Increased Pho regulon activation correlates with decreased virulence of an avian pathogenic Escherichia coli O78 strain

Characteristics, pathogenic mechanism, zoonotic potential, drug resistance, and prevention of avian pathogenic Escherichia coli (APEC)

Although most Escherichia coli (E. coli) strains are commensal and abundant, certain pathogenic strains cause severe diseases from gastroenteritis to extraintestinal infections. Extraintestinal pathogenic E. coli (ExPEC) contains newborn meningitis E. coli (NMEC), uropathogenic E. coli (UPEC), avian pathogenic E. coli (APEC), and septicemic E. coli (SEPEC) based on their original host and clinical symptom. APEC is a heterogeneous group derived from human ExPEC. APEC causes severe respiratory and systemic diseases in a variety of avians, threatening the poultry industries, food security, and avian welfare worldwide. APEC has many serotypes, and it is a widespread pathogenic bacterium in poultry. In addition, ExPEC strains share significant genetic similarities and similar pathogenic mechanisms, indicating that APEC potentially serves as a reservoir of virulence and resistance genes for human ExPEC, and the virulence and resistance genes can be transferred to humans through food animals. Due to economic losses, drug resistance, and zoonotic potential, APEC has attracted heightened awareness. Various virulence factors and resistance genes involved in APEC pathogenesis and drug resistance have been identified. Here, we review the characteristics, epidemiology, pathogenic mechanism zoonotic potential, and drug resistance of APEC, and summarize the current status of diagnosis, alternative control measures, and vaccine development, which may help to have a better understanding of the pathogenesis and resistance of APEC, thereby reducing economic losses and preventing the spread of multidrug-resistant APEC to humans.

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.

Proteomics insights into the Burkholderia cenocepacia phosphorus stress response

The Burkholderia cepacia complex is a group of Burkholderia species that are opportunistic pathogens causing high mortality rates in patients with cystic fibrosis. An environmental stress often encountered by these soil-dwelling and pathogenic bacteria is phosphorus limitation, an essential element for cellular processes. Here, we describe cellular and extracellular proteins differentially regulated between phosphate-deplete (0 mM, no added phosphate) and phosphate-replete (1 mM) growth conditions using a comparative proteomics (LC-MS/MS) approach. We observed a total of 128 and 65 unique proteins were downregulated and upregulated respectively, in the B. cenocepacia proteome. Of those downregulated proteins, many have functions in amino acid transport/metabolism. We have identified 24 upregulated proteins that are directly/indirectly involved in inorganic phosphate or organic phosphorus acquisition. Also, proteins involved in virulence and antimicrobial resistance were differentially regulated, suggesting B. cenocepacia experiences a dramatic shift in metabolism under these stress conditions. Overall, this study provides a baseline for further research into the biology of Burkholderia in response to phosphorus stress.

The two-component system, BasSR, is involved in the regulation of biofilm and virulence in avian pathogenic Escherichia coli

Avian pathogenic Escherichia coli (APEC) is a subgroup of extra-intestinal pathogenic E. coli (ExPEC) strains that cause avian colibacillosis, resulting in significant economic losses to the poultry industry worldwide. It has been reported that a few two-component signal transduction systems (TCS) participate in the regulation of the virulence factors of APEC infection. In this study, a basSR-deficient mutant strain was constructed from its parent strain APECX40 (WT), and high-throughput sequencing (RNA-seq) was performed to analyse the transcriptional profile of WT and its mutant strain XY1. Results showed that the deletion of basSR down-regulated the transcript levels of a series of biofilm- and virulence-related genes. Results of biofilm formation assays and bird model experiments indicated that the deletion of basSR inhibited biofilm formation in vitro and decreased bacterial virulence and colonization in vivo. In addition, electrophoretic mobility shift assays confirmed that the BasR protein could bind to the promoter regions of several biofilm- and virulence-related genes, including ais, opgC and fepA. This study suggests that the BasSR TCS might be a global regulator in the pathogenesis of APEC infection. RESEARCH HIGHLIGHTS Transcriptional profiling showed that BasSR might be a global regulator in APEC. BasSR increases APEC pathogenicity in vivo. BasSR positively regulates biofilm- and the virulence-associated genes. BasSR can bind to the promoter regions of virulence-associated genes ais, opgC and fepA.

