Gene l0017 encodes a second chaperone for EspA of enterohaemorrhagic Escherichia coli O157 : H7

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.

Contribution of glutaminases to glutamine metabolism and acid resistance in Lactobacillus reuteri and other vertebrate host adapted lactobacilli

The bacterial conversion of glutamine to glutamate is catalyzed by glutamine-amidotransferases or glutaminases. Glutamine deamination contributes to the formation of the bioactive metabolites glutamate, γ-aminobutyrate (GABA) and γ-glutamyl peptides, and to acid resistance. This study aimed to investigate the distribution of glutaminase(s) in lactobacilli, and to evaluate their contribution in L. reuteri to amino acid metabolism and acid resistance. Phylogenetic analysis of the glutaminases gls1, gls2 and gls3 in the genus Lactobacillus demonstrated that glutaminase is exclusively present in host-adapted species of lactobacilli. The disruption gls1, gls2 and gls3 in L. reuteri 100-23 had only a limited effect on the conversion of glutamine to glutamate, GABA, or γ-glutamyl peptides in sourdough. The disruption of all glutaminases in L. reuteri 100-23Δgls1Δgls2Δgls3 but not disruption of gls2 and gls3 eliminated the protective effect of glutamine on the survival of the strain at pH 2.5. Glutamine also enhanced acid resistance of L. reuteri 100-23ΔgadB and L. taiwanensis 107q, strains without glutamate decarboxylase activity. Taken together, the study demonstrates that glutaminases of lactobacilli do not contribute substantially to glutamine metabolism but enhance acid resistance. Their exclusive presence in host-adapted lactobacilli provides an additional link between the adaptation of lactobacilli to specific habitats and their functionality when used as probiotics and starter cultures.

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.

A Plant-Produced Candidate Subunit Vaccine Reduces Shedding of Enterohemorrhagic Escherichia coli in Ruminants

Enterohemorrhagic Escherichia coli (EHEC) are commonly present in the gastrointestinal tract of cattle and cause serious infectious disease in humans. Immunizing cattle against EHEC is a promising strategy to decrease the risk of food contamination; however, veterinary vaccines against EHEC such as Econiche have not been widely adopted by the agricultural industry, and have been discontinued, prompting the need for more cost-effective EHEC vaccines. The objective of this project is to develop a platform to produce plant-made antigens for oral vaccination of ruminants against EHEC. Five recombinant proteins were designed as vaccine candidates and expressed transiently in Nicotiana benthamiana and transplastomically in Nicotiana tabacum. Three of these EHEC proteins, NleA, Stx2b, and a fusion of EspA accumulated when transiently expressed. Transient protein accumulation was the highest when EHEC proteins were fused to an elastin-like polypeptide (ELP) tag. In the transplastomic lines, EspA accumulated up to 479 mg kg-1 in lyophilized leaf material. Sheep that were administered leaf tissue containing recombinant EspA shed less E. coli O157:H7 when challenged, as compared to control animals. These results suggest that plant-made, transgenic EspA has the potential to reduce EHEC shedding in ruminants.

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.

Co-expression with the Type 3 Secretion Chaperone CesT from Enterohemorrhagic E. coli Increases Accumulation of Recombinant Tir in Plant Chloroplasts

