The Type III Secretion System of Pathogenic Escherichia coli

Genomic island-encoded regulatory proteins in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) is an important zoonotic pathogen that is a major cause of foodborne diseases in most developed and developing countries and can cause uncomplicated diarrhoea, haemorrhagic colitis, and haemolytic uraemic syndrome. O islands (OIs), which are unique genomic islands in EHEC O157:H7, are composed of 177 isolated genomic features and harbour 26% of the total genes that are absent in the non-pathogenic E. coli K-12 genome. In the last twenty years, many OI-encoded proteins have been characterized, including proteins regulating virulence, motility, and acid resistance. Given the critical role of regulatory proteins in the systematic and hierarchical regulation of bacterial biological processes, this review summarizes the OI-encoded regulatory proteins in EHEC O157:H7 characterized to date, emphasizing OI-encoded regulatory proteins for bacterial virulence, motility, and acid resistance. This summary will be significant for further exploration and understanding of the virulence and pathogenesis of EHEC O157:H7.

Integrated Information for Pathogenicity and Treatment of Spiroplasma

Spiroplasma, belonging to the class Mollicutes, is a small, helical, motile bacterium lacking a cell wall. Its host range includes insects, plants, and aquatic crustaceans. Recently, a few human cases of Spiroplasma infection have been reported. The diseases caused by Spiroplasma have brought about serious economic losses and hindered the healthy development of agriculture. The pathogenesis of Spiroplasma involves the ability to adhere, such as through the terminal structure of Spiroplasma, colonization, and invasive enzymes. However, the exact pathogenic mechanism of Spiroplasma remains a mystery. Therefore, we systematically summarize all the information about Spiroplasma in this review article. This provides a reference for future studies on virulence factors and treatment strategies of Spiroplasma.

Emergence of potentially disinfection-resistant, naturalized Escherichia coli populations across food- and water-associated engineered environments

The Escherichia coli species is comprised of several 'ecotypes' inhabiting a wide range of host and natural environmental niches. Recent studies have suggested that novel naturalized ecotypes have emerged across wastewater treatment plants and meat processing facilities. Phylogenetic and multilocus sequence typing analyses clustered naturalized wastewater and meat plant E. coli strains into two main monophyletic clusters corresponding to the ST635 and ST399 sequence types, with several serotypes identified by serotyping, potentially representing distinct lineages that have naturalized across wastewater treatment plants and meat processing facilities. This evidence, taken alongside ecotype prediction analyses that distinguished the naturalized strains from their host-associated counterparts, suggests these strains may collectively represent a novel ecotype that has recently emerged across food- and water-associated engineered environments. Interestingly, pan-genomic analyses revealed that the naturalized strains exhibited an abundance of biofilm formation, defense, and disinfection-related stress resistance genes, but lacked various virulence and colonization genes, indicating that their naturalization has come at the cost of fitness in the original host environment.

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

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

Expression of the locus of enterocyte effacement genes during the invasion process of the atypical enteropathogenic Escherichia coli 1711-4 strain of serotype O51:H40

Atypical enteropathogenic Escherichia coli (aEPEC) is a significant cause of diarrhea in developing countries. Some aEPEC strains, including the Brazilian representative strain of serotype O51:H40 called aEPEC 1711-4, can use flagella to attach to, invade, and persist in T84 and Caco-2 intestinal cells. They can even translocate from the gut to extraintestinal sites in a rat model. Although various aspects of the virulence of this strain were studied and the requirement of the T3SS for the efficiency of the invasion process was demonstrated, the expression of the LEE genes during the invasion and intracellular persistence remains unclear. To address this, the expression of flagella and the different LEE operons was evaluated during kinetic experiments of the interaction of aEPEC 1711-4 with enterocytes in vitro. The genome of the strain was also sequenced. The results showed that flagella expression remained unchanged, but the expression of eae and escJ increased during the early interaction and invasion of aEPEC 1711-4 into Caco-2 cells, and there was no change 24 hours post-infection during the persistence period. The number of pedestal-like structures formed on HeLa cells also increased during the 24-hour analysis. No known gene related to the invasion process was identified in the genome of aEPEC 1711-4, which was shown to belong to the global EPEC lineage 10. These findings suggest that LEE components and the intimate adherence promoted by intimin are necessary for the invasion and persistence of aEPEC 1711-4, but the detailed mechanism needs further study.

Virtual Screening Assisted Search for Inhibitors of the Translocated Intimin Receptor of Enteropathogenic Escherichia Coli

This study aimed to identify inhibitors of the translocated intimin receptor (Tir) of enteropathogenic Escherichia coli (EPEC). EPEC is an intestinal pathogen that causes diarrhea and is a major health concern worldwide. Because Tir is a key virulence factor involved in EPEC pathogenesis, inhibiting its function is a potential strategy for controlling EPEC infections. Virtual screening was applied to chemical libraries to search for compounds that inhibit Tir-mediated bacterial adherence to host cells. Three sites were targeted using the cocrystal structure published earlier. A selection of compounds was then assessed in a cell-based infection model and fluorescence microscopy assay. The results of this study provide a basis for further optimization and testing of Tir inhibitors as potential therapeutic agents for EPEC infections.

