Magnesium Sensing Regulates Intestinal Colonization of Enterohemorrhagic Escherichia coli O157:H7

Genomic island-encoded LmiA regulates acid resistance and biofilm formation in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is an important intestinal pathogen that causes severe foodborne diseases. We previously demonstrated that the genomic island-encoded regulator LmiA activates the locus of enterocyte effacement (LEE) genes to promote EHEC O157:H7 adherence and colonization in the host intestine. However, whether LmiA is involved in the regulation of any other biological processes in EHEC O157:H7 remains largely unexplored. Here, we compared global gene expression differences between the EHEC O157:H7 wild-type strain and an lmiA mutant strain using RNA-seq technology. Genes whose expression was affected by LmiA were identified and classified using the Cluster of Orthologous Groups (COG) database. Specifically, the expression of acid resistance genes (including gadA, gadB, and gadC) was significantly downregulated, whereas the transcript levels of biofilm-related genes (including Z_RS00105, yadN, Z_RS03020, and fdeC) were increased, in the ΔlmiA mutant compared to the EHEC O157:H7 wild-type strain. Further investigation revealed that LmiA enhanced the acid resistance of EHEC O157:H7 by directly activating the transcription of gadA and gadBC. In contrast, LmiA reduced EHEC O157:H7 biofilm formation by indirectly repressing the expression of biofilm-related genes. Furthermore, LmiA-mediated regulation of acid resistance and biofilm formation is highly conserved and widespread among EHEC and enteropathogenic E. coli (EPEC). Our findings provide essential insight into the regulatory function of LmiA in EHEC O157:H7, particularly its role in regulating acid resistance and biofilm formation.

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.

Key roles of two-component systems in intestinal signal sensing and virulence regulation in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is a foodborne pathogen that infects humans by colonizing the large intestine. Upon reaching the large intestine, EHEC mediates local signal recognition and the transcriptional regulation of virulence genes to promote adherence and colonization in a highly site-specific manner. Two-component systems (TCSs) represent an important strategy used by EHEC to couple external stimuli with the regulation of gene expression, thereby allowing EHEC to rapidly adapt to changing environmental conditions. An increasing number of studies published in recent years have shown that EHEC senses a variety of host- and microbiota-derived signals present in the human intestinal tract and coordinates the expression of virulence genes via multiple TCS-mediated signal transduction pathways to initiate the disease-causing process. Here, we summarize how EHEC detects a wide range of intestinal signals and precisely regulates virulence gene expression through multiple signal transduction pathways during the initial stages of infection, with a particular emphasis on the key roles of TCSs. This review provides valuable insights into the importance of TCSs in EHEC pathogenesis, which has relevant implications for the development of antibacterial therapies against EHEC infection.

Vibrio cholerae virulence is blocked by chitosan oligosaccharide-mediated inhibition of ChsR activity

Vibrio cholerae causes cholera, an important cause of death worldwide. A fuller understanding of how virulence is regulated offers the potential for developing virulence inhibitors, regarded as efficient therapeutic alternatives for cholera treatment. Here we show using competitive infections of wild-type and mutant bacteria that the regulator of chitosan utilization, ChsR, increases V. cholerae virulence in vivo. Mechanistically, RNA sequencing, chromatin immunoprecipitation with sequencing and molecular biology approaches revealed that ChsR directly upregulated the expression of the virulence regulator, TcpP, which promoted expression of the cholera toxin and the toxin co-regulated pilus, in response to low O2 levels in the small intestine. We also found that chitosan degradation products inhibit the ChsR-tcpP promoter interaction. Consistently, administration of chitosan oligosaccharide, particularly when delivered via sodium alginate microsphere carriers, reduced V. cholerae intestinal colonization and disease severity in mice by blocking the chsR-mediated pathway. These data reveal the potential of chitosan oligosaccharide as supplemental therapy for cholera treatment and prevention.

Innovative nebulization delivery of lipid nanoparticle-encapsulated siRNA: a therapeutic advance for Staphylococcus aureus-induced pneumonia

BACKGROUND

Integrin α5β1 plays a crucial role in the invasion of nonphagocytic cells by Staphylococcus aureus (S. aureus), thereby facilitating infection development. Lipid nanoparticles (LNPs) serve as an effective vehicle for delivering small interfering ribonucleic acids (siRNA) that represent a method to knockdown integrin α5β1 in the lungs through nebulization, thereby potentially mitigating the severity of S. aureus pneumonia. The aim of this study was to harness LNP-mediated targeting to precisely knockdown integrin α5β1, thus effectively addressing S. aureus-induced pneumonia.

METHODS

C57 mice (8 week-old females) infected with S. aureus via an intratracheal nebulizing device were utilized for the experiments. The LNPs were synthesized via microfluidic mixing and characterized by their size, polydispersity index, and encapsulation efficiency. Continuous intratracheal nebulization was employed for consistent siRNA administration, with the pulmonary function metrics affirming biosafety. The therapeutic efficacy of LNP-encapsulated siRNAs against pneumonia was assessed through western blotting, bacterial count measurement, quantitative polymerase chain reaction, and histological analyses.

