The Tat Substrate SufI Is Critical for the Ability of Yersinia pseudotuberculosis To Cause Systemic Infection

The Assessment on Synergistic Activity of Ebselen and Silver Ion Against Yersinia pseudotuberculosis

Yersinia pseudotuberculosis is a foodborne zoonotic bacterium that is pathogenic to guinea pigs, rabbits, and mice. It also causes pseudotuberculosis in humans. However, it still lacked the scientific basis for control. Here, we found out that Ebselen (EbSe) exhibited synergistic antibacterial activity with silver nitrate (Ag⁺) against Y. pseudotuberculosis YpIII strain with high efficacy in vitro using UV-visible light absorption spectrum, 5,5’-dithiobis-(2-nitrobenzoic acid), laser scanning confocal microscope, flow cytometry, transmission electron microscopy and Western blotting assays. The depletion of total glutathione (GSH) amount and inhibition of thioredoxin reductase (TrxR) activity in thiol-dependent redox system revealed the destructiveness of EbSe-Ag⁺-caused intracellular oxidative stress. Furthermore, a YpIII-caused mice gastroenteritis model was constructed. EbSe-Ag⁺ significantly reduced bacterial loads with low toxicity. It also down-regulated the expression levels of interferon (IL)-1β and tumor necrosis factor-α, up-regulated the expression level of IL-10 on-site. All the in vivo results demonstrated the antibacterial activity and immune-modulatory property of EbSe-Ag⁺. Collectively, these results provided academic fundament for further analysis and development of EbSe-Ag⁺ as the antibacterial agents for pseudotuberculosis control.

The Rcs System Contributes to the Motility Defects of the Twin-Arginine Translocation System Mutant of Extraintestinal Pathogenic Escherichia coli

Flagellum-mediated bacterial motility is important for bacteria to take up nutrients, adapt to environmental changes, and establish infection. The twin-arginine translocation system (Tat) is an important protein export system, playing a critical role in bacterial physiology and pathogenesis. It has been observed for a long time that the Tat system is critical for bacterial motility. However, the underlying mechanism remains unrevealed. In this study, a comparative transcriptomics analysis was performed with extraintestinal pathogenic Escherichia coli (ExPEC), which identified a considerable number of genes differentially expressed when the Tat system was disrupted. Among them, a large proportion of flagellar biosynthesis genes showed downregulation, indicating that transcription regulation plays an important role in mediating the motility defects. We further identified three Tat substrate proteins, MdoD, AmiA, and AmiC, that were responsible for the nonmotile phenotype. The Rcs system was deleted in the Δtat, the ΔmdoD, and the ΔamiAΔamiC strains, which restored the motility of ΔmdoD and partially restored the motility of Δtat and ΔamiAΔamiC. The flagella were also observed in all of the ΔtatΔrcsDB, ΔmdoDΔrcsDB, and ΔamiAΔamiCΔrcsDB strains, but not in the Δtat, ΔmdoD, and ΔamiAΔamiC strains, by using transmission electron microscopy. Quantitative reverse transcription-PCR data revealed that the regulons of the Rcs system displayed differential expression in the tat mutant, indicating that the Rcs signaling was activated. Our results suggest that the Rcs system plays an important role in mediating the motility defects of the tat mutant of ExPEC. IMPORTANCE The Tat system is an important protein export system critical for bacterial physiology and pathogenesis. It has been observed for a long time that the Tat system is critical for bacterial motility. However, the underlying mechanism remains unrevealed. In this study, we combine transcriptomics analysis and bacterial genetics, which reveal that transcription regulation plays an important role in mediating the motility defects of the tat mutant of extraintestinal pathogenic Escherichia coli. The Tat substrate proteins responsible for the motility defects are identified. We further show that the Rcs system contributes to the motility suppression. We for the first time reveal the link between the Tat system and bacterial motility, which is important for understanding the physiological functions of the Tat system.

