Interplay between genetic regulation of phosphate homeostasis and bacterial virulence

Abstract

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.

Keywords:

• phosphate limitation
• Pho regulon
• PhoB
• Pst
• virulence
• gene regulation
• bacterial pathogens
• Vibrio cholerae
• Pseudomonas spp
• Escherichia coli (E. coli)

Introduction

Phosphorous (P) is the sixth most abundant element in living organisms. Its primary biological importance in bacteria, in the form of phosphate (Pi), is as a component of nucleotides, which serve as energy storage within cells (ATP) or when linked together to form the nucleic acids DNA and RNA. The Pi group is also an important component of membrane phospholipids and it is incorporated into proteins during post-translational modifications, particularly as a regulatory device such as employed during signal transduction of bacterial two-component systems (TCS). For bacteria, the primary sources of P are inorganic phosphate, pyrophosphate and metaphosphate. They are acquired by the Pi-specific transport system (Pst) and low Pi-affinity transporter (Pit). Secondary P sources used by bacteria are phosphonates and organophosphates (i.e., Glycerol-3-P) that require separate transporters (Ugp and Phn systems) and enzymatic hydrolysis by alkaline phosphatase (PhoA).

In this review, we focus on recent findings in the mechanisms of gene regulation that underlie the virulence responses to Pi stress in Vibrio cholerae, Pseudomonas spp., and pathogenic E. coli. We also briefly discuss the relationship between Pi regulation and bacterial virulence in additional non-enteric pathogens.

The Pho Regulon and Pi Availability

TCSs are signal transduction pathways commonly used by prokaryotes to sense and adapt to stimuli in the environment. As many as 50 different TCSs may exist in a single bacterium such as E. coli which has at least 30 TCSs.^1-3 These systems are characterized by a sensor histidine-kinase (HK) that undergoes auto phosphorylation on a histidine residue. This phosphoryl group is then transferred to an aspartate residue of a response regulator protein that usually acts as a transcriptional activator (Fig. 1). In E. coli, there are at least 40 gene members of the Pho regulon that are primarily involved in Pi assimilation and metabolism.^4-6 The core members of the Pho regulon comprise 9 transcriptional units: eda, phnCDEFGHIJKLMNOP, phoA, phoBR, phoE, phoH, psiE, pstSCAB-phoU, and ugpBAECQ. These units encode proteins which ensure Pi homeostasis including the PhoB-R TCS, alkaline phosphatase and transporters of Pi, carbon-P, glycerol-3-P, and phosphonates. Transcription of genes within the Pho regulon is regulated in response to extracellular Pi concentrations via the TCS PhoB-R. The Pst system also plays an important role in Pi homeostasis. It has two related but distinct functions: high-affinity uptake of Pi (in low Pi conditions = Pho ON) and the maintainance of Pho regulon expression at basal level (in high Pi conditions = Pho OFF).⁷ The pst operon (pstSCAB–phoU) encodes a transport complex belonging to the ABC transporter superfamily. The periplasmic protein PstS binds Pi that is able to freely diffuse across the Gram-negative outer membrane, whereas PstA and PstC are membrane channel-forming proteins. The ATPase PstB provides the energy for translocation and interacts with PstA and PstC.⁸ PhoU has no evident role in Pi transport, but is required for Pho regulon expression.

Figure 1. Model for transcription regulation by PhoB and transmembrane signal transduction by environmental Pi and the Pst system in E. coli (adapted from refs. 5 and 56). Two processes are involved in Pho regulon control: inhibition when Pi is in excess and activation under Pi limitation. The Pst transport system is associated with the PhoR histidine kinase/phosphatase, which controls the phosphorylation state of the response regulator PhoB. At high Pi concentrations (>4 μM), PhoR might interact with the PstSCAB-PhoU complex, which represses its autophosphorylation and acts as a PhoB phosphatase. In contrast, low extracellular Pi concentrations sensed by Pst system, which increases the kinase activity of PhoR, resulting in the accumulation of phosphorylated PhoB∼P. Likewise, loss-of-function mutations in the pst genes lead to the constitutive phosphorylation of PhoB, regardless of the external Pi concentration. The Pi binding protein PstS acts as the primary sensor of external Pi concentration. When the Pi concentration is low (<4 μM), PhoR undergoes conformational change, resulting in its release from the repressor complex and autophosphorylation. PhoR is the histidine kinase/phosphatase that donates a phosphoryl group to PhoB when environmental Pi is limiting and removes the phosphoryl group from PhoB∼P when environmental Pi is abundant. PhoB is a typical winged-helix response regulator that upon aspartyl phosphorylation forms a dimer, which binds to DNA sequences upstream of Pho regulon genes to recruit RNA polymerase (RNAP) and initiate transcription. In some cases the transcription is repressed possibly by disruption of RNAP interaction with other activators.