RESULTS

LNPs, which have an onion-like structure, retained integrity post-nebulization, ensuring prolonged siRNA stability and in vivo safety. Intratracheal nebulization delivery markedly alleviated the severity of S. aureus-induced pneumonia, as indicated by reduced bacterial load and bolstered immune response, thereby localizing the infection to the lungs and averting systemic dissemination.

CONCLUSIONS

Intratracheal nebulization of LNP-encapsulated siRNAs targeting integrin α5β1 significantly diminished the S. aureus-mediated cellular invasion and disease progression in the lungs, presenting a viable therapeutic approach for respiratory infections.

Association Between Serum Magnesium Levels and Glycemic Control in Type 2 Diabetes

Introduction

Serum magnesium is a crucial mineral within the human body. It is imperative for diabetic patients to maintain optimal serum magnesium levels. We focus on the relationship between glycemic control and serum magnesium in type 2 diabetes mellitus (T2DM).

Methods

The retrospective, observational, cross-sectional study comprised 1694 patients recruited from the People's Hospital of Yuxi. Fasting blood samples were collected for analysis, accompanied by the recording of participants' demographic characteristics. Patients were categorized into two groups based on whether their glycosylated hemoglobin (HbA1c) levels < 7%. A t-test was employed to identify significant differences between the two groups. Correlation coefficients were calculated using Pearson's method. A Logistic regression analysis was conducted to assess the association between variables and glycemic control. A linear regression analysis was performed to assess the relationship between serum magnesium levels and HbA1c.

Results

Patients with poor glycemic control exhibited elevated age, low-density lipoprotein (LDL-C), fasting plasma glucose (FPG), and homeostasis model assessment (HOMA-IR) compared to those with good glycemic control (P < 0.001). Additionally, total cholesterol (TC) levels were significantly higher in patients with poor glycemic control. Conversely, high-density lipoprotein (HDL-C) and serum magnesium levels were lower in patients with poor glycemic control. Serum magnesium levels exhibited negative correlations with HOMA-IR (r = -0.05, P < 0.05), HbA1c (r = -0.29, P < 0.05), and FPG (r = -0.20, P < 0.05). Moreover, serum magnesium was significantly associated with reduced odds of glycemic control (OR = 0.0005, 95% CI 0.0001-0.0027, P < 0.001).

Conclusion

The serum magnesium level in patients with T2DM is closely associated with glycemic control.

Adaptive attenuation of virulence in hypervirulent carbapenem-resistant Klebsiella pneumoniae

The emergence of nosocomial infections caused by hypervirulent and carbapenem-resistant K. pneumoniae (hv-CRKP) has become a significant public health challenge. The genetic traits of virulence and resistance plasmids in hv-CRKP have been extensively studied; however, research on the adaptive evolution strategies of clinical strains inside the host was scarce. This study aimed to understand the effects of antibiotic treatment on the phenotype and genotype characteristics of hv-CRKP. We investigated the evolution of hv-CRKP strains isolated from the same patient to elucidate the transition between hospital invasion and colonization. A comparative genomics analysis was performed to identify single nucleotide polymorphisms in the rmpA promoter. Subsequent validation through RNA-seq and gene deletion confirmed that distinct rmpA promoter sequences exert control over the mucoid phenotype. Additionally, biofilm experiments, cell adhesion assays, and animal infection models were conducted to illuminate the influence of rmpA promoter diversity on virulence changes. We demonstrated that the P12T and P11T promoters of rmpA possess strong activity, which leads to the evolution of CRKP into infectious and virulent strains. Meanwhile, the specific sequence of polyT motifs in the rmpA promoter led to a decrease in the lethality of hv-CRKP and enhanced cell adhesion and colonization. To summarize, the rmpA promoter of hv-CRKP is utilized to control capsule production, thereby modifying pathogenicity to better suit the host's ecological environment.IMPORTANCEThe prevalence of hospital-acquired illness caused by hypervirulent carbapenem-resistant Klebsiella pneumoniae (hv-CRKP) is significant, leading to prolonged antibiotic treatment. However, there are few reports on the phenotypic changes of hv-CRKP in patients undergoing antibiotic treatment. We performed a comprehensive examination of the genetic evolutionary traits of hv-CRKP obtained from the same patient and observed variations in the promoter sequences of the virulence factor rmpA. The strong activity of the promoter sequences P11T and P12T enhances the consistent production of capsule polysaccharides, resulting in an invasive strain. Conversely, weak promoter activity of P9T and P10T is advantageous for exposing pili, hence improving bacterial cell attachment ability and facilitating bacterial colonization. This finding also explains the confusion of some clinical strains carrying wild-type rmpA but exhibiting a low mucoid phenotype. This adaptive alteration facilitates the dissemination of K. pneumoniae within the hospital setting.