Transcriptional analysis of RstA/RstB in avian pathogenic Escherichia coli identifies its role in the regulation of hdeD-mediated virulence and survival in chicken macrophages

Avian pathogenic Escherichia coli (APEC) causes avian colibacillosis in poultry, which is characterized by systemic infections such as septicemia, air sacculitis, and pericarditis. APEC uses two-component regulatory systems (TCSs) to handle the stressful environments present in infected hosts. While many TCSs in E. coli have been well characterized, the RstA/RstB system in APEC has not been thoroughly investigated. The involvement of the RstA regulator in APEC pathogenesis was demonstrated during previous studies investigating its role in APEC persistence in chicken macrophages and respiratory infections. However, the mechanism underlying this phenomenon has not been clarified. Transcriptional analysis of the effect of rstAB deletion was therefore performed to improve the understanding of the RstA/RstB regulatory mechanism, and particularly its role in virulence. The transcriptomes of the rstAB mutant and the wild-type strain E058 were compared during their growth in the bloodstreams of challenged chickens. Overall, 198 differentially expressed (DE) genes were identified, and these indicated that RstA/RstB mainly regulates systems involved in nitrogen metabolism, iron acquisition, and acid resistance. Phenotypic assays indicated that the rstAB mutant responded more to an acidic pH than the wild-type strain did, possibly because of the repression of the acid-resistance operons hdeABD and gadABE by the deletion of rstAB. Based on the reported RstA box motif TACATNTNGTTACA, we identified four possible RstA target genes (hdeD, fadE, narG, and metE) among the DE genes. An electrophoretic mobility shift assay confirmed that RstA binds directly to the promoter of hdeD, and β-galactosidase assays showed that hdeD expression was reduced by rstAB deletion, indicating that RstA directly regulates hdeD expression. The hdeD mutation resulted in virulence attenuation in both cultured chicken macrophages and experimentally infected chickens. In conclusion, our data suggest that RstA affects APEC E058 virulence partly by directly regulating the acidic resistance gene hdeD.

yqhG Contributes to Oxidative Stress Resistance and Virulence of Uropathogenic Escherichia coli and Identification of Other Genes Altering Expression of Type 1 Fimbriae

Urinary tract infections (UTIs) are common bacterial infections and the vast majority of UTIs are caused by extraintestinal pathogenic Escherichia coli (ExPEC) strains referred to as uropathogenic E. coli (UPEC). Successful colonization of the human urinary tract by UPEC is mediated by secreted or surface exposed virulence factors-toxins, iron transport systems, and adhesins, such as type 1 fimbriae (pili). To identify factors involved in the expression of type 1 fimbriae, we constructed a chromosomal transcriptional reporter consisting of lux under the control of the fimbrial promoter region, fimS and this construct was inserted into the reference UPEC strain CFT073 genome at the attTn7 site. This fimS reporter strain was used to generate a Tn10 transposon mutant library, coupled with high-throughput sequencing to identify genes that affect the expression of type 1 fimbriae. Transposon insertion sites were linked to genes involved in protein fate and synthesis, energy metabolism, adherence, transcriptional regulation, and transport. We showed that YqhG, a predicted periplasmic protein, is one of the important mediators that contribute to the decreased expression of type 1 fimbriae in UPEC strain CFT073. The ΔyqhG mutant had reduced expression of type 1 fimbriae and a decreased capacity to colonize the murine urinary tract. Reduced expression of type 1 fimbriae correlated with an increased bias for orientation of the fim switch in the OFF position. Interestingly, the ΔyqhG mutant was more motile than the WT strain and was also significantly more sensitive to hydrogen peroxide. Taken together, loss of yqhG may decrease virulence in the urinary tract due to a decrease in production of type 1 fimbriae and a greater sensitivity to oxidative stress.