Type 3 secretion systems (T3SSs) are utilized by pathogenic Escherichia coli to infect their hosts and many proteins from these systems are affected by chaperones specific to T3SS-containing bacteria. Toward developing a recombinant vaccine against enterohaemorrhagic E. coli (EHEC), we expressed recombinant T3SS and related proteins from predominant EHEC serotypes in Nicotiana chloroplasts. Nicotiana benthamiana were transiently transformed to express chloroplast-targeted Tir, NleA, and EspD from the EHEC serotype O157:H7; a fusion of EspA proteins from serotypes O157:H7 and O26:H11; and a fusion of epitopes of Tir (Tir-ep) from serotypes O157:H7, O26:H11, O45:H2, and O111:H8. C-terminal GFP reporter fusion constructs were also developed and transiently expressed to confirm subcellular localization and quantify relative expression levels in situ. Recombinant proteins were co-expressed with chaperones specific to each T3SS protein with the goal of increasing their accumulation in the chloroplast. We found that co-expression with the chloroplast-targeted chaperone CesT significantly increases accumulation of recombinant Tir when the latter is either transiently expressed in the nucleus and targeted to the chloroplast of N. benthamiana or stably expressed in transplastomic Nicotiana tabacum. CesT also helped maintain higher levels of Tir:GFP fusion protein over time both in vivo and ex vivo, indicating that the favorable effect of CesT on accumulation of Tir is not specific to a single time point or to fresh material. By contrast, T3SS chaperones CesT, CesAB, CesD, and CesD2 did not increase accumulation of NleA:GFP, EspA:GFP, or EspD:GFP, which suggests dissimilar functioning of these chaperone-substrate combinations. CesT did not increase accumulation of Tir-ep:GFP, which may be due to the absence of the CesT binding domain from this fusion protein. The fusion to GFP improved accumulation of Tir-ep relative to the unfused protein, but not for the other recombinant proteins. These results emphasize the importance of native chaperones and stabilizing fusions as potential tools for the production of higher levels of recombinant proteins in plants; and may have implications for understanding interactions between T3SS chaperones and their substrates. In particular, our findings highlight the potential of T3SS chaperones to increase accumulation of recombinant T3SS proteins in heterologous systems.

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.

Protein interactions and regulation of EscA in enterohemorrhagic E. coli

Infections caused by enterohemorrhagic Escherichia coli (EHEC) can lead to diarrhea with abdominal cramps and sometimes are complicated by severe hemolytic uremic syndrome. EHEC secretes effector proteins into host cells through a type III secretion system that is composed of proteins encoded by a chromosomal island, locus for the enterocyte effacement (LEE). EspA is the major component of the filamentous structure connecting the bacteria and the host's cells. Synthesis and secretion of EspA must be carefully controlled since the protein is prone to polymerize. CesAB, CesA2, and EscL have been identified as being able to interact with EspA. Furthermore, the intracellular level of EspA declines when cesAB, cesA2, and escL are individually deleted. Here, we report a LEE gene named l0033, which also affects the intracellular level of EspA. We renamed l0033 as escA since its counterpart in enteropathogenic E. coli has been recently described. Similar to CesAB, EscL, and CesA2, EscA interacts with EspA and enhances the protein stability of EspA. However, EscA is also able to interact with inner membrane-associated EscL, CesA2, and EscN, but not with cytoplasmic CesAB. In terms of gene organizations, escA locates in LEE3. Expression of EscA is faithfully regulated via Mpc, the first gene product of LEE3. Since Mpc is tightly regulated to low level, we suggest that EscA is highly synchronized and critical to the process of escorting EspA to its final destination.

An O island 172 encoded RNA helicase regulates the motility of Escherichia coli O157:H7

Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 is a major cause of zoonotic food- and water-borne intestinal infections worldwide with clinical consequences ranging from mild diarrhoea to hemolytic uraemic syndrome. The genome of EHEC O157:H7 contains many regions of unique DNA that are referred to as O islands including the Shiga toxin prophages and pathogenicity islands encoding key virulence factors. However many of these O islands are of unknown function. In this study, genetic analysis was conducted on OI-172 which is a 44,434 bp genomic island with 27 open reading frames. Comparative genome analysis showed that O1-72 is a composite island with progressive gain of genes since O157:H7 evolved from its ancestral O55:H7. A partial OI-172 island was also found in 2 unrelated E. coli strains and 2 Salmonella strains. OI-172 encodes several putative helicases, one of which (Z5898) is a putative DEAH box RNA helicase. To investigate the function of Z5898, a deletion mutant (EDL933ΔZ5898) was constructed in the O157:H7 strain EDL933. Comparative proteomic analysis of the mutant with the wild-type EDL933 found that flagellin was down-regulated in the Z5898 mutant. Motility assay showed that EDL933ΔZ5898 migrated slower than the wild-type EDL933 and electron microscopy found no surface flagella. Quantitative reverse transcription PCR revealed that the fliC expression of EDL933ΔZ5898 was significantly lower while the expression of its upstream regulator gene, fliA, was not affected. Using a fliA and a fliC promoter - green fluorescent protein fusion contruct, Z5898 was found to affect only the fliC promoter activity. Therefore, Z5898 regulates the flagella based motility by exerting its effect on fliC. We conclude that OI-172 is a motility associated O island and hereby name it the MAO island.

