Characterization of translocation pores inserted into plasma membranes by type III-secreted Esp proteins of enteropathogenic Escherichia coli

The sequence of events of enteropathogenic E. coli's type III secretion system translocon assembly

Many bacterial pathogens employ the type III secretion system (T3SS), a specialized complex that transports effector proteins that manipulate various cellular processes. The T3SS forms a translocon pore within the host-cell membrane consisting of two secreted proteins that transition from a soluble state into a transmembrane complex. Still, the exact sequence of events leading to the formation of a membranous functional pore remains uncertain. Here, we utilized the translocon proteins of enteropathogenic E. coli (EPEC) to investigate the sequence of those steps leading to translocon assembly, including self-oligomerization, hetero-oligomerization, interprotein interaction, and membrane insertion. We found that in EPEC, EspD (SctE) plays a dominant role in pore formation as it assembles into an oligomeric state, regardless of pH, membrane contact, or the presence of EspB (SctB). Subsequently, EspB subunits integrate into EspD homo-oligomers to create EspB-EspD hetero-oligomers that adopt a transmembrane orientation to create a functional pore complex.

Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome

Two of the most fascinating bacterial nanomachines-the broadly disseminated rotary flagellum at the heart of cellular motility and the eukaryotic cell-puncturing injectisome essential to specific pathogenic species-utilize at their core a conserved export machinery called the type III secretion system (T3SS). The T3SS not only secretes the components that self-assemble into their extracellular appendages but also, in the case of the injectisome, subsequently directly translocates modulating effector proteins from the bacterial cell into the infected host. The injectisome is thought to have evolved from the flagellum as a minimal secretory system lacking motility, with the subsequent acquisition of additional components tailored to its specialized role in manipulating eukaryotic hosts for pathogenic advantage. Both nanomachines have long been the focus of intense interest, but advances in structural and functional understanding have taken a significant step forward since 2015, facilitated by the revolutionary advances in cryo-electron microscopy technologies. With several seminal structures of each nanomachine now captured, we review here the molecular similarities and differences that underlie their diverse functions.

Synthetic and natural antimicrobials as a control against food borne pathogens: A review

Food borne pathogens are one of the most common yet concerning cause of illnesses around the globe. These microbes invade the body via food items, through numerous mediums of contamination and it is impossible to completely eradicate these organisms from food. Extensive research has been made regarding their treatment. Unfortunately, the only available treatment currently is by antibiotics. Recent exponential increase in antibiotic resistance and the side effect of synthetic compounds have established a need for alternate therapies that could be utilized either on their own or along with antibiotics to provide protection against food-borne diseases. The aim of this review is to provide information regarding some common food borne diseases, their current and possible natural treatment. It will include details regarding some common foodborne pathogens, the disease they cause, prevalence, manifestations and treatment of the respective disease. Some natural modes of potential treatment will be summarized, which including phytochemicals, derived from plants either as crude extracts or as purified form and Bacteriocins as microbial based treatment, obtained from various types of bacteria. The paper will describe their mechanism of action, classification, susceptible organisms, some antimicrobial compounds and producing organisms, application in food systems and as potential treatment. Along with that, synthetic treatment i.e., antibiotics will be discussed including the first-line treatment of some common food borne infections, prevalence and mechanism of resistance against antibiotics in the pathogens.

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.

The Diversity of Escherichia coli Pathotypes and Vaccination Strategies against This Versatile Bacterial Pathogen

Escherichia coli (E. coli) is a gram-negative bacillus and resident of the normal intestinal microbiota. However, some E. coli strains can cause diseases in humans, other mammals and birds ranging from intestinal infections, for example, diarrhea and dysentery, to extraintestinal infections, such as urinary tract infections, respiratory tract infections, meningitis, and sepsis. In terms of morbidity and mortality, pathogenic E. coli has a great impact on public health, with an economic cost of several billion dollars annually worldwide. Antibiotics are not usually used as first-line treatment for diarrheal illness caused by E. coli and in the case of bloody diarrhea, antibiotics are avoided due to the increased risk of hemolytic uremic syndrome. On the other hand, extraintestinal infections are treated with various antibiotics depending on the site of infection and susceptibility testing. Several alarming papers concerning the rising antibiotic resistance rates in E. coli strains have been published. The silent pandemic of multidrug-resistant bacteria including pathogenic E. coli that have become more difficult to treat favored prophylactic approaches such as E. coli vaccines. This review provides an overview of the pathogenesis of different pathotypes of E. coli, the virulence factors involved and updates on the major aspects of vaccine development against different E. coli pathotypes.

Transmembrane substrates of type three secretion system injectisomes

The type three secretion system injectisome of Gram-negative bacterial pathogens injects virulence proteins, called effectors, into host cells. Effectors of mammalian pathogens carry out a range of functions enabling bacterial invasion, replication, immune suppression and transmission. The injectisome secretes two translocon proteins that insert into host cell membranes to form a translocon pore, through which effectors are delivered. A subset of effectors also integrate into infected cell membranes, enabling a unique range of biochemical functions. Both translocon proteins and transmembrane effectors avoid cytoplasmic aggregation and integration into the bacterial inner membrane. Translocated transmembrane effectors locate and integrate into the appropriate host membrane. In this review, we focus on transmembrane translocon proteins and effectors of bacterial pathogens of mammals. We discuss what is known about the mechanisms underlying their membrane integration, as well as the functions conferred by the position of injectisome effectors within membranes.

Enhanced protein translocation to mammalian cells by expression of EtgA transglycosylase in a synthetic injector E. coli strain

Background

Bacterial type III secretion systems (T3SSs) assemble a multiprotein complex termed the injectisome, which acts as a molecular syringe for translocation of specific effector proteins into the cytoplasm of host cells. The use of injectisomes for delivery of therapeutic proteins into mammalian cells is attractive for biomedical applications. With that aim, we previously generated a non-pathogenic Escherichia coli strain, called Synthetic Injector E. coli (SIEC), which assembles functional injectisomes from enteropathogenic E. coli (EPEC). The assembly of injectisomes in EPEC is assisted by the lytic transglycosylase EtgA, which degrades the peptidoglycan layer. As SIEC lacks EtgA, we investigated whether expression of this transglycosylase enhances the protein translocation capacity of the engineered bacterium.

Results

The etgA gene from EPEC was integrated into the SIEC chromosome under the control of the inducible tac promoter, generating the strain SIEC-eEtgA. The controlled expression of EtgA had no effect on the growth or viability of bacteria. Upon induction, injectisome assembly was ~ 30% greater in SIEC-eEtgA than in the parental strain, as determined by the level of T3SS translocon proteins, the hemolytic activity of the bacterial strain, and the impairment in flagellar motility. The functionality of SIEC-eEtgA injectisomes was evaluated in a derivative strain carrying a synthetic operon (eLEE5), which was capable of delivering Tir effector protein into the cytoplasm of HeLa cells triggering F-actin polymerization beneath the attached bacterium. Lastly, using β-lactamase as a reporter of T3SS-protein injection, we determined that the protein translocation capacity was ~ 65% higher in the SIEC-EtgA strain than in the parental SIEC strain.

Conclusions

We demonstrate that EtgA enhances the assembly of functional injectisomes in a synthetic injector E. coli strain, enabling the translocation of greater amounts of proteins into the cytoplasm of mammalian cells. Accordingly, EtgA expression may boost the protein translocation of SIEC strains programmed as living biotherapeutics.

HrpA anchors meningococci to the dynein motor and affects the balance between apoptosis and pyroptosis

BACKGROUND

In Neisseria meningitidis the HrpA/HrpB two-partner secretion system (TPS) was implicated in diverse functions including meningococcal competition, biofilm formation, adherence to epithelial cells, intracellular survival and vacuolar escape. These diverse functions could be attributed to distinct domains of secreted HrpA.

METHODS

A yeast two-hybrid screening, in vitro pull-down assay and immunofluorescence microscopy experiments were used to investigate the interaction between HrpA and the dynein light-chain, Tctex-type 1 (DYNLT1). In silico modeling was used to analyze HrpA structure. Western blot analysis was used to investigate apoptotic and pyroptotic markers.

