Two Functional Type VI Secretion Systems in Avian Pathogenic Escherichia coli Are Involved in Different Pathogenic Pathways

Abstract

Type VI secretion systems (T6SSs) are involved in the pathogenicity of several Gram-negative bacteria. The VgrG protein, a core component and effector of T6SS, has been demonstrated to perform diverse functions. The N-terminal domain of VgrG protein is a homologue of tail fiber protein gp27 of phage T4, which performs a receptor binding function and determines the host specificity. Based on sequence analysis, we found that two putative T6SS loci exist in the genome of the avian pathogenic Escherichia coli (APEC) strain TW-XM. To assess the contribution of these two T6SSs to TW-XM pathogenesis, the crucial clpV clusters of these two T6SS loci and their vgrG genes were deleted to generate a series of mutants. Consequently, T6SS1-associated mutants presented diminished adherence to and invasion of several host cell lines cultured in vitro, decreased pathogenicity in duck and mouse infection models in vivo, and decreased biofilm formation and bacterial competitive advantage. In contrast, T6SS2-associated mutants presented a significant decrease only in the adherence to and invasion of mouse brain microvascular endothelial cell (BMEC) line bEnd.3 and brain tissue of the duck infection model. These results suggested that T6SS1 was involved in the proliferation of APEC in systemic infection, whereas VgrG-T6SS2 was responsible only for cerebral infection. Further study demonstrated that VgrG-T6SS2 was able to bind to the surface of bEnd.3 cells, whereas it did not bind to DF-1 (chicken embryo fibroblast) cells, which further proved the interaction of VgrG-T6SS2 with the surface of BMECs.

The Type VI Secretion System Modulates Flagellar Gene Expression and Secretion in Citrobacter freundii and Contributes to Adhesion and Cytotoxicity to Host Cells

“…Our results are consistent with findings in other bac- terial pathogens of the role of T6SS in adherence and cytotoxicity to host cells. It has been reported that the T6SS in APEC is involved in adherence to host cells (46), the Bordetella bronchiseptica T6SS mediates cytotoxicity in murine macrophages (47), and the Campylobacter jejuni T6SS confers cytotoxicity to red blood cells (48). Since FliC contributes to cytotoxicity (49), it is possible that the T6SS of CF74 contributes to the apoptotic death of host cells in vitro indirectly by affecting the production and secretion of FliC.…”

The Hcp proteins fused with diverse extended-toxin domains represent a novel pattern of antibacterial effectors in type VI secretion systems

“…27,28 The T6SS2 clusters encoded by the intestinal pathogenic E. coli (including enterohemorrhagic E. coli, enterotoxigenic E. coli, enteropathogenic E. coli and so on) and extraintestinal pathogenic E. coli, show significant difference in their sequence identity, while share similar gene organizations. 27,29 Notably, the T6SS2 from E. coli has never been verified to mediate interbacterial antagonism.…”

Investigating the Relatedness of Enteroinvasive Escherichia coli to Other E. coli and Shigella Isolates by Using Comparative Genomics

“…The role of T2SS in the pathogenicity of EIEC is unknown. Another secretion system of interest for its potential contribution to virulence is the T6SS (69)(70)(71)(72). Genes with similarity to those of the T6SS of EPEC isolate B171 were identified in nearly all of the EIEC genomes of phylogroups A and B1 but were absent from all of the EIEC genomes of phylogroup E (Table 3).…”

“…In contrast, the T6SS genes were absent from all of the Shigella genomes investigated except S. sonnei 046 (Table 3). While the T6SS has been demonstrated to contribute to virulence in avian pathogenic E. coli (69) and EAEC (71,72), its role in the virulence of EIEC or Shigella has not been investigated. The T6SS has also been reported to have roles other than those directly involved in virulence, such as providing a competitive advantage in the presence of other bacteria in a community structure (73)(74)(75).…”