Cyclic dinucleotides (CDNs) have been previously recognized as important secondary signaling molecules in bacteria and, more recently, in mammalian cells. In the former case, they represent secondary ...messengers affecting numerous responses of the prokaryotic cell, whereas in the latter, they act as agonists of the innate immune response. Remarkable new discoveries have linked these two patterns of utilization of CDNs as secondary messengers and have revealed unexpected influences they likely had on shaping human genetic variation. This Review summarizes these recent insights and provides a perspective on future unanswered questions in this exciting field.
The bacterial type VI secretion system (T6SS) is a dynamic organelle that bacteria use to target prey cells for inhibition via translocation of effector proteins. Time-lapse fluorescence microscopy ...has documented striking dynamics of opposed T6SS organelles in adjacent sister cells of Pseudomonas aeruginosa. Such cell-cell interactions have been termed “T6SS dueling” and likely reflect a biological process that is driven by T6SS antibacterial attack. Here, we show that T6SS dueling behavior strongly influences the ability of P. aeruginosa to prey upon heterologous bacterial species. We show that, in the case of P. aeruginosa, T6SS-dependent killing of either Vibrio cholerae or Acinetobacter baylyi is greatly stimulated by T6SS activity occurring in those prey species. Our data suggest that, in P. aeruginosa, T6SS organelle assembly and lethal counterattack are regulated by a signal that corresponds to the point of attack of the T6SS apparatus elaborated by a second aggressive T6SS+ bacterial cell.
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► P. aeruginosa targets T6SS-positive bacteria for T6SS-mediated counterattack ► Phosphorylation signaling cascade regulates P. aeruginosa response to attack ► P. aeruginosa T6SS delivers Tse-1 effector to target cells ► Tit-for-tat evolutionary strategy controls interactions between bacterial species
When P. aeruginosa is attacked by a type VI secretion system (T6SS) from a neighboring cell of another species, it mounts its own lethal T6SS counterattack from the point where it was targeted.
The bacterial type VI secretion system (T6SS) is an organelle that is structurally and mechanistically analogous to an intracellular membrane-attached contractile phage tail. Recent studies ...determined that a rapid conformational change in the structure of a sheath protein complex propels T6SS spike and tube components along with antibacterial and antieukaryotic effectors out of predatory T6SS+ cells and into prey cells. The contracted organelle is then recycled in an ATP-dependent process. T6SS is regulated at transcriptional and posttranslational levels, the latter involving detection of membrane perturbation in some species. In addition to directly targeting eukaryotic cells, the T6SS can also target other bacteria coinfecting a mammalian host, highlighting the importance of the T6SS not only for bacterial survival in environmental ecosystems, but also in the context of infection and disease. This review highlights these and other advances in our understanding of the structure, mechanical function, assembly, and regulation of the T6SS.
Ho et al. examine the structure, mechanical function, assembly, and regulation of the bacterial type VI secretion system.
Vibrio cholerae is the causative agent of cholera, a potentially lethal enteric bacterial infection
. Cholera toxin (CTX), a protein complex that is secreted by V. cholerae, is required for ...V. cholerae to cause severe disease. CTX is also thought to promote transmission of the organism, as infected individuals shed many litres of diarrhoeal fluid that typically contains in excess of 10
organisms per litre. How the pathogen is able to reach such high concentrations in the intestine during infection remains poorly understood. Here we show that CTX increases pathogen growth and induces a distinct V. cholerae transcriptomic signature that is indicative of an iron-depleted gut niche. During infection, bacterial pathogens need to acquire iron, which is an essential nutrient for growth
. Most iron in the mammalian host is found in a chelated form within the porphyrin structure of haem, and the ability to use haem as a source of iron is genetically encoded by V. cholerae
. We show that the genes that enable V. cholerae to obtain iron via haem and vibriobactin confer a growth advantage to the pathogen only when CTX is produced. Furthermore, we found that CTX-induced congestion of capillaries in the terminal ileum correlated with an increased bioavailability of luminal haem. CTX-induced disease in the ileum also led to increased concentrations of long-chain fatty acids and L-lactate metabolites in the lumen, as well as the upregulation of V. cholerae genes that encode enzymes of the tricarboxylic acid (TCA) cycle that contain iron-sulfur clusters. Genetic analysis of V. cholerae suggested that pathogen growth was dependent on the uptake of haem and long-chain fatty acids during infection, but only in a strain capable of producing CTX in vivo. We conclude that CTX-induced disease creates an iron-depleted metabolic niche in the gut, which selectively promotes the growth of V. cholerae through the acquisition of host-derived haem and fatty acids.
