Genetic and molecular determinants of polymicrobial interactions in <i>Fusobacterium nucleatum</i>

Abstract

A gram-negative colonizer of the oral cavity, <i>Fusobacterium nucleatum</i> not only interacts with many pathogens in the oral microbiome but also has the ability to spread to extraoral sites including placenta and amniotic fluid, promoting preterm birth. To date, however, the molecular mechanism of interspecies interactions-termed coaggregation-by <i>F. nucleatum</i> and how coaggregation affects bacterial virulence remain poorly defined. Here, we employed genome-wide transposon mutagenesis to uncover fusobacterial coaggregation factors, revealing the intertwined function of a two-component signal transduction system (TCS), named CarRS, and a lysine metabolic pathway in regulating the critical coaggregation factor RadD. Transcriptome analysis shows that CarR modulates a large regulon including <i>radD</i> and lysine metabolic genes, such as <i>kamA</i> and <i>kamD</i>, the expression of which are highly up-regulated in the Δ<i>carR</i> mutant. Significantly, the native culture medium of Δ<i>kamA</i> or Δ<i>kamD</i> mutants builds up abundant amounts of free lysine, which blocks fusobacterial coaggregation with streptococci. Our demonstration that lysine-conjugated beads trap RadD from the membrane lysates suggests that lysine utilizes RadD as its receptor to act as a metabolic inhibitor of coaggregation. Lastly, using a mouse model of preterm birth, we show that fusobacterial virulence is significantly attenuated with the Δ<i>kamA</i> and Δ<i>carR</i> mutants, in contrast to the enhanced virulence phenotype observed upon diminishing RadD (Δ<i>radD</i> or Δ<i>carS</i> mutant). Evidently, <i>F. nucleatum</i> employs the TCS CarRS and environmental lysine to modulate RadD-mediated interspecies interaction, virulence, and nutrient acquisition to thrive in the adverse environment of oral biofilms and extraoral sites.