiratory health challenging.33 Several previous studies have investigated the association between the composition of the immediate living environment and health, and found that the composition and diversity of environmental microbiota seem to differ among different land use types.31, 34, 35 Environments, such as traditional farms 36, 37 and green spaces32, 38 which contain enriched and specific microbial exposures, may be protective against asthma and allergies. More recently a systematic review and meta-analysis reported that the associations between exposure to green spaces and asthma (current and ever) and allergic rhinitis were inconsistent.39 The authors suggested that their result may be explained by a variable balance between the positive and negative effects related to biodiversity exposure.39 According to Hanski et al.31, environmental biodiversity, human microbiota, and the function of the immune system are dynamic and complex systems including different components which interact with each other. Their study hypothesised that the association between environmental biodiversity and atopy reflects the immunologic responses that have been developed by individuals with long-term exposure to specific environmental microbiota and allergens.31 Ruff et al.40 have also suggested that the drastic changes in modern environments and lifestyles may have reduced microbial biodiversity and led to an imbalance of the evolutionarily processes, which in turn may have led to more unstable and less resilient microbiota. This change in the microbiota – dysbiosis – may, consequently, alter the balance maintained in the gut, skin and airway microbiomes, impair immune homeostasis and increase the risk of many chronic inflammatory diseases, such as asthma and allergic diseases.
Furthermore, the diversity and composition of human microbiota is also influenced by several environmental exposures.7-9 A study conducted in the Russian Karelia and the Finnish Karelia showed significant differences in the skin and nasal microbiota composition between the countries.41 The microbial diversity was higher in the Russian samples than in the Finnish samples. However, no significant associations were observed between nasal and skin microbiota diversity and asthma among the Finnish individuals.41Consistently, a study including 72 adults participants (20 with asthma exacerbation, 31 with non-exacerbated asthma and 21 healthy individuals) found no statistically significant difference in Faith’s phylogenetic diversity among these three study groups.42 A recent cohort of mother-infant pairs from USA also showed that alpha diversity in gut microbiome was not significantly enriched in atopic compared to non-atopic infants.43 On the other hand, Espuela-Ortiz et al. 44 reported significant differences in both Shannon diversity index and Pielou index between individuals with asthma and those without asthma. These results are consistent with the studies conducted by Huang et al. 45 and Marri et al.46 regarding alpha diversity in airway samples. Huang et al. 45 reported a significantly higher bacterial diversity among individuals having asthma compared with control individuals. The authors suggested that bacterial diversity (variation in composition and relative abundance of specific phylotypes) is associated with the degree of bronchial hyperresponsiveness in individuals having asthma treated with inhaled corticosteroids.45 Consistently with our systematic review and meta-analysis, the results of these previous studies on microbiota diversity and respiratory outcomes are variable.
In addition to diversity, several studies have suggested that the composition of human and environmental microbiota may contribute to the development of asthma and allergies.11, 22, 47, 48 The microbiota composition may be related to a decrease in diversity, promoting less resilient microbiota. This would alter the ecosystem provided by the microbiota and the balance of the immune system response.49 Furthermore, most studies addressing the role of biodiversity in development of respiratory outcomes have analysed only single point in time in cross-sectional studies, which has not allowed assessment of responses of the immune system to changes in human microbiome caused by exposure to environmental and biological factors.50 Another important limitation in the previous studies has been that the link between biodiversity and pathophysiological mechanisms underlying asthma may have been confounded by the asthma subtype and the inflammatory process.50Based on this systematic review and meta-analyses, there is a need for population-based longitudinal studies, including: (i) cohort studies, especially in previously under-represented populations (e.g., in Asian and African regions); (ii) studies that apply the same or similar definition of the outcome; (iii) studies with recruitment at an early developmental phase (e.g. from preconception) and having a longitudinal follow-up to identify critical periods of exposure in the life course and to better understand mechanisms linking environmental exposures and changes in microbiome composition, diversity and/or function to development of asthma and allergic sensitization. Furthermore, climate change is affecting biodiversity of both outer and inner layers. Climate change is responsible for environmental degradation and loss of biodiversity in plants, animals, and microorganisms, thus affecting the distribution, composition, and interactions between microorganisms. Climate change can also disrupt the relation between environmental microorganisms and humans, resulting in loss of inner layer biodiversity.51, 52 Therefore, understanding how the interactions between outer and inner layer, biodiversity, human being, and immune system respond to climate change is also needed for assessing the role of biodiversity in the development of asthma, wheezing, and allergic sensitization.