During Pi limitation, the HK PhoR phosphorylates PhoB (PhoB∼P). PhoB∼P is then able to activate the Pho regulon by binding to a consensus Pho box sequence within the promoters of Pho regulon genes.⁵ The Pho boxes are formed by direct repeat units consisting of 11 nucleotides, where the first 7 nucleotides are well conserved (consensus sequence GTTCACC). Conversely, when Pi is abundant, the Pho regulon is not induced (OFF state = basal expression), which is mediated by an interaction between the Pst-PhoU complex and PhoR, that prevents PhoR-mediated phosphorylation of PhoB.⁹ In these high Pi conditions, PhoB is maintained in the non-phosphorylated form by PhoR phosphatase activity, and expression of the Pst system becomes deactivated while the expression of the Pit system becomes de-repressed. Pit does not require chemical energy to drive Pi transport when it is abundant, but when conditions are limiting, the cell uses ATP to scavenge Pi and similar P-containing compounds from the environment via the Pst system.¹⁰

PhoB is a typical winged-helix response regulator that upon aspartyl phosphorylation forms a dimer, which binds in head-to-tail arrangement to DNA sequences upstream of Pho regulon genes to recruit RNA polymerase (RNAP)^11-14 (Fig. 1). PhoB∼P interacts specifically with the σ⁷⁰ subunit of RNAP, and this interaction facilitates the entry of the RNA polymerase into the pho promoters for transcription initiation.¹⁵ In most cases PhoB∼P acts as a positive transcriptional factor, but there are a few promoters in which PhoB works as a repressor. This is the case for hrdA and rpoZ encoding respectively the σ-factor and ω-factor of RNAP in Streptomyces species.^16,17 This repression is due to promoter occlusion when PhoB∼P binds to the −10 to +1 region of the promoter thus interfering with RNAP binding. Another example is PhoB∼P repressing tcpPH by binding to a Pho-box located between the promoter and the binding sites for the AphA and AphB positive regulators (Fig. 2). In this case, the repression may be due to disruption of RNAP interaction with other transcriptional activators.¹⁸

Figure 2. The V. cholerae virulence cascade and the mechanisms of Pi/Pho regulation of virulence, motility, and biofilm formation. (A) V. cholerae virulence cascade. In high Pi conditions (Pho OFF) and in conditions favoring gene expression of AphA and AphB regulators, AphA cooperates with AphB to activate the tcpPH promoter. TcpPH cooperatively with ToxR/S activates the toxT promoter. ToxT then activates the tcpA-F and ctxAB promoters. This increases colonization through formation of the toxin co-regulated pilus (TCP) and cytotoxicity through Cholera toxin (CT) release. (B) In low Pi conditions (or Δpst mutation), PhoB∼P binds to the Pho-box located between the tcpPH promoter and the binding sites for AphA and AphB. This results in tcpPH repression which reduces the downstream ToxT-dependent virulence cascade. C-di-GMP is synthesized from 2 GTP molecules by diguanylate cyclases (DGCs) and degraded to 2 GMP molecules by phosphodiesterases (PDEs). Studies in V. cholerae have shown that c-di-GMP increases biofilm formation and reduces the motility and toxT expression. In low Pi (or Δpst mutation) PhoB induces indirectly the expression of acgAB which results in degradation of c-di-GMP leading to an increase in motility and a decrease of biofilm formation.