Protein export according to schedule: architecture, assembly, and regulation of type III secretion systems from plant- and animal-pathogenic bacteria

Flagellar and translocation-associated type III secretion (T3S) systems are present in most gram-negative plant- and animal-pathogenic bacteria and are often essential for bacterial motility or pathogenicity. The architectures of the complex membrane-spanning secretion apparatuses of both systems are similar, but they are associated with different extracellular appendages, including the flagellar hook and filament or the needle/pilus structures of translocation-associated T3S systems. The needle/pilus is connected to a bacterial translocon that is inserted into the host plasma membrane and mediates the transkingdom transport of bacterial effector proteins into eukaryotic cells. During the last 3 to 5 years, significant progress has been made in the characterization of membrane-associated core components and extracellular structures of T3S systems. Furthermore, transcriptional and posttranscriptional regulators that control T3S gene expression and substrate specificity have been described. Given the architecture of the T3S system, it is assumed that extracellular components of the secretion apparatus are secreted prior to effector proteins, suggesting that there is a hierarchy in T3S. The aim of this review is to summarize our current knowledge of T3S system components and associated control proteins from both plant- and animal-pathogenic bacteria.

Contribution of glutamate decarboxylase in Lactobacillus reuteri to acid resistance and persistence in sourdough fermentation

Background

Acid stress impacts the persistence of lactobacilli in industrial sourdough fermentations, and in intestinal ecosystems. However, the contribution of glutamate to acid resistance in lactobacilli has not been demonstrated experimentally, and evidence for the contribution of acid resistance to the competitiveness of lactobacilli in sourdough is lacking. It was therefore the aim of this study to investigate the ecological role of glutamate decarboxylase in L. reuteri.

Results

A gene coding for a putative glutamate decarboxylase, gadB, was identified in the genome of L. reuteri 100-23. Different from the organization of genetic loci coding for glutamate decarboxylase in other lactic acid bacteria, gadB was located adjacent to a putative glutaminase gene, gls3. An isogenic deletion mutant, L. reuteri ∆gadB, was generated by a double crossover method. L. reuteri 100-23 but not L. reuteri ∆gadB converted glutamate to γ-aminobutyrate (GABA) in phosphate butter (pH 2.5). In sourdough, both strains converted glutamine to glutamate but only L. reuteri 100-23 accumulated GABA. Glutamate addition to phosphate buffer, pH 2.5, improved survival of L. reuteri 100-23 100-fold. However, survival of L. reuteri ∆gadB remained essentially unchanged. The disruption of gadB did not affect growth of L. reuteri in mMRS or in sourdough. However, the wild type strain L. reuteri 100-23 displaced L. reuteri ∆gadB after 5 cycles of fermentation in back-slopped sourdough fermentations.

Conclusions

The conversion of glutamate to GABA by L. reuteri 100-23 contributes to acid resistance and to competitiveness in industrial sourdough fermentations. The organization of the gene cluster for glutamate conversion, and the availability of amino acids in cereals imply that glutamine rather than glutamate functions as the substrate for GABA formation. The exceptional coupling of glutamine deamidation to glutamate decarboxylation in L. reuteri likely reflects adaptation to cereal substrates.