Oral magnesium prevents acetaminophen-induced acute liver injury by modulating microbial metabolism

Acetaminophen overuse is a common cause of acute liver failure (ALF). During ALF, toxins are metabolized by enzymes such as CYP2E1 and transformed into reactive species, leading to oxidative damage and liver failure. Here, we found that oral magnesium (Mg) alleviated acetaminophen-induced ALF through metabolic changes in gut microbiota that inhibit CYP2E1. The gut microbiota from Mg-supplemented humans prevented acetaminophen-induced ALF in mice. Mg exposure modulated Bifidobacterium metabolism and enriched indole-3-carboxylic acid (I3C) levels. Formate C-acetyltransferase (pflB) was identified as a key Bifidobacterium enzyme involved in I3C generation. Accordingly, a Bifidobacterium pflB knockout showed diminished I3C generation and reduced the beneficial effects of Mg. Conversely, treatment with I3C or an engineered bacteria overexpressing Bifidobacterium pflB protected against ALF. Mechanistically, I3C bound and inactivated CYP2E1, thus suppressing formation of harmful reactive intermediates and diminishing hepatocyte oxidative damage. These findings highlight how interactions between Mg and gut microbiota may help combat ALF.

CobB-mediated deacetylation of the chaperone CesA regulates Escherichia coli O157:H7 virulence

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a common food-borne pathogen that can cause acute diseases. Lysine acetylation is a post-translational modification (PTM) that occurs in various prokaryotes and is regulated by CobB, the only deacetylase found in bacteria. Here, we demonstrated that CobB plays an important role in the virulence of EHEC O157:H7 and that deletion of cobB significantly decreased the intestinal colonization ability of bacteria. Using acetylation proteomic studies, we systematically identified several proteins that could be regulated by CobB in EHEC O157:H7. Among these CobB substrates, we found that acetylation at the K44 site of CesA, a chaperone for the type-III secretion system (T3SS) translocator protein EspA, weakens its binding to EspA, thereby reducing the stability of this virulence factor; this PTM ultimately attenuating the virulence of EHEC O157:H7. Furthermore, we showed that deacetylation of the K44 site, which is deacetylated by CobB, promotes the interaction between CesA and EspA, thereby increasing bacterial virulence in vitro and in animal experiments. In summary, we showed that acetylation influences the virulence of EHEC O157:H7, and uncovered the mechanism by which CobB contributes to bacterial virulence based on the regulation of CesA deacetylation.

Lcn2 deficiency accelerates the infection of Escherichia coli O157:H7 by disrupting the intestinal barrier function

Bacterial infections result in intestinal inflammation and injury, which affects gut health and nutrient absorption. Lipocalin 2 (Lcn2) is a protein that reacts to microbial invasion, inflammatory responses, and tissue damage. However, it remains unclear whether Lcn2 has a protective effect against bacterial induced intestinal inflammation. Therefore, this study endeavors to investigate the involvement of Lcn2 in the intestinal inflammation of mice infected with Enterohemorrhagic Escherichia coli O157:H7 (E. coli O157:H7). Lcn2 knockout (Lcn2-/-) mice were used to evaluate the changes of inflammatory responses. Lcn2 deficiency significantly exacerbated clinical symptoms of E. coli O157:H7 infection by reducing body weight and encouraging bacterial colonization of. Compared to infected wild type mice, infected Lcn2-/- mice had significantly elevated levels of pro-inflammatory cytokines in serum and ileum, including interleukin (IL)-6, IL-1β, and tumor necrosis factor-α (TNF-α), as well as severe villi destruction in the jejunum. Furthermore, Lcn2 deficiency aggravated intestinal barrier degradation by significantly reducing the expression of tight junction proteins occludin and claudin 1, the content of myeloperoxidase (MPO) in the ileum, and the number of goblet cells in the colon. Our findings indicated that Lcn2 could alleviate inflammatory damage caused by E. coli O157:H7 infection in mice by enhancing intestinal barrier function.

Enterohaemorrhagic E. coli utilizes host- and microbiota-derived L-malate as a signaling molecule for intestinal colonization

The mammalian gastrointestinal tract is a complex environment that hosts a diverse microbial community. To establish infection, bacterial pathogens must be able to compete with the indigenous microbiota for nutrients, as well as sense the host environment and modulate the expression of genes essential for colonization and virulence. Here, we found that enterohemorrhagic Escherichia coli (EHEC) O157:H7 imports host- and microbiota-derived L-malate using the DcuABC transporters and converts these substrates into fumarate to fuel anaerobic fumarate respiration during infection, thereby promoting its colonization of the host intestine. Moreover, L-malate is important not only for nutrient metabolism but also as a signaling molecule that activates virulence gene expression in EHEC O157:H7. The complete virulence-regulating pathway was elucidated; the DcuS/DcuR two-component system senses high L-malate levels and transduces the signal to the master virulence regulator Ler, which in turn activates locus of enterocyte effacement (LEE) genes to promote EHEC O157:H7 adherence to epithelial cells of the large intestine. Disruption of this virulence-regulating pathway by deleting either dcuS or dcuR significantly reduced colonization by EHEC O157:H7 in the infant rabbit intestinal tract; therefore, targeting these genes and altering physiological aspects of the intestinal environment may offer alternatives for EHEC infection treatment.