Host-Pathogen Interaction: Biology and Public Health

Interactions between host and pathogenic microorganisms are common in nature and have a significant impact on host health, often leading to several types of infections. These interactions have evolved as a result of the ongoing battle between the host's defense mechanisms and the pathogens' invasion strategies. In this chapter, we will explore the evolution of host-pathogen interactions, explore their molecular mechanisms, examine the different stages of interaction, and discuss the development of pharmacological treatments. Understanding these interactions is crucial for improving public health, as it enables us to develop effective strategies to prevent and control infectious diseases. By gaining insights into the intricate dynamics between pathogens and their hosts, we can work towards reducing the burden of such diseases on society.

Phylogenomic Analysis of CTX-M-15-Positive Escherichia coli from Companion Animal Reveals Intercontinental Dissemination of ST90 Within a One Health Framework

The global dissemination of extended-spectrum-β-lactamase (ESBL)-producing Escherichia coli has been considered a critical issue within a One Health framework. The aim of this study was to perform a genomic investigation of an ESBL-producing E. coli strain belonging to the globally spread sequence type/clonal complex ST90/CC23, isolated from gastrointestinal tract of a dog, in Brazil. Besides CTX-M-15 ESBL, this E. coli isolate carried mutations conferring resistance to human and veterinary fluoroquinolones (GyrA [Ser83Leu, Asp87Asn], ParC [Ser80Ile] and ParE [Ser458Ala]), and resistance determinants to disinfectants and pesticides. Noteworthy, phylogenomic analysis revealed that this multidrug E. coli strain clustered with ST90 lineages isolated from human, dog, and livestock in Brazil. The phylogenetic tree also revealed that this E. coli strain shares a common ancestor with isolates from the United States, Russia, Germany, and China, highlighting the potential global spreading of this clone. In summary, we report genomic data of CTX-M-15-positive E.coli ST90 colonizing a pet. Colonization of companion animals by critical resistant pathogens highlights the need for close monitoring to better understand the epidemiology and genetic factors contributing for successful adaptation of global clones at the human-animal interface.

Chicken Production and Human Clinical Escherichia coli Isolates Differ in Their Carriage of Antimicrobial Resistance and Virulence Factors

Contamination of food animal products by Escherichia coli is a leading cause of foodborne disease outbreaks, hospitalizations, and deaths in humans. Chicken is the most consumed meat both in the United States and across the globe according to the U.S. Department of Agriculture. Although E. coli is a ubiquitous commensal bacterium of the guts of humans and animals, its ability to acquire antimicrobial resistance (AMR) genes and virulence factors (VFs) can lead to the emergence of pathogenic strains that are resistant to critically important antibiotics. Thus, it is important to identify the genetic factors that contribute to the virulence and AMR of E. coli. In this study, we performed in-depth genomic evaluation of AMR genes and VFs of E. coli genomes available through the National Antimicrobial Resistance Monitoring System GenomeTrackr database. Our objective was to determine the genetic relatedness of chicken production isolates and human clinical isolates. To achieve this aim, we first developed a massively parallel analytical pipeline (Reads2Resistome) to accurately characterize the resistome of each E. coli genome, including the AMR genes and VFs harbored. We used random forests and hierarchical clustering to show that AMR genes and VFs are sufficient to classify isolates into different pathogenic phylogroups and host origin. We found that the presence of key type III secretion system and AMR genes differentiated human clinical isolates from chicken production isolates. These results further improve our understanding of the interconnected role AMR genes and VFs play in shaping the evolution of pathogenic E. coli strains. IMPORTANCE Pathogenic Escherichia coli causes disease in both humans and food-producing animals. E. coli pathogenesis is dependent on a repertoire of virulence factors and antimicrobial resistance genes. Food-borne outbreaks are highly associated with the consumption of undercooked and contaminated food products. This association highlights the need to understand the genetic factors that make E. coli virulent and pathogenic in humans and poultry. This research shows that E. coli isolates originating from human clinical settings and chicken production harbor different antimicrobial resistance genes and virulence factors that can be used to classify them into phylogroups and host origins. In addition, to aid in the repeatability and reproducibility of the results presented in this study, we have made a public repository of the Reads2Resistome pipeline and have provided the accession numbers associated with the E. coli genomes analyzed.