RESULTS

The HrpA carboxy-terminal region acts as a manganese-dependent cell lysin, while the results of a yeast two-hybrid screening demonstrated that the HrpA middle region has the ability to bind the dynein light-chain, Tctex-type 1 (DYNLT1). This interaction was confirmed by in vitro pull-down assay and immunofluorescence microscopy experiments showing co-localization of N. meningitidis with DYNLT1 in infected epithelial cells. In silico modeling revealed that the HrpA-M interface interacting with the DYNLT1 has similarity with capsid proteins of neurotropic viruses that interact with the DYNLT1. Indeed, we found that HrpA plays a key role in infection of and meningococcal trafficking within neuronal cells, and is implicated in the modulation of the balance between apoptosis and pyroptosis.

CONCLUSIONS

Our findings revealed that N. meningitidis is able to effectively infect and survive in neuronal cells, and that this ability is dependent on HrpA, which establishes a direct protein-protein interaction with DYNLTI in these cells, suggesting that the HrpA interaction with dynein could be fundamental for N. meningitidis spreading inside the neurons. Moreover, we found that the balance between apoptotic and pyroptotic pathways is heavily affected by HrpA.

Live imaging of Yersinia translocon formation and immune recognition in host cells

Yersinia enterocolitica employs a type three secretion system (T3SS) to translocate immunosuppressive effector proteins into host cells. To this end, the T3SS assembles a translocon/pore complex composed of the translocator proteins YopB and YopD in host cell membranes serving as an entry port for the effectors. The translocon is formed in a Yersinia-containing pre-phagosomal compartment that is connected to the extracellular space. As the phagosome matures, the translocon and the membrane damage it causes are recognized by the cell-autonomous immune system. We infected cells in the presence of fluorophore-labeled ALFA-tag-binding nanobodies with a Y. enterocolitica strain expressing YopD labeled with an ALFA-tag. Thereby we could record the integration of YopD into translocons and its intracellular fate in living host cells. YopD was integrated into translocons around 2 min after uptake of the bacteria into a phosphatidylinositol-4,5-bisphosphate enriched pre-phagosomal compartment and remained there for 27 min on average. Damaging of the phagosomal membrane as visualized with recruitment of GFP-tagged galectin-3 occurred in the mean around 14 min after translocon formation. Shortly after recruitment of galectin-3, guanylate-binding protein 1 (GBP-1) was recruited to phagosomes, which was accompanied by a decrease in the signal intensity of translocons, suggesting their degradation or disassembly. In sum, we were able for the first time to film the spatiotemporal dynamics of Yersinia T3SS translocon formation and degradation and its sensing by components of the cell-autonomous immune system.

A Macroporous Magnesium Oxide-Templated Carbon Adsorbs Shiga Toxins and Type III Secretory Proteins in Enterohemorrhagic Escherichia coli, Which Attenuates Virulence

Enterohemorrhagic Escherichia coli (EHEC) is one of the most common foodborne pathogens. However, no drug that prevents the severe complications caused by this bacterium has been approved yet. This study showed that a macroporous magnesium oxide (MgO)-templated carbon material (MgOC₁₅₀) adsorbs Shiga toxins, and Type III secretory EspA/EspB proteins responsible for EHEC pathogenesis, and decreases the extracellular levels of these proteins. On the other hand, this material did not affect the growth of EHEC. Citrobacter rodentium traditionally used to estimate Type III secretion system-associated virulence in mice is highly virulent. The survival period of infected mice was prolonged when MgOC₁₅₀ was administered. This adsorbent disturbed neither mammalian cells nor normal intestinal bacteria, such as Enterococcus hirae, Lactobacillus acidophilus, and Lactobacillus casei. In contrast, MgOC₁₅₀ adsorbed antimicrobial agents, including β-lactams, quinolones, tetracyclines, and trimethoprim/sulfamethoxazole. However, fosfomycin and amikacin were not adsorbed. Thus, MgOC₁₅₀ can be used with fosfomycin and amikacin to treat infections. MgOC₁₅₀ is used for industrial purposes, such as an electrode catalyst, a bioelectrode, and enzyme immobilization. The study proposed another potential application of MgOC₁₅₀, assisting anti-EHEC chemotherapy.

Intestinal Enteroid Monolayers Model the Human Intestinal Environment for Escherichia coli Infection

Enterohemorrhagic Escherichia coli O157:H7 is an enteric pathogen responsible for bloody diarrhea, hemolytic uremic syndrome, and in severe cases, even death. The study of O157:H7 is difficult due to the high specificity of the bacteria for the human intestine, along with our lack of sufficiently complex human cell culture models. The recent development of human intestinal enteroids derived from intestinal crypt multipotent stem cells has allowed us to construct two-dimensional differentiated epithelial monolayers grown in transwells that mimic the human intestine. Unlike previous studies, saline was added to the apical surface, while maintaining culture media in the basolateral well. The monolayers continued to grow and differentiate with apical saline. Apical infection with O157:H7 or commensal E. coli resulted in robust bacterial growth from 10⁵ to over 10⁸ over 24 h. Despite this robust bacterial growth, commensal E. coli neither adhered to nor damaged the epithelial barrier over 30 h. However, O157:H7 was almost fully adhered (>90%) by 18 h with epithelial damage observed by 30 h. O157:H7 contains the locus of enterocyte effacement (LEE) pathogenicity island responsible for attachment and damage to the intestinal epithelium. Previous studies report the ability of nutrients such as biotin, d-serine, and L-fucose to downregulate LEE gene expression. O157:H7 treated with biotin or L-fucose, but not d-serine displayed both decreased attachment and reduced epithelial damage over 36 h. These data illustrate enteroid monolayers can serve as a suitable model for the study of O157:H7 pathogenesis, and identification of potential therapeutics. IMPORTANCE O157:H7 is difficult to study due to its high specificity for the human intestine and the lack of sufficiently complex human cell culture models. The recent development of human intestinal enteroids derived from intestinal crypt multipotent stem cells has allowed us to construct two-dimensional differentiated epithelial monolayers grown in transwells that mimic the human intestine. Our data illustrates enteroid monolayers can serve as a suitable model for the study of O157:H7 pathogenesis, and allow for identification of potential therapeutics.

Topology and Contribution to the Pore Channel Lining of Plasma Membrane-Embedded Shigella flexneri Type 3 Secretion Translocase IpaB

Shigella spp. are human bacterial pathogens that cause bacillary dysentery. Virulence depends on a type 3 secretion system (T3SS), a highly conserved structure present in multiple important human and plant pathogens. Upon host cell contact, the T3SS translocon is delivered to the host membrane, facilitates bacterial docking to the membrane, and enables delivery of effector proteins into the host cytosol. The Shigella translocon is composed of two proteins, IpaB and IpaC, which together form this multimeric structure within host plasma membranes. Upon interaction of IpaC with host intermediate filaments, the translocon undergoes a conformational change that allows for bacterial docking onto the translocon and, together with host actin polymerization, enables subsequent effector translocation through the translocon pore. To generate additional insights into the translocon, we mapped the topology of IpaB in plasma membrane-embedded pores using cysteine substitution mutagenesis coupled with site-directed labeling and proximity-enabled cross-linking by membrane-permeant sulfhydryl reactants. We demonstrate that IpaB function is dependent on posttranslational modification by a plasmid-encoded acyl carrier protein. We show that the first transmembrane domain of IpaB lines the interior of the translocon pore channel such that the IpaB portion of the channel forms a funnel-like shape leading into the host cytosol. In addition, we identify regions of IpaB within its cytosolic domain that protrude into and are closely associated with the pore channel. Taken together, these results provide a framework for how IpaB is arranged within translocons natively delivered by Shigella during infection.

