Mobilization and prevalence of a fusobacterial plasmid.
Brianna M. Claypool, Sean C. Yoder, Diane M. Citron, Sydney M. Finegold, Ellie J.C. Goldstein and Susan Kinder Haake
Plasmid 2010 63:11-19
Fusobacterium nucleatum is a Gram-negative anaerobic rod found in dental plaque biofilms, and is an opportunistic pathogen implicated in periodontitis as well as a wide range of systemic abscesses and infections.
Studies indicate considerable phenotypic variability between F. nucleatum strains, and biochemical analyses have suggested that F. nucleatum represents a “species complex”. Comparative genomic analyses revealed that 25% of the genes encoded by F. nucleatum ATCC 10953 ssp. polymorphum are not found in either of the sequenced genomes of F. nucleatum ssp. nucleatum and vincentii). In addition, 21% of these unique ORFs mapped to clusters of five or more genes, suggesting that they may have been introduced into the genome by horizontal gene transfer. Several plasmids were found in Fusobacterium but they are too small to be self transmissible.
In this paper authors defined the minimal “mobilon” of plasmid pFN1 that included the relaxase, and DNA directly upstream containing ORF4 and ORF7 with the oriT region located upstream of ORF4. The pFN1 mobilon is related by sequence to the mobilons of the staphylococcal plasmids pC221 and pC223. Authors screened 94 clinical and 4 laboratory isolates of F. nucleatum for plasmids and plasmid encoded relaxase gene and they found that 11.5% of isolates.
As the main discovery authors demonstrated for the first time that the fusobacterial plasmid pFN1 encodes genes enabling efficient mobilization between strains of E. coli by the broad-host range plasmid RP4. I found this experiment really interesting. It is true that IncP plasmid RP4 can mobilize plasmid pFN1 between E. coli strains but if we look closer we can find that plasmid pFN1 cannot replicate in E. coli as oriP15A was used for stable replication. On the other hand there is no evidence that pRP4 can transfer to and/or replicate in F. nucleatum. This can raise a question, how is it possible for pRP4 to mobilize the pFN1 plasmid in vivo. Unfortunately, authors did not discuss that point. On the other hand they realize that the main aim to really prove the role of Fusobacterium plasmids in horizontal gene transfer is to show that plasmid transfer occurs in vivo and find what can support the missing conjugation functions.
Our dual reporter system for detection plasmid transfer in situ in anaerobic conditions can help in this research.