Genomic Island-Encoded Histidine Kinase and Response Regulator Coordinate Mannose Utilization with Virulence in Enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is a highly adaptive pathogen and has acquired diverse genetic elements, such as genomic islands and prophages, via horizontal gene transfer to promote fitness in vivo. Two-component signaling systems (TCSs) allow bacteria to sense, respond to, and adapt to various environments. This study identified a putative two-component signaling system composed of the histidine kinase EDL5436 (renamed LmvK) and the response regulator EDL5428 (renamed LmvR) in EHEC. lmvK and lmvR along with EDL5429 to EDL5434 (EDL5429-5434) between them constitute the OI167 genomic island and are highly associated with the EHEC pathotype. EDL5429-5434 encode transporters and metabolic enzymes that contribute to growth on mannose and are directly upregulated by LmvK/LmvR in the presence of mannose, as revealed by quantitative PCR (qPCR) and DNase I footprint assays. Moreover, LmvR directly activates the expression of the type III secretion system in response to mannose and promotes the formation of attaching and effacing lesions on HeLa cells. Using human colonoid and mouse infection models, we show that lmvK and lmvR contributed greatly to adherence and microcolony (MC) formation ex vivo and colonization in vivo. Finally, RNA sequencing and chromatin immunoprecipitation coupled with sequencing analyses identified additional direct targets of LmvR, most of which are involved in metabolism. Given that mannose is a mucus-derived sugar that induces virulence and is preferentially used by EHEC during infection, our data revealed a previously unknown mechanism by which EHEC recognizes the host metabolic landscape and regulates virulence expression accordingly. Our findings provide insights into how pathogenic bacteria evolve by acquiring genetic elements horizontally to adapt to host environments. IMPORTANCE The gastrointestinal tract represents a complex and challenging environment for enterohemorrhagic Escherichia coli (EHEC). However, EHEC is a highly adaptable pathogen, requiring only 10 to 100 CFUs to cause infection. This ability was achieved partially by acquiring mobile genetic elements, such as genomic islands, that promote overall fitness. Mannose is an intestinal mucus-derived sugar that stimulates virulence and is preferentially used by EHEC during infection. Here, we characterize the OI167 genomic island of EHEC, which encodes a novel two-component signaling system (TCS) and transporters and metabolic enzymes (EDL5429-5434) involved in mannose utilization. The TCS directly upregulates EDL5429-5434 and genes encoding the type III secretion system in the presence of mannose. Moreover, the TCS contributes greatly to EHEC virulence ex vivo and in vivo. Our data demonstrate an elegant example in which EHEC strains evolve by acquiring genetic elements horizontally to recognize the host metabolic landscape and regulate virulence expression accordingly, leading to successful infections.

Transcriptional profiling of extraintestinal Escherichia coli exposed to cranberry pomace extracts alone or in combination with ceftriaxone

Extraintestinal pathogenic Escherichia coli (ExPEC) includes several serotypes that have been associated with colibacillosis in poultry, as well as urinary tract infections and newborn meningitis in humans. This study investigated the antimicrobial activities of ceftriaxone (AXO) and cranberry pomace extracts (CRAN) alone or in combination (CC) against multidrug-resistant (MDR) ExPEC from broiler. The growth-inhibitory activity of CRAN and synergy tests by a checkerboard method were determined in cation-adjusted Mueller–Hinton broth (CAMHB). The transcriptomic profile of the MDR E. coli O7:H18 (ST38) grown in CAMHB supplemented with sub-inhibitory concertation of CRAN and AXO alone or in combination was obtained by RNA-seq. The MIC of CRAN for all isolates was 16 mg/mL. An additive activity was observed between 4 mg/mL of CRAN and 4 μg/mL of AXO. Compared to the control, the transcriptomic analysis revealed that 4 mg/ml of (1/4MIC) CRAN and its combination with 4 μg/mL of (1/8MIC) AXO (CC) exposures resulted in 727 and 712 differentially expressed genes, respectively (false discovery rate < 0.001 and log2-fold change > 2), in the studied E. coli. Major virulence genes including adhesins (fim, flg, csg, and yad), protectins (omp, tra, waa, and hly), secretion systems (hof, pho, and vir), and quorum sensing (lsr), which are energetically expensive for bacteria, were downregulated. Most importantly, 1/4MIC of CRAN or CC downregulated the β-lactamase blaCMY-2 and efflux pump including tolC, mdtEIJ, gadEW, and their regulator gene evgS, while upregulating the cysteine biosynthesis and oxidative stress-related regulatory genes including cys, dmlA, sbp, nrdGHI, soxSR, and rpoH. Downregulation of multiple enzymes involved in TCA cycles and upregulation of Fe–S cluster coordinated by Cys and Isc proteins reflect the regulation of energy metabolism of the studied E. coli upon CRAN or CC exposure. The downregulation of outer membrane protein genes that control permeability barriers, along with different antimicrobial resistance genes, demonstrates that CRAN may have the unique potential to enhance the antimicrobial activities of third-generation cephalosporins such as AXO against MDR E. coli.

A toxin-deformation dependent inhibition mechanism in the T7SS toxin-antitoxin system of Gram-positive bacteria

Toxin EsaD secreted by some S. aureus strains through the type VII secretion system (T7SS) specifically kills those strains lacking the antitoxin EsaG. Here we report the structures of EsaG, the nuclease domain of EsaD and their complex, which together reveal an inhibition mechanism that relies on significant conformational change of the toxin. To inhibit EsaD, EsaG breaks the nuclease domain of EsaD protein into two independent fragments that, in turn, sandwich EsaG. The originally well-folded ββα-metal finger connecting the two fragments is stretched to become a disordered loop, leading to disruption of the catalytic site of EsaD and loss of nuclease activity. This mechanism is distinct from that of the other Type II toxin-antitoxin systems, which utilize an intrinsically disordered region on the antitoxins to cover the active site of the toxins. This study paves the way for developing therapeutic approaches targeting this antagonism.

