Signature Microbiology in Indoor Air and House Dust: A Meta-analysis of Bacterial–Fungal Functional–Taxonomic Indicators with Respiratory Health Implications

Background: Indoor microbiomes are an emerging determinant of respiratory health. Environmental factors such as humidity, ventilation, and material type influence the diversity, abundance, and activity of airborne fungi and bacteria. Objective: This meta-analysis identifies bacterial and fungal signature microbiology patterns in indoor air and household dust, linking their ecological and functional indicators with respiratory health outcomes. Methods: Twenty peer-reviewed studies (2019–2025) were analysed using random-effects models (REML). Extracted data included log₂ fold-changes or standardised mean differences (SMDs) for gene expression (sporulation, biofilm, toxin pathways), hydrophilic/mesophilic fungal ratios, and Gram-negative/positive bacterial ratios. Results: Elevated equilibrium relative humidity (ERH ≥ 85 %) was associated with increased hydrophilic fungal taxa and higher expression of growth and sporulation genes (brlA, rodA, catA), secondary metabolites (stcC, Alt a 7), and bacterial inflammation markers (LPS, rpoS). The pooled SMD for hydrophilic fungi was +0.84 (95 % CI 0.59–1.08; p < 0.001); bacterial Gram-negative enrichment showed SMD +0.67 (95 % CI 0.41–0.93). Conclusion: Functional and ecological indicators are more reliable than single-species markers. Combined bacterial–fungal signatures can serve as robust biomarkers for assessing indoor air microbiological quality and respiratory risk.