The type II secretion system as an underappreciated and understudied mediator of interbacterial antagonism.

Interbacterial antagonism involves all major phyla, occurs across the full range of ecological niches, and has great significance for the environment, clinical arena, and agricultural and industrial sectors. Though the earliest insight into interbacterial antagonism traces back to the discovery of antibiotics, a paradigm shift happened when it was learned that protein secretion systems (e.g., types VI and IV secretion systems) deliver toxic "effectors" against competitors. However, a link between interbacterial antagonism and the Gram-negative type II secretion system (T2SS), which exists in many pathogens and environmental species, is not evident in prior reviews on bacterial competition or T2SS function. A current examination of the literature revealed four examples of a T2SS or one of its known substrates having a bactericidal activity against a Gram-positive target or another Gram-negative. When further studied, the T2SS effectors proved to be peptidases that target the peptidoglycan of the competitor. There are also reports of various bacteriolytic enzymes occurring in the culture supernatants of some other Gram-negative species, and a link between these bactericidal activities and T2SS is suggested. Thus, a T2SS can be a mediator of interbacterial antagonism, and it is possible that many T2SSs have antibacterial outputs. Yet, at present, the T2SS remains relatively understudied for its role in interbacterial competition. Arguably, there is a need to analyze the T2SSs of a broader range of species for their role in interbacterial antagonism. Such investigation offers, among other things, a possible pathway toward developing new antimicrobials for treating disease.

The twin-arginine translocation system is vital for cell adhesion and uptake of iron in the cystic fibrosis pathogen Achromobacter xylosoxidans.

BACKGROUND: Achromobacter xylosoxidans is an emerging pathogen that causes airway infections in patients with cystic fibrosis. Knowledge of virulence factors and protein secretion systems in this bacterium is limited. Twin arginine translocation (Tat) is a protein secretion system that transports folded proteins across the inner cell membranes of gram-negative bacteria. Tat has been shown to be important for virulence and cellular processes in many different bacterial species. This study aimed to investigate the role of Tat in iron metabolism and host cell adhesion in A. xylosoxidans. METHODS: Putative Tat substrates in A. xylosoxidans were identified using the TatFind, TatP, and PRED-Tat prediction tools. An isogenic tatC deletion mutant (ΔtatC) was generated and phenotypically characterized. The wild-type and ΔtatC A. xylosoxidans were fractionated into cytosolic, membrane, and periplasmic fractions, and the expressed proteome of the different fractions was analyzed using liquid chromatography-mass spectrometry (LC-MS/MS). RESULTS: A total of 128 putative Tat substrates were identified in the A. xylosoxidans proteome. The ΔtatC mutant showed attenuated host cell adhesion, growth rate, and iron acquisition. Twenty predicted Tat substrates were identified as expressed proteins in the periplasmic compartment, nine of which were associated with the wild type. CONCLUSION: The data indicate that Tat secretion is important for iron acquisition and host cell adhesion in A. xylosoxidans.

Secretory expression of recombinant small laccase genes in Gram-positive bacteria.

BACKGROUND: Laccases are multicopper enzymes that oxidize a wide range of aromatic and non-aromatic compounds in the presence of oxygen. The majority of industrially relevant laccases are derived from fungi and are produced in eukaryotic expression systems such as Pichia pastoris and Saccharomyces cerevisiae. Bacterial laccases for research purposes are mostly produced intracellularly in Escherichia coli, but secretory expression systems are needed for future applications. Bacterial laccases from Streptomyces spp. are of interest for potential industrial applications because of their lignin degrading activities. RESULTS: In this study, we expressed small laccases genes from Streptomyces coelicolor, Streptomyces viridosporus and Amycolatopsis 75iv2 with their native signal sequences in Gram-positive Bacillus subtilis and Streptomyces lividans host organisms. The extracellular activities of ScLac, SvLac and AmLac expressed in S. lividans reached 1950 ± 99 U/l, 812 ± 57 U/l and 12 ± 1 U/l in the presence of copper supplementation. The secretion of the small laccases was irrespective of the copper supplementation; however, activities upon reconstitution with copper after expression were significantly lower, indicating the importance of copper during laccase production. The production of small laccases in B. subtilis resulted in extracellular activity that was significantly lower than in S. lividans. Unexpectedly, AmLac and ScLac were secreted without their native signal sequences in B. subtilis, indicating that B. subtilis secretes some heterologous proteins via an unknown pathway. CONCLUSIONS: Small laccases from S. coelicolor, S. viridosporus and Amycolatopsis 75iv2 were secreted in both Gram-positive expression hosts B. subtilis and S. lividans, but the extracellular activities were significantly higher in the latter.