EspH interacts with the host active Bcr related (ABR) protein to suppress RhoGTPases

Enteropathogenic Escherichia coli are bacterial pathogens that colonize the gut and cause severe diarrhea in humans. Upon intimate attachment to the intestinal epithelium, these pathogens translocate via a type III secretion system virulent proteins, termed effectors, into the host cells. These effectors manipulate diverse host cell organelles and functions for the pathogen's benefit. However, the precise mechanisms underlying their activities are not fully understood despite intensive research. EspH, a critical effector protein, has been previously reported to disrupt the host cell actin cytoskeleton by suppressing RhoGTPase guanine exchange factors. However, native host proteins targeted by EspH to mediate these activities remained unknown. Here, we identified the active Bcr related (ABR), a protein previously characterized to possess dual Rho guanine nucleotide exchange factor and GTPase activating protein (GAP) domains, as a native EspH interacting partner. These interactions are mediated by the effector protein's C-terminal 38 amino acid segment. The effector primarily targets the GAP domain of ABR to suppress Rac1 and Cdc42, host cell cytotoxicity, bacterial invasion, and filopodium formation at infection sites. Knockdown of ABR expression abolished the ability of EspH to suppress Rac1, Cdc42. Our studies unravel a novel mechanism by which host RhoGTPases are hijacked by bacterial effectors.

icaR and icaT Are Ancient Chromosome Genes Encoding Substrates of the Type III Secretion Apparatus in Shigella flexneri

Shigella is an Escherichia coli pathovar that colonizes the cytosol of mucosal cells in the human large intestine. To do this, Shigella uses a Type III Secretion Apparatus (T3SA) to translocate several proteins into host cells. The T3SA and its substrates are encoded by genes of the virulence plasmid pINV or by chromosomal genes derived thereof. We recently discovered two chromosomal genes, which seem unrelated to pINV, although they are activated by MxiE and IpgC similarly to some of the canonical substrates of the T3SA. Here, we showed that the production of the corresponding proteins depended on the conservation of a MxiE box in their cognate promoters. Furthermore, both proteins were secreted by the T3SA in a chaperone-independent manner through the recognition of their respective amino-terminal secretion signal. Based on these observations, we named these new genes icaR and icaT, which stand for invasion chromosome antigen with homology for a transcriptional regulator and a transposase, respectively. icaR and icaT have orthologs in commensal and pathogenic E. coli strains belonging mainly to phylogroups A, B1, D and E. Finally, we demonstrated that icaR and icaT orthologs could be activated by the coproduction of IpgC and MxiE in strains MG1655 K-12 (phylogroup A) and O157:H7 ATCC 43888 (phylogroup E). In contrast, the coproduction of EivF and YgeG, which are homologs of MxiE and IpgC in the E. coli T3SS 2 (ETT2), failed to activate icaR and icaT.

IMPORTANCEicaR and icaT are the latest members of the MxiE regulon discovered in the chromosome. The proteins IcaR and IcaT, albeit produced in small amounts, are nonetheless secreted by the T3SA comparably to canonical substrates. The high occurrence of icaR and icaT in phylogroups A, B1, D, and E coupled with their widespread absence in their B2 counterparts agree with the consensus E. coli phylogeny. The widespread conservation of the MxiE box among icaR and icaT orthologs supports the notion that both genes had already undergone coevolution with transcriptional activators ipgC and mxiE- harbored in pINV or a relative- in the last common ancestor of Shigella and of E. coli from phylogroups A, B1, D, and E. The possibility that icaR and icaT may contribute to Shigella pathogenesis cannot be excluded, although some of their characteristics suggest they are fossil genes.

YqeH contributes to avian pathogenic Escherichia coli pathogenicity by regulating motility, biofilm formation, and virulence

Avian pathogenic Escherichia coli (APEC) is a pathotype of extraintestinal pathogenic E. coli and one of the most serious infectious diseases of poultry. It not only causes great economic losses to the poultry industry, but also poses a serious threat to public health worldwide. Here, we examined the role of YqeH, a transcriptional regulator located at E. coli type III secretion system 2 (ETT2), in APEC pathogenesis. To investigate the effects of YqeH on APEC phenotype and virulence, we constructed a yqeH deletion mutant (APEC40-ΔyqeH) and a complemented strain (APEC40-CΔyqeH) of APEC40. Compared with the wild type (WT), the motility and biofilm formation of APEC40-ΔyqeH were significantly reduced. The yqeH mutant was highly attenuated in a chick infection model compared with WT, and showed severe defects in its adherence to and invasion of chicken embryo fibroblast DF-1 cells. However, the mechanisms underlying these phenomena were unclear. Therefore, we analyzed the transcriptional effects of the yqeH deletion to clarify the regulatory mechanisms of YqeH, and the role of YqeH in APEC virulence. The deletion of yqeH downregulated the transcript levels of several flagellum-, biofilm-, and virulence-related genes. Our results demonstrate that YqeH is involved in APEC pathogenesis, and the reduced virulence of APEC40-ΔyqeH may be related to its reduced motility and biofilm formation.