The regulation of bacterial two-partner secretion systems

Two-partner secretion (TPS) systems, also known as Type Vb secretion systems, allow the translocation of effector proteins across the outer membrane of Gram-negative bacteria. By secreting different classes of effectors, including cytolysins and adhesins, TPS systems play important roles in bacterial pathogenesis and host interactions. Here, we review the current knowledge on TPS systems regulation and highlight specific and common regulatory mechanisms across TPS functional classes. We discuss in detail the specific regulatory networks identified in various bacterial species and emphasize the importance of understanding the context-dependent regulation of TPS systems. Several regulatory cues reflecting host environment during infection, such as temperature and iron availability, are common determinants of expression for TPS systems, even across relatively distant species. These common regulatory pathways often affect TPS systems across subfamilies with different effector functions, representing conserved global infection-related regulatory mechanisms.

Enterohemorrhagic Escherichia coli senses microbiota-derived nicotinamide to increase its virulence and colonization in the large intestine

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a foodborne pathogen that specifically colonizes and infects the human large intestine. EHEC O157:H7 engages intricate regulatory pathways to detect host intestinal signals and regulate virulence-related gene expression during colonization and infection. However, the overall EHEC O157:H7 virulence regulatory network in the human large intestine remains incompletely understood. Here, we report a complete signal regulatory pathway where the EvgSA two-component system responds to high-nicotinamide levels produced by microbiota in the large intestine and directly activates loci of enterocyte effacement genes to promote EHEC O157:H7 adherence and colonization. This EvgSA-mediated nicotinamide signaling regulatory pathway is conserved and widespread among several other EHEC serotypes. Moreover, disruption of this virulence-regulating pathway by the deletion of evgS or evgA significantly decreased EHEC O157:H7 adherence and colonization in the mouse intestinal tract, indicating that these genes could be potential targets for the development of new therapeutics for EHEC O157:H7 infection.

Salmonella Invasion Is Controlled by Competition among Intestinal Chemical Signals

The intestine is a complex, ever-changing environment replete with an array of signaling molecules. To colonize such a complex organ, pathogens have adapted to utilize specific cues from the local environment to intricately regulate the expression of their virulence determinants. Salmonella preferentially colonizes the distal ileum, a niche enriched in the metabolite formic acid. Here, we show that the relatively higher concentration of this metabolite in the distal ileum prevents other signals from repressing Salmonella invasion in that region. We show that imported and unmetabolized formic acid functions as a cytoplasmic signal that competitively binds to HilD, the master transcriptional regulator of Salmonella invasion, thus preventing repressive fatty acids from binding to the protein. This results in an increased lifetime of HilD and subsequent derepression of invasion genes. This study demonstrates an important mechanism by which Salmonella utilizes competition among signals in the gut to its advantage as a pathogen. IMPORTANCE Enteric pathogens acutely sense their environment for signals to regulate their virulence functions. We demonstrate here that the enteric pathogen Salmonella utilizes the competition among certain regional intestinal constituents to modulate its virulence determinants in that region. We show that the high concentration of formic acid in the ileum outcompetes other signals and triggers the activation of virulence genes in the ileum. This study shows a delicate spatial and temporal mechanism by which enteric pathogens may utilize the competition among environmental cues to optimize their pathogenicity.

Synthetic bacterial therapies for intestinal diseases based on quorum- sensing circuits

Bacterial therapy has become a key strategy against intestinal infectious diseases in recent years. Moreover, regulating the gut microbiota through traditional fecal microbiota transplantation and supplementation of probiotics faces controllability, efficacy, and safety challenges. The infiltration and emergence of synthetic biology and microbiome provide an operational and safe treatment platform for live bacterial biotherapies. Synthetic bacterial therapy can artificially manipulate bacteria to produce and deliver therapeutic drug molecules. This method has the advantages of solid controllability, low toxicity, strong therapeutic effects, and easy operation. As an essential tool for dynamic regulation in synthetic biology, quorum sensing (QS) has been widely used for designing complex genetic circuits to control the behavior of bacterial populations and achieve predefined goals. Therefore, QS-based synthetic bacterial therapy might become a new direction for the treatment of diseases. The pre-programmed QS genetic circuit can achieve a controllable production of therapeutic drugs on particular ecological niches by sensing specific signals released from the digestive system in pathological conditions, thereby realizing the integration of diagnosis and treatment. Based on this as well as the modular idea of synthetic biology, QS-based synthetic bacterial therapies are divided into an environmental signal sensing module (senses gut disease physiological signals), a therapeutic molecule producing module (plays a therapeutic role against diseases), and a population behavior regulating module (QS system). This review article summarized the structure and function of these three modules and discussed the rational design of QS gene circuits as a novel intervention strategy for intestinal diseases. Moreover, the application prospects of QS-based synthetic bacterial therapy were summarized. Finally, the challenges faced by these methods were analyzed to make the targeted recommendations for developing a successful therapeutic strategy for intestinal diseases.