The cellular surveillance-activated detoxification and defenses (cSADD) theory postulates the presence of host surveillance mechanisms that monitor the integrity of common cellular processes and ...components targeted by pathogen effectors. Being organelles essential for multiple cellular processes, including innate immune responses, mitochondria represent an attractive target for pathogens. We describe a Vibrio cholerae Type 3 secretion system effector VopE that localizes to mitochondria during infection and acts as a specific GTPase-activating protein to interfere with the function of mitochondrial Rho GTPases Miro1 and Miro2. Miro GTPases modulate mitochondrial dynamics and interfering with this functionality effectively blocks innate immune responses that presumably require mitochondria as signaling platforms. Our data indicate that interference with mitochondrial dynamics may be an unappreciated strategy that pathogens use to block host innate immune responses that would otherwise control these bacterial infections. VopE might represent a bacterial effector that targets the cSADD surveillance response.
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•V. cholerae T3SS effector VopE is targeted to mitochondria during infection•VopE binds and activates GTP hydrolysis of Miro GTPases•Miro promotes perinuclear mitochondrial clustering during infection, which VopE inhibits•VopE modulation of mitochondrial dynamics inhibits host inflammatory responses
Bacterial virulence effectors have been long appreciated as powerful probes for uncovering host cell functions ranging from signal transduction pathways to organelle functions. Suzuki et al. describe VopE as a Vibrio cholerae effector that inactivates mitochondrial Rho GTPases to modulate mitochondrial dynamics and block innate immune responses during infection.
A type VI secretion system (T6SS) was recently shown to be required for full virulence of Vibrio cholerae O37 serogroup strain V52. In this study, we systematically mutagenized each individual gene ...in T6SS locus and characterized their functions based on expression and secretion of the hemolysin co-regulated protein (Hcp), virulence towards amoebae of Dictyostelium discoideum and killing of Escherichia coli bacterial cells. We group the 17 proteins characterized in the T6SS locus into four categories: twelve (VipA, VipB, VCA0109-VCA0115, ClpV, VCA0119, and VasK) are essential for Hcp secretion and bacterial virulence, and thus likely function as structural components of the apparatus; two (VasH and VCA0122) are regulators that are required for T6SS gene expression and virulence; another two, VCA0121 and valine-glycine repeat protein G 3 (VgrG-3), are not essential for Hcp expression, secretion or bacterial virulence, and their functions are unknown; the last group is represented by VCA0118, which is not required for Hcp expression or secretion but still plays a role in both amoebae and bacterial killing and may therefore be an effector protein. We also showed that the clpV gene product is required for Dictyostelium virulence but is less important for killing E. coli. In addition, one vgrG gene (vgrG-2) outside of the T6SS gene cluster was required for bacterial killing but another (vgrG-1) was not. However, a bacterial killing defect was observed when vgrG-1 and vgrG-3 were both deleted. Several genes encoded in the same putative operon as vgrG-1 and vgrG-2 also contribute to virulence toward Dictyostelium but have a smaller effect on bacterial killing. Our results provide new insights into the functional requirements of V. cholerae's T6SS in the context of secretion as well as killing of bacterial and eukaryotic phagocytic cells.
Analysis of genes required for host infection will provide clues to the drivers of evolutionary fitness of pathogens like Vibrio cholerae, a mounting threat to global heath. We used transposon ...insertion site sequencing (Tn-seq) to comprehensively assess the contribution of nearly all V. cholerae genes toward growth in the infant rabbit intestine. Four hundred genes were identified as critical to V. cholerae in vivo fitness. These included most known colonization factors and several new genes affecting the bacterium’s metabolic properties, resistance to bile, and ability to synthesize cyclic AMP-GMP. Notably, a mutant carrying an insertion in tsiV3, encoding immunity to a bacteriocidal type VI secretion system (T6SS) effector VgrG3, exhibited a colonization defect. The reduced in vivo fitness of tsiV3 mutants depends on their cocolonization with bacterial cells carrying an intact T6SS locus and VgrG3 gene, suggesting that the V. cholerae T6SS is functional and mediates antagonistic interbacterial interactions during infection.
