Differences in virulence attributes between cytolethal distending toxin positive and negative Campylobacter jejuni strains

Outer membrane vesicle-mediated release of cytolethal distending toxin (CDT) from Campylobacter jejuni

Background

Background: Cytolethal distending toxin (CDT) is one of the well-characterized virulence factors of Campylobacter jejuni, but it is unknown how CDT becomes surface-exposed or is released from the bacterium to the surrounding environment.

Results

Our data suggest that CDT is secreted to the bacterial culture supernatant via outer membrane vesicles (OMVs) released from the bacteria. All three subunits (the CdtA, CdtB, and CdtC proteins) were detected by immunogold labeling and electron microscopy of OMVs. Subcellular fractionation of the bacteria indicated that, apart from the majority of CDT detected in the cytoplasmic compartment, appreciable amounts (20-50%) of the cellular pool of CDT proteins were present in the periplasmic compartment. In the bacterial culture supernatant, we found that a majority of the extracellular CDT was tightly associated with the OMVs. Isolated OMVs could exert the cell distending effects typical of CDT on a human intestinal cell line, indicating that CDT is present there in a biologically active form.

Conclusion

Our results strongly suggest that the release of outer membrane vesicles is functioning as a route of C. jejuni to deliver all the subunits of CDT toxin (CdtA, CdtB, and CdtC) to the surrounding environment, including infected host tissue.

Genomic analysis of Campylobacter fetus subspecies: identification of candidate virulence determinants and diagnostic assay targets

Background

Campylobacter fetus subspecies venerealis is the causative agent of bovine genital campylobacteriosis, asymptomatic in bulls the disease is spread to female cattle causing extensive reproductive loss. The microbiological and molecular differentiation of C. fetus subsp. venerealis from C. fetus subsp. fetus is extremely difficult. This study describes the analysis of the available C. fetus subsp. venerealis AZUL-94 strain genome (~75–80%) to identify elements exclusively found in C. fetus subsp. venerealis strains as potential diagnostic targets and the characterisation of subspecies virulence genes.

Results

Eighty Kb of genomic sequence (22 contigs) was identified as unique to C. fetus subsp. venerealis AZUL-94 and consisted of type IV secretory pathway components, putative plasmid genes and hypothetical proteins. Of the 9 PCR assays developed to target C. fetus subsp. venerealis type IV secretion system genes, 4 of these were specific for C. fetus subsp. venerealis biovar venerealis and did not detect C. fetus subsp. venerealis biovar intermedius. Two assays were specific for C. fetus subsp. venerealis AZUL-94 strain, with a further single assay specific for the AZUL-94 strain and C. fetus subsp. venerealis biovar intermedius (and not the remaining C. fetus subsp. venerealis biovar venerealis strains tested). C. fetus subsp. fetus and C. fetus subsp. venerealis were found to share most common Campylobacter virulence factors such as SAP, chemotaxis, flagellar biosynthesis, 2-component systems and cytolethal distending toxin subunits (A, B, C). We did not however, identify in C. fetus the full complement of bacterial adherence candidates commonly found in other Campylobacter spp.

Conclusion

The comparison of the available C. fetus subsp. venerealis genome sequence with the C. fetus subsp. fetus genome identified 80 kb of unique C. fetus subsp. venerealis AZUL94 sequence, with subsequent PCR confirmation demonstrating inconsistent amplification of these targets in all other C. fetus subsp. venerealis strains and biovars tested. The assays developed here highlight the complexity of targeting strain specific virulence genes for field studies for the molecular identification and epidemiology of C. fetus.

In vivo virulence properties of bacterial cytolethal-distending toxin

Multiple pathogenic Gram-negative bacteria produce cytolethal-distending toxins (CDTs). CDT is typically composed of three subunits: the catalytic subunit CdtB has DNase I-like activity, whereas CdtA and CdtC are binding proteins for delivering CdtB into target cells. Translocation of CdtB to the nucleus induces genotoxic effects on host DNA, triggering DNA repair cascades that lead to cell cycle arrest and eventual cell death. Several lines of evidence indicate that this toxin contributes to the pathogenicity of CDT-producing bacteria in vivo. Helicobacter hepaticus and Campylobacter jejuni CDTs are essential for persistent infection of the gastrointestinal tract and increase the severity of mucosal inflammation or liver disease in susceptible mouse strains. Haemophilus ducreyi CDT may contribute to the pathogenesis of chancroid in rabbits. Recently, H. hepaticus CDT has been shown to play a crucial role in promoting the progression of infectious hepatitis to pre-malignant, dysplastic lesions via activation of a pro-inflammatory NF-kappaB pathway and increased proliferation of hepatocytes, providing the first evidence that CDT has carcinogenic potential in vivo. Thus, both in vitro and in vivo data indicate that CDT is a bacterial virulence factor.