Heat-Stable Enterotoxin Secretions Assessed via ICP-MS Reveal Iron-Mediated Regulation of Virulence in CFA/I- and CS6-Expressing ETEC Isolates

Enterotoxigenic Escherichia coli (ETEC) are a significant cause of childhood diarrhea in low-resource settings. ETEC are defined by the production of heat-stable enterotoxin (ST) and/or heat-labile enterotoxin (LT), which alter intracellular cyclic nucleotide signaling and cause the secretion of water and electrolytes into the intestinal lumen. ETEC take cues from chemicals (e.g., glycans, bile salts, and solutes) that may be liberated following enterotoxin activity to recognize entrance into the host. ETEC then alter the expression of surface adhesins called colonization factors (CFs) to attach to the intestinal epithelium, proliferate, and cause disease. Here, we used an in vivo model of oral ST intoxication to determine its impact on luminal ion concentrations via ICP-MS. We also used functional assays, including Western blots, qPCR, and toxin activity assays, to assess the impact of luminal ion flux on CF and toxin expression. Finally, we assessed ETEC strains with CFs CFA/I or CS6 in a streptomycin mouse model of ETEC colonization. ST causes rapid and significant increases in luminal chloride but significant decreases in luminal magnesium and iron. We confirmed that increased sodium chloride suppresses CFA/I production in ETEC H10407 but does not affect CS6 production in ETEC 214-4. CFA/I production in ETEC H10407 is increased when magnesium becomes limiting, although it does not affect CS6 production in ETEC 214-4. Iron restriction via deferoxamine induces CFA/I expression in ETEC H10407 but not CS6 expression in ETEC 214-4. We demonstrate that ST production is suppressed via iron restriction in H10407, 214-4, and over 50 other ETEC clinical isolates. Lastly, we demonstrate that the iron restriction of mice using oral deferoxamine pre-treatment extends the duration of ETEC H10407 (CFA/I+) fecal shedding while accelerating ETEC 214-4 (CS6+) fecal shedding. Combined, these data suggest that enterotoxins modulate luminal ion flux to influence ETEC virulence including toxin and CF production.

A Novel Role of the Two-Component System Response Regulator UvrY in Enterohemorrhagic Escherichia coli O157:H7 Pathogenicity Regulation

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is an important human pathogen causing severe diseases, such as hemorrhagic colitis and lethal hemolytic uremic syndrome. The signal-sensing capability of EHEC O157:H7 at specific host colonization sites via different two-component systems (TCSs) is closely related to its pathogenicity during infection. However, the types of systems involved and the regulatory mechanisms are not fully understood. Here, we investigated the function of the TCS BarA/UvrY regulator UvrY in the pathogenicity regulation of EHEC O157:H7. Our results showed that UvrY acts as a positive regulator of EHEC O157:H7 for cellular adherence and mouse colonization through the transcriptional activation of the locus for enterocyte effacement (LEE) pathogenic genes. Furthermore, this regulation is mediated by the LEE island master regulator, Ler. Our results highlight the significance of UvrY in EHEC O157:H7 pathogenicity and underline the unknown importance of BarA/UvrY in colonization establishment and intestinal adaptability during infection.

Nutritional immunity: the battle for nutrient metals at the host-pathogen interface

Trace metals are essential micronutrients required for survival across all kingdoms of life. From bacteria to animals, metals have critical roles as both structural and catalytic cofactors for an estimated third of the proteome, representing a major contributor to the maintenance of cellular homeostasis. The reactivity of metal ions engenders them with the ability to promote enzyme catalysis and stabilize reaction intermediates. However, these properties render metals toxic at high concentrations and, therefore, metal levels must be tightly regulated. Having evolved in close association with bacteria, vertebrate hosts have developed numerous strategies of metal limitation and intoxication that prevent bacterial proliferation, a process termed nutritional immunity. In turn, bacterial pathogens have evolved adaptive mechanisms to survive in conditions of metal depletion or excess. In this Review, we discuss mechanisms by which nutrient metals shape the interactions between bacterial pathogens and animal hosts. We explore the cell-specific and tissue-specific roles of distinct trace metals in shaping bacterial infections, as well as implications for future research and new therapeutic development.

Protocol to detect nucleotide-protein interaction in vitro using a non-radioactive competitive electrophoretic mobility shift assay

Electrophoretic mobility shift assay (EMSA) is a classical and popular approach for DNA/RNA protein-binding affinity detection in vitro. This protocol describes a competitive EMSA assay using digoxigenin (DIG)-labeled probe, which solves the safety issues and limitations attributed to the short lifespan of the ³²P-radiolabeled DNA probe. We detail steps for DNA probe preparation, protein-DNA mixture coincubation, EMSA, and competitive EMSA process. We optimize the standard DIG-ddUTP-labeling EMSA protocol to high sensitivity with reproducible results. For complete details on the use and execution of this protocol, please refer to Feng et al. (2022).

