Sequelae of Bacteroides fragilis infection and carriage

Student thesis: Doctoral ThesisDoctor of Philosophy


B. fragilis is considered an opportunistic pathogen, often isolated from abdominal abscesses, bloodstream infections and peritonitis. B. fragilis can produce multiple capsular polysaccharides including an immunomodulatory, zwitterionic, polysaccharide A (PSA) capable of stimulating anti-inflammatory interleukin-10 (IL-10) production. Conversely, some strains can produce a putative carcinogenic toxin, the metalloprotease Bacteroides fragilis toxin (BFT). BFT has been demonstrated to promote colonic cell proliferation and DNA damage in mammalian cell culture and animal models. An additional putative B. fragilis virulence factor present in select strains is a eukaryotic-like ubiquitin protein (BfUbb). BfUbb is capable of interfering with the host ubiquitination cascade this protein but the consequence of this on host health in unknown.
This study describes the robust design and validation of PCR-based assay to target specific bacterial taxa and putative virulence genes. The PCR assay was subsequently used to determine prevalence in a collection of gastrointestinal tissue samples from individuals with and without disease. The bft gene was found to have a significantly higher prevalence in individuals newly diagnosed with polyps/cancer compared with a healthy patient group. This finding further points towards the importance of BFT in colonic tumorigenesis. Contrary to previous CRC literature the prevalence of Fusobacterium and fadA were not significant in the cohorts investigated in this study. Colonic location and histological type of Fusobacterium-positive tumours did not result in any significant associations, but the trends observed support previous suggestions of an association between Fusobacterium species and right-sided colon cancer.
Presented here is the first reported determination of B. fragilis capsular PSA promoter orientation in vivo. Furthermore, individuals with IBD had a significantly lower percentage of the B. fragilis population PSA orientated on in comparison with a healthy cohort. Similarly, bft-positivity was significantly associated with a lower proportion of the PSA promoter orientated on.
In conclusion, overall the results presented indicate that the common gastrointestinal species, B. fragilis, can have wide ranging effects on gastrointestinal health. Between-strain differences and within-strain antigenic variation were shown to have significant associations with patient populations and argue for a gene-centric, and not taxonomic-centric, approach to microbiome research.
Date of Award2018
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorAlistair Lax (Supervisor), Jeremy Sanderson (Supervisor) & William Wade (Supervisor)

Cite this