Developing Cyclic Peptomers as Broad-Spectrum Type III Secretion System Inhibitors in Gram-Negative Bacteria

Antibiotic-resistant bacteria are an emerging global health threat. New antimicrobials are urgently needed. The injectisome type III secretion system (T3SS), required by dozens of Gram-negative bacteria for virulence but largely absent from nonpathogenic bacteria, is an attractive antimicrobial target. We previously identified synthetic cyclic peptomers, inspired by the natural product phepropeptin D, that inhibit protein secretion through the Yersinia Ysc and Pseudomonas aeruginosa Psc T3SSs but do not inhibit bacterial growth. Here, we describe the identification of an isomer, 4EpDN, that is 2-fold more potent (50% inhibitory concentration [IC₅₀] of 4 μM) than its parental compound. Furthermore, 4EpDN inhibited the Yersinia Ysa and the Salmonella SPI-1 T3SSs, suggesting that this cyclic peptomer has broad efficacy against evolutionarily distant injectisome T3SSs. Indeed, 4EpDN strongly inhibited intracellular growth of Chlamydia trachomatis in HeLa cells, which requires the T3SS. 4EpDN did not inhibit the unrelated twin arginine translocation (Tat) system, nor did it impact T3SS gene transcription. Moreover, although the injectisome and flagellar T3SSs are evolutionarily and structurally related, the 4EpDN cyclic peptomer did not inhibit secretion of substrates through the Salmonella flagellar T3SS, indicating that cyclic peptomers broadly but specifically target the injectisome T3SS. 4EpDN reduced the number of T3SS needles detected on the surface of Yersinia pseudotuberculosis as detected by microscopy. Collectively, these data suggest that cyclic peptomers specifically inhibit the injectisome T3SS from a variety of Gram-negative bacteria, possibly by preventing complete T3SS assembly.

The Tat system and its dependent cell division proteins are critical for virulence of extra-intestinal pathogenic Escherichia coli

The twin-arginine translocation (Tat) system is involved in a variety of important bacterial physiological processes. Conserved among bacteria and crucial for virulence, the Tat system is deemed as a promising anti-microbial drug target. However, the mechanism of how the Tat system functions in bacterial pathogenesis has not been fully understood. In this study, we showed that the Tat system was critical for the virulence of an extra-intestinal pathogenic E. coli (ExPEC) strain PCN033. A total of 20 Tat-related mutant strains were constructed, and competitive infection assays were performed to evaluate the relative virulence of these mutants. The results demonstrated that several Tat substrate mutants, including the ΔsufI, ΔamiAΔamiC double mutant as well as each single mutant, ΔyahJ, ΔcueO, and ΔnapG, were significantly outcompeted by the WT strain, among which the ΔsufI and ΔamiAΔamiC strains showed the lowest competitive index (CI) value. Results of individual mouse infection assay, in vitro cell adhesion assay, whole blood bactericidal assay, and serum bactericidal assay further confirmed the virulence attenuation phenotype of the ΔsufI and ΔamiAΔamiC strains. Moreover, the two mutants displayed chained morphology in the log phase resembling the Δtat and were defective in stress response. Our results suggest that the Tat system and its dependent cell division proteins SufI, AmiA, and AmiC play critical roles during ExPEC pathogenesis.

The Twin-Arginine Translocation System Is Important for Stress Resistance and Virulence of Brucella melitensis

Brucella, the causative agent of brucellosis, is a stealthy intracellular pathogen that is highly pathogenic to a range of mammals, including humans. The twin-arginine translocation (Tat) pathway transports folded proteins across the cytoplasmic membrane and has been implicated in virulence in many bacterial pathogens. However, the roles of the Tat system and related substrates in Brucella remain unclear. We report here that disruption of Tat increases the sensitivity of Brucella melitensis M28 to the membrane stressor sodium dodecyl sulfate (SDS), indicating cell envelope defects, as well as to EDTA. In addition, mutating Tat renders M28 bacteria more sensitive to oxidative stress caused by H₂O₂ Further, loss of Tat significantly attenuates B. melitensis infection in murine macrophages ex vivo Using a mouse model for persistent infection, we demonstrate that Tat is required for full virulence of B. melitensis M28. Genome-wide in silico prediction combined with an in vivo amidase reporter assay indicates that at least 23 proteins are authentic Tat substrates, and they are functionally categorized into solute-binding proteins, oxidoreductases, cell envelope biosynthesis enzymes, and others. A comprehensive deletion study revealed that 6 substrates contribute significantly to Brucella virulence, including an l,d-transpeptidase, an ABC transporter solute-binding protein, and a methionine sulfoxide reductase. Collectively, our work establishes that the Tat pathway plays a critical role in Brucella virulence.

