Interactions and substrate selectivity within the SctRST complex of the type III secretion system of enteropathogenic Escherichia coli

Comparative transcriptome reveals importance of export apparatus subunit (ascR) in type III secretion system and its roles on biological properties, gene expression profiles, virulence and colonization of Aeromonas veronii

Aeromonas veronii, an opportunistic pathogen, is known to cause serious infections across various species. In our previous study, we discovered that A. veronii GL2 exhibited a virulence up to ten times greater than that of FO1. To ascertain the factors contributing to the disparity in virulence between the two strains, we conducted a comparative transcriptome analysis. This analysis reveals a significant upregulation (P < 0.05) of the ascR gene in GL2 compared with FO1. Additionally, six differentially expressed genes (DEGs) were identified within the "Bacterial secretion system" pathway (map03070), with ascR being an essential component of type III secretion system (T3SS). AscR, considered as SctR family export apparatus subunit within the T3SS, has ambiguous roles in the biological properties, gene expression profiles, virulence and colonization of A. veronii. Therefore, we constructed a mutant strain (ΔascR) by homologous recombination. Comparative analysis with the wide-type GL2 reveals no significant differences in terms of colony morphology, growth curve, hemolytic activity and protease activity. However, significant reductions (P < 0.01) were observed in the abilities of biofilm formation and swimming mobility. No remarkable difference was noted in the lengths of flagella. The LD50 value of ΔascR was to be 5.15 times higher than that of GL2. Interestingly, the mRNA expression of ascC, ascD, ascJ and ascI genes in the T3SS, and mshB, mshE, mshK and mshP genes in the MSHA type pili were significantly upregulated (P < 0.05) in ΔascR, potentially due to transcriptional compensation. Further analysis of enzymatic biomarkers revealed that ΔascR might not destruct the recognition of innate immune response in host remarkably, but the colonization levels of A.veronii were significantly suppressed (P < 0.01) in ΔascR group. In conclusion, the ascR gene may be a key determinant in regulating the virulence of A. veronii, and the destruction of the T3SS caused by ascR deficiency results in these notable changes.

Genetic distribution, characterization, and function of Escherichia coli type III secretion system 2 (ETT2)

Many Gram-negative bacteria use type Ⅲ secretion system (T3SS) to inject effector proteins and subvert host signaling pathways, facilitating the growth, survival, and virulence. Notably, some bacteria harbor multiple distinct T3SSs with different functions. An extraordinary T3SS, the Escherichia coli Type III Secretion System 2 (ETT2), is widespread among Escherichia coli (E. coli) strains. Since many ETT2 carry genetic mutations or deletions, it is thought to be nonfunctional. However, increasing studies highlight ETT2 contributes to E. coli pathogenesis. Here, we present a comprehensive overview of genetic distribution and characterization of ETT2. Subsequently, we outline its functional potential, contending that an intact ETT2 may retain the capacity to translocate effector proteins and manipulate the host's innate immune response. Given the potential zoonotic implications associated with ETT2-carrying bacteria, further investigations into the structure, function and regulation of ETT2 are imperative for comprehensive understanding of E. coli pathogenicity and the development of effective control strategies.

Recent insights into type-3 secretion system injectisome structure and mechanism of human enteric pathogens

Type-3 secretion system injectisomes are multiprotein complexes that translocate bacterial effector proteins from the cytoplasm of gram-negative bacteria directly into the cytosol of eukaryotic host cells. These systems are present in more than 30 bacterial species, including numerous human, animal, and plant pathogens. We review recent discoveries of structural and molecular mechanisms of effector protein translocation through the injectisomes and recent advances in the understanding of mechanisms of activation of effector protein secretion.

Indole intercepts the communication between enteropathogenic E. coli and Vibrio cholerae

Reported numbers of diarrheal samples exhibiting co-infections or multiple infections, with two or more infectious agents, are rising, likely due to advances in bacterial diagnostic techniques. Bacterial species detected in these samples include Vibrio cholerae (V. cholerae) and enteropathogenic Escherichia coli (EPEC), which infect the small intestine and are associated with high mortality rates. It has previously been reported that EPEC exhibit enhanced virulence in the presence of V. cholerae owing to their ability to sense and respond to elevated concentrations of cholera autoinducer 1 (CAI-1), which is the primary quorum-sensing (QS) molecule produced by V. cholerae. In this study, we examined this interspecies bacterial communication in the presence of indole, a major microbiome-derived metabolite found at high concentrations in the human gut. Interestingly, we discovered that although indole did not affect bacterial growth or CAI-1 production, it impaired the ability of EPEC to enhance its virulence activity in response to the presence of V. cholerae. Furthermore, the co-culture of EPEC and V. cholerae in the presence of B. thetaiotaomicron, an indole-producing commensal bacteria, ablated the enhancement of EPEC virulence. Together, these results suggest that microbiome compositions or diets that influence indole gut concentrations may differentially impact the virulence of pathogens and their ability to sense and respond to competing bacteria.