PNPase and RhlB Interact and Reduce the Cellular Availability of Oxidized RNA in Deinococcus radiodurans

8-Oxo-7,8-dihydroguanine (8-oxoG) is a major RNA modification caused by oxidative stresses and has been implicated in carcinogenesis, neurodegeneration, and aging. Several RNA-binding proteins have been shown to have a binding preference for 8-oxoG-modified RNA in eukaryotes and protect cells from oxidative stress. To date, polynucleotide phosphorylase (PNPase) is one of the most well-characterized proteins in bacteria that recognize 8-oxoG-modified RNA, but how PNPase cooperates with other proteins to process oxidized RNA is still unclear. Here, we use RNA affinity chromatography and mass spectrometry to search for proteins that preferably bind 8-oxoG-modified RNA in Deinococcus radiodurans, an extremophilic bacterium with extraordinary resistance to oxidative stresses. We identified four proteins that preferably bind to oxidized RNA: PNPase (DR_2063), DEAD box RNA helicase (DR_0335/RhlB), ribosomal protein S1 (DR_1983/RpsA), and transcriptional termination factor (DR_1338/Rho). Among these proteins, PNPase and RhlB exhibit high-affinity binding to 8-oxoG-modified RNA in a dose-independent manner. Deletions of PNPase and RhlB caused increased sensitivity of D. radiodurans to oxidative stress. We further showed that PNPase and RhlB specifically reduce the cellular availability of 8-oxoG-modified RNA but have no effect on oxidized DNA. Importantly, PNPase directly interacts with RhlB in D. radiodurans; however, no additional phenotypic effect was observed for the double deletion of pnp and rhlB compared to the single deletions. Overall, our findings suggest the roles of PNPase and RhlB in targeting 8-oxoG-modified RNAs and thereby constitute an important component of D. radiodurans resistance to oxidative stress. IMPORTANCE Oxidative RNA damage can be caused by oxidative stress, such as hydrogen peroxide, ionizing radiation, and antibiotic treatment. 8-oxo-7,8-dihydroguanine (8-oxoG), a major type of oxidized RNA, is highly mutagenic and participates in a variety of disease occurrences and development. Although several proteins have been identified to recognize 8-oxoG-modified RNA, the knowledge of how RNA oxidative damage is controlled largely remains unclear, especially in nonmodel organisms. In this study, we identified four RNA binding proteins that show higher binding affinity to 8-oxoG-modified RNA compared to unmodified RNA in the extremophilic bacterium Deinococcus radiodurans, which can endure high levels of oxidative stress. Two of the proteins, polynucleotide phosphorylase (PNPase) and DEAD-box RNA helicase (RhlB), interact with each other and reduce the cellular availability of 8-oxoG-modified RNA under oxidative stress. As such, this work contributes to our understanding of how RNA oxidation is influenced by RNA binding proteins in bacteria.

Regulation of flagellar motility and biosynthesis in enterohemorrhagic Escherichia coli O157:H7

ABSTARCTEnterohemorrhagic Escherichia coli (EHEC) O157:H7 is a human pathogen that causes a variety of diseases, such as hemorrhagic colitis and lethal hemolytic uremic syndrome. Flagellum-dependent motility plays diverse roles in the pathogenesis of EHEC O157:H7, including its migration to an optimal host site, adherence and colonization, survival at the infection site, and post-infection dispersal. However, it is very expensive for cellular economy in terms of the number of genes and the energy required for flagellar biosynthesis and functioning. Furthermore, the flagellar filament bears strong antigenic properties that induce a strong host immune response. Consequently, the flagellar gene expression and biosynthesis are highly regulated to occur at the appropriate time and place by different regulatory influences. The present review focuses on the regulatory mechanisms of EHEC O157:H7 motility and flagellar biosynthesis, especially in terms of flagellar gene regulation by environmental factors, regulatory proteins, and small regulatory RNAs.

Escherichia coli 0157:H7 virulence factors and the ruminant reservoir

PURPOSE OF REVIEW

This review updates recent findings about Escherichia coli O157:H7 virulence factors and its bovine reservoir. This Shiga toxin (Stx)-producing E. coli belongs to the Enterohemorrhagic E. coli (EHEC) pathotype causing hemorrhagic colitis. Its low infectious dose makes it an efficient, severe, foodborne pathogen. Although EHEC remains in the intestine, Stx can translocate systemically and is cytotoxic to microvascular endothelial cells, especially in the kidney and brain. Disease can progress to life-threatening hemolytic uremic syndrome (HUS) with hemolytic anemia, acute kidney failure, and thrombocytopenia. Young children, the immunocompromised, and the elderly are at the highest risk for HUS. Healthy ruminants are the major reservoir of EHEC and cattle are the primary source of human exposure.

RECENT FINDINGS

Advances in understanding E. coli O157:H7 pathogenesis include molecular mechanisms of virulence, bacterial adherence, type three secretion effectors, intestinal microbiome, inflammation, and reservoir maintenance.

SUMMARY

Many aspects of E. coli O157:H7 disease remain unclear and include the role of the human and bovine intestinal microbiomes in infection. Therapeutic strategies involve controlling inflammatory responses and/or intestinal barrier function. Finally, elimination/reduction of E. coli O157:H7 in cattle using CRISPR-engineered conjugative bacterial plasmids and/or on-farm management likely hold solutions to reduce infections and increase food safety/security.