In certain circumstances, PhoB is activated in response to other environmental signals through cross-activation by non-partner HK proteins. Indeed, in addition to the well- known CreC, it has been shown that 5 other non-partner HK (ArcB, KdpD, QseC, BaeS, and VanS) could also activate PhoB.^19,20 Within a genetically homogeneous cell population, bacteria evolved strategies that display a stochastic character of gene expression in a cross-regulation network of TCSs. This would allow for non-genetic diversity, and thereby a rapid adaption to different environments.²¹

Mechanisms by Which PhoB Controls the Virulence of Vibrio cholerae

PhoB-mediated regulation of V. cholerae toxicity and colonization

In order to cause the cholera diarrheal disease, V. cholerae induces expression of genes encoding the colonization factor toxin-co-regulated pilus (TCP) and the ADP ribosylating cholera toxin (CT).^22-24 Dependent on a transcriptional cascade, this is triggered by a pair of independent activators, AphA and AphB (Fig. 2A). These 2 activators cooperatively induce the tcpPH operon encoding TcpP/H, a transcriptional activator of toxT expression. ToxT in turn increases gene transcription of the ctx (CT) and tcp (TCP) genes. The low Pi response regulator PhoB was capable of shorting this regulation cascade by binding and repressing the tcpPH promoter (Fig. 2B). This resulted in impaired colonization by a V. cholerae Δpst strain in the infant mouse and decreased expression of CT and TCP.¹⁸ However, under the normally high Pi conditions of the gut, the AphA/AphB regulatory cascade would serve to promote TCP and CT production.

Cyclic-di-GMP: A Pi/PhoB messenger of motility and biofilm formation

During the last decade, several investigations point-out the implication of c-di-GMP in various bacterial phenotypes and fundamental processes including Pi homeostasis and bacterial virulence. c-di-GMP [bis-(3′-5′)-cyclic dimeric guanosine monophosphate] is a ubiquitous soluble molecule and a bacterial second messenger. c-di-GMP induces biofilm formation by stimulating the biosynthesis of various adhesins and exopolysaccharides and inhibits bacterial motility by controlling the switch between planktonic and sedentary lifestyles (for a review, see refs. 25 and 26). The level of c-di-GMP is enzymatically and spatio-temporally controlled in bacterial cells by the GGDEF and the EAL domains of many diguanylate cyclases (DGC) and phosphodiesterases (PDE) (for a review, see ref. 27).

The link between Pi/Pho and bacterial virulence via c-di-GMP signaling has been investigated in V. cholerae. Twenty-two V. cholerae genes have been predicted to encode an EAL domain protein including VieA.²⁸ It has been shown that expression of the PDE VieA positively regulates virulence gene expression (ctx and toxT),²⁹ while ectopic expression of a DGC reduced the induction of virulence genes during infection using the infant mouse model of cholera.³⁰ VieA also positively regulates motility and flagellar synthesis genes,³¹ and negatively regulates the expression of Vibrio exopolysaccharide synthesis (vps) genes.³² These findings suggest that c-di-GMP assists in the transition from environment to host via regulation of V. cholerae behavior. Indeed, Tischler et al.³² and Pratt et al.³³ have shown that the vieA(E170A) mutation abrogates PDE activity, and increases intracellular c-di-GMP levels leading to hypo-motility and hyper-biofilm phenotypes. Sometime after, the same team demonstrated that a Δpst mutation could restore motility and decrease biofilm formation of the vieA(E170A) strain.³⁴ In the study, the authors also demonstrated that the V. cholerae WT strain grown in low Pi increased its motility and decreased biofilm formation (Fig. 2B). This was due to degradation of [c-di-GMP] by AcgA, another EAL domain containing PDE. In this condition, PhoB indirectly activated the transcription of the acgAB operon that encodes two enzymes with opposing activities on the intracellular c-di-GMP. It seems that in low Pi, the PDE activity (AcgA) overrides the DGC activity (AcgB).³⁴

The link between Pi homeostasis and c-di-GMP signaling, suggests a regulatory model in which biofilm formation would be promoted during the course of infection. However, in late infection stages, PhoB activation shuts down virulence gene expression. This also occurs when extracellular Pi becomes limiting and results in a concomitant induction of gene expression that promotes bacterial survival and dissemination in low Pi aquatic environments.³⁴

Roles of PhoB and Pi Response in Various Virulence-Related Phenotypes of Pseudomonas spp.

Many bacterial species belonging to the genus Pseudomonas have been characterized, demonstrate metabolic variability, and are able to colonize a wide range of niches such as soil, water, and plant or animal tissue. The most-studied Pseudomonas species include the opportunistic pathogen P. aeruginosa and the plant growth-promoting Pseudomonas fluorescens.