The accessory protein TagV is required for full Type VI secretion system activity in Serratia marcescens.

The bacterial Type VI secretion system (T6SS) is a dynamic macromolecular structure that promotes inter- and intra-species competition through the delivery of toxic effector proteins into neighbouring cells. The T6SS contains 14 well-characterised core proteins necessary for effector delivery (TssA-M, PAAR). In this study, we have identified a novel accessory component required for optimal T6SS activity in the opportunistic pathogen Serratia marcescens, which we name TagV. Deletion of tagV, which encodes an outer membrane lipoprotein, caused a reduction in the T6SS-dependent antibacterial activity of S. marcescens Db10. Mutants of S. marcescens lacking the core component TssJ, a distinct outer membrane lipoprotein previously considered essential for T6SS firing, retained a modest T6SS activity that could be abolished through deletion of tagV. TagV did not interact with the T6SS membrane complex proteins TssL or TssM, but is proposed to bind to peptidoglycan, indicating that the mechanism by which TagV promotes T6SS firing differs from that of TssJ. Homologues of tagV were identified in several other bacterial genera, suggesting that the accessory function of TagV is not restricted to S. marcescens. Together, our findings support the existence of a second, TssJ-independent mechanism for T6SS firing that is dependent upon the activity of TagV proteins.

Antimicrobial Weapons of Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a robust and versatile organism capable of surviving and prospering in a diverse array of environments and is an opportunistic pathogen of humans. One reason for the success of this pathogen is the large arsenal of antimicrobial weapons that it possesses. Here we focus our attention on these antimicrobial weapons and how they give P. aeruginosa a survival edge in polymicrobial environments. We define antimicrobial weapons as components produced by P. aeruginosa that are used to kill, inhibit growth and/or subvert key cellular functions in other microbes. P. aeruginosa has a large and complex genome and encodes an armament of antimicrobial weapons that fall into two subclasses; those that are delivered directly to competing microbes using a contact-dependent method, and those that are secreted in a contact-independent manner into the environment to then be available to target neighbouring cells. This chapter provides an overview of the major antimicrobial weapons possessed by P. aeruginosa, captures recent advances in the field and discusses how these could be targeted as a therapeutic intervention, or potentially harnessed to combat infection.

Structure of the Extracellular Region of the Bacterial Type VIIb Secretion System Subunit EsaA.

Gram-positive bacteria use type VII secretion systems (T7SSs) to export effector proteins that manipulate the physiology of nearby prokaryotic and eukaryotic cells. Several mycobacterial T7SSs have established roles in virulence. By contrast, the genetically distinct T7SSb pathway found in Firmicutes bacteria more often functions to mediate bacterial competition. A lack of structural information on the T7SSb has limited the understanding of effector export by this protein secretion apparatus. Here, we present the 2.4 Å crystal structure of the extracellular region of the T7SSb subunit EsaA from Streptococcus gallolyticus. Our structure reveals that homodimeric EsaA is an elongated, arrow-shaped protein with a surface-accessible "tip", which in some species of bacteria serves as a receptor for lytic bacteriophages. Because it is the only T7SSb subunit large enough to traverse the peptidoglycan layer of Firmicutes, we propose that EsaA plays a critical role in transporting effectors across the entirety of the Gram-positive cell envelope.

Cryo-EM reveals species-specific components within the Helicobacter pylori Cag type IV secretion system core complex.

The pathogenesis of Helicobacter pylori-associated gastric cancer is dependent on delivery of CagA into host cells through a type IV secretion system (T4SS). The H. pylori Cag T4SS includes a large membrane-spanning core complex containing five proteins, organized into an outer membrane cap (OMC), a periplasmic ring (PR) and a stalk. Here, we report cryo-EM reconstructions of a core complex lacking Cag3 and an improved map of the wild-type complex. We define the structures of two unique species-specific components (Cag3 and CagM) and show that Cag3 is structurally similar to CagT. Unexpectedly, components of the OMC are organized in a 1:1:2:2:5 molar ratio (CagY:CagX:CagT:CagM:Cag3). CagX and CagY are components of both the OMC and the PR and bridge the symmetry mismatch between these regions. These results reveal that assembly of the H. pylori T4SS core complex is dependent on incorporation of interwoven species-specific components.