Assay for Type III Secretion in Escherichia coli

The type III secretion system (T3SS) is crucial for the virulence of several pathogenic Escherichia coli species as well as for other gram-negative bacterial strains. Therefore, the ability to monitor this system constitutes a valuable tool for assessing the involvement of different proteins in bacterial virulence, for identifying critical domains and specific mutations, and for evaluating the antivirulence activities of various drugs. The major advantage of the T3SS secretion assay for E. coli over assays for other gram-negative pathogens is that it does not necessarily require specific antibodies. Here, we describe how to grow enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC) strains under T3SS-inducing conditions, separate the supernatant fraction from the bacterial pellet, analyze this fraction on sodium dodecyl sulfate (SDS)-polyacrylamide gels, and evaluate the level of T3SS activity. We describe a qualitative analysis using Coomassie staining and a quantitative assay using western blotting.

The Regulatory Functions of σ54 Factor in Phytopathogenic Bacteria

σ⁵⁴ factor (RpoN), a type of transcriptional regulatory factor, is widely found in pathogenic bacteria. It binds to core RNA polymerase (RNAP) and regulates the transcription of many functional genes in an enhancer-binding protein (EBP)-dependent manner. σ⁵⁴ has two conserved functional domains: the activator-interacting domain located at the N-terminal and the DNA-binding domain located at the C-terminal. RpoN directly binds to the highly conserved sequence, GGN₁₀GC, at the −24/−12 position relative to the transcription start site of target genes. In general, bacteria contain one or two RpoNs but multiple EBPs. A single RpoN can bind to different EBPs in order to regulate various biological functions. Thus, the overlapping and unique regulatory pathways of two RpoNs and multiple EBP-dependent regulatory pathways form a complex regulatory network in bacteria. However, the regulatory role of RpoN and EBPs is still poorly understood in phytopathogenic bacteria, which cause economically important crop diseases and pose a serious threat to world food security. In this review, we summarize the current knowledge on the regulatory function of RpoN, including swimming motility, flagella synthesis, bacterial growth, type IV pilus (T4Ps), twitching motility, type III secretion system (T3SS), and virulence-associated phenotypes in phytopathogenic bacteria. These findings and knowledge prove the key regulatory role of RpoN in bacterial growth and pathogenesis, as well as lay the groundwork for further elucidation of the complex regulatory network of RpoN in bacteria.

Serum resistance factors in avian pathogenic Escherichia coli mediated by ETT2 gene cluster

Serum resistance is a poorly understood but common trait of some systemic disease pathogenic strains of bacteria. In this study, we analysed the role Escherichia coli type three secretion system 2 (ETT2) of avian pathogenic Escherichia coli (APEC) in serum resistance by bacteria survival number in serum culture, mRNA Seq and Tandem Mass Tag™ (TMT™) detection, lipopolysaccharide (LPS) extraction, and biofilm formation detection. We found that the ETT2 gene cluster deletion strain (ΔETT2) is more resistant to the killing effect of serum than wild type APEC40. The analysis of ΔETT2 compared to APEC40 in the transcriptomics and proteomics data showed that ETT2 has a negative effect in the ATP-binding cassette (ABC) translator system and quorum sensing system and a positive effect in purine metabolism. ETT2 may affect the LPS, biofilm, flagella, and fimbriae which may affect the serum resistance. These results could lead to effective strategies for managing the infection of APEC. The mRNA Seq data of this study are available in the Sequence Read Archive of the National Center for Biotechnology Information under the BioProject PRJNA757182, and proteomic raw data have been deposited under the accession number IPX0003420000 at iProX.

Secondary-structure switch regulates the substrate binding of a YopJ family acetyltransferase

The Yersinia outer protein J (YopJ) family effectors are widely deployed through the type III secretion system by both plant and animal pathogens. As non-canonical acetyltransferases, the enzymatic activities of YopJ family effectors are allosterically activated by the eukaryote-specific ligand inositol hexaphosphate (InsP6). However, the underpinning molecular mechanism remains undefined. Here we present the crystal structure of apo-PopP2, a YopJ family member secreted by the plant pathogen Ralstonia solanacearum. Structural comparison of apo-PopP2 with the InsP6-bound PopP2 reveals a substantial conformational readjustment centered in the substrate-binding site. Combining biochemical and computational analyses, we further identify a mechanism by which the association of InsP6 with PopP2 induces an α-helix-to-β-strand transition in the catalytic core, resulting in stabilization of the substrate recognition helix in the target protein binding site. Together, our study uncovers the molecular basis governing InsP6-mediated allosteric regulation of YopJ family acetyltransferases and further expands the paradigm of fold-switching proteins.