Roles of OmpA in Type III Secretion System-Mediated Virulence of Enterohemorrhagic Escherichia coli

Outer membrane proteins are commonly produced by gram-negative bacteria, and they have diverse functions. A subgroup of proteins, which includes OmpA, OmpW and OmpX, is often involved in bacterial pathogenesis. Here we show that OmpA, rather than OmpW or OmpX, contributes to the virulence of enterohemorrhagic Escherichia coli (EHEC) through its type III secretion system (T3SS). Deletion of ompA decreased secretion of the T3SS proteins EspA and EspB; however, the expression level of the LEE genes that encode a set of T3SS proteins did not decrease. The ompA mutant had less abilities to form A/E lesions in host epithelial cells and lyse human red blood cells than the parent strain. Moreover, the virulence of an ompA mutant of Citrobacter rodentium (traditionally used to estimate T3SS-associated virulence in mice) was attenuated. Mice infected with the ompA mutant survived longer than those infected with the parent strain. Furthermore, mice infected with ompA developed symptoms of diarrhea more slowly than mice infected with the parent strain. Altogether, these results suggest that OmpA sustains the activity of the T3SS and is required for optimal virulence in EHEC. This work expands the roles of outer membrane proteins in bacterial pathogenesis.

Identification of Translocation Inhibitors Targeting the Type III Secretion System of Enteropathogenic Escherichia coli

Infections with enteropathogenic Escherichia coli (EPEC) cause severe diarrhea in children. The noninvasive bacteria adhere to enterocytes of the small intestine and use a type III secretion system (T3SS) to inject effector proteins into host cells to modify and exploit cellular processes in favor of bacterial survival and replication. Several studies have shown that the T3SSs of bacterial pathogens are essential for virulence. Furthermore, the loss of T3SS-mediated effector translocation results in increased immune recognition and clearance of the bacteria. The T3SS is, therefore, considered a promising target for antivirulence strategies and novel therapeutics development. Here, we report the results of a high-throughput screening assay based on the translocation of the EPEC effector protein Tir (translocated intimin receptor). Using this assay, we screened more than 13,000 small molecular compounds of six different compound libraries and identified three substances which showed a significant dose-dependent effect on translocation without adverse effects on bacterial or eukaryotic cell viability. In addition, these substances reduced bacterial binding to host cells, effector-dependent cell detachment, and abolished attaching and effacing lesion formation without affecting the expression of components of the T3SS or associated effector proteins. Moreover, no effects of the inhibitors on bacterial motility or Shiga-toxin expression were observed. In summary, we have identified three new compounds that strongly inhibit T3SS-mediated translocation of effectors into mammalian cells, which could be valuable as lead substances for treating EPEC and enterohemorrhagic E. coli infections.

The type 3 secretion system requires actin polymerization to open translocon pores

Many bacterial pathogens require a type 3 secretion system (T3SS) to establish a niche. Host contact activates bacterial T3SS assembly of a translocon pore in the host plasma membrane. Following pore formation, the T3SS docks onto the translocon pore. Docking establishes a continuous passage that enables the translocation of virulence proteins, effectors, into the host cytosol. Here we investigate the contribution of actin polymerization to T3SS-mediated translocation. Using the T3SS model organism Shigella flexneri, we show that actin polymerization is required for assembling the translocon pore in an open conformation, thereby enabling effector translocation. Opening of the pore channel is associated with a conformational change to the pore, which is dependent upon actin polymerization and a coiled-coil domain in the pore protein IpaC. Analysis of an IpaC mutant that is defective in ruffle formation shows that actin polymerization-dependent pore opening is distinct from the previously described actin polymerization-dependent ruffles that are required for bacterial internalization. Moreover, actin polymerization is not required for other pore functions, including docking or pore protein insertion into the plasma membrane. Thus, activation of the T3SS is a multilayered process in which host signals are sensed by the translocon pore leading to the activation of effector translocation.

CesL Regulates Type III Secretion Substrate Specificity of the Enteropathogenic E. coli Injectisome

The type III secretion system (T3SS) is a complex molecular device used by several pathogenic bacteria to translocate effector proteins directly into eukaryotic host cells. One remarkable feature of the T3SS is its ability to secrete different categories of proteins in a hierarchical manner, to ensure proper assembly and timely delivery of effectors into target cells. In enteropathogenic Escherichia coli, the substrate specificity switch from translocator to effector secretion is regulated by a gatekeeper complex composed of SepL, SepD, and CesL proteins. Here, we report a characterization of the CesL protein using biochemical and genetic approaches. We investigated discrepancies in the phenotype among different cesL deletion mutants and showed that CesL is indeed essential for translocator secretion and to prevent premature effector secretion. We also demonstrated that CesL engages in pairwise interactions with both SepL and SepD. Furthermore, while association of SepL to the membrane does not depended on CesL, the absence of any of the proteins forming the heterotrimeric complex compromised the intracellular stability of each component. In addition, we found that CesL interacts with the cytoplasmic domains of the export gate components EscU and EscV. We propose a mechanism for substrate secretion regulation governed by the SepL/SepD/CesL complex.

Transmembrane domains of type III-secreted proteins affect bacterial-host interactions in enteropathogenic E. coli

Many bacterial pathogens utilize a specialized secretion system, termed type III secretion system (T3SS), to translocate effector proteins into host cells and establish bacterial infection. The T3SS is anchored within the bacterial membranes and contains a long needle/filament that extends toward the host-cell and forms, at its distal end, a pore complex within the host membrane. The T3SS pore complex consists of two bacterial proteins, termed SctB and SctE, which have conflicting targeting indications; a signal sequence that targets to secretion to the extracellular environment via the T3SS, and transmembrane domains (TMDs) that target to membrane localization. In this study, we investigate whether the TMD sequences of SctB and SctE have special features that differentiate them from classical TMDs and allow them to escape bacterial membrane integration. For this purpose, we exchanged the SctB and SctE native TMDs for alternative hydrophobic sequences and found that the TMD sequences of SctB and SctE dictate membrane destination (bacterial versus host membrane). Moreover, we examined the role of the SctB TMD sequence in the activity of the full-length protein, post secretion, and found that the TMD does not serve only as a hydrophobic segment, but is also involved in the ability of the protein to translocate itself and other proteins into and across the host cell membrane.

Molecular Targets and Strategies for Inhibition of the Bacterial Type III Secretion System (T3SS); Inhibitors Directly Binding to T3SS Components

The type III secretion system (T3SS) is a virulence apparatus used by many Gram-negative pathogenic bacteria to cause infections. Pathogens utilizing a T3SS are responsible for millions of infections yearly. Since many T3SS knockout strains are incapable of causing systemic infection, the T3SS has emerged as an attractive anti-virulence target for therapeutic design. The T3SS is a multiprotein molecular syringe that enables pathogens to inject effector proteins into host cells. These effectors modify host cell mechanisms in a variety of ways beneficial to the pathogen. Due to the T3SS’s complex nature, there are numerous ways in which it can be targeted. This review will be focused on the direct targeting of components of the T3SS, including the needle, translocon, basal body, sorting platform, and effector proteins. Inhibitors will be considered a direct inhibitor if they have a binding partner that is a T3SS component, regardless of the inhibitory effect being structural or functional.

Cryo-EM structure of the EspA filament from enteropathogenic Escherichia coli: Revealing the mechanism of effector translocation in the T3SS

The type III secretion system (T3SS) is a virulence mechanism employed by Gram-negative pathogens. The T3SS forms a proteinaceous channel that projects a needle into the extracellular medium where it interacts with the host cell to deliver virulence factors. Enteropathogenic Escherichia coli (EPEC) is unique in adopting a needle extension to the T3SS-a filament formed by EspA-which is absolutely required for efficient colonization of the gut. Here, we describe the cryoelectron microscopy structure of native EspA filaments from EPEC at 3.6-Å resolution. Within the filament, positively charged residues adjacent to a hydrophobic groove line the lumen of the filament in a spiral manner, suggesting a mechanism of substrate translocation mediated via electrostatics. Using structure-guided mutagenesis, in vivo studies corroborate the role of these residues in secretion and translocation function. The high-resolution structure of the EspA filament could aid in structure-guided drug design of antivirulence therapeutics.