Pi limitation switches on harmful and lethal phenotypes in P. aeruginosa

P. aeruginosa is responsible for severe nosocomial infections and chronically colonizes lungs of cystic fibrosis (CF) patients leading to morbidity and mortality.^35,36 Infection with Gram-negative pathogens, leads to Pi reduction in the plasma to a suboptimal level for bacterial growth,³⁷ and the Pi concentration measured from sputum of CF patients ranged from 12 to 14 mM.³⁸ In these conditions, P. aeruginosa synthesizes several exo-products, including the hemolytic and non-hemolytic C phospholipases, PLC-H and PLC-N.³⁹ These PLCs are also induced by osmoprotectant compounds from eukaryotic and prokaryotic cells.⁴⁰ It was demonstrated that induction of PLC-H by the osmoprotective compounds is independent of Pi concentration or PhoB, while induction of PLC-N by these compounds requires Pi-deficient conditions and PhoB.⁴¹ This indicates a relevant pathogenic potential of PLC in the hyperosmotic conditions of the lungs of cystic fibrosis patients. Furthermore, PLC is also known to degrade phosphatidylcholine, a component of lung surfactant that may provide essential Pi nutrients with the potential to enhance lung colonization and cause atelectasis. In addition, mucoidy has been shown to be induced in P. aeruginosa by growth on minimal medium with phosphorylcholine as the sole carbon and Pi source.⁴²

It was reported that Pi-starvation of P. aeruginosa in the distal gut coupled with the administration of analgesic opioids led to PhoB induction of the expression of phzA1/2 (phenazine biosynthesis), and rhlR, and lasR (which encode response regulators of the quorum sensing systems Rhl and Las). This was directly linked to production of the toxic phenazine, pyocyanin and drug resistance during development of human sepsis.^43-45 Increased pyocyanin toxin production was PhoB-dependent via RhlR and PhzA1/2 when Pi was limited. However, in Pi-sufficient conditions, RhlR stimulated pyocyanin production in a PhoB-independent way through PQS (Pseudomonas quinolone signal) and/or low iron availability.⁴³ By contrast, the host supplemented with excess Pi was less susceptible to P. aeruginosa infection⁴⁶. The excess of Pi attenuated bacterial virulence by decreasing opioid-induced pyocyanin production.

Highly virulent multidrug-resistant clinical strains of P. aeruginosa showed an over-production of PstS and the ability to kill 60% of surgically injured mice.⁴⁷ This virulence phenotype was associated with formation of PstS-rich surface structures induced by PhoB in low Pi that allows scavenging of Pi within the host while remaining less accessible to the immune system. The mechanism by which these PstS-containing structures are formed remains to be determined.⁴⁸ In contrast, the Δpst strain was harmless and did not produce PstS.^47,49 Furthermore, the rough cells (producing PstS-rich appendages at cell surfaces) formed less biofilm than smooth cells.⁴⁹ P. aeruginosa could also cause red death in 60% of Caenorhabditis elegans nematodes.⁵⁰ This lethal phenotype appears to be activated by a triangulated response to low Pi between PhoB, MvfR-PQS quorum sensing, and the pyoverdine iron acquisition system.

All these reports emphasize the central role of Pi in switching on the virulence of P. aeruginosa. They also suggest that this opportunistic pathogen can use Pi deficiency as an environmental signal of host injury and then shift to virulent and lethal phenotypes.

Pi stress changes the motility and biofilm formation of Pseudomonas spp.

The studies on motility and biofilm formation phenotypes of Pseudomonas spp. due to Pi stress reported changes similar to those occurring in V. cholerae. In P. aeruginosa, Pi limitation promoted bacterial hyper-swarming through PhoB upregulation of rhlR expression resulting in increased rhamnolipid production.⁵¹ The rhamnolipid biosurfactants facilitate swarming motility. This may explain the hyper-motile phenotype exhibited by the Δpst mutant and the motility defect observed in the ΔphoB mutant.⁵²

Moreover, Monds et al.⁵³ showed that P. fluorescens lost its ability to form biofilm in response to Pi limitation through Pho regulon activation. For biofilm formation, P. fluorescens requires production of the LapA adhesin that is responsible for the transition from reversible to irreversible attachment, and that is stimulated by c-di-GMP. In low Pi conditions, the level of c-di-GMP was decreased through PhoB inducing the PDE activity of RapA. This inhibition of P. fluorescens biofilm formation in low Pi also requires the Pho-dependent activation of ApaH, an enzyme that degrades Ap4A, the dinucleoside polyphosphate suggested to be an “alarmone” that signals the onset of oxidative stress.⁵⁴

Effect of Phosphate Availability on Virulence Gene Expression in Pathogenic E. coli

We previously reviewed the importance of the Pho regulon for Escherichia coli virulence and the relationship between the Pi metabolism and pathogenicity.^55,56 More recently, other studies have reported virulence mechanisms depending on the environmental Pi or the Pho/Pst systems.