Virulence Factors of Enteric Pathogenic Escherichia coli: A Review

Escherichia coli are remarkably versatile microorganisms and important members of the normal intestinal microbiota of humans and animals. This harmless commensal organism can acquire a mixture of comprehensive mobile genetic elements that contain genes encoding virulence factors, becoming an emerging human pathogen capable of causing a broad spectrum of intestinal and extraintestinal diseases. Nine definite enteric E. coli pathotypes have been well characterized, causing diseases ranging from various gastrointestinal disorders to urinary tract infections. These pathotypes employ many virulence factors and effectors subverting the functions of host cells to mediate their virulence and pathogenesis. This review summarizes new developments in our understanding of diverse virulence factors associated with encoding genes used by different pathotypes of enteric pathogenic E. coli to cause intestinal and extraintestinal diseases in humans.

DctR contributes to the virulence of avian pathogenic Escherichia coli through regulation of type III secretion system 2 expression

Pathogens could precisely alter their gene expression to facilitate their survival and successful infection. The LuxR family transcriptional regulator DctR (also known as YhiF) was shown to participate in the regulation of acid fitness and adhesion of enterohemorrhagic E. coli (EHEC) O157:H7. Avian pathogenic Escherichia coli (APEC) causes significant economic losses to the poultry industries and also potentially threatens human health. However, the effects of DctR on the fitness and virulence of APEC have not been investigated yet. To assess the function of DctR in APEC, the dctR gene mutant and complemented strains were constructed and biologically characterized. Our results show that inactivation of the dctR gene led to decreased biofilm formation, diminished serum resistance, reduced adherence capacity, attenuated colonization and virulence of APEC in ducks. The altered capacities of the mutant strain were restored by genetic complementation. In addition, we found that DctR positively regulates the expression of E. coli type III secretion system 2 (ETT2) core genes in APEC. The expression of the inflammatory cytokines interleukin (IL)-1β and IL-8 were decreased in HD-11 macrophages infected with the mutant strain compared with the wild-type strain. These observations indicate that regulator DctR contributes to the virulence of APEC through regulation of ETT2 expression.

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.

Type III secretion system effectors form robust and flexible intracellular virulence networks

Infections with many Gram-negative pathogens, including Escherichia coli, Salmonella, Shigella, and Yersinia, rely on type III secretion system (T3SS) effectors. We hypothesized that while hijacking processes within mammalian cells, the effectors operate as a robust network that can tolerate substantial contractions. This was tested in vivo using the mouse pathogen Citrobacter rodentium (encoding 31 effectors). Sequential gene deletions showed that effector essentiality for infection was context dependent and that the network could tolerate 60% contraction while maintaining pathogenicity. Despite inducing very different colonic cytokine profiles (e.g., interleukin-22, interleukin-17, interferon-γ, or granulocyte-macrophage colony-stimulating factor), different networks induced protective immunity. Using data from >100 distinct mutant combinations, we built and trained a machine learning model able to predict colonization outcomes, which were confirmed experimentally. Furthermore, reproducing the human-restricted enteropathogenic E. coli effector repertoire in C. rodentium was not sufficient for efficient colonization, which implicates effector networks in host adaptation. These results unveil the extreme robustness of both T3SS effector networks and host responses.

Citrobacter rodentium Infection Induces Persistent Molecular Changes and Interferon Gamma-Dependent Major Histocompatibility Complex Class II Expression in the Colonic Epithelium

Most studies of infections at mucosal surfaces have focused on the acute phase of the disease. Consequently, little is known about the molecular processes that underpin tissue recovery and the long-term consequences postinfection. Here, we conducted temporal deep quantitative proteomic analysis of colonic intestinal epithelial cells (cIECs) from mice infected with the natural mouse pathogen Citrobacter rodentium over time points corresponding to the late steady-state phase (10 days postinfection [DPI]), the clearance phase (13 to 20 DPI), and 4 weeks after the pathogen has been cleared (48 DPI). C. rodentium, which relies on a type III secretion system to infect, is used to model infections with enteropathogenic and enterohemorrhagic Escherichia coli. We observe a strong upregulation of inflammatory signaling and nutritional immunity responses during the clearance phase of the infection. Despite morphological tissue recovery, chromogranin B (ChgB)-positive endocrine cells remained significantly below baseline levels at 48 DPI. In contrast, we observed an increased abundance of proteins involved in antigen processing and presentation 4 weeks after pathogen clearance. In particular, long-term changes were characterized by a persistent interferon gamma (IFN-γ) response and the expression of major histocompatibility complex class II (MHCII) molecules in 60% of the EpCAM⁺ cIECs, which were not seen in Ifnγ^-/- mice. Nonetheless, both wild-type and Ifnγ^-/- mice mounted similar systemic and colonic IgG responses to C. rodentium and were equally protected from rechallenge, suggesting that cIEC MHCII is not necessary for protective immunity against C. rodentium. IMPORTANCE Mucosal surfaces respond to infection by mounting an array of metabolic, inflammatory, and tissue repair responses. While these have been well studied during acute infection, less is known about tissue recovery after pathogen clearance. We employ the mouse pathogen Citrobacter rodentium, which binds colonic intestinal epithelial cells (cIECs), to investigate the long-term effects of bacterial infection on gut physiology. Using global proteomic analysis, we study cIEC temporal responses during and after the clearance phase of infection. While the overall tissue morphology recovered, cIECs showed persistent signs of infection 4 weeks after pathogen clearance. These were characterized by a strong IFN-γ signature, including the upregulation of major histocompatibility complex class II (MHCII) antigen presentation proteins, suggesting that the tissue remains on "high alert" for weeks after the acute insult is resolved. However, we demonstrate that cIEC MHCII expression, which is induced by IFN-γ, is not required for protective IgG-mediated immunity against C. rodentium; instead, it may play a role in mucosal recovery.