Outer membrane proteins YbjX and PagP co-regulate motility in Escherichia coli via the bacterial chemotaxis pathway

Mutation of the PhoP/Q two-component system decreases the expression of ybjX and pagP encoding outer membrane proteins, and mutation of ybjX or pagP attenuates avian pathogenic Escherichia coli (APEC) pathogenicity. However, whether ybjX/pagP mutation (double-deletion mutant) has a synergistic effect on pathogenicity remains unknown. Herein, electrophoresis mobility shift assay (EMSA) experiments showed that the PhoP/Q system regulated ybjX and pagP transcription indirectly. The APECΔybjX/pagP mutant strain, constructed using the Red recombination method, exhibited reduced invasion of chicken embryo fibroblast (DF-1) cells, but had no effect on virulence in a chicken model. Using RNA sequencing to identify differential mRNAs in APECΔybjXΔpagP and native strains, we revealed up-regulation of genes involved in the bacterial chemotaxis pathway. The ybjX/pagP mutant strain displayed significantly increased motility, suggesting that double deletion of ybjX and pagP enhances motility via the bacterial chemotaxis pathway.

Baicalin alleviated APEC-induced acute lung injury in chicken by inhibiting NF-κB pathway activation

Bacterial pneumonia is a leading cause of death in the animal husbandry. Acute lung injury (ALI), most often seen as a part of systemic inflammatory process, characterized by progressive hypoxemia, edema, and neutrophil accumulation in the lung. Baicalin has been reported to inhibit inflammatory response, but its role in ALI remains unknown. The purpose of our study was to determine the protective effect and possible mechanism of baicalin against avian pathogenic Escherichia coli (APEC)-induced ALI in chicken. Chickens were conditioned with baicalin 1 week before intratracheally instilled with APEC. Then, chickens were sacrificed by CO₂ inhalation 12 h later and the lung tissues were collected for examining histopathological changes, wet/dry (W/D) ratio, myeloperoxidase (MPO) activity, levels of pro-inflammatory cytokines and activation of NF-κB signaling pathway. The results showed that pre-treatment of chickens with baicalin significantly alleviated the death rate, histopathological changes in lung tissues. The W/D ratio, MPO activity and production of cytokines, such as IL-1β, TNF-α, IL-6 of lung tissues were also decreased following treatment with baicalin. Furthermore, the mechanism responsible for these effects was attributed to the inhibitory effect of baicalin on nuclear factor-κB (NF-κB) signaling activation. These data thus support the application of baicalin as a potential medicine for the treatment of E. coli-induced ALI by regulating NF-κB signaling pathway.

PhoPR Contributes to Staphylococcus aureus Growth during Phosphate Starvation and Pathogenesis in an Environment-Specific Manner

Microbial pathogens must obtain all essential nutrients, including phosphate, from the host. To optimize phosphate acquisition in diverse and dynamic environments, such as mammalian tissues, many bacteria use the PhoPR two-component system. Despite the necessity of this system for virulence in several species, PhoPR has not been studied in the major human pathogen Staphylococcus aureus To illuminate its role in staphylococcal physiology, we initially assessed whether PhoPR controls the expression of the three inorganic phosphate (P_i) importers (PstSCAB, NptA, and PitA) in S. aureus This analysis revealed that PhoPR is required for the expression of pstSCAB and nptA and can modulate pitA expression. Consistent with a role in phosphate homeostasis, PhoPR-mediated regulation of the transporters is influenced by phosphate availability. Further investigations revealed that PhoPR is necessary for growth under P_i-limiting conditions, and in some environments, its primary role is to induce the expression of pstSCAB or nptA Interestingly, in other environments, PhoPR is necessary for growth independent of P_i transporter expression, indicating that additional PhoPR-regulated factors promote S. aureus adaptation to low-P_i conditions. Together, these data suggest that PhoPR differentially contributes to growth in an environment-specific manner. In a systemic infection model, a mutant of S. aureus lacking PhoPR is highly attenuated. Further investigation revealed that PhoPR-regulated factors, in addition to P_i transporters, are critical for staphylococcal pathogenesis. Cumulatively, these findings point to an important role for PhoPR in orchestrating P_i acquisition as well as transporter-independent mechanisms that contribute to S. aureus virulence.

