Gut and respiratory microbiomes in asthma and allergic diseases: a narrative review of mechanistic insights, gut-lung axis interactions and therapeutic opportunities.

Background and Objective: Asthma and allergic diseases are increasingly prevalent chronic inflammatory disorders characterized by immune dysregulation, epithelial barrier impairment, and marked clinical heterogeneity. Increasing evidence suggests that both the gut microbiome and the respiratory microbiome are associated with disease initiation, phenotype expression, and exacerbation risk. This narrative review aims to synthesize current evidence on microbiome alterations associated with asthma and allergic diseases, with particular emphasis on mechanistic pathways, bidirectional gut-lung axis interactions, and microbiome-targeted therapeutic opportunities. Methods: We conducted a narrative review of recent English-language literature on the gut microbiome, respiratory microbiome, asthma, allergic diseases, microbial metabolites, and microbiome-based interventions. Relevant studies and reviews were identified through literature screening and were selected for their relevance to early-life microbial colonization, disease-associated dysbiosis, immune regulation, gut-lung axis biology, and translational strategies. Key Content and Findings: Current evidence indicates that early-life gut microbial colonization, airway microbial dysbiosis, and altered metabolite production are associated with allergic susceptibility, inflammatory phenotype, exacerbation risk, and disease progression. The strength of evidence differs across domains: human cohort and clinical studies most strongly support associations between early-life microbial patterns, airway dysbiosis, and disease phenotypes, whereas many mechanistic pathways remain supported primarily by preclinical or experimental data. Key mechanisms include mucosal microbiome-immune crosstalk, local airway epithelial-microbial interactions, short-chain fatty acid-mediated immune regulation, tryptophan and bile acid signaling, epithelial barrier dysfunction, viral-microbiome interactions, and epigenetic modulation. The gut-lung axis provides a bidirectional framework linking intestinal and airway microbial ecosystems through immune, metabolic, inflammatory, infectious, and treatment-related pathways. Emerging interventions show different levels of evidence and should not be interpreted as equally mature therapeutic strategies. Conclusions: The gut and respiratory microbiomes are important components of the pathogenic network underlying asthma and allergic diseases and may represent future targets for prevention and therapy. However, many reported microbial signatures remain associative, and stronger standardization, longitudinal validation, functional studies, and evidence-stratified clinical trials are needed before microbiome-informed precision medicine can be broadly implemented in routine care.