There is accumulating evidence to suggest that the environmental microbiome plays a significant role in asthma development. The very low prevalence of asthma in populations highly exposed to microbial environments (farm children and Amish populations) highlights its preventive potential. This microbial diversity might be necessary to instruct a well-adapted immune response and regulated inflammatory responses to other inhaled and ingested environmental elements, such as allergens, particles, and viruses. Like the internal gut microbiome, which is increasingly recognized as an important instructor of immune maturation, the external environmental microbiome might shape immune responses on the skin, airway mucosal surfaces, and potentially also the gut early in life. The diversity of the external microbial world will ensure that of the many maladapted pathways leading to asthma development, most, if not all, will be counterbalanced. Likewise, important contributors to asthma, such as allergen sensitization and allergic manifestations early in life, are being suppressed. Thus the facets of innate immunity targeted by microbes and their compounds and metabolites might be the master switch to asthma and allergy protection, which has been found in environments rich in microbial exposures.
Key words
•Microbiome;
•asthma;
•environment;
•children;
•epidemiology
Abbreviations used
•ALEX, Protection Against the Development of Atopy: Relevant Factors from Farming Environments;
•aOR, Adjusted odds ratio;
•EPS, Extracellular polysaccharide;
•PAMP, Pathogen-associated molecular pattern;
•PARSIFAL, Prevention of Allergy–Risk Factors for Sensitisation in Children Related to Farming and Anthroposophic Lifestyle
The microbial world is a vast universe with innumerable species. Microbes have been found in the deep sea, caverns, hot springs, and other places not inhabitable by human subjects. However, microbes also populate our daily indoor and outdoor environments in urban and rural areas in great abundance.1 This abundance had been underestimated as long as culture methods were the only possible means of investigation. Yet isolation and cultivation have strong limitations because for many microbes, it is unknown under which conditions they grow.2 For certain environments, only about 1% of all microbes can be cultured. Furthermore, cultivation methods are tedious and personnel intensive, which hampers their application to large population-based surveys.
Alternatively, exposure to microbes can indirectly be assessed by detecting substances derived from their cell walls, such as LPS and peptidoglycan. Although LPS or endotoxin is found on the cell walls of gram-negative bacteria, peptidoglycan is a component of the cell wall of gram-positive and also, to a smaller extent, gram-negative bacteria. The most commonly used test (the limulus amebocyte lysate test) to detect endotoxin in dust samples has been largely applied in epidemiologic surveys. Others have measured levels of 3-hydroxy fatty acids from LPS, but levels correlate poorly with limulus amebocyte lysate assay measurements. The same problem applies to different methods detecting muramic acid, a component of peptidoglycan.3 Other compounds reflecting exposure to fungi, such as extracellular polysaccharides (EPSs) and β- glucans, have also been measured.4 However, the specificity of these individual markers is limited.
The advent of DNA-based high-throughput analyses has revolutionized the field. These novel study instruments are comparable with the invention of the microscope, which also allowed making the invisible world seen. In 1977, Woese and colleagues5 discovered that the nucleotide sequence of 16S ribosomal RNA (16S rRNA) is present in every bacterial and archaeal cell but not in human or animal cells. 16S rRNA contains highly conserved regions among all bacteria. Thus detection of 16S rRNA signals the presence of bacteria and archaea but not protozoa, virus, vertebrate, and other animals or human subjects. These conserved regions flank sequence regions that are variable among different bacterial species and thus allow phylogenetic classification. Several techniques combining high-throughput methods, such as microarrays or next-generation sequencing, with 16S rRNA gene analysis are being increasingly applied to microbial investigations of environmental and human samples.6 The taxonomic classification of the bacteria is obtained by comparing the sequences against databases, such as the National Center for Biotechnology Information (http://www.ncbi.nlm.nih.gov/nucleotide), the Ribosomal Database Project (http://rdp.cme.msu.edu), Greengenes (http://greengenes.lbl.gov/cgi-bin/nph-index.cgi), or SILVA (http://www.arb-silva.de). These techniques are advancing fast, and it is now possible to study all genetic material (human, animal, plant, microbes, and protozoa) recovered from environmental samples by using metagenomic shotgun sequencing. Given the expone