Transposon-insertion sequencing screens unveil requirements for EHEC growth and intestinal colonization

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) is an important food-borne pathogen that colonizes the colon. Transposon-insertion sequencing (TIS) was used to identify genes required for EHEC and E. coli K-12 growth in vitro and for EHEC growth in vivo in the infant rabbit colon. Surprisingly, many conserved loci contribute to EHEC's but not to K-12's growth in vitro. There was a restrictive bottleneck for EHEC colonization of the rabbit colon, which complicated identification of EHEC genes facilitating growth in vivo. Both a refined version of an existing analytic framework as well as PCA-based analysis were used to compensate for the effects of the infection bottleneck. These analyses confirmed that the EHEC LEE-encoded type III secretion apparatus is required for growth in vivo and revealed that only a few effectors are critical for in vivo fitness. Over 200 mutants not previously associated with EHEC survival/growth in vivo also appeared attenuated in vivo, and a subset of these putative in vivo fitness factors were validated. Some were found to contribute to efficient type-three secretion while others, including tatABC, oxyR, envC, acrAB, and cvpA, promote EHEC resistance to host-derived stresses. cvpA is also required for intestinal growth of several other enteric pathogens, and proved to be required for EHEC, Vibrio cholerae and Vibrio parahaemolyticus resistance to the bile salt deoxycholate, highlighting the important role of this previously uncharacterized protein in pathogen survival. Collectively, our findings provide a comprehensive framework for understanding EHEC growth in the intestine.

Disruption of the NlpD lipoprotein of the plague pathogen Yersinia pestis affects iron acquisition and the activity of the twin-arginine translocation system

We have previously shown that the cell morphogenesis NlpD lipoprotein is essential for virulence of the plague bacteria, Yersinia pestis. To elucidate the role of NlpD in Y. pestis pathogenicity, we conducted a whole-genome comparative transcriptome analysis of the wild-type Y. pestis strain and an nlpD mutant under conditions mimicking early stages of infection. The analysis suggested that NlpD is involved in three phenomena: (i) Envelope stability/integrity evidenced by compensatory up-regulation of the Cpx and Psp membrane stress-response systems in the mutant; (ii) iron acquisition, supported by modulation of iron metabolism genes and by limited growth in iron-deprived medium; (iii) activity of the twin-arginine (Tat) system, which translocates folded proteins across the cytoplasmic membrane. Virulence studies of Y. pestis strains mutated in individual Tat components clearly indicated that the Tat system is central in Y. pestis pathogenicity and substantiated the assumption that NlpD essentiality in iron utilization involves the activity of the Tat system. This study reveals a new role for NlpD in Tat system activity and iron assimilation suggesting a modality by which this lipoprotein is involved in Y. pestis pathogenesis.

We have previously shown that the NlpD lipoprotein, which is involved in the regulation of cell morphogenesis, is essential for virulence of the plague bacteria, Yersinia pestis. To uncover the role of NlpD in Y. pestis pathogenicity, we conducted a whole-genome comparative transcriptome analysis as well as phenotypic and virulence evaluation analyses of the nlpD and related mutants. The study reveals a new role for the Y. pestis NlpD lipoprotein in iron assimilation and Tat system activity.