Functional characterization of SsaE, a novel chaperone protein of the type III secretion system encoded by Salmonella pathogenicity island 2

The type III secretion system (T3SS) encoded by Salmonella pathogenicity island 2 (SPI-2) is involved in systemic infection and intracellular replication of Salmonella enterica serovar Typhimurium. In this study, we investigated the function of SsaE, a small cytoplasmic protein encoded within the SPI-2 locus, which shows structural similarity to the T3SS class V chaperones. An S. enterica serovar Typhimurium ssaE mutant failed to secrete SPI-2 translocator SseB and SPI-2-dependent effector PipB proteins. Coimmunoprecipitation and mass spectrometry analyses using an SsaE-FLAG fusion protein indicated that SsaE interacts with SseB and a putative T3SS-associated ATPase, SsaN. A series of deleted and point-mutated SsaE-FLAG fusion proteins revealed that the C-terminal coiled-coil domain of SsaE is critical for protein-protein interactions. Although SseA was reported to be a chaperone for SseB and to be required for its secretion and stability in the bacterial cytoplasm, an sseA deletion mutant was able to secrete the SseB in vitro when plasmid-derived SseB was overexpressed. In contrast, ssaE mutant strains could not transport SseB extracellularly under the same assay conditions. In addition, an ssaE(I55G) point-mutated strain that expresses the SsaE derivative lacking the ability to form a C-terminal coiled-coil structure showed attenuated virulence comparable to that of an SPI-2 T3SS null mutant, suggesting that the coiled-coil interaction of SsaE is absolutely essential for the functional SPI-2 T3SS and for Salmonella virulence. Based on these findings, we propose that SsaE recognizes translocator SseB and controls its secretion via SPI-2 type III secretion machinery.

Post-transcriptional processing of the LEE4 operon in enterohaemorrhagic Escherichia coli

Enterohaemorrhagic Escherichia coli (EHEC) employs a type III secretion system (T3SS) to export translocator and effector proteins required for mucosal colonization. The T3SS is encoded in a pathogenicity island called the locus of enterocyte effacement (LEE) that is organized in five major operons, LEE1 to LEE5. LEE4 encodes a regulator of secretion (SepL), translocators (EspA, D and B), two chaperones (CesD2 and L0017), a T3SS component (EscF) and an effector protein (EspF). It was originally proposed that the esp transcript is transcribed from a promoter located at the end of sepL but other authors suggested that this transcript is the result of a post-transcriptional processing event. In this study, we established that the espADB mRNA is generated by post-transcriptional processing at the end of the sepL coding sequence. RNase E is the endonuclease involved in the cleavage, but the interaction of this enzyme with other proteins through its C-terminal half is dispensable. A putative transcription termination event in the cesD2 coding region would generate the 3' end of the transcript. Similar to what has been described for other processed transcripts, the cleavage of LEE4 seems a mechanism to differentially regulate SepL and Esp protein production.

Identification of a third EspA-binding protein that forms part of the type III secretion system of enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli utilizes a type III secretion system to deliver virulent effectors into cells. The secretion apparatus comprises a membrane basal body and an external needle complex of which EspA is the major component. An l0050-deletion (DeltaL50) mutation was found to impair type III secretion and bacterial adherence. These phenotypes and the localization of the gene product to the inner membrane support the hypothesis that L0050, renamed EscL, forms part of the secretion apparatus. Furthermore, in DeltaL50, the amount of EspA present within the cell lysate was found to have diminished, whereas the EspA co-cistron-expressed partner protein EspB remained unaffected. The decreased EspA level appeared to result from instability of the newly synthesized EspA protein in DeltaL50 rather than a decrease in EspA mRNA. Using both biochemical co-purification and a bacterial two-hybrid interaction system, we were able to conclude that EscL is a third protein that, in addition to CesAB and CesA2, interacts with EspA and enhances the stability of intracellular EspA.