Abstract
RadD, a major adhesin of oral fusobacteria, is part of a four-gene operon encoding the small lipoprotein FAD-I and two currently uncharacterized small proteins encoded by the rapA and rapB genes. Previously, we described a role for FAD-I in the induction of human B-defensin 2 (hBD2) upon contact with oral epithelial cells. Here, we investigated potential roles for fad-I, rapA, and rapB in interspecies interaction and biofilm formation. Gene inactivation mutants were generated for each of these genes in the nucleatum and polymorphum subspecies of Fusobacterium nucleatum and characterized for their adherence to partner species, biofilm formation, and operon transcription. Binding to Streptococcus gordonii was increased in all mutant strains with Δfad-I having the most significant effect. This increased adherence was directly proportional to elevated radD transcript levels and resulted in significantly different architecture and height of the biofilms formed by Δfad-I and S. gordonii compared to the wild-type parent. In conclusion, FAD-I is important for fusobacterial interspecies interaction as its lack leads to increased production of the RadD adhesin suggesting a role of FAD-I in its regulation. This regulatory effect does not require the presence of functional RadD.
Highlights
Bacterial adhesion is an essential process in inter-species interaction that enables the development and maturation of multi-species biofilm
Our studies revealed that lack of fad-I led to increased binding of fusobacteria to Streptococcus gordonii by triggering significant overexpression of radD, which resulted in an increase in thickness and density of the biofilm formed with this important partner species
In fusobacteria, including the genetically tractable strains of F. nucleatum ssp nucleatum ATCC 23726 and F. nucleatum ssp polymorphum ATCC 10953, the adhesin RadD is encoded by the last gene of a four-gene operon [15], comprised of the homologs of FN1529, FN1528, FN1527 [21], and FN1526
Summary
Bacterial adhesion is an essential process in inter-species interaction that enables the development and maturation of multi-species biofilm. Adhesion has been observed among all the genetically distinct bacteria isolated from a number of human sites including the gut, urogenital tract [1], and the oral cavity [2] It is mainly mediated by the interaction of bacterial adhesins to a specific co-receptor in the partner species [3]. Fusobacterium nucleatum, an anaerobic, gram-negative commensal and opportunistic pathogen, is one of the most abundant microorganisms of the oral cavity [10,11,12,13] It is an important species in dental biofilm ecology as it interacts with both early and late colonizers. FAD-I of F. nucleatum ssp nucleatum, type strains ATCC 25586 and ATCC 23726, induce expression of hBD-2, while FAD-I of F. nucleatum ssp polymorphum, type strain ATCC 10953, fails to do so This differential antimicrobial peptide induction could have a profound influence on oral community composition. We demonstrated that the presence of the RadD adhesin is not needed for FAD-I-mediated regulation of radD expression and that the binding to S. gordonii leads to suppression of radD transcription in a FAD-I-independent manner
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