Across three independent human cohorts, lean individuals consistently harbored enriched populations of methanogenic archaea — specifically Methanobrevibacter smithii and Methanobrevibacter millerae — while obesity was characterized by elevated Caudoviricetes bacteriophages. In an elderly sub-cohort, Blastocystis spp. eukaryotes also associated with lean status. Ecological network analyses revealed that hub taxa — the microbial species occupying central interaction roles — differed substantially between obese and non-obese microbiomes, even when the same kingdoms were represented, pointing to rewired inter-kingdom communication rather than simple abundance shifts.
The field has spent a decade mapping bacterial dysbiosis in obesity while the archaeome and virome remained largely uncharacterized noise. This multicohort design gives the findings unusual reproducibility — cross-cohort consistency is the gold standard for microbiome claims, which are notoriously non-replicable. Methanobrevibacter smithii is known to enhance caloric extraction efficiency from fermentation substrates, so its depletion in obesity initially seems counterintuitive, but its role in hydrogen disposal and short-chain fatty acid metabolism is more nuanced than simple caloric contribution. The Caudoviricetes elevation may reflect phage predation reshaping bacterial communities downstream. Limitations include inherent observational design — causality is unresolved — and fecal sampling capturing luminal rather than mucosal communities. Still, this is more than incremental: positioning archaea and phages as ecologically central players in metabolic disease opens genuinely new intervention hypotheses, including archaeome-targeted therapeutics and phage-mediated microbiome modulation.