The Type IX Secretion System: Advances in Structure, Function and Organisation.

The type IX secretion system (T9SS) is specific to the Bacteroidetes phylum. Porphyromonas gingivalis, a keystone pathogen for periodontitis, utilises the T9SS to transport many proteins-including its gingipain virulence factors-across the outer membrane and attach them to the cell surface. Additionally, the T9SS is also required for gliding motility in motile organisms, such as Flavobacterium johnsoniae. At least nineteen proteins have been identified as components of the T9SS, including the three transcription regulators, PorX, PorY and SigP. Although the components are known, the overall organisation and the molecular mechanism of how the T9SS operates is largely unknown. This review focusses on the recent advances made in the structure, function, and organisation of the T9SS machinery to provide further insight into this highly novel secretion system.

The Helicobacter pylori Cag Type IV Secretion System.

Colonization of the human stomach with Helicobacter pylori strains containing the cag pathogenicity island is a risk factor for development of gastric cancer. The cag pathogenicity island contains genes encoding a secreted effector protein (CagA) and components of a type IV secretion system (Cag T4SS). The molecular architecture of the H. pylori Cag T4SS is substantially more complex than that of prototype T4SSs in other bacterial species. In this review, we discuss recent discoveries pertaining to the structure and function of the Cag T4SS and its role in gastric cancer pathogenesis.

Contact-Dependent Interbacterial Antagonism Mediated by Protein Secretion Machines.

To establish and maintain an ecological niche, bacteria employ a wide range of pathways to inhibit the growth of their microbial competitors. Some of these pathways, such as those that produce antibiotics or bacteriocins, exert toxicity on nearby cells in a cell contact-independent manner. More recently, however, several mechanisms of interbacterial antagonism requiring cell-to-cell contact have been identified. This form of microbial competition is mediated by antibacterial protein toxins whose delivery to target bacteria uses protein secretion apparatuses embedded within the cell envelope of toxin-producing bacteria. In this review, we discuss recent work implicating the bacterial Type I, IV, VI, and VII secretion systems in the export of antibacterial 'effector' proteins that mediate contact-dependent interbacterial antagonism.

Genomic insights into cyanobacterial protein translocation systems.

Cyanobacteria are ubiquitous oxygenic photosynthetic bacteria with a versatile metabolism that is highly dependent on effective protein targeting. Protein sorting in diderm bacteria is not trivial and, in cyanobacteria, even less so due to the presence of a complex membrane system: the outer membrane, the plasma membrane and the thylakoid membrane. In cyanobacteria, protein import into the thylakoids is essential for photosynthesis, export to the periplasm fulfills a multifunctional role in maintaining cell homeostasis, and secretion mediates motility, DNA uptake and environmental interactions. Intriguingly, only one set of genes for the general secretory and the twin-arginine translocation pathways seem to be present. However, these systems have to operate in both plasma and thylakoid membranes. This raises the question of how substrates are recognized and targeted to their correct, final destination. Additional complexities arise when a protein has to be secreted across the outer membrane, where very little is known regarding the mechanisms involved. Given their ecological importance and biotechnological interest, a better understanding of protein targeting in cyanobacteria is of great value. This review will provide insights into the known knowns of protein targeting, propose hypotheses based on available genomic sequences and discuss future directions.

Life After Secretion-Yersinia enterocolitica Rapidly Toggles Effector Secretion and Can Resume Cell Division in Response to Changing External Conditions.

Many pathogenic bacteria use the type III secretion system (T3SS) injectisome to manipulate host cells by injecting virulence-promoting effector proteins into the host cytosol. The T3SS is activated upon host cell contact, and its activation is accompanied by an arrest of cell division; hence, many species maintain a T3SS-inactive sibling population to propagate efficiently within the host. The enteric pathogen Yersinia enterocolitica utilizes the T3SS to prevent phagocytosis and inhibit inflammatory responses. Unlike other species, almost all Y. enterocolitica are T3SS-positive at 37°C, which raises the question, how these bacteria are able to propagate within the host, that is, when and how they stop secretion and restart cell division after a burst of secretion. Using a fast and quantitative in vitro secretion assay, we have examined the initiation and termination of type III secretion. We found that effector secretion begins immediately once the activating signal is present, and instantly stops when this signal is removed. Following effector secretion, the bacteria resume division within minutes after being introduced to a non-secreting environment, and the same bacteria are able to re-initiate effector secretion at later time points. Our results indicate that Y. enterocolitica use their type III secretion system to promote their individual survival when necessary, and are able to quickly switch their behavior toward replication afterwards, possibly gaining an advantage during infection.

