Ronit Naor

Bacteriophage isolation from human saliva.

Aims: To detect bacteriophages for Gram‐positive oral pathogens in human saliva.

Fap2 of Fusobacterium nucleatum Is a Galactose-Inhibitable Adhesin Involved in Coaggregation, Cell Adhesion, and Preterm Birth

ABSTRACT Fusobacterium nucleatum is a common oral anaerobe involved in periodontitis that is known to translocate and cause intrauterine infections. In the oral environment, F. nucleatum adheres to a large diversity of species, facilitating their colonization and creating biological bridges that stabilize the multispecies dental biofilm. Many of these interactions (called coadherences or coaggregations) are galactose sensitive. Galactose-sensitive interactions are also involved in the binding of F. nucleatum to host cells. Hemagglutination of some F. nucleatum strains is also galactose sensitive, suggesting that a single galactose-sensitive adhesin might mediate the interaction of fusobacteria with many partners and targets. In order to identify the fusobacterial galactose-sensitive adhesin, a system for transposon mutagenesis in fusobacteria was created. The mutant library was screened for hemagglutination deficiency, and three clones were isolated. All three clones were found to harbor the transposon in the gene coding for the Fap2 outer membrane autotransporter. The three fap2 mutants failed to show galactose-inhibitable coaggregation with Porphyromonas gingivalis and were defective in cell binding. A fap2 mutant also showed a 2-log reduction in murine placental colonization compared to that of the wild type. Our results suggest that Fap2 is a galactose-sensitive hemagglutinin and adhesin that is likely to play a role in the virulence of fusobacteria.

Natural Killer Cell-Mediated Host Defense against Uropathogenic E. coli Is Counteracted by Bacterial HemolysinA-Dependent Killing of NK Cells

Uropathogenic Escherichia coli (UPEC) are a common cause of urinary tract infections (UTIs) in humans. While the importance of natural killer (NK) cells in innate immune protection against tumors and viral infections is well documented, their role in defense against bacterial infections is still emerging, and their involvement in UPEC-mediated UTI is practically unknown. Using a systematic mutagenesis approach, we found that UPEC adheres to NK cells primarily via its type I fimbriae and employs its hemolysinA toxin to kill NK cells. In the absence of hemolysinA, NK cells directly respond to the bacteria and secrete the cytokine TNF-α, which results in decreased bacterial numbers in vitro and reduction of bacterial burden in the infected bladders. Thus, NK cells control UPEC via TNF-α production, which UPEC counteracts by hemolysinA-mediated killing of NK cells, representing a previously unrecognized host defense and microbial counterattack mechanism in the context of UTI.

Garlic Allicin as a Potential Agent for Controlling Oral Pathogens

Garlic has been used medicinally throughout human history. Allicin is considered the most therapeutic constituent of garlic. This study tested the antimicrobial activity of garlic allicin on oral pathogens associated with dental caries and periodontitis. Allicin was found effective against all the tested bacteria. The broth dilution method revealed that planktonic growth of the cariogenic, gram-positive species Streptococcus mutans, S. sobrinus, and Actinomyces oris was inhibited by an allicin concentration of 600 μg/mL or higher. Planktonic growth of the tested gram-negative periopathogenic species Aggregatibacter actinomycetemcomitans and Fusobacterium nucleatum was inhibited by a minimum allicin concentration of 300 μg/mL. Porphyromonas gingivalis, an anaerobic, gram-negative pathogen and the bacterium most associated with chronic periodontitis, demonstrated the lowest sensitivity to allicin (2,400 μg/mL). Gel zymography and the synthetic chromogenic substrate N(α)-benzoyl-L-arginine 4-nitroanilide hydrochloride demonstrated that allicin inhibits the proteases of P. gingivalis, including the arginine and lysine gingipains known as major virulence factors of this organism. A gingipain-inactivated mutant demonstrated high sensitivity to allicin (<300 μg/mL), revealing that gingipains confer resistance to allicin. Live/dead staining followed by analysis with confocal laser scanning microscopy revealed that allicin was bactericidal to S. mutans grown in mature biofilms. However, this bactericidal effect was reduced as biofilm depth increased. In conclusion, these results support the traditional medicinal use of garlic and suggest the use of allicin for alleviating dental diseases.

Characterization of the Novel Fusobacterium nucleatum Plasmid pKH9 and Evidence of an Addiction System

ABSTRACT Fusobacterium nucleatum is an important oral anaerobic pathogen involved in periodontal and systemic infections. Studies of the molecular mechanisms involved in fusobacterial virulence and adhesion have been limited by lack of systems for efficient genetic manipulation. Plasmids were isolated from eight strains of F. nucleatum. The smallest plasmid, pKH9 (4,975 bp), was characterized and used to create new vectors for fusobacterial genetic manipulation. DNA sequence analysis of pKH9 revealed an open reading frame (ORF) encoding a putative autonomous rolling circle replication protein (Rep), an ORF predicted to encode a protein homologous to members of the FtsK/SpoIIIE cell division-DNA segregation protein family, and an operon encoding a putative toxin-antitoxin plasmid addiction system (txf-axf). Deletion analysis localized the pKH9 replication region in a 0.96-kbp fragment. The pKH9 rep gene is not present in this fragment, suggesting that pKH9 can replicate in fusobacteria independently of the Rep protein. A pKH9-based, compact Escherichia coli-F. nucleatum shuttle plasmid was constructed and found to be compatible with a previously described pFN1-based fusobacterial shuttle plasmid. Deletion of the pKH9 putative addiction system (txf-axf) reduced plasmid stability in fusobacteria, indicating its addiction properties and suggesting it to be the first plasmid addiction system described for fusobacteria. pKH9, its genetic elements, and its shuttle plasmid derivatives can serve as useful tools for investigating fusobacterial properties important in biofilm ecology and pathogenesis.

Identification and Characterization of Fusolisin, the Fusobacterium nucleatum Autotransporter Serine Protease

Fusobacterium nucleatum is an oral anaerobe associated with periodontal disease, adverse pregnancy outcomes and colorectal carcinoma. A serine endopeptidase of 61–65 kDa capable of damaging host tissue and of inactivating immune effectors was detected previously in F. nucleatum. Here we describe the identification of this serine protease, named fusolisin, in three oral F. nucleatum sub-species. Gel zymogram revealed fusobacterial proteolytic activity with molecular masses ranging from 55–101 kDa. All of the detected proteases were inhibited by the serine protease inhibitor PMSF. analysis revealed that all of the detected proteases are encoded by genes encoding an open reading frame (ORF) with a calculated mass of approximately 115 kDa. Bioinformatics analysis of the identified ORFs demonstrated that they consist of three domains characteristic of autotransporters of the type Va secretion system. Our results suggest that the F. nucleatum fusolisins are derived from a precursor of approximately 115 kDa. After crossing the cytoplasmic membrane and cleavage of the leader sequence, the C-terminal autotransporter domain of the remaining 96–113 kDa protein is embedded in the outer membrane and delivers the N-terminal S8 serine protease passenger domain to the outer cell surface. In most strains the N-terminal catalytic 55–65 kDa domain self cleaves and liberates itself from the autotransporter domain after its transfer across the outer cell membrane. In F. nucleatum ATCC 25586 this autocatalytic activity is less efficient resulting in a full length membrane-anchored serine protease. The mature serine protease was found to cleave after Thr, Gly, Ala and Leu residues at the P1 position. Growth of F. nucleatum in complex medium was inhibited when serine protease inhibitors were used. Additional experiments are needed to determine whether fusolisin might be used as a target for controlling fusobacterial infections.