Exploration of factors in response to low acid tolerance using random mutagenesis in Cronobacter malonaticus

Cronobacter species are associated with rare but severe infections in newborns, and their tolerance to environmental stress such as acid stress has been described. However, the factors involved in low acid tolerance in Cronobacter are poorly understood. Here, a transposon mutagenesis approach was used to explore the factors involved in acid tolerance in C. malonaticus. Eight mutants from mutant library (n = 215) were successfully screened through a comparison of growth with wild type (WT) strain under acid stress (pH 4.0). Eight mutating sites including glucosyltransferase MdoH, extracellular serine protease, sulfate transporter, phosphate transporter permease subunit PstC, lysine transporter, nitrogen regulation protein NR (II), D-alanine-D-alanine ligase, glucan biosynthesis protein G were successfully identified by arbitrary polymerase chain reaction and sequencing. The biomass of biofilm of eight mutants were significantly reduced using crystal violet staining (CVS) compared with that of WT. furthermore, the more compact biofilms of WT was observed than those of eight mutants through scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM). Disassembly of biofilms appeared among mutants and WT strain from 48 h to 72 h through the increasing of dead cells and reduction of viable cells and exopolysaccharide. The study reveals the molecular basis involved in acid tolerance of C. malonaticus and a possible relationship between biofilm formation and acid tolerance, which provides valuable information for survival of C. malonaticus under acid stress.

Altered Regulation of the Diguanylate Cyclase YaiC Reduces Production of Type 1 Fimbriae in a Pst Mutant of Uropathogenic Escherichia coli CFT073

The pst gene cluster encodes the phosphate-specific transport (Pst) system. Inactivation of the Pst system constitutively activates the two-component regulatory system PhoBR and attenuates the virulence of pathogenic bacteria. In uropathogenic Escherichia coli strain CFT073, attenuation by inactivation of pst is predominantly attributed to the decreased expression of type 1 fimbriae. However, the molecular mechanisms connecting the Pst system and type 1 fimbriae are unknown. To address this, a transposon library was constructed in the pst mutant, and clones were tested for a regain in type 1 fimbrial production. Among them, the diguanylate cyclase encoded by yaiC (adrA in Salmonella) was identified to connect the Pst system and type 1 fimbrial expression. In the pst mutant, the decreased expression of type 1 fimbriae is connected by the induction of yaiC This is predominantly due to altered expression of the FimBE-like recombinase genes ipuA and ipbA, affecting at the same time the inversion of the fim promoter switch (fimS). In the pst mutant, inactivation of yaiC restored fim-dependent adhesion to bladder cells and virulence. Interestingly, the expression of yaiC was activated by PhoB, since transcription of yaiC was linked to the PhoB-dependent phoA-psiF operon. As YaiC is involved in cyclic di-GMP (c-di-GMP) biosynthesis, an increased accumulation of c-di-GMP was observed in the pst mutant. Hence, the results suggest that one mechanism by which deletion of the Pst system reduces the expression of type 1 fimbriae is through PhoBR-mediated activation of yaiC, which in turn increases the accumulation of c-di-GMP, represses the fim operon, and, consequently, attenuates virulence in the mouse urinary tract infection model.IMPORTANCE Urinary tract infections (UTIs) are common bacterial infections in humans. They are mainly caused by uropathogenic Escherichia coli (UPEC). We previously showed that interference with phosphate homeostasis decreases the expression of type 1 fimbriae and attenuates UPEC virulence. Herein, we identified that alteration of the phosphate metabolism increases production of the signaling molecule c-di-GMP, which in turn decreases the expression of type 1 fimbriae. We also determine the regulatory cascade leading to the accumulation of c-di-GMP and identify the Pho regulon as new players in c-di-GMP-mediated cell signaling. By understanding the molecular mechanisms leading to the expression of virulence factors, we will be in a better position to develop new therapeutics.

Identification of the PhoB Regulon and Role of PhoU in the Phosphate Starvation Response of Caulobacter crescentus

An ability to sense and respond to changes in extracellular phosphate is critical for the survival of most bacteria. For Caulobacter crescentus, which typically lives in phosphate-limited environments, this process is especially crucial. Like many bacteria, Caulobacter responds to phosphate limitation through a conserved two-component signaling pathway called PhoR-PhoB, but the direct regulon of PhoB in this organism is unknown. Here we used chromatin immunoprecipitation-DNA sequencing (ChIP-Seq) to map the global binding patterns of the phosphate-responsive transcriptional regulator PhoB under phosphate-limited and -replete conditions. Combined with genome-wide expression profiling, our work demonstrates that PhoB is induced to regulate nearly 50 genes under phosphate-starved conditions. The PhoB regulon is comprised primarily of genes known or predicted to help Caulobacter scavenge for and import inorganic phosphate, including 15 different membrane transporters. We also investigated the regulatory role of PhoU, a widely conserved protein proposed to coordinate phosphate import with expression of the PhoB regulon by directly modulating the histidine kinase PhoR. However, our studies show that it likely does not play such a role in Caulobacter, as PhoU depletion has no significant effect on PhoB-dependent gene expression. Instead, cells lacking PhoU exhibit striking accumulation of large polyphosphate granules, suggesting that PhoU participates in controlling intracellular phosphate metabolism.