An Escherichia coli Effector Protein EspF May Induce Host DNA Damage via Interaction With SMC1

Enterohemorrhagic Escherichia coli (EHEC) O157: H7 is an important foodborne pathogen that causes human diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome. EspF is one of the most important effector proteins injected by the Type III Secretion System. It can target mitochondria and nucleoli, stimulate host cells to produce ROS, and promote host cell apoptosis. However, the mechanism of the host-pathogen interaction leading to host oxidative stress and cell cytotoxic effects such as DNA damage remains to be elucidated. Here, we used Cell Counting Kit-8 (CCK-8) assays and 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-OHdG) ELISA to study cell viability and DNA oxidative damage level after exposure to EspF. Western blot and immunofluorescence were also used to determine the level of the DNA damage target protein p-H2AX and cell morphology changes after EspF infection. Moreover, we verified the toxicity in intestinal epithelial cells mediated by EspF infection in vivo. In addition, we screened the host proteins that interact with EspF using CoIP-MS. We found that EspF may more depend on its C-terminus to interact with SMC1, and EspF could activate SMC1 phosphorylation and migrate it to the cytoplasm. In summary, this study revealed that EspF might mediate host cell DNA damage and found a new interaction between EspF and the DNA damage repair protein SMC1. Thus, EspF may mediate DNA damage by regulating the subcellular localization and phosphorylation of SMC1.

Comparison of Enzyme Secretion and Ferulic Acid Production by Escherichia coli Expressing Different Lactobacillus Feruloyl Esterases

Construction of recombinant Escherichia coli strains carrying feruloyl esterase genes for secretory expression offers an attractive way to facilitate enzyme purification and one-step production of ferulic acid from agricultural waste. A total of 10 feruloyl esterases derived from nine Lactobacillus species were expressed in E. coli BL21 (DE3) to investigate their secretion and ferulic acid production. Extracellular activity determination showed all these Lactobacillus feruloyl esterases could be secreted out of E. coli cells. However, protein analysis indicated that they could be classified as three types. The first type presented a low secretion level, including feruloyl esterases derived from Lactobacillus acidophilus and Lactobacillus johnsonii. The second type showed a high secretion level, including feruloyl esterases derived from Lactobacillus amylovorus, Lactobacillus crispatus, Lactobacillus gasseri, and Lactobacillus helveticus. The third type also behaved a high secretion level but easy degradation, including feruloyl esterases derived from Lactobacillus farciminis, Lactobacillus fermentum, and Lactobacillus reuteri. Moreover, these recombinant E. coli strains could directly release ferulic acid from agricultural waste. The highest yield was 140 μg on the basis of 0.1 g de-starched wheat bran by using E. coli expressed L. amylovorus feruloyl esterase. These results provided a solid basis for the production of feruloyl esterase and ferulic acid.

Virulence factors impair epithelial junctions during bacterial infection

Epithelial cells are typically connected through different types of cell junctions that are localized from the apical membrane to the basal surface. In this way, epithelium cells form the first barrier against pathogenic microorganisms and prevent their entry into internal organs and the circulatory system. Recent studies demonstrate that bacterial pathogens disrupt epithelial cell junctions through targeting junctional proteins by secreted virulence factors. In this review, we discuss the diverse strategies used by common bacterial pathogens, including Pseudomonas aeruginosa, Helicobacter pylori, and enteropathogenic Escherichia coli, to disrupt epithelial cell junctions during infection. We also discuss the potential of targeting the pathogenic mechanisms in the treatment of pathogen-associated diseases.

The interaction of Escherichia coli O157 :H7 and Salmonella Typhimurium flagella with host cell membranes and cytoskeletal components

Bacterial flagella have many established roles beyond swimming motility. Despite clear evidence of flagella-dependent adherence, the specificity of the ligands and mechanisms of binding are still debated. In this study, the molecular basis of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium flagella binding to epithelial cell cultures was investigated. Flagella interactions with host cell surfaces were intimate and crossed cellular boundaries as demarcated by actin and membrane labelling. Scanning electron microscopy revealed flagella disappearing into cellular surfaces and transmission electron microscopy of S. Typhiumurium indicated host membrane deformation and disruption in proximity to flagella. Motor mutants of E. coli O157:H7 and S. Typhimurium caused reduced haemolysis compared to wild-type, indicating that membrane disruption was in part due to flagella rotation. Flagella from E. coli O157 (H7), EPEC O127 (H6) and S. Typhimurium (P1 and P2 flagella) were shown to bind to purified intracellular components of the actin cytoskeleton and directly increase in vitro actin polymerization rates. We propose that flagella interactions with host cell membranes and cytoskeletal components may help prime intimate attachment and invasion for E. coli O157:H7 and S. Typhimurium, respectively.