The Transcription of Flagella of Enteropathogenic Escherichia coli O127:H6 Is Activated in Response to Environmental and Nutritional Signals

The flagella of enteropathogenic Escherichia coli (EPEC) O127:H6 E2348/69 mediate adherence to host proteins and epithelial cells. What environmental and nutritional signals trigger or down-regulate flagella expression in EPEC are largely unknown. In this study, we analyzed the influence of pH, oxygen tension, cationic and anionic salts (including bile salt), carbon and nitrogen sources, and catecholamines on the expression of the flagellin gene (fliC) of E2348/69. We found that sodium bicarbonate, which has been shown to induce the expression of type III secretion effectors, down-regulated flagella expression, explaining why E2348/69 shows reduced motility and flagellation when growing in Dulbecco’s Minimal Essential Medium (DMEM). Further, growth under a 5% carbon dioxide atmosphere, in DMEM adjusted to pH 8.2, in M9 minimal medium supplemented with 80 mM glucose or sucrose, and in DMEM containing 150 mM sodium chloride, 0.1% sodium deoxycholate, or 30 µM epinephrine significantly enhanced fliC transcription to different levels in comparison to growth in DMEM alone. When EPEC was grown in the presence of HeLa cells or in supernatants of cultured HeLa cells, high levels (4-fold increase) of fliC transcription were detected in comparison to growth in DMEM alone. Our data suggest that nutritional and host signals that EPEC may encounter in the intestinal niche activate fliC expression in order to favor motility and host colonization.

Elucidation of a complete mechanical signaling and virulence activation pathway in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is an important extracellular human pathogen. The initial adherence of EHEC to host cells is a major cue for transcriptional induction of the locus of enterocyte effacement (LEE) genes to promote colonization and pathogenesis, but the mechanism through which this adherence is sensed and the LEE is induced remains largely elusive. Here, we report a complete signal transduction pathway for this virulence activation process. In this pathway, the outer-membrane lipoprotein NlpE senses a mechanical cue generated from initial host adherence and activates the BaeSR two-component regulatory system; the response regulator BaeR then directly activates the expression of airA located on O-island-134 and encoding a LEE transcriptional activator. Disruption of this pathway severely attenuates EHEC O157:H7 virulence both in vitro and in vivo. This study provides further insights into the evolution of EHEC pathogenesis and the host-pathogen interaction.

Virulence-related O islands in enterohemorrhagic Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a principally foodborne pathogen linked to serious diseases, including bloody diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome. Comparative genomics analysis revealed that EHEC O157 contains 177 unique genomic islands, termed O islands, compared with the nonpathogenic E. coli K-12 laboratory strain. These O islands contribute largely to the pathogenicity of EHEC O157:H7 by providing numerous virulence factors, effectors, virulence regulatory proteins, and virulence regulatory sRNAs. The present review aimed to provide a comprehensive understanding of the research progress on the function of O islands, especially focusing on virulence-related O islands.

What Flips the Switch? Signals and Stress Regulating Extraintestinal Pathogenic Escherichia coli Type 1 Fimbriae (Pili)

Pathogens are exposed to a multitude of harmful conditions imposed by the environment of the host. Bacterial responses against these stresses are pivotal for successful host colonization and pathogenesis. In the case of many E. coli strains, type 1 fimbriae (pili) are an important colonization factor that can contribute to diseases such as urinary tract infections and neonatal meningitis. Production of type 1 fimbriae in E. coli is dependent on an invertible promoter element, fimS, which serves as a phase variation switch determining whether or not a bacterial cell will produce type 1 fimbriae. In this review, we present aspects of signaling and stress involved in mediating regulation of type 1 fimbriae in extraintestinal E. coli; in particular, how certain regulatory mechanisms, some of which are linked to stress response, can influence production of fimbriae and influence bacterial colonization and infection. We suggest that regulation of type 1 fimbriae is potentially linked to environmental stress responses, providing a perspective for how environmental cues in the host and bacterial stress response during infection both play an important role in regulating extraintestinal pathogenic E. coli colonization and virulence.

Polyphosphate Kinase 1 Is a Pathogenesis Determinant in Enterohemorrhagic Escherichia coli O157:H7

The ppk1 gene encodes polyphosphate kinase (PPK1), which is the major catalytic enzyme that Escherichia coli utilizes to synthesize inorganic polyphosphate (polyP). The aim of this study was to explore the role of PPK1 in the pathogenesis of Enterohemorrhagic E. coli O157:H7 (EHEC O157:H7). An isogenic in-frame ppk1 deletion mutant (Δppk1) and ppk1 complemented mutant (Cppk1) were constructed and characterized in comparison to wild-type (WT) EHEC O157:H7 strain EDL933w by microscope observation and growth curve analysis. Survival rates under heat stress and acid tolerance, both of which the bacteria would face during pathogenesis, were compared among the three strains. LoVo cells and a murine model of intestinal colitis were used as the in vitro and in vivo models, respectively, to evaluate the effect of PPK1 on adhesion and invasion during the process of pathogenesis. Real-time reverse-transcription PCR of regulatory gene rpoS, adhesion gene eae, and toxin genes stx1 and stx2 was carried out to corroborate the results from the in vitro and in vivo models. The ppk1 deletion mutant exhibited disrupted polyP levels, but not morphology and growth characteristics. The survival rate of the Δppk1 strain under stringent environmental conditions was lower as compared with WT and Cppk1. The in vitro assays showed that deletion of the ppk1 gene reduced the adhesion, formation of attaching and effacing (A/E) lesions, and invasive ability of EHEC O157:H7. Moreover, the virulence of the Δppk1 in BALB/c mice was weaker as compared with the other two strains. Additionally, mRNA expression of rpoS, eae, stx1 and stx2 were consistent with the in vitro and in vivo results. In conclusion: EHEC O157:H7 requires PPK1 for both survival under harsh environmental conditions and virulence in vivo.