IMPORTANCE

The transcription factor PhoB is widely conserved throughout the bacterial kingdom, where it helps organisms respond to phosphate limitation by driving the expression of a battery of genes. Most of what is known about PhoB and its target genes is derived from studies of Escherichia coli. Our work documents the PhoB regulon in Caulobacter crescentus, and comparison to the regulon in E. coli reveals significant differences, highlighting the evolutionary plasticity of transcriptional responses driven by highly conserved transcription factors. We also demonstrated that the conserved protein PhoU, which is implicated in bacterial persistence, does not regulate PhoB activity, as previously suggested. Instead, our results favor a model in which PhoU affects intracellular phosphate accumulation, possibly through the high-affinity phosphate transporter.

Avian colibacillosis: still many black holes

Avian pathogenic Escherichia coli (APEC) strains cause severe respiratory and systemic diseases, threatening food security and avian welfare worldwide. Intensification of poultry production and the quick expansion of free-range production systems will increase the incidence of colibacillosis through greater exposure of birds to pathogens and stress. Therapy is mainly based on antibiotherapy and current vaccines have poor efficacy. Serotyping remains the most frequently used diagnostic method, only allowing the identification of a limited number of APEC strains. Several studies have demonstrated that the most common virulence factors studied in APEC are all rarely present in the same isolate, showing that APEC strains constitute a heterogeneous group. Different isolates may harbor different associations of virulence factors, each one able to induce colibacillosis. Despite its economical relevance, pathogenesis of colibacillosis is poorly understood. Our knowledge on the host response to APEC is based on very descriptive studies, mostly restricted to bacteriological and histopathological analysis of infected organs such as lungs. Furthermore, only a small number of APEC isolates have been used in experimental studies. In the present review, we discuss current knowledge on APEC diversity and virulence, including host response to infection and the associated inflammatory response with a focus on pulmonary colibacillosis.

ArcA Controls Metabolism, Chemotaxis, and Motility Contributing to the Pathogenicity of Avian Pathogenic Escherichia coli

Avian pathogenic Escherichia coli (APEC) strains cause one of the three most significant infectious diseases in the poultry industry and are also potential food-borne pathogens threating human health. In this study, we showed that ArcA (aerobic respiratory control), a global regulator important for E. coli's adaptation from anaerobic to aerobic conditions and control of that bacterium's enzymatic defenses against reactive oxygen species (ROS), is involved in the virulence of APEC. Deletion of arcA significantly attenuates the virulence of APEC in the duck model. Transcriptome sequencing (RNA-Seq) analyses comparing the APEC wild type and the arcA mutant indicate that ArcA regulates the expression of 129 genes, including genes involved in citrate transport and metabolism, flagellum synthesis, and chemotaxis. Further investigations revealed that citCEFXG contributed to APEC's microaerobic growth at the lag and log phases when cultured in duck serum and that ArcA played a dual role in the control of citrate metabolism and transportation. In addition, deletion of flagellar genes motA and motB and chemotaxis gene cheA significantly attenuated the virulence of APEC, and ArcA was shown to directly regulate the expression of motA, motB, and cheA. The combined results indicate that ArcA controls metabolism, chemotaxis, and motility contributing to the pathogenicity of APEC.

Environmental phosphate differentially affects virulence phenotypes of uropathogenic Escherichia coli isolates causative of prostatitis

K-12 Escherichia coli cells grown in static media containing a critical phosphate (Pi) concentration ≥25 mM maintained a high polyphosphate (polyP) level in stationary phase, impairing biofilm formation, a phenomenon that is triggered by polyP degradation. Pi concentration in human urine fluctuates according to health state. Here, the influence of environmental Pi concentration on the occurrence of virulence traits in uropathogenic E. coli (UPEC) isolated from acute prostatitis patients was evaluated. After a first screening, 3 isolates were selected according to differential biofilm formation profiles depending on media Pi concentration. For each isolate, biofilm positive and negative conditions were established. Regardless of the isolate, biofilm formation capacity was accompanied with curli and cellulose production and expression of some key virulence factors associated with adhesion. When the selected isolates were grown in their non-biofilm-forming condition, low concentrations of nalidixic acid and ciprofloxacin induced biofilm formation. Interestingly, similar to laboratory strains, polyP degradation induced biofilm formation in the selected isolates. Data demonstrated the complexity of UPEC responses to environmental Pi and the importance of polyP metabolism in the virulence of clinical isolates.