Genomic Properties and Temporal Analysis of the Interaction of an Invasive Escherichia albertii With Epithelial Cells

Diarrhea is one of the main causes of infant mortality worldwide, mainly in the developing world. Among the various etiologic agents, Escherichia albertii is emerging as an important human enteropathogen. E. albertii promote attaching and effacing (AE) lesions due to the presence of the locus of enterocyte effacement (LEE) that encodes a type three secretion system (T3SS), the afimbrial adhesin intimin and its translocated receptor, Tir, and several effector proteins. We previously showed that E. albertii strain 1551-2 invades several epithelial cell lineages by a process that is dependent on the intimin-Tir interaction. To understand the contribution of T3SS-dependent effectors present in E. albertii 1551-2 during the invasion process, we performed a genetic analysis of the LEE and non-LEE genes and evaluated the expression of the LEE operons in various stages of bacterial interaction with differentiated intestinal Caco-2 cells. The kinetics of the ability of the 1551-2 strain to colonize and form AE lesions was also investigated in epithelial HeLa cells. We showed that the LEE expression was constant during the early stages of infection but increased at least 4-fold during bacterial persistence in the intracellular compartment. An in silico analysis indicated the presence of a new tccP/espF_U subtype, named tccP3. We found that the encoded protein colocalizes with Tir and polymerized F-actin during the infection process in vitro. Moreover, assays performed with Nck null cells demonstrated that the 1551-2 strain can trigger F-actin polymerization in an Nck-independent pathway, despite the fact that TccP3 is not required for this phenotype. Our study highlights the importance of the T3SS during the invasion process and for the maintenance of E. albertii 1551-2 inside the cells. In addition, this work may help to elucidate the versatility of the T3SS for AE pathogens, which are usually considered extracellular and rarely reach the intracellular environment.

Analysis on the virulomes and resistomes of multi-drug resistance clinical Escherichia coli isolates, as well as the interactome with gut microbiome

Escherichia coli is one of the most diverse microbial species. Pathogenic E. coli is capable of causing various diseases in humans, including several types of diarrhea, urinary tract infections, sepsis, and meningitis. This study focused on the antibiotic susceptibility profile and genomic analysis of a clinical E. coli Guangzhou-Eco330 isolated from a hospitalized 8-year-old female patient suffered from pulmonary infection in 2017. Susceptibility to 15 antibiotics were determined using Vitek2™ Automated Susceptibility System and Etest strips and interpreted based on CLSI guidelines. The genome was sequenced using Illumina Hiseq 2500 platform and assembled de novo using Velvet, followed by bioinformatics analysis. The genome has a length of 5,132,642 bp and contains 4989 predicted genes with an average GC content of 50.51%. The carriage of rfbE gene suggested the strain belonging to O157. In the genome, 70 non-coding RNAs, 50 repeat sequences, 18 transposons, 78 GIs, 9 CRISPRs, and 3 large prophages were identified. 37 PHI related genes and 108 virulence genes were determined to contribute to its pathogenicity. Specifically, the acquisition of multiple antibiotic resistance genes including bla_CTX-M-55, bla_OXA-10, bla_CMY-48, tetB, and qnrS1 contributed to its resistance to penicillins, telracyclines, cephalosporin, and quinolones. The understanding of the genome may aid in further study on the clinical control of multi-drug resistance E. coli.

Comparative Analysis of Virulence Profiles of Serratia marcescens Isolated from Diverse Clinical Origins in Mexican Patients

Background: Serratia marcescens is an enteric bacterium with increasing incidence in clinical settings, attributed mainly to the opportune expression of diverse virulence determinants plus a wide intrinsic and acquired antibiotic resistance. Methods: The aim of this study was to compare the virulence factor profiles of 185 Serratia marcescens isolates from different clinical origins. In vitro proteolytic and hemolytic activities, biofilm formation, and motility were assessed in each strain. Additionally, the pathogenicity of four hypervirulent strains was analyzed in vivo in Galleria mellonella. Results: We found that bacterial isolates from wound/abscess and respiratory tract specimens exhibited the highest protease activity along with a strong biofilm production, while uropathogenic isolates showed the highest hemolytic activity. Swarming and swimming motilities were similar among all the strains. However, respiratory tract isolates showed the most efficient motility. Two hyperhemolytic and two hyperproteolytic strains were detected; the latter were more efficient killing Galleria mellonella with a 50%-60% larval mortality 48 hours after challenge. Conclusion: A correlation was found between biofilm formation and proteolytic and hemolytic activities in biopsy specimens and bloodstream isolates, respectively. Overall, it becomes critical to evaluate and compare the clinical strains virulence diversity in order to understand the underlying mechanisms that allow the establishment and persistence of opportunistic bacterial infections in the host.

Dynamics of expression, secretion and translocation of type III effectors during enteropathogenic Escherichia coli infection

Enteropathogenic Escherichia coli (EPEC) is an important cause of infant diarrhea and mortality worldwide. The locus of enterocyte effacement (LEE) pathogenicity island in the EPEC genome encodes a type 3 secretion system (T3SS). This nanomachine directly injects a sophisticated arsenal of effectors into host cells, which is critical for EPEC pathogenesis. To colonize the gut mucosa, EPEC alters its gene expression in response to host environmental signals. Regulation of the LEE has been studied extensively, revealing key mechanisms of transcriptional regulation, and more recently at the posttranscriptional and posttranslational levels. Moreover, the T3SS assembly and secretion is a highly coordinated process that ensures hierarchical delivery of effectors upon cell contact. EPEC effectors and virulence factors not only manipulate host cellular processes, but also modulate effector translocation by controlling T3SS formation. In this review, we focus on the regulation of EPEC virulence genes and modulation of effector secretion and translocation.

Activation of Shigella flexneri type 3 secretion requires a host-induced conformational change to the translocon pore

Type 3 secretion systems (T3SSs) are conserved bacterial nanomachines that inject virulence proteins (effectors) into eukaryotic cells during infection. Due to their ability to inject heterologous proteins into human cells, these systems are being developed as therapeutic delivery devices. The T3SS assembles a translocon pore in the plasma membrane and then docks onto the pore. Docking activates effector secretion through the pore and into the host cytosol. Here, using Shigella flexneri, a model pathogen for the study of type 3 secretion, we determined the molecular mechanisms by which host intermediate filaments trigger docking and enable effector secretion. We show that the interaction of intermediate filaments with the translocon pore protein IpaC changed the pore's conformation in a manner that was required for docking. Intermediate filaments repositioned residues of the Shigella pore protein IpaC that are located on the surface of the pore and in the pore channel. Restricting these conformational changes blocked docking in an intermediate filament-dependent manner. These data demonstrate that a host-induced conformational change to the pore enables T3SS docking and effector secretion, providing new mechanistic insight into the regulation of type 3 secretion.

The type III secretion system needle, tip, and translocon

Many Gram-negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.

The Injectisome, a Complex Nanomachine for Protein Injection into Mammalian Cells

Type III protein secretion systems (T3SSs), or injectisomes, are multiprotein nanomachines present in many Gram-negative bacteria that have a sustained long-standing close relationship with a eukaryotic host. These secretion systems have evolved to modulate host cellular functions through the activity of the effector proteins they deliver. To reach their destination, T3SS effectors must cross the multibarrier bacterial envelope and the eukaryotic cell membrane. Passage through the bacterial envelope is mediated by the needle complex, a central component of T3SSs that expands both the inner and outer membranes of Gram-negative bacteria. A set of T3SS secreted proteins, known as translocators, form a channel in the eukaryotic plasma membrane through which the effector proteins are delivered to reach the host cell cytosol. While the effector proteins are tailored to the specific lifestyle of the bacterium that encodes them, the injectisome is conserved among the different T3SSs. The central role of T3SSs in pathogenesis and their high degree of conservation make them a desirable target for the development of antimicrobial therapies against several important bacterial pathogens.

The Type III Secretion System (T3SS)-Translocon of Atypical Enteropathogenic Escherichia coli (aEPEC) Can Mediate Adherence

The intimin protein is the major adhesin involved in the intimate adherence of atypical enteropathogenic Escherichia coli (aEPEC) strains to epithelial cells, but little is known about the structures involved in their early colonization process. A previous study demonstrated that the type III secretion system (T3SS) plays an additional role in the adherence of an Escherichia albertii strain. Therefore, we assumed that the T3SS could be related to the adherence efficiency of aEPEC during the first stages of contact with epithelial cells. To test this hypothesis, we examined the adherence of seven aEPEC strains and their eae (intimin) isogenic mutants in the standard HeLa adherence assay and observed that all wild-type strains were adherent while five isogenic eae mutants were not. The two eae mutant strains that remained adherent were then used to generate the eae/escN double mutants (encoding intimin and the T3SS ATPase, respectively) and after the adherence assay, we observed that one strain lost its adherence capacity. This suggested a role for the T3SS in the initial adherence steps of this strain. In addition, we demonstrated that this strain expressed the T3SS at significantly higher levels when compared to the other wild-type strains and that it produced longer translocon-filaments. Our findings reveal that the T3SS-translocon can play an additional role as an adhesin at the beginning of the colonization process of aEPEC.