•Four hundred genes critical to in vivo fitness of V. cholerae were identified through Tn-seq•Inactivation of a cyclic dinucleotide synthetase regulator enhances intestinal colonization•Tn mutant of tsiV3, encoding immunity to a T6SS effector, VgrG3, is colonization defective•Reduced tsiV3 mutant fitness depends on cocolonization with T6SS and VgrG3 carrying bacteria
The bacterial type VI secretion system (T6SS) is a nanomachine that delivers toxic effector proteins into target cells, killing them. In mice, we found that the
T6SS attacks members of the host ...commensal microbiota in vivo, facilitating the pathogen's colonization of the gut. This microbial antagonistic interaction drives measurable changes in the pathogenicity of
through enhanced intestinal colonization, expression of bacterial virulence genes, and activation of host innate immune genes. Because ablation of mouse commensals by this enteric pathogen correlated with more severe diarrheal symptoms, we conclude that antagonism toward the gut microbiota could improve the fitness of
as a pathogen by elevating its transmission to new susceptible hosts.
Outer membrane vesicles (OMVs) produced by Gram-negative bacteria provide an interesting research material for defining cell-envelope proteins without experimental cell disruption. OMVs are also ...promising immunogenic platforms and may play important roles in bacterial survival and pathogenesis. We used in-solution trypsin digestion coupled to mass spectrometry to identify 90 proteins present in OMVs of Vibrio cholerae when grown under conditions that activate the TCP pilus virulence regulatory protein (ToxT) virulence regulon. The ToxT expression profile and potential contribution to virulence of these proteins were assessed using ToxT and in vivo RNA-seq, Tn-seq, and cholera stool proteomic and other genome-wide data sets. Thirteen OMV-associated proteins appear to be essential for cell growth, and therefore may represent antibacterial drug targets. Another 12 nonessential OMV proteins, including DegP protease, were required for intestinal colonization in rabbits. Comparative proteomics of a degP mutant revealed the importance of DegP in the incorporation of nine proteins into OMVs, including ones involved in biofilm matrix formation and various substrates of the type II secretion system. Taken together, these results suggest that DegP plays an important role in determining the content of OMVs and also affects phenotypes such as intestinal colonization, proper function of the type II secretion system, and formation of biofilm matrix.
Enteroendocrine cells (EEs) are interspersed between enterocytes and stem cells in the Drosophila intestinal epithelium. Like enterocytes, EEs express components of the immune deficiency (IMD) innate ...immune pathway, which activates transcription of genes encoding antimicrobial peptides. The discovery of large lipid droplets in intestines of IMD pathway mutants prompted us to investigate the role of the IMD pathway in the host metabolic response to its intestinal microbiota. Here we provide evidence that the short-chain fatty acid acetate is a microbial metabolic signal that activates signaling through the enteroendocrine IMD pathway in a PGRP-LC-dependent manner. This, in turn, increases transcription of the gene encoding the endocrine peptide Tachykinin (Tk), which is essential for timely larval development and optimal lipid metabolism and insulin signaling. Our findings suggest innate immune pathways not only provide the first line of defense against infection but also afford the intestinal microbiota control over host development and metabolism.
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•The innate immune deficiency (IMD) pathway regulates tachykinin transcription•Microbial activation of enteroendocrine IMD signaling mediates metabolic homeostasis•Acetate restores IMD pathway activation and metabolic homeostasis in axenic flies•Acetate signaling through IMD depends on the membrane-associated receptor PGRP-LC
Here Kamareddine et al. show that an innate immune pathway in enteroendocrine cells of the Drosophila melanogaster intestine is activated by the intestinal microbiota and the microbial metabolite acetate. Activation of this pathway increases expression of the endocrine peptide tachykinin to promote host metabolic homeostasis.
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