RstA is required for the virulence of an avian pathogenic Escherichia coli O2 strain E058

Certain strains of avian pathogenic Escherichia coli (APEC) cause severe extraintestinal infections in poultry, including acute fatal septicemia, subacute pericarditis, and airsacculitis. These bacteria contain an RstA/RstB regulatory system, a two-component system that may help APEC strains adapt to the extra-intestinal environment and survive under stressful conditions. Whether RstA correlates with APEC pathogenesis or acts as an APEC virulence factor has not been established. Here we provide the first evidence for an important role of rstA in APEC virulence. We generated an rstA-deficient mutant from the highly virulent APEC strain E058. Virulence of the mutant strain was evaluated in vivo and in vitro through bird infection assays, a cytotoxicity assay on chicken macrophage cell line HD-11, and a bactericidal assay to serum complement. Based on lethality assays in 1-day-old birds, rstA deletion from APEC E058 reduced the bacterial virulence in birds. The deletion also deeply impaired the capacity of APEC E058 to colonize deeper tissues of 5-week-old specific pathogen free chickens. No obvious gross or histopathological lesions were observed in the visceral organs of chickens challenged with the rstA-deficient strain. Also, rstA inactivation reduced the survival of APEC E058 within chicken macrophages. However, no significant differences were observed between the mutant and the wild-type strain in resistance to serum. Our data collectively show that the rstA gene functions in the pathogenesis of diseases caused by avian pathogenic E. coli.

Interplay between genetic regulation of phosphate homeostasis and bacterial virulence

Bacterial pathogens, including those of humans, animals, and plants, encounter phosphate (Pi)-limiting or Pi-rich environments in the host, depending on the site of infection. The environmental Pi-concentration results in modulation of expression of the Pho regulon that allows bacteria to regulate phosphate assimilation pathways accordingly. In many cases, modulation of Pho regulon expression also results in concomitant changes in virulence phenotypes. Under Pi-limiting conditions, bacteria use the transcriptional-response regulator PhoB to translate the Pi starvation signal sensed by the bacterium into gene activation or repression. This regulator is employed not only for the maintenance of bacterial Pi homeostasis but also to differentially regulate virulence. The Pho regulon is therefore not only a regulatory circuit of phosphate homeostasis but also plays an important adaptive role in stress response and bacterial virulence. Here we focus on recent findings regarding the mechanisms of gene regulation that underlie the virulence responses to Pi stress in Vibrio cholerae, Pseudomonas spp., and pathogenic E. coli.

PhoB activates Escherichia coli O157:H7 virulence factors in response to inorganic phosphate limitation

Enterohemorrhagic Escherichia coli (EHEC), an emerging food- and water-borne hazard, is highly pathogenic to humans. In the environment, EHEC must survive phosphate (Pi) limitation. The response to such Pi starvation is an induction of the Pho regulon including the Pst system that senses Pi variation. The interplay between the virulence of EHEC, Pho-Pst system and environmental Pi remains unknown. To understand the effects of Pi deprivation on the molecular mechanisms involved in EHEC survival and virulence under Pho regulon control, we undertook transcriptome profiling of the EDL933 wild-type strain grown under high Pi and low Pi conditions and its isogenic ΔphoB mutant grown in low Pi conditions. The differentially expressed genes included 1067 Pi-dependent genes and 603 PhoB-dependent genes. Of these 131 genes were both Pi and PhoB-dependent. Differentially expressed genes that were selected included those involved in Pi homeostasis, cellular metabolism, acid stress, oxidative stress and RpoS-dependent stress responses. Differentially expressed virulence systems included the locus of enterocyte effacement (LEE) encoding the type-3 secretion system (T3SS) and its effectors, as well as BP-933W prophage encoded Shiga toxin 2 genes. Moreover, PhoB directly regulated LEE and stx2 gene expression through binding to specific Pho boxes. However, in Pi-rich medium, constitutive activation of the Pho regulon decreased LEE gene expression and reduced adherence to HeLa cells. Together, these findings reveal that EHEC has evolved a sophisticated response to Pi limitation involving multiple biochemical strategies that contribute to its ability to respond to variations in environmental Pi and to coordinating the virulence response.