The Type III Secretion System of Pathogenic Escherichia coli

Infection with enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC), enteroinvasive E. coli (EIEC) and Shigella relies on the elaboration of a type III secretion system (T3SS). Few strains also encode a second T3SS, named ETT2. Through the integration of coordinated intracellular and extracellular cues, the modular T3SS is assembled within the bacterial cell wall, as well as the plasma membrane of the host cell. As such, the T3SS serves as a conduit, allowing the chaperone-regulated translocation of effector proteins directly into the host cytosol to subvert eukaryotic cell processes. Recent technological advances revealed high structural resolution of the T3SS apparatus and how it could be exploited to treat enteric disease. This chapter summarises the current knowledge of the structure and function of the E. coli T3SSs.

Topological Analysis of the Type 3 Secretion System Translocon Pore Protein IpaC following Its Native Delivery to the Plasma Membrane during Infection

The type 3 secretion system (T3SS) is a nanomachine required for virulence of many bacterial pathogens that infect humans. The system delivers bacterial virulence proteins into the cytosol of human cells, where the virulence proteins promote bacterial infection. The T3SS forms a translocon pore in the membranes of target cells. This pore is the portal through which bacterial virulence proteins are delivered by the T3SS into the eukaryotic cytosol. The pore also regulates secretion of these virulence proteins. Our work defines the topology of translocon pore proteins in their native context during infection, resolves previously conflicting reports about the topology of the Shigella translocon pore protein IpaC, and provides new insights into how interactions of the pore with the T3SS likely produce signals that activate secretion of virulence proteins.

Many Gram-negative bacterial pathogens require a type 3 secretion system (T3SS) to deliver effector proteins into eukaryotic cells. Contact of the tip complex of the T3SS with a target eukaryotic cell initiates secretion of the two bacterial proteins that assemble into the translocon pore in the plasma membrane. The translocon pore functions to regulate effector protein secretion and is the conduit for effector protein translocation across the plasma membrane. To generate insights into how the translocon pore regulates effector protein secretion, we defined the topology of the Shigella translocon pore protein IpaC in the plasma membrane following its native delivery by the T3SS. Using single cysteine substitution mutagenesis and site-directed labeling with a membrane-impermeant chemical probe, we mapped residues accessible from the extracellular surface of the cell. Our data support a model in which the N terminus of IpaC is extracellular and the C terminus of IpaC is intracellular. These findings resolve previously conflicting data on IpaC topology that were based on nonnative delivery of IpaC to membranes. Salmonella enterica serovar Typhimurium also requires the T3SS for effector protein delivery into eukaryotic cells. Although the sequence of IpaC is closely related to the Salmonella translocon pore protein SipC, the two proteins have unique functional attributes during infection. We showed a similar overall topology for SipC and IpaC and identified subtle topological differences between their transmembrane α-helixes and C-terminal regions. Together, our data suggest that topological differences among distinct translocon pore proteins may dictate organism-specific functional differences of the T3SSs during infection.

Thinking Outside the Bug: Molecular Targets and Strategies to Overcome Antibiotic Resistance

Since their discovery in the early 20th century, antibiotics have been used as the primary weapon against bacterial infections. Due to their prophylactic effect, they are also used as part of the cocktail of drugs given to treat complex diseases such as cancer or during surgery, in order to prevent infection. This has resulted in a decrease of mortality from infectious diseases and an increase in life expectancy in the last 100 years. However, as a consequence of administering antibiotics broadly to the population and sometimes misusing them, antibiotic-resistant bacteria have appeared. The emergence of resistant strains is a global health threat to humanity. Highly-resistant bacteria like Staphylococcus aureus (methicillin-resistant) or Enterococcus faecium (vancomycin-resistant) have led to complications in intensive care units, increasing medical costs and putting patient lives at risk. The appearance of these resistant strains together with the difficulty in finding new antimicrobials has alarmed the scientific community. Most of the strategies currently employed to develop new antibiotics point towards novel approaches for drug design based on prodrugs or rational design of new molecules. However, targeting crucial bacterial processes by these means will keep creating evolutionary pressure towards drug resistance. In this review, we discuss antibiotic resistance and new options for antibiotic discovery, focusing in particular on new alternatives aiming to disarm the bacteria or empower the host to avoid disease onset.

Electron Cryotomography of Bacterial Secretion Systems

In biology, function arises from form. For bacterial secretion systems, which often span two membranes, avidly bind to the cell wall, and contain hundreds of individual proteins, studying form is a daunting task, made possible by electron cryotomography (ECT). ECT is the highest-resolution imaging technique currently available to visualize unique objects inside cells, providing a three-dimensional view of the shapes and locations of large macromolecular complexes in their native environment. Over the past 15 years, ECT has contributed to the study of bacterial secretion systems in two main ways: by revealing intact forms for the first time and by mapping components into these forms. Here we highlight some of these contributions, revealing structural convergence in type II secretion systems, structural divergence in type III secretion systems, unexpected structures in type IV secretion systems, and unexpected mechanisms in types V and VI secretion systems. Together, they offer a glimpse into a world of fantastic forms-nanoscale rotors, needles, pumps, and dart guns-much of which remains to be explored.

Enteropathogens: Tuning Their Gene Expression for Hassle-Free Survival

Enteropathogenic bacteria have been the cause of the majority of foodborne illnesses. Much of the research has been focused on elucidating the mechanisms by which these pathogens evade the host immune system. One of the ways in which they achieve the successful establishment of a niche in the gut microenvironment and survive is by a chain of elegantly regulated gene expression patterns. Studies have shown that this process is very elaborate and is also regulated by several factors. Pathogens like, enteropathogenic Escherichia coli (EPEC), Salmonella Typhimurium, Shigellaflexneri, Yersinia sp. have been seen to employ various regulated gene expression strategies. These include toxin-antitoxin systems, quorum sensing systems, expression controlled by nucleoid-associated proteins (NAPs), several regulons and operons specific to these pathogens. In the following review, we have tried to discuss the common gene regulatory systems of enteropathogenic bacteria as well as pathogen-specific regulatory mechanisms.

Assembly and Post-assembly Turnover and Dynamics in the Type III Secretion System

The type III secretion system (T3SS) is one of the largest transmembrane complexes in bacteria, comprising several intricately linked and embedded substructures. The assembly of this nanomachine is a hierarchical process which is regulated and controlled by internal and external cues at several critical points. Recently, it has become obvious that the assembly of the T3SS is not a unidirectional and deterministic process, but that parts of the T3SS constantly exchange or rearrange. This article aims to give an overview on the assembly and post-assembly dynamics of the T3SS, with a focus on emerging general concepts and adaptations of the general assembly pathway.

Visualization of translocons in Yersinia type III protein secretion machines during host cell infection

Type III secretion systems (T3SSs) are essential virulence factors of numerous bacterial pathogens. Upon host cell contact the T3SS machinery—also named injectisome—assembles a pore complex/translocon within host cell membranes that serves as an entry gate for the bacterial effectors. Whether and how translocons are physically connected to injectisome needles, whether their phenotype is related to the level of effector translocation and which target cell factors trigger their formation have remained unclear. We employed the superresolution fluorescence microscopy techniques Stimulated Emission Depletion (STED) and Structured Illumination Microscopy (SIM) as well as immunogold electron microscopy to visualize Y. enterocolitica translocons during infection of different target cell types. Thereby we were able to resolve translocon and needle complex proteins within the same injectisomes and demonstrate that these fully assembled injectisomes are generated in a prevacuole, a PI(4,5)P2 enriched host cell compartment inaccessible to large extracellular proteins like antibodies. Furthermore, the operable translocons were produced by the yersiniae to a much larger degree in macrophages (up to 25% of bacteria) than in HeLa cells (2% of bacteria). However, when the Rho GTPase Rac1 was activated in the HeLa cells, uptake of the yersiniae into the prevacuole, translocon formation and effector translocation were strongly enhanced reaching the same levels as in macrophages. Our findings indicate that operable T3SS translocons can be visualized as part of fully assembled injectisomes with superresolution fluorescence microscopy techniques. By using this technology, we provide novel information about the spatiotemporal organization of T3SS translocons and their regulation by host cell factors.

