Proteomic Characterization of the Oral Pathogen Filifactor alocis Reveals Key Inter-Protein Interactions of Its RTX Toxin: FtxA

Abstract

Filifactor alocis is a Gram-positive asaccharolytic, obligate anaerobic rod that has been isolated from a variety of oral infections including periodontitis, peri-implantitis, and odontogenic abscesses. As a newly emerging pathogen, its type strain has been investigated for pathogenic properties, yet little is known about its virulence variations among strains. We previously screened the whole genome of nine clinical oral isolates and a reference strain of F. alocis, and they expressed a novel RTX toxin, FtxA. In the present study, we aimed to use label-free quantification proteomics to characterize the full proteome of those ten F. alocis strains. A total of 872 proteins were quantified, and 97 among them were differentially expressed in FtxA-positive strains compared with the negative strains. In addition, 44 of these differentially expressed proteins formed 66 pairs of associations based on their predicted functions, which included clusters of proteins with DNA repair/mediated transformation and catalytic activity-related function, indicating different biosynthetic activities among strains. FtxA displayed specific interactions with another six intracellular proteins, forming a functional cluster that could discriminate between FtxA-producing and non-producing strains. Among them were FtxB and FtxD, predicted to be encoded by the same operon as FtxA. While revealing the broader qualitative and quantitative proteomic landscape of F. alocis, this study also sheds light on the deeper functional inter-relationships of FtxA, thus placing this RTX family member into context as a major virulence factor of this species.

Periodontal microbiology and microbial etiology of periodontal diseases: Historical concepts and contemporary perspectives

“…It is suggested that F. alocis is an important organism in the structural organization of subgingival biofilm, 38 whereas Swedish researchers recently identified that it expresses a unique protein exotoxin. 40,41…”

Aggregatibacter actinomycetemcomitans and Filifactor alocis: Two exotoxin-producing oral pathogens

“…Taken together, ftxA was carried by six of the ten tested strains and is therefore not a universal property of this bacterium. The expression of this gene and its role in F. alocis virulence is still not known, yet proteomic characterization of available strains has identified intra-species differences, as well as clustering of FtxA with another six intracellular proteins [36]. S1).…”

“…Most recently, FtxA, has been identified as a putative ~250 kDa exotoxin of F. alocis [ 35 ], and similar to LtxA, belongs to the large family of RTX proteins, found in both gram-negative and gram-positive bacteria [ 45 ]. The activity of FtxA is not yet known; interestingly it appeared to be encoded and expressed only by six of the 10 assessed F. alocis strains [ 35 , 36 ], suggesting potentially different capabilities to modulate host functions by FtxA-expressing strains, as compared to those that do not encode this protein.…”

Association of Filifactor alocis and its RTX toxin gene ftxA with periodontal attachment loss, and in synergy with Aggregatibacter actinomycetemcomitans

“…F. alocis was recently found to encode and express a putative repeats-in-toxin (RTX) protein, FtxA ( Bao et al., 2022 ; Oscarsson et al., 2020 ). Interestingly, only approximately 50% of the isolated F. alocis strains appeared to carry the ftxA gene ( Oscarsson et al., 2020 ), suggesting the presence of a potentially more virulent genotype of this species.…”