Tat-exported peptidoglycan amidase-dependent cell division contributes to Salmonella Typhimurium fitness in the inflamed gut

Salmonella enterica serovar Typhimurium (S. Tm) is a cause of food poisoning accompanied with gut inflammation. Although mucosal inflammation is generally thought to be protective against bacterial infection, S. Tm exploits the inflammation to compete with commensal microbiota, thereby growing up to high densities in the gut lumen and colonizing the gut continuously at high levels. However, the molecular mechanisms underlying the beneficial effect of gut inflammation on S. Tm competitive growth are poorly understood. Notably, the twin-arginine translocation (Tat) system, which enables the transport of folded proteins outside bacterial cytoplasm, is well conserved among many bacterial pathogens, with Tat substrates including virulence factors and virulence-associated proteins. Here, we show that Tat and Tat-exported peptidoglycan amidase, AmiA- and AmiC-dependent cell division contributes to S. Tm competitive fitness advantage in the inflamed gut. S. Tm tatC or amiA amiC mutants feature a gut colonization defect, wherein they display a chain form of cells. The chains are attributable to a cell division defect of these mutants and occur in inflamed but not in normal gut. We demonstrate that attenuated resistance to bile acids confers the colonization defect on the S. Tm amiA amiC mutant. In particular, S. Tm cell chains are highly sensitive to bile acids as compared to single or paired cells. Furthermore, we show that growth media containing high concentrations of NaCl and sublethal concentrations of antimicrobial peptides induce the S. Tm amiA amiC mutant chain form, suggesting that gut luminal conditions such as high osmolarity and the presence of antimicrobial peptides impose AmiA- and AmiC-dependent cell division on S. Tm. Together, our data indicate that Tat and the Tat-exported amidases, AmiA and AmiC, are required for S. Tm luminal fitness in the inflamed gut, suggesting that these proteins might comprise effective targets for novel antibacterial agents against infectious diarrhea.

For proteins residing outside the bacterial cytoplasm, transport is an essential step for adequate function. The twin-arginine translocation (Tat) system enables the transport of folded proteins across the cytoplasmic membrane in prokaryotes. It has recently become clear that this system plays a pivotal role in the detrimental effects of many bacterial pathogens, suggesting Tat as a novel therapeutic target against their infection. In particular, the bacterial enteropathogen Salmonella Typhimurium causes foodborne diarrhea by colonizing the gut interior space. Here, we describe that the S. Typhimurium Tat system contributes to intestinal infection by facilitating colonization of the gut by this pathogen. We also identify that two Tat-exported enzymes, peptidoglycan amidase AmiA and AmiC, are responsible for the Tat-dependent colonization. S. Typhimurium strains having nonfunctional Tat systems or lacking these enzymes undergo filamentous growth in the gut interior owing to defective cell division. Notably, this chain form of S. Typhimurium cells is highly sensitive to bile acids, rendering it less competitive with native bacteria in the gut. The data presented here suggest that the Tat system and associated amidases may comprise promising therapeutic targets for Salmonella diarrhea, and that controlling bacterial shape might be new strategy for regulating intestinal enteropathogen infection.

Citrobacter freundii fitness during bloodstream infection

Sepsis resulting from microbial colonization of the bloodstream is a serious health concern associated with high mortality rates. The objective of this study was to define the physiologic requirements of Citrobacter freundii in the bloodstream as a model for bacteremia caused by opportunistic Gram-negative pathogens. A genetic screen in a murine host identified 177 genes that contributed significantly to fitness, the majority of which were broadly classified as having metabolic or cellular maintenance functions. Among the pathways examined, the Tat protein secretion system conferred the single largest fitness contribution during competition infections and a putative Tat-secreted protein, SufI, was also identified as a fitness factor. Additional work was focused on identifying relevant metabolic pathways for bacteria in the bloodstream environment. Mutations that eliminated the use of glucose or mannitol as carbon sources in vitro resulted in loss of fitness in the murine model and similar results were obtained upon disruption of the cysteine biosynthetic pathway. Finally, the conservation of identified fitness factors was compared within a cohort of Citrobacter bloodstream isolates and between Citrobacter and Serratia marcescens, the results of which suggest the presence of conserved strategies for bacterial survival and replication in the bloodstream environment.