Many human, animal and plant pathogenic bacteria employ a molecular machine termed injectisome to inject their toxins into host cells. Because injectisomes are crucial for these bacteria’s infectious potential they have been considered as targets for antiinfective drugs. Injectisomes are highly similar between the different bacterial pathogens and most of their overall structure is well established at the molecular level. However, only little information is available for a central part of the injectisome named the translocon. This pore-like assembly integrates into host cell membranes and thereby serves as an entry gate for the bacterial toxins. We used state of the art fluorescence microscopy to watch translocons of the diarrheagenic pathogen Yersinia enterocolitica during infection of human host cells. Thereby we could for the first time—with fluorescence microscopy—visualize translocons connected to other parts of the injectisome. Furthermore, because translocons mark functional injectisomes we could obtain evidence that injectisomes only become active for secretion of translocators when the bacteria are almost completely enclosed by host cells. These findings provide a novel view on the organization and regulation of bacterial translocons and may thus open up new strategies to block the function of infectious bacteria.

Microbiota and Pathogen Proteases Modulate Type III Secretion Activity in Enterohemorrhagic Escherichia coli

The gut microbiota is usually regarded as providing colonization resistance against enteric pathogens. However, some pathogens evolved to thrive with the aid of certain members of the microbiota. Several Gram-negative bacteria employ type three secretion systems (T3SSs), which are molecular syringes that deliver effector proteins to host cells, hijacking host cell function. Here we show that the T3SS of enterohemorrhagic E. coli (EHEC) is cleaved by self and microbiota-derived proteases. Self-cleavage limits effector translocation, while cleavage by the microbiota member Bacteroides thetaiotamicron (Bt) exacerbates effector translocation and lesion formation on epithelial cells.

Enteric pathogens have complex interactions with the gut microbiota. Most of what is known about them has focused on microbiota-derived metabolites or small molecules that serve as nutrients and/or signals to aid in growth or transcriptionally regulate virulence gene expression. A common virulence strategy is to express a type III secretion system (T3SS), which is a molecular syringe deployed by many Gram-negative pathogens to hijack host cell function. Enterohemorrhagic Escherichia coli (EHEC) requires its T3SS to colonize the intestinal tract and cause disease. Here we report that a prominent member of the intestinal microbiota, Bacteroides thetaiotamicron (Bt), secretes proteases that cleave the translocon of the T3SS of EHEC to enhance effector translocation into host cells. This is in contrast from an endogenous protease from EHEC itself (namely, EspP) that cleaves the translocon protein EspB in a different site to limit effector translocation. The EspB protein forms the T3SS pore in mammalian cells, and pore proteins are conserved in the T3SSs from several pathogens. This is the first demonstration of a commensal species directly processing a pathogen’s T3SS, posing a new paradigm for how the microbiota can influence the severity of disease caused by bacterial pathogens. Because T3SSs are employed by many pathogens, this phenomenon has broad implications to commensal-pathogen relationships.

Visualization of the type III secretion mediated Salmonella-host cell interface using cryo-electron tomography

Many important gram-negative bacterial pathogens use highly sophisticated type III protein secretion systems (T3SSs) to establish complex host-pathogen interactions. Bacterial-host cell contact triggers the activation of the T3SS and the subsequent insertion of a translocon pore into the target cell membrane, which serves as a conduit for the passage of effector proteins. Therefore the initial interaction between T3SS-bearing bacteria and host cells is the critical step in the deployment of the protein secretion machine, yet this process remains poorly understood. Here, we use high-throughput cryo-electron tomography (cryo-ET) to visualize the T3SS-mediated Salmonella-host cell interface. Our analysis reveals the intact translocon at an unprecedented level of resolution, its deployment in the host cell membrane, and the establishment of an intimate association between the bacteria and the target cells, which is essential for effector translocation. Our studies provide critical data supporting the long postulated direct injection model for effector translocation.

Optimization of type 3 protein secretion in enteropathogenic Escherichia coli

The type 3 secretion system (T3SS) is a protein export pathway common to Gram-negative pathogens. It comprises a trans-envelope syringe, the injectisome, with a cytoplasm-facing translocase channel. In enteropathogenic Escherichia coli, exported substrates are chaperone-delivered to the major translocase component, EscV, and cross the membrane in strict hierarchical manner, e.g. first 'translocators', then 'effectors'. The in vitro dissection of the T3SS and the determination of its structure are hampered by the low numbers of the injectisomes per cell. We have now defined an optimal M9 minimal medium and established that the per transcriptional regulator enhances the number of filamented cells, the number of injectisomes per cell and the secretion of T3S substrates. Our findings provide a valuable tool for further biochemical and biophysical analysis of the T3SS and suggest that additional improvement to maximize injectisome production is possible in future efforts.

An RNA-dependent mechanism for transient expression of bacterial translocation filaments

The prokaryotic RNA chaperone Hfq mediates sRNA-mRNA interactions and plays a significant role in post-transcriptional regulation of the type III secretion (T3S) system produced by a range of Escherichia coli pathotypes. UV-crosslinking was used to map Hfq-binding under conditions that promote T3S and multiple interactions were identified within polycistronic transcripts produced from the locus of enterocyte effacement (LEE) that encodes the T3S system. The majority of Hfq binding was within the LEE5 and LEE4 operons, the latter encoding the translocon apparatus (SepL-EspADB) that is positively regulated by the RNA binding protein, CsrA. Using the identified Hfq-binding sites and a series of sRNA deletions, the sRNA Spot42 was shown to directly repress translation of LEE4 at the sepL 5' UTR. In silico and in vivo analyses of the sepL mRNA secondary structure combined with expression studies of truncates indicated that the unbound sepL mRNA is translationally inactive. Based on expression studies with site-directed mutants, an OFF-ON-OFF toggle model is proposed that results in transient translation of SepL and EspA filament assembly. Under this model, the nascent mRNA is translationally off, before being activated by CsrA, and then repressed by Hfq and Spot42.

Vaccination with killed whole-cells of Escherichia coli O157:H7 hha mutant emulsified with an adjuvant induced vaccine strain-specific serum antibodies and reduced E. coli O157:H7 fecal shedding in cattle

Escherichia coli O157:H7 (O157) can cause from a mild diarrheal illness to hemorrhagic colitis and hemolytic uremic syndrome in humans. Cattle are the primary reservoir for O157 and fecal shedding of O157 by these animals is a major risk factor in contamination of cattle hides and carcasses at slaughter. Vaccination is an important strategy to reduce fecal shedding of O157 in cattle. In this study, we evaluated the immunogenicity and efficacy of an inactivated vaccine strain of O157 formulated with an adjuvant. This vaccine strain was deleted of the hha gene enabling high level expression of the locus of enterocyte effacement (LEE) encoded proteins required for O157 colonization in cattle. The inactivated vaccine strain emulsified with the adjuvant or suspended in the phosphate-buffered saline (PBS) was injected in the neck muscles of two groups of weaned calves followed by a booster three weeks later with the corresponding formulation. Animals in groups 3 and 4 were injected similarly with the adjuvant and PBS, respectively. All animals were orally inoculated three weeks post-booster vaccination with a live culture of O157. The animals vaccinated with the adjuvanted vaccine showed higher serum antibody titers to the vaccine strain and shed O157 for a shorter duration and at lower numbers compared to the animals vaccinated with the non-adjuvanted vaccine, adjuvant-only, or PBS. Western blotting of the vaccine strain lysates showed higher immunoreactivity of serum IgG in vaccinated animals to several O157-specific proteins and lipopolysaccharides (LPS). The vaccination induced IgG showed specificity to LEE-encoded proteins and outer membrane LPS as LEE and waaL deletion mutants, unable to produce LEE proteins and synthesize high molecular weight LPS, respectively, yielded significantly lower antibody titers compared to the parent vaccine strain. The positive reactivity of the immune serum was also observed for purified LEE-encoded proteins EspA and EspB. In conclusion, the results of this animal study showed that a two-dose regimen of an adjuvanted vaccine is capable of inducing O157-specific immune response that directly or indirectly reduced fecal shedding of O157 in cattle.

