How Mouth Bacteria Choose Their Home: Oral Porphyromonas Mapped Across 1,242 Samples

The human mouth is not a single habitat but a collection of microenvironments — tongue, teeth, gums, tonsils, palate — each home to distinct bacterial communities. Understanding which bacteria live where, and why, is fundamental to linking the oral microbiome to both dental disease and systemic health. This study applied metapangenomics, a method combining large-scale pangenome construction with competitive metagenomic read recruitment, to systematically map the Porphyromonas genus across the healthy and diseased human oral cavity at unprecedented resolution.

The research team assembled a curated reference set of 84 dereplicated Porphyromonas genomes from an initial pool of 377 publicly available genomes, filtering for human oral origin, genome quality, and 98% average nucleotide identity (ANI) dereplication. These genomes were then used as competitive mapping targets for 1,242 metagenomes spanning nine healthy oral sites — tongue dorsum, palatine tonsils, saliva, throat, hard palate, buccal mucosa, keratinized gingiva, supragingival plaque, and subgingival plaque — plus 24 subgingival plaque samples from individuals with periodontitis. Breadth of coverage and mean depth of coverage metrics were used to assess genome presence and relative abundance, respectively.

The dominant finding was clear niche partitioning. Porphyromonas pasteri was the most abundant and widespread species in healthy individuals, found broadly across all nine sites. Critically, it resolved into two ecological subtypes: one mucosal ecotype favoring tongue dorsum and soft tissues, and one plaque-associated ecotype. Despite this ecological divergence, the two subtypes showed minimal gene-content and functional differences, suggesting that habitat specialization can emerge before strong genomic divergence becomes apparent. This challenges simple interpretations of the site-specialist hypothesis and implies that subtle regulatory or expression-level differences — not just gene presence — may drive niche adaptation.

P. gingivalis, the most intensively studied periodontitis pathogen, was rare in healthy subjects and detected in only 10 of 24 (42%) periodontitis metagenomes — a striking finding that complicates its status as a universal driver of gum disease. P. catoniae was restricted almost entirely to healthy dental plaque and was found to lack biosynthetic pathways for cobalamin (vitamin B12), biotin, and serine, indicating metabolic dependency on co-resident bacteria or the host. P. endodontalis occupied both healthy and diseased subgingival plaque and also lacked several metabolic pathways, consistent with a niche-dependent lifestyle. Strong negative abundance correlations (Spearman rho = −0.34 to −0.64; FDR < 4×10⁻⁸) among plaque specialists indicated competitive dominance within individual samples even when co-occurrence was frequent at the population level.

A particularly notable discovery was a ~44 kilobase conjugative mobile genetic element first identified in P. gingivalis that was detected across healthy and periodontitis subgingival plaque metagenomes independently of the P. gingivalis chromosome. This element encodes the full machinery for autonomous horizontal transfer and was found in healthy samples even where P. gingivalis itself was absent, suggesting it moves between bacterial genera. If this element carries virulence-associated genes, its independent mobility could have significant implications for how pathogenic traits disseminate through the oral microbiome without requiring establishment of P. gingivalis itself. Together, these findings reframe Porphyromonas ecology from a simple pathogen-versus-commensal dichotomy to a complex, niche-structured system with implications for periodontitis diagnosis, treatment, and microbiome-based interventions.