The gut microbiome carries its own epigenetic clock, measurable by age-related DNA methylation patterns in bacterial genomes. I hypothesize that this microbial epigenetic age, not just taxonomic composition, directly fuels host inflammaging by shaping the cargo of microbiota‑derived extracellular vesicles (MVs). When microbial DNA becomes hypomethylated with age, MVs enriched for unmethylated CpG motifs and specific small RNAs are released into the gut lumen. These MV contents act as pathogen‑associated molecular patterns that activate Toll‑like receptor 9 (TLR9) and cytosolic nucleic‑acid sensors on intestinal epithelial and immune cells, triggering NF‑κB signaling and IL‑6/TNF‑α release. The resulting cytokine surge impairs vagal afferent firing and propagates systemic inflammation, accelerating host epigenetic clocks such as GrimAge and PhenoAge. Crucially, because MV biogenesis depends on microbial metabolic state, interventions that reset bacterial methylation—e.g., targeted expression of a bacterial DNA demethylase or supplementation with methyl‑donor precursors—should normalize MV immunostimulatory potential without altering overall taxa abundance.

Testable predictions:

1. Aged mice receiving fecal microbiota transplants (FMT) from donors whose bacterial DNA methylation has been chemically restored to a youthful state will show no increase in serum IL‑6/TNF‑α, hippocampal microglial activation, or host epigenetic age acceleration compared to recipients of untreated aged FMT, despite identical taxonomic profiles.
2. Isolating MVs from youth‑like versus aged microbiota and administering them germ‑free young mice will recapitulate the inflammaging phenotype only when the MVs carry elevated hypomethylated CpG loads, an effect blocked by TLR9 antagonist or RNase treatment.
3. Longitudinal sampling of human stool will reveal that individual microbial epigenetic age predicts future changes in plasma inflammatory markers and epigenetic age acceleration better than relative abundance of any single taxon.

Experimental approach:

• Use bisulfite sequencing of fecal bacterial DNA to compute a microbial epigenetic age metric.
• Apply CRISPR‑dCas9‑TET1 demethylase or 5‑azacytidine pulses in vitro to reverse age‑related methylation in cultured aged‑donor microbiota.
• Perform FMT into young recipient mice, measure MV CpG content by qPCR after MV isolation, and assess host outcomes (cytokines, vagal nerve activity via electrophysiology, hippocampal LTP, and mouse GrimAge).
• Include controls: live‑vs‑heat‑killed microbiota, MV‑depleted filtrate, and TLR9‑knockout recipients.

If confirmed, this hypothesis shifts focus from merely correcting dysbiosis to resetting the epigenetic state of the microbiome itself, offering a precise leverage point to decouple microbial aging from host inflammaging.