UCLA

Fusobacterium nucleatum is a Gram-negative opportunistic pathogen that is indigenous to the human oral cavity. It is a prevalent member of the oral microbial community and considered a key organism in biofilm formation due to its ability to adhere to a large variety of microbial species. While present in healthy oral biofilms, F. nucleatum is also dominant in periodontal disease and has been implicated in a number of invasive human infections, acute and chronic inflammatory conditions as well as adverse pregnancy outcomes. F. nucleatum pathogenicity can be partially attributed to its function as a “bridging organism” that supports the integration of periodontal pathogens into oral biofilms. Despite the extensive exploration of F. nucleatum interspecies interactions and the identification of a number of binding partners, only one fusobacterial large outer membrane protein (OMP), RadD, has been extensively characterized at a molecular level for its role as an adhesin in binding to a variety of Gram-positive species.

The identification and characterization of fusobacterial adhesins involved in interspecies interactions addressed in the following aims:

(1) We have identified an additional fusobacterial OMP, Fap2, a galactose-inhibitable adhesin that mediates attachment with at least two P. gingivalis strains. We also demonstrate that RadD is an additional strain-specific fusobacterial adhesin for interaction with P. gingivalis and provide evidence that additional adhesins exist that have yet to be identified.

(2) An in-depth investigation of the four-gene operon encoding the RadD adhesin revealed that inactivation of the gene directly upstream of radD, named fad-I, resulted in increased binding of F. nucleatum to both Streptococcus gordonii and Porphyromonas gingivalis as a result of the overexpression of radD. The mutant lacking FAD-I also exhibited the enhanced ability to form robust biofilms with S. gordonii. We propose that the protein encoded by fad-I is an element acting as a repressor for radD expression.

(3) We also took a global approach to examine contact-induced transcriptional changes that occur when F. nucleatum is in contact with partner species. We report partner-specific responses, suggesting F. nucleatum can differentially regulate its genes based on contact with neighboring species. Furthermore, we report that there are a subset of genes regulated based on the interacting partner and the adhesin mediating attachment.

In conclusion, this thesis describes a comprehensive study of fusobacterial adhesins involved in interspecies interactions and further highlights the prominence of F. nucleatum in the oral microbiome warranting future studies that continue to investigate this organism on molecular and “omic” levels.