Ecological and stochastic determinants of the growth and persistence of the oral pathogen Porphyromonas gingivalis.

Population density plays a critical role in microbial fitness. However, its influence on pathogen colonization remains incompletely understood. Porphyromonas gingivalis (Pg) is a pathogen that plays a major role in periodontitis. It exhibits Allee-type growth and requires a quorum threshold to replicate. Yet, it is frequently detected at low abundance in vivo. We integrate quantitative growth experiments with mathematical modeling to identify ecological and stochastic determinants of Pg persistence. A cubic Allee-effect model quantifies a quorum threshold below which populations collapse. Conditioned medium from Veillonella parvula (Vp) lowers this threshold, indicating early-colonizer facilitation. Stochastic extensions and Fokker-Planck analysis show that microenvironmental noise enables persistence below the Allee barrier. This behavior is consistent with long-term subthreshold experiments that yield persistent survival. Pg-Vp co-cultures further demonstrate replicate rescue outcomes for subcritical inocula. Vp reliably reaches capacity, constraining terminal phases within the experimental horizon to coexistence or Pg extinction. A two-species replicator model maps these outcomes onto a (β, γ) plane. This mapping restricts accessible regions once Vp is established and suggests interventions that reduce facilitation and limit Pg-associated inflammation.