Multitalented EspB of enteropathogenic Escherichia coli (EPEC) enters cells autonomously and induces programmed cell death in human monocytic THP-1 cells

Enteropathogenic Escherichia coli (EPEC) subvert host cell signaling pathways by injecting effector proteins via a Type 3 Secretion System (T3SS). The T3SS-dependent EspB protein is a multi-functional effector protein, which contributes to adherence and translocator pore formation and after injection exhibits several intracellular activities. In addition, EspB is also secreted into the environment. Effects of secreted EspB have not been reported thus far. As a surrogate for secreted EspB we employed recombinant EspB (rEspB) derived from the prototype EPEC strain E2348/69 and investigated the interactions of the purified protein with different human epithelial and immune cells including monocytic THP-1 cells, macrophages, dendritic cells, U-937, epithelial T84, Caco-2, and HeLa cells. To assess whether these proteins might exert a cytotoxic effect we monitored the release of lactate dehydrogenase (LDH) as well as propidium iodide (PI) uptake. For comparison, we also investigated several homologs of EspB such as IpaD of Shigella, and SipC, SipD, SseB, and SseD of Salmonella as purified recombinant proteins. Interestingly, cytotoxicity was only observed in THP-1 cells and macrophages, whereas epithelial cells remained unaffected. Cell fractionation and immune fluorescence experiments showed that rEspB enters cells autonomously, which suggests that EspB might qualify as a novel cell-penetrating effector protein (CPE). Using specific organelle tracers and inhibitors of signaling pathways we found that rEspB destroys the mitochondrial membrane potential - an indication of programmed cell death induction in THP-1 cells. Here we show that EspB not only constitutes an essential part of the T3SS-nanomachine and contributes to the arsenal of injected effector proteins but, furthermore, that secreted (recombinant) EspB autonomously enters host cells and selectively induces cell death in immune cells.

Effect of RNase E deficiency on translocon protein synthesis in an RNase E-inducible strain of enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) is a food-borne pathogen that assembles a type III secretion system (T3SS) on its surface. The last portion of the T3SS, called the 'translocon', is composed of a filament and a pore complex that is inserted into the membrane of intestinal epithelial cells. The genes encoding the translocon (espADB) are part of the LEE4 operon. Their expression is regulated by a complex post-transcriptional mechanism that involves the processing of LEE4 mRNA by the essential endoribonuclease RNase E. Here, we report the construction of an EHEC strain (TEA028-rne) in which RNase E can be induced by adding IPTG to the culture medium. EHEC cells deficient in RNase E displayed an abnormal morphology and slower growth, in agreement with published observations in E. coli K-12. Under those conditions, EspA and EspB were produced at higher concentrations, and protein secretion still occurred. These results indicate that RNase E negatively regulates translocon protein synthesis and demonstrate the utility of E. coli strain TEA028-rne as a tool for investigating the influence of this ribonuclease on EHEC gene expression in vitro.

Functional Characterization of EscK (Orf4), a Sorting Platform Component of the Enteropathogenic Escherichia coli Injectisome

The type III secretion system (T3SS) is a supramolecular machine used by many bacterial pathogens to translocate effector proteins directly into the eukaryotic host cell cytoplasm. Enteropathogenic Escherichia coli (EPEC) is an important cause of infantile diarrheal disease in underdeveloped countries. EPEC virulence relies on a T3SS encoded within a chromosomal pathogenicity island known as the locus of enterocyte effacement (LEE). In this work, we pursued the functional characterization of the LEE-encoded protein EscK (previously known as Orf4). We provide evidence indicating that EscK is crucial for efficient T3S and belongs to the SctK (OrgA/YscK/MxiK) protein family, whose members have been implicated in the formation of a sorting platform for secretion of T3S substrates. Bacterial fractionation studies showed that EscK localizes to the inner membrane independently of the presence of any other T3SS component. Combining yeast two-hybrid screening and pulldown assays, we identified an interaction between EscK and the C-ring/sorting platform component EscQ. Site-directed mutagenesis of conserved residues revealed amino acids that are critical for EscK function and for its interaction with EscQ. In addition, we found that T3S substrate overproduction is capable of compensating for the absence of EscK. Overall, our data suggest that EscK is a structural component of the EPEC T3SS sorting platform, playing a central role in the recruitment of T3S substrates for boosting the efficiency of the protein translocation process.

IMPORTANCE

The type III secretion system (T3SS) is an essential virulence determinant for enteropathogenic Escherichia coli (EPEC) colonization of intestinal epithelial cells. Multiple EPEC effector proteins are injected via the T3SS into enterocyte cells, leading to diarrheal disease. The T3SS is encoded within a genomic pathogenicity island termed the locus of enterocyte effacement (LEE). Here we unravel the function of EscK, a previously uncharacterized LEE-encoded protein. We show that EscK is central for T3SS biogenesis and function. EscK forms a protein complex with EscQ, the main component of the cytoplasmic sorting platform, serving as a docking site for T3S substrates. Our results provide a comprehensive functional analysis of an understudied component of T3SSs.

Type Three Secretion System in Attaching and Effacing Pathogens

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli are diarrheagenic bacterial human pathogens that cause severe gastroenteritis. These enteric pathotypes, together with the mouse pathogen Citrobacter rodentium, belong to the family of attaching and effacing pathogens that form a distinctive histological lesion in the intestinal epithelium. The virulence of these bacteria depends on a type III secretion system (T3SS), which mediates the translocation of effector proteins from the bacterial cytosol into the infected cells. The core architecture of the T3SS consists of a multi-ring basal body embedded in the bacterial membranes, a periplasmic inner rod, a transmembrane export apparatus in the inner membrane, and cytosolic components including an ATPase complex and the C-ring. In addition, two distinct hollow appendages are assembled on the extracellular face of the basal body creating a channel for protein secretion: an approximately 23 nm needle, and a filament that extends up to 600 nm. This filamentous structure allows these pathogens to get through the host cells mucus barrier. Upon contact with the target cell, a translocation pore is assembled in the host membrane through which the effector proteins are injected. Assembly of the T3SS is strictly regulated to ensure proper timing of substrate secretion. The different type III substrates coexist in the bacterial cytoplasm, and their hierarchical secretion is determined by specialized chaperones in coordination with two molecular switches and the so-called sorting platform. In this review, we present recent advances in the understanding of the T3SS in attaching and effacing pathogens.

The intranasal vaccination of pregnant dams with Intimin and EspB confers protection in neonatal mice from Escherichia coli (EHEC) O157:H7 infection

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is responsible for intestinal disease and hemolytic uremic syndrome (HUS), a serious systemic complication which particularly affects children. In this study, we evaluated whether passive immunization protects from EHEC O157:H7 colonization and renal damage, by using a weaned BALB/c mouse model of infection. Recombinant proteins EspB and the carboxyl-terminal fragment of 280 amino acids of γ-intimin (γ-IntC280) were used in combination with a macrophage-activating lipopeptide-2 (MALP) adjuvant to immunize pregnant mice by the intranasal route. Neonatal mice were allowed to suckle vaccinated or sham-vaccinated dams until weaning when they were challenged by the oral route with a suspension of an E. coli O157:H7 Stx2+ strain. The excretion of the inoculated strain was followed for 72h. All vaccinated dams exhibited elevated serum IgG response against both γ-Int C280 and EspB. Passive immunization of newborn mice resulted in a significant increase in serum IgG titers against γ-Int C280 and a slight increase in EspB-specific antibodies. The neonates from vaccinated dams showed a significant reduction in EHEC O157:H7 colonization 48h post challenge. In addition, the level of plasma urea concentration, a marker of renal failure, was significantly higher in offsprings of sham-vaccinated mice. In conclusion, vaccination of pregnant dams with γ-Int C280 and EspB could reduce colonization and systemic toxicity of EHEC O157:H7 in their suckling offsprings.

