AI-2 Production in Fusobacterium nucleatum Is Subspecies-Specific and Uncoupled from Quorum Sensing

Autoinducer-2 (AI-2) is a LuxS-dependent product of the activated methyl cycle (AMC) that functions as a quorum-sensing signal in diverse bacteria. Fusobacterium nucleatum is a genetically heterogeneous oral anaerobe comprising four subspecies: nucleatum (FNN), vincentii (FNV), polymorphum (FNP), and animalis (FNA). Previous studies have reported that FNN and FNP strains produce AI-2 and have proposed that AI-2-mediated quorum sensing contributes to biofilm formation and virulence. However, the distribution and functional relevance of AI-2 across all subspecies have not been systematically examined. Here, we show that AI-2 production is restricted to FNA strains. Genomic analysis revealed that FNN and FNV lack luxS, whereas FNP carries a disrupted luxS homolog. Consistent with these findings, AI-2 bioassays using the Vibrio harveyi BB170 reporter detected AI-2 exclusively in FNA strains. Deletion of luxS in FNA abolished AI-2 production but resulted in minimal transcriptional changes, and exogenous AI-2 failed to elicit global gene expression responses in non-producing subspecies. These results demonstrate that AI-2 production in F. nucleatum is subspecies-specific and uncoupled from quorum sensing. Our findings revise current assumptions regarding AI-2-mediated communication in F. nucleatum and reveal previously unrecognized metabolic divergence within the species complex. IMPORTANCEPeriodontitis affects nearly half of adults in the United States and remains a leading cause of tooth loss worldwide. Fusobacterium nucleatum is a central member of oral biofilms and has also been linked to adverse pregnancy outcomes and colorectal cancer. Although AI-2-mediated quorum sensing has been proposed to contribute to its biofilm formation and virulence, our study demonstrates that AI-2 production is confined to subsp. animalis and is absent in other subspecies. Moreover, AI-2 does not function as a conserved quorum-sensing regulator in this species. These findings fundamentally revise prevailing assumptions about AI-2 signaling in F. nucleatum and suggest that subspecies-specific metabolic traits, rather than universal quorum sensing, may underlie ecological adaptation and host association.