Sequencing and functional annotation of avian pathogenic Escherichia coli serogroup O78 strains reveal the evolution of E. coli lineages pathogenic for poultry via distinct mechanisms

Avian pathogenic Escherichia coli (APEC) causes respiratory and systemic disease in poultry. Sequencing of a multilocus sequence type 95 (ST95) serogroup O1 strain previously indicated that APEC resembles E. coli causing extraintestinal human diseases. We sequenced the genomes of two strains of another dominant APEC lineage (ST23 serogroup O78 strains χ7122 and IMT2125) and compared them to each other and to the reannotated APEC O1 sequence. For comparison, we also sequenced a human enterotoxigenic E. coli (ETEC) strain of the same ST23 serogroup O78 lineage. Phylogenetic analysis indicated that the APEC O78 strains were more closely related to human ST23 ETEC than to APEC O1, indicating that separation of pathotypes on the basis of their extraintestinal or diarrheagenic nature is not supported by their phylogeny. The accessory genome of APEC ST23 strains exhibited limited conservation of APEC O1 genomic islands and a distinct repertoire of virulence-associated loci. In light of this diversity, we surveyed the phenotype of 2,185 signature-tagged transposon mutants of χ7122 following intra-air sac inoculation of turkeys. This procedure identified novel APEC ST23 genes that play strain- and tissue-specific roles during infection. For example, genes mediating group 4 capsule synthesis were required for the virulence of χ7122 and were conserved in IMT2125 but absent from APEC O1. Our data reveal the genetic diversity of E. coli strains adapted to cause the same avian disease and indicate that the core genome of the ST23 lineage serves as a chassis for the evolution of E. coli strains adapted to cause avian or human disease via acquisition of distinct virulence genes.

PhoB regulates the survival of Bacteroides fragilis in peritoneal abscesses

In response to phosphate limitation, bacteria employ the Pho regulon, a specific regulatory network for phosphate acquisition. The two-component signal transduction system of PhoRB plays a crucial role in the induction of Pho regulon genes, leading to the adaptation to phosphate starvation. Herein, we identified the PhoRB system in Bacteroides fragilis, a commensal gut bacterium, and evaluated its role in gut colonization and survival in peritoneal abscesses. BF1575 and BF1576 encoded PhoR (sensor histidine kinase) and PhoB (response regulator) in the sequenced B. fragilis strain YCH46, respectively. Transcriptome analysis revealed that deletion of phoB affected the expression of 585 genes (more than 4-fold change) in B. fragilis, which included genes for stress response (chaperons and heat shock proteins), virulence (capsular polysaccharide biosynthesis) and phosphate metabolism. Deletion of phoB reduced the ability of the bacterium to persist in peritoneal abscesses induced by an intra-abdominal challenge of B. fragilis. Furthermore, PhoB was necessary for survival of this anaerobe in peritoneal abscesses but not for in vitro growth in rich media or in intestinal colonization. These results indicate that PhoB plays an important role in the survival of B. fragilis under stressful extraintestinal conditions.

Survival of enterohemorrhagic Escherichia coli in the presence of Acanthamoeba castellanii and its dependence on Pho regulon

Enterohemorrhagic Escherichia coli (EHEC) are involved in outbreaks of food-borne illness and transmitted to humans through bovine products or water contaminated by cattle feces. Microbial interaction is one of the strategies used by pathogenic bacteria to survive in the environment. Among protozoa, the free-living amoebae are known to host and protect several water-borne pathogens. In this study, the interaction between EHEC and the predacious protozoa Acanthamoeba castellanii was investigated. Using monoculture and cocultures, growth of both organisms was estimated for 3 weeks by total and viable cell counts. The numbers of EHEC were significantly higher when cultured with amoebae than without, and less EHEC shifted into a viable but nonculturable state in the presence of amoebae. Using several mutants, we observed that the Pho regulon is required for EHEC growth when cocultured with amoebae. In contrast, the Shiga toxins (Stx) were not involved in this association phenotype. Cocultures monitored by electron microscopy revealed a loss of the regular rod shape of EHEC and the secretion of multilamellar vesicles by the amoebae, which did not contain bacteria. As the interaction between A. castellanii and EHEC appears beneficial for bacterial growth, this supports a potential role for protozoa in promoting the persistence of EHEC in the environment.