The Type III Secretion Translocation Pore Senses Host Cell Contact

Type III secretion systems (T3SS) are nano-syringes used by a wide range of Gram-negative pathogens to promote infection by directly injecting effector proteins into targeted host cells. Translocation of effectors is triggered by host-cell contact and requires assembly of a pore in the host-cell plasma membrane, which consists of two translocator proteins. Our understanding of the translocation pore, how it is assembled in the host cell membrane and its precise role in effector translocation, is extremely limited. Here we use a genetic technique to identify protein-protein contacts between pore-forming translocator proteins, as well as the T3SS needle-tip, that are critical for translocon function. The data help establish the orientation of the translocator proteins in the host cell membrane. Analysis of translocon function in mutants that break these contacts demonstrates that an interaction between the pore-forming translocator PopD and the needle-tip is required for sensing host cell contact. Moreover, tethering PopD at a dimer interface also specifically prevents host-cell sensing, arguing that the translocation pore is actively involved in detecting host cell contact. The work presented here therefore establishes a signal transduction pathway for sensing host cell contact that is initiated by a conformational change in the translocation pore, and is subsequently transmitted to the base of the apparatus via a specific contact between the pore and the T3SS needle-tip.

Type III secretion systems (T3SSs) are molecular syringes used by a wide variety of Gram-negative pathogens to directly deliver proteins (effectors) into host cells, allowing the bacteria to cause disease. Injection of proteins is triggered by host-cell contact, but how the machinery to deliver effectors is assembled (the translocon), or indeed, how cell contact is perceived, is unclear. Here we identify protein-protein contacts that are critical for translocon function. Our analysis sheds light on the organization of the translocon, and reveals that host cell contact is perceived by a change in the structure of the translocation pore. This signal is then transmitted to the tip of the T3SS needle, and down to the base of the apparatus.

Bacterial Control of Pores Induced by the Type III Secretion System: Mind the Gap

Type III secretion systems (T3SSs) are specialized secretion apparatus involved in the virulence of many Gram-negative pathogens, enabling the injection of bacterial type III effectors into host cells. The T3SS-dependent injection of effectors requires the insertion into host cell membranes of a pore-forming "translocon," whose effects on cell responses remain ill-defined. As opposed to pore-forming toxins that damage host cell plasma membranes and induce cell survival mechanisms, T3SS-dependent pore formation is transient, being regulated by cell membrane repair mechanisms or bacterial effectors. Here, we review host cell responses to pore formation induced by T3SSs associated with the loss of plasma membrane integrity and regulation of innate immunity. We will particularly focus on recent advances in mechanisms controlling pore formation and the activity of the T3SS linked to type III effectors or bacterial proteases. The implications of the regulation of the T3SS translocon activity during the infectious process will be discussed.

The Locus of Enterocyte Effacement and Associated Virulence Factors of Enterohemorrhagic Escherichia coli

A subset of Shiga toxin-producing Escherichia coli strains, termed enterohemorrhagic E. coli (EHEC), is defined in part by the ability to produce attaching and effacing (A/E) lesions on intestinal epithelia. Such lesions are characterized by intimate bacterial attachment to the apical surface of enterocytes, cytoskeletal rearrangements beneath adherent bacteria, and destruction of proximal microvilli. A/E lesion formation requires the locus of enterocyte effacement (LEE), which encodes a Type III secretion system that injects bacterial proteins into host cells. The translocated proteins, termed effectors, subvert a plethora of cellular pathways to the benefit of the pathogen, for example, by recruiting cytoskeletal proteins, disrupting epithelial barrier integrity, and interfering with the induction of inflammation, phagocytosis, and apoptosis. The LEE and selected effectors play pivotal roles in intestinal persistence and virulence of EHEC, and it is becoming clear that effectors may act in redundant, synergistic, and antagonistic ways during infection. Vaccines that target the function of the Type III secretion system limit colonization of reservoir hosts by EHEC and may thus aid control of zoonotic infections. Here we review the features and functions of the LEE-encoded Type III secretion system and associated effectors of E. coli O157:H7 and other Shiga toxin-producing E. coli strains.

The Epinephrine/Norepinephrine/Autoinducer-3 Interkingdom Signaling System in Escherichia coli O157:H7

Epinephrine/norepinephrine/AI-3 signaling is used as an interkingdom chemical signaling system between microbes and their hosts. This system is also exploited by pathogens to regulate virulence traits. In enterohemorrhagic E. coli (EHEC) O157:H7, it is essential for pathogenesis and flagella motility. These three signals activate expression of a pathogenicity island named locus of enterocyte effacement (LEE), Shiga toxin, and the flagella regulon. These signals are sensed by the two-component system QseBC, whereas the bacterial membrane receptor QseC autophosphorylates and phosphorylates the QseB response regulator initiating a complex phosphorelay signaling cascade that activates the expression of a second two-component system, QseEF. The QseEF two-component system is also involved in the expression of the virulence genes, and it senses epinephrine, phosphate, and sulfate. This complex signaling cascade still needs to be completely elucidated.

Effects of lactoferrin treatment on Escherichia coli O157:H7 rectal colonization in cattle

The terminal rectal mucosa has been identified as the predominant colonization site of Escherichia coli O157:H7 in cattle, thus a possible intervention approach should directly target this colonization site. To determine the effect of lactoferrin on E. coli O157:H7 mucosal colonization at the rectum, five 6-month-old Holstein-Friesian calves were experimentally infected with E. coli O157:H7 and received daily rectal treatment with bovine lactoferrin. Three calves that did not receive the lactoferrin served as control group. The treatment decreased faecal shedding of E. coli O157:H7 and completely eliminated the infection in all animals (n=5) after 19 days administration. The rectal mucosa of all animals (n=5) was cleared from E. coli O157:H7 within 13 days of lactoferrin treatment. To evaluate the local immune responses, three calves treated previously with lactoferrin and three calves of the control group were re-infected when E. coli O157:H7 excretion was no longer detected. The rectal administration of lactoferrin resulted in an EspA- and EspB-specific IgA responses at the rectal mucosa. These mucosal antibodies were not detected in the animals which did not receive the lactoferrin powder. Interestingly, no serum IgA antibodies could be found in animals of the group that received the lactoferrin. These findings emphasize the ability of bovine lactoferrin to clear E. coli O157:H7 colonization in cattle, where lactoferrin may influence the local immune processes against E. coli O157:H7 infection. Thus, bovine lactoferrin treatment could be used in the field to eliminate high-level faecal excretion of E. coli O157:H7 in cattle.

Type III Secretion-Dependent Sensitivity of Escherichia coli O157 to Specific Ketolides

A subset of Gram-negative bacterial pathogens uses a type III secretion system (T3SS) to open up a conduit into eukaryotic cells in order to inject effector proteins. These modulate pathways to enhance bacterial colonization. In this study, we screened established bioactive compounds for any that could repress T3SS expression in enterohemorrhagic Escherichia coli (EHEC) O157. The ketolides telithromycin and, subsequently, solithromycin both demonstrated repressive effects on expression of the bacterial T3SS at sub-MICs, leading to significant reductions in bacterial binding and actin-rich pedestal formation on epithelial cells. Preincubation of epithelial cells with solithromycin resulted in significantly less attachment of E. coli O157. Moreover, bacteria expressing the T3SS were more susceptible to solithromycin, and there was significant preferential killing of E. coli O157 bacteria when they were added to epithelial cells that had been preexposed to the ketolide. This killing was dependent on expression of the T3SS. Taken together, this research indicates that the ketolide that has accumulated in epithelial cells may traffic back into the bacteria via the T3SS. Considering that neither ketolide induces the SOS response, nontoxic members of this class of antibiotics, such as solithromycin, should be considered for future testing and trials evaluating their use for treatment of EHEC infections. These antibiotics may also have broader significance for treating infections caused by other pathogenic bacteria, including intracellular bacteria, that express a T3SS.