Structure of the Helicobacter pylori Cag type IV secretion system.

Bacterial type IV secretion systems (T4SSs) are molecular machines that can mediate interbacterial DNA transfer through conjugation and delivery of effector molecules into host cells. The Helicobacter pylori Cag T4SS translocates CagA, a bacterial oncoprotein, into gastric cells, contributing to gastric cancer pathogenesis. We report the structure of a membrane-spanning Cag T4SS assembly, which we describe as three sub-assemblies: a 14-fold symmetric outer membrane core complex (OMCC), 17-fold symmetric periplasmic ring complex (PRC), and central stalk. Features that differ markedly from those of prototypical T4SSs include an expanded OMCC and unexpected symmetry mismatch between the OMCC and PRC. This structure is one of the largest bacterial secretion system assemblies ever reported and illustrates the remarkable structural diversity that exists among bacterial T4SSs.

Dynamics of Mycobacterium tuberculosis Ag85B Revealed by a Sensitive Enzyme-Linked Immunosorbent Assay.

Secretion of specific proteins contributes to pathogenesis and immune responses in tuberculosis and other bacterial infections, yet the kinetics of protein secretion and fate of secreted proteins in vivo are poorly understood. We generated new monoclonal antibodies that recognize the Mycobacteriumtuberculosis secreted protein Ag85B and used them to establish and characterize a sensitive enzyme-linked immunosorbent assay (ELISA) to quantitate Ag85B in samples generated in vitro and in vivo We found that nutritional or culture conditions had little impact on the secretion of Ag85B and that there is considerable variation in Ag85B secretion by distinct strains in the M. tuberculosis complex: compared with the commonly used H37Rv strain (lineage 4), Mycobacteriumafricanum (lineage 6) secretes less Ag85B, and two strains from lineage 2 secrete more Ag85B. We also used the ELISA to determine that the rate of secretion of Ag85B is 10- to 100-fold lower than that of proteins secreted by Gram-negative and Gram-positive bacteria, respectively. ELISA quantitation of Ag85B in lung homogenates of M. tuberculosis H37Rv-infected mice revealed that although Ag85B accumulates in the lungs as the bacterial population expands, the amount of Ag85B per bacterium decreases nearly 10,000-fold at later stages of infection, coincident with the development of T cell responses and arrest of bacterial population growth. These results indicate that bacterial protein secretion in vivo is dynamic and regulated, and quantitation of secreted bacterial proteins can contribute to the understanding of pathogenesis and immunity in tuberculosis and other infections.IMPORTANCE Bacterial protein secretion contributes to host-pathogen interactions, yet the process and consequences of bacterial protein secretion during infection are poorly understood. We developed a sensitive ELISA to quantitate a protein (termed Ag85B) secreted by M. tuberculosis and used it to find that Ag85B secretion occurs with slower kinetics than for proteins secreted by Gram-positive and Gram-negative bacteria and that accumulation of Ag85B in the lungs is markedly regulated as a function of the bacterial population density. Our results demonstrate that quantitation of bacterial proteins during infection can reveal novel insights into host-pathogen interactions.

The Type VI secretion system: a versatile bacterial weapon.

The Type VI secretion system (T6SS) is a protein nanomachine that is widespread in Gram-negative bacteria and is used to translocate effector proteins directly into neighbouring cells. It represents a versatile bacterial weapon that can deliver effectors into distinct classes of target cells, playing key roles in inter-bacterial competition and bacterial interactions with eukaryotic cells. This versatility is underpinned by the ability of the T6SS to deliver a vast array of effector proteins, with many distinct activities and modes of interaction with the secretion machinery. Recent work has highlighted the importance and diversity of interactions mediated by T6SSs within polymicrobial communities, and offers new molecular insights into effector delivery and action in target cells.

Effector Translocation Assay: Differential Solubilization.