AraC-type regulators HilC and RtsA are directly controlled by an intestinal fatty acid to regulate Salmonella invasion

Invasion of the intestinal epithelium is an essential but energetically expensive survival strategy and is, therefore, tightly regulated by using specific cues from the environment. The enteric pathogen Salmonella controls its invasion machinery through the elegant coordination of three AraC-type transcription activators, HilD, HilC, and RtsA. Most environmental signals target HilD to control invasion, whereas HilC and RtsA are known only to augment these effects on HilD. Here we show that a fatty acid found in the murine colon, cis-2-hexadecenoic acid (c2-HDA), represses Salmonella invasion by directly targeting HilC and RtsA, in addition to HilD. c2-HDA directly binds each of these regulators and inhibits their attachment to DNA targets, repressing invasion even in the absence of HilD. Fatty acid binding, however, does not affect HilC and RtsA protein stability, unlike HilD. Importantly, we show that HilC and RtsA are highly effective in restoring HilD production and invasion gene expression after elimination of the repressive fatty acid c2-HDA. Together, these results illuminate a precise mechanism by which HilC and RtsA may modulate invasion as Salmonella navigates through different regions of the intestine, contributing to our understanding of how this enteric pathogen senses and adapts to a diverse intestinal environment while maintaining its virulence.

cAMP Receptor Protein Positively Regulates the Expression of Genes Involved in the Biosynthesis of Klebsiella oxytoca Tilivalline Cytotoxin

Klebsiella oxytoca is a resident of the human gut. However, certain K. oxytoca toxigenic strains exist that secrete the nonribosomal peptide tilivalline (TV) cytotoxin. TV is a pyrrolobenzodiazepine that causes antibiotic-associated hemorrhagic colitis (AAHC). The biosynthesis of TV is driven by enzymes encoded by the aroX and NRPS operons. In this study, we determined the effect of environmental signals such as carbon sources, osmolarity, and divalent cations on the transcription of both TV biosynthetic operons. Gene expression was enhanced when bacteria were cultivated in tryptone lactose broth. Glucose, high osmolarity, and depletion of calcium and magnesium diminished gene expression, whereas glycerol increased transcription of both TV biosynthetic operons. The cAMP receptor protein (CRP) is a major transcriptional regulator in bacteria that plays a key role in metabolic regulation. To investigate the role of CRP on the cytotoxicity of K. oxytoca, we compared levels of expression of TV biosynthetic operons and synthesis of TV in wild-type strain MIT 09-7231 and a Δcrp isogenic mutant. In summary, we found that CRP directly activates the transcription of the aroX and NRPS operons and that the absence of CRP reduced cytotoxicity of K. oxytoca on HeLa cells, due to a significant reduction in TV production. This study highlights the importance of the CRP protein in the regulation of virulence genes in enteric bacteria and broadens our knowledge on the regulatory mechanisms of the TV cytotoxin.

How the PhoP/PhoQ System Controls Virulence and Mg2+ Homeostasis: Lessons in Signal Transduction, Pathogenesis, Physiology, and Evolution

The PhoP/PhoQ two-component system governs virulence, Mg2+ homeostasis, and resistance to a variety of antimicrobial agents, including acidic pH and cationic antimicrobial peptides, in several Gram-negative bacterial species. Best understood in Salmonella enterica serovar Typhimurium, the PhoP/PhoQ system consists o-regulated gene products alter PhoP-P amounts, even under constant inducing conditions. PhoP-P controls the abundance of hundreds of proteins both directly, by having transcriptional effects on the corresponding genes, and indirectly, by modifying the abundance, activity, or stability of other transcription factors, regulatory RNAs, protease regulators, and metabolites. The investigation of PhoP/PhoQ has uncovered novel forms of signal transduction and the physiological consequences of regulon evolution.

A diffusible signal factor of the intestine dictates Salmonella invasion through its direct control of the virulence activator HilD

Successful intestinal infection by Salmonella requires optimized invasion of the gut epithelium, a function that is energetically costly. Salmonella have therefore evolved to intricately regulate the expression of their virulence determinants by utilizing specific environmental cues. Here we show that a powerful repressor of Salmonella invasion, a cis-2 unsaturated long chain fatty acid, is present in the murine large intestine. Originally identified in Xylella fastidiosa as a diffusible signal factor for quorum sensing, this fatty acid directly interacts with HilD, the master transcriptional regulator of Salmonella, and prevents hilA activation, thus inhibiting Salmonella invasion. We further identify the fatty acid binding region of HilD and show it to be selective and biased in favour of signal factors with a cis-2 unsaturation over other intestinal fatty acids. Single mutation of specific HilD amino acids to alanine prevented fatty acid binding, thereby alleviating their repressive effect on invasion. Together, these results highlight an exceedingly sensitive mechanism used by Salmonella to colonize its host by detecting and exploiting specific molecules present within the complex intestinal environment.