Staying out or Going in? The Interplay between Type 3 and Type 5 Secretion Systems in Adhesion and Invasion of Enterobacterial Pathogens

Enteric pathogens rely on a variety of toxins, adhesins and other virulence factors to cause infections. Some of the best studied pathogens belong to the Enterobacterales order; these include enteropathogenic and enterohemorrhagic Escherichia coli, Shigella spp., and the enteropathogenic Yersiniae. The pathogenesis of these organisms involves two different secretion systems, a type 3 secretion system (T3SS) and type 5 secretion systems (T5SSs). The T3SS forms a syringe-like structure spanning both bacterial membranes and the host cell plasma membrane that translocates toxic effector proteins into the cytoplasm of the host cell. T5SSs are also known as autotransporters, and they export part of their own polypeptide to the bacterial cell surface where it exerts its function, such as adhesion to host cell receptors. During infection with these enteropathogens, the T3SS and T5SS act in concert to bring about rearrangements of the host cell cytoskeleton, either to invade the cell, confer intracellular motility, evade phagocytosis or produce novel structures to shelter the bacteria. Thus, in these bacteria, not only the T3SS effectors but also T5SS proteins could be considered "cytoskeletoxins" that bring about profound alterations in host cell cytoskeletal dynamics and lead to pathogenic outcomes.

Vying for the control of inflammasomes: The cytosolic frontier of enteric bacterial pathogen-host interactions

Enteric pathogen-host interactions occur at multiple interfaces, including the intestinal epithelium and deeper organs of the immune system. Microbial ligands and activities are detected by host sensors that elicit a range of immune responses. Membrane-bound toll-like receptors and cytosolic inflammasome pathways are key signal transducers that trigger the production of pro-inflammatory molecules, such as cytokines and chemokines, and regulate cell death in response to infection. In recent years, the inflammasomes have emerged as a key frontier in the tussle between bacterial pathogens and the host. Inflammasomes are complexes that activate caspase-1 and are regulated by related caspases, such as caspase-11, -4, -5 and -8. Importantly, enteric bacterial pathogens can actively engage or evade inflammasome signalling systems. Extracellular, vacuolar and cytosolic bacteria have developed divergent strategies to subvert inflammasomes. While some pathogens take advantage of inflammasome activation (e.g. Listeria monocytogenes, Helicobacter pylori), others (e.g. E. coli, Salmonella, Shigella, Yersinia sp.) deploy a range of virulence factors, mainly type 3 secretion system effectors, that subvert or inhibit inflammasomes. In this review we focus on inflammasome pathways and their immune functions, and discuss how enteric bacterial pathogens interact with them. These studies have not only shed light on inflammasome-mediated immunity, but also the exciting area of mammalian cytosolic immune surveillance.

Diet-derived galacturonic acid regulates virulence and intestinal colonization in enterohaemorrhagic Escherichia coli and Citrobacter rodentium

Enteric pathogens sense the complex chemistry within the gastrointestinal (GI) tract to efficiently compete with the resident microbiota and establish a colonization niche. Here we show that enterohemorrhagic E. coli (EHEC), and its surrogate murine infection model Citrobacter rodentium, sense galacturonic-acid to initiate a multi-layered program towards successful mammalian infection. Galacturonic-acid utilization as a carbon source aids the initial pathogen expansion. The main source of galacturonic-acid is dietary pectin, which is broken into galacturonic-acid by the prominent member of the microbiota, Bacteroides thetaiotamicron (Bt). This regulation occurs through the ExuR transcription factor. However, galacturonic-acid is also sensed as a signal through ExuR to modulate the expression of the genes encoding a molecular syringe known as a type three secretion system (T3SS) leading to infectious colitis and inflammation. Galacturonic-acid moonlights as a nutrient and a signal directing the exquisite microbiota-pathogen relationships within the GI tract. Importantly, this work highlights that differential dietary sugar availability impacts the relationship between the microbiota and enteric pathogens, as well as disease outcomes.

Draft Genome Sequence of Escherichia albertii Strain Mex-12/320a, Isolated from an Infant with Diarrhea and Harboring Virulence Genes Associated with Diarrheagenic Strains of Enteropathogenic Escherichia coli

Escherichia albertii is an emerging human enteropathogen. We report the draft genome sequence of E. albertii strain Mex-12/320a, isolated from an infant with diarrhea. The presence of the pathogenic island O122/IE6 and the nleA gene, previously found in diarrheagenic enteropathogenic Escherichia coli strains, suggests that E. albertii may cause acute diarrhea.