The identification of effector proteins delivered into mammalian host cells by bacterial pathogens possessing syringelike nanomachines is an important step toward understanding the mechanisms underlying the virulence of these pathogens. In this chapter, we describe a method based on mammalian tissue culture infection models where incubation with a nonionic detergent (Triton X-100) enables solubilization of host cell membranes but not of bacterial membranes. This allows the isolation of a Triton-soluble fraction lacking bacteria but enriched in proteins present in the host cell cytoplasm and plasma membrane. Using appropriate controls, this fraction can be probed by immunoblotting for the presence of bacterial effector proteins delivered into host cells.

A secretome view of colonisation factors in Shiga toxin-encoding Escherichia coli (STEC): from enterohaemorrhagic E. coli (EHEC) to related enteropathotypes.

Shiga toxin-encoding Escherichia coli (STEC) regroup strains that carry genes encoding Shiga toxin (Stx). Among intestinal pathogenic E. coli, enterohaemorrhagic E. coli (EHEC) constitute the major subgroup of virulent STEC. EHEC cause serious human disease such as haemorrhagic colitis and haemolytic-uremic syndrome. While EHEC have evolved from enteropathogenic E. coli, hybrids with enteroaggregative E. coli have recently emerged. Of note, some enteroinvasive E. coli also belong to the STEC group. While the LEE (locus of enterocyte effacement) is a key and prominent molecular determinant in the pathogenicity, neither all EHEC nor STEC contain the LEE, suggesting that they possess additional virulence and colonisation factors. Currently, nine protein secretion systems have been described in diderm-lipopolysaccharide bacteria (archetypal Gram-negative) and can be involved in the secretion of extracellular effectors, cell-surface proteins or assembly of cell-surface organelles, such as flagella or pili. In this review, we focus on the secretome of STEC and related enteropathotypes, which are relevant to the colonisation of biotic and abiotic surfaces. Considering the wealth of potential protein trafficking mechanisms, the different combinations of colonisation factors and modulation of their expression is further emphasised with regard to the ecophysiology of STEC.

Aim, Load, Fire: The Type VI Secretion System, a Bacterial Nanoweapon.

Bacteria utilise specialised protein secretion systems to interact with host organisms, competitor bacteria, and the environment. The Type VI secretion system (T6SS) is a versatile weapon deployed by many bacterial species to target either host cells or rival bacteria. The widespread occurrence and significance of the T6SS is becoming increasingly appreciated, as is its intriguing mode of action. The T6SS delivers multiple, diverse effector proteins directly into target cells using a dynamic 'firing' mechanism related to the action of contractile bacteriophage tails. Here, we summarise the contribution of recent findings to our developing picture of how the T6SS assembles and fires, how it is loaded with different types of effectors, and how it can be aimed towards an incoming assault.

Growth phase-dependent composition of the Helicobacter pylori exoproteome.

Helicobacter pylori colonizes the human stomach and is associated with an increased risk of gastric cancer and peptic ulcer disease. Analysis of H. pylori protein secretion is complicated by the occurrence of bacterial autolysis. In this study, we analyzed the exoproteome of H. pylori at multiple phases of bacterial growth and identified 74 proteins that are selectively released into the extracellular space. These include proteins known to cause alterations in host cells, antigenic proteins, and additional proteins that have not yet been studied in any detail. The composition of the H. pylori exoproteome is dependent on the phase of bacterial growth. For example, the proportional abundance of the vacuolating toxin VacA in culture supernatant is higher during late growth phases than early growth phases, whereas the proportional abundance of many other proteins is higher during early growth phases. We detected marked variation in the subcellular localization of putative secreted proteins within soluble and membrane fractions derived from intact bacteria. By providing a comprehensive view of the H. pylori exoproteome, these results provide new insights into the array of secreted H. pylori proteins that may cause alterations in the gastric environment.

A method to investigate protein association with intact sealed mycobacterial membrane vesicles.

In mycobacteria, probing the association of cytoplasmic proteins with the membrane itself, as well as with integral or peripheral membrane proteins, is limited by the difficulty in extracting intact sealed membrane vesicles due to the complex cell wall structure. Here we tested the association of Mycobacterium tuberculosis SecA1 and SecA2 proteins with intact membrane vesicles by a flotation assay using iodixanol density gradients. These protocols have wide applications for studying the association of other mycobacterial cytoplasmic proteins with the membrane and membrane-associated proteins.