Decreased expression of type 1 fimbriae by a pst mutant of uropathogenic Escherichia coli reduces urinary tract infection

The pstSCAB-phoU operon encodes the phosphate-specific transport system (Pst). Loss of Pst constitutively activates the Pho regulon and decreases bacterial virulence. However, specific mechanisms underlying decreased bacterial virulence through inactivation of Pst are poorly understood. In uropathogenic Escherichia coli (UPEC) strain CFT073, inactivation of pst decreased urinary tract colonization in CBA/J mice. The pst mutant was deficient in production of type 1 fimbriae and showed decreased expression of the fimA structural gene which correlated with differential expression of the fimB, fimE, ipuA, and ipbA genes, encoding recombinases, mediating inversion of the fim promoter. The role of fim downregulation in attenuation of the pst mutant was confirmed using a fim phase-locked-on derivative, which demonstrated a significant gain in virulence. In addition, the pst mutant was less able to invade human bladder epithelial cells. Since type 1 fimbriae contribute to UPEC virulence by promoting colonization and invasion of bladder cells, the reduced bladder colonization by the pst mutant is predominantly attributed to downregulation of these fimbriae. Elucidation of mechanisms mediating the control of type 1 fimbriae through activation of the Pho regulon in UPEC may open new avenues for therapeutics or prophylactics against urinary tract infections.

Induction of the Pho regulon suppresses the growth defect of an Escherichia coli sgrS mutant, connecting phosphate metabolism to the glucose-phosphate stress response

Some bacteria experience stress when glucose-6-phosphate or analogues like α-methyl glucoside-6-phosphate (αMG6P) accumulate in the cell. In Escherichia coli, the small SgrS RNA is vital to recovery from glucose-phosphate stress; the growth of sgrS mutants is strongly inhibited by αMG. SgrS helps to restore growth in part through inhibiting translation of the ptsG mRNA, which encodes the major glucose transporter EIICB(Glc). While the regulatory mechanism of SgrS has been characterized, little is known about how glucose-phosphate stress connects to other aspects of cell physiology. In the present study, we discovered that mutation of pitA, which encodes the low-affinity transporter of inorganic phosphate, partially suppresses the αMG growth defect of an sgrS mutant. Induction of the stress response was also reduced in the sgrS pitA mutant compared to its sgrS parent. Microarray analysis suggested that expression of phosphate (Pho) regulon genes is increased in the sgrS pitA mutant compared to the sgrS parent. Consistent with this, we found increased PhoA (alkaline phosphatase) activity in the sgrS pitA mutant compared to the sgrS strain. Further, direct induction of the Pho regulon (in a pitA(+) background) also resulted in partial suppression of the sgrS growth defect. The suppression was reversed when Pho induction was prevented by mutation of phoB, which encodes the Pho transcriptional activator. Deletion of individual Pho structural genes in suppressed strains did not identify a single gene responsible for suppression. Altogether, this work describes one of the first studies of glucose-phosphate stress physiology and suggests a novel connection of carbon and phosphate metabolism.

The Pho regulon and the pathogenesis of Escherichia coli

During the course of infection, bacteria must coordinately regulate gene expression in response to environmental stimuli. The phosphate (Pho) regulon is controlled by the two component-regulatory system PhoBR. PhoBR is activated during starvation and regulates genes involved in phosphate homeostasis. Several studies have highlighted the importance of the Pho regulon in bacterial pathogenesis, showing how induction of PhoBR, in addition to regulating genes participating in phosphate metabolism, leads to modulation of many cellular processes. The pleiotropic effects of Pho regulon activation include attenuated virulence and alteration of many virulence traits, including adhesion to host cells and resistance to cationic antimicrobial peptides, acidity and oxidative stresses. This review provides an overview of the relationship between the Pho regulon and virulence in Escherichia coli and illustrates that, in addition to regulating phosphate homeostasis, the Pho regulon plays a key role in regulating stress responses and virulence.