Extra-intestinal pathogenic E. coli (ExPEC)

ExPEC are an important group of pathogenic E. coli that cause a wide range of human and animal diseases. The constitutive expression of the Pho regulon through inactivation of the Pst system was demonstrated in ExPEC strain 5131 causing porcine septicemia,^57,58 the avian pathogenic E. coli (APEC) serogroup O78 strain χ7122,⁴⁵ and uropathogenic E. coli (UPEC) strain CFT073.⁵⁹ In all these strains, the Pho constitutive expression resulting from deletion of the Pst system attenuated virulence and virulence attributes, including sensitivity to hydrogen peroxide and serum, decreased production of type 1 fimbriae and cell surface modification. In the case of UPEC strain CFT073, the constitutive activation of the Pho regulon altered expression of regulators fimB, ipuA, and ipbA (encoding recombinases, mediating inversion of the fim promoter), and decreased the level of the alarmone ppGpp that led to reduced type 1 fimbriae production and mouse bladder colonization.⁵⁹ However, in APEC, the PhoR-mediated constitutive activation of the Pho regulon was specifically shown to be critical for virulence, rather than inactivation of the Pst system alone. Indeed, a point mutation in phoR, which resulted in constitutive activation of the Pho regulon, independent of Pi transport and or inactivation of the Pst system, also resulted in attenuation of APEC virulence.⁶⁰

Diarrheagenic E. coli and Citrobacter rodentium

For intestinal pathogenic E. coli, the implication of the Pho regulon in virulence modulation was demonstrated for enteropathogenic E. coli (EPEC) and enterohemorrhagic E. coli (EHEC). These 2 diarrheagenic E. coli groups, together with C. rodentium, belong to a class labeled as attaching-effacing E. coli (AEEC). They share the locus of enterocyte effacement (LEE) encoding the type III-secretion system (T3SS) and its effectors and the ability to cause the A/E phenotype. A deletion of the Pst system in different strains among the EPEC showed either an in vitro or ex vivo defect of the attachment and adhesion to epithelial cells. Moreover, it has been shown that a pst mutant of C. rodentium is less virulent for mice, its natural host,⁶¹ and recent studies reported that the Pho regulon regulates the virulence of C. rodentium through PhoB control of DegP (serine protease) and NleG8 (a T3SS protein).⁶² In EHEC, PhoB directly modulated LEE gene expression as well as proteins secreted by T3SS.⁶³ Moreover, EHEC can release Shiga toxins (Stx) that cause bloody diarrhea and hemolytic uremic syndrome (HUS). In vitro, growth in Pi-limiting media seems to increase production and secretion of Stx2 through increased stx2 transcription mediated by PhoB.⁶³ Also, we previously showed that the PhoB protein is critical for EHEC persistence in co-culture with amoebae.⁶⁴ This could be linked to an adaptive response of EHEC to low Pi after exit from the mammalian host, as EHEC has been found in aquatic poor nutrient environments,⁶⁵ where it might co-exist with protozoan predators that would promote its survival outside of the mammalian gut.

Other Phenotypes and Bacterial Virulence Tied to Phosphate and the Pho Regulon

In recent years, additional reports have linked extracellular Pi availability, the PhoB regulator and/or the Pst system to different bacterial phenotypes and virulence-related determinants. Here we describe some virulence phenotypes where the genetic mechanisms are not yet fully elucidated.

Biofilm formation and antibiotic resistance

Biofilm formation by the plant pathogen Agrobacterium tumefaciens is enhanced in low Pi conditions through increased attachment by unipolar polysaccharide (UPP), and is controlled by the PhoB-PhoR regulatory system that is also essential for viability of A. tumefaciens.^66,67 Proteus mirabilis is the most common etiological agent responsible for complicated urinary tract infections (UTIs). Its survival in the urinary tract depends to its ability to produce a battery of virulence factors, including urease, flagella, fimbriae, hemolysin and formation of crystalline biofilm on indwelling catheters.^68-70 The abolition of the Pst system in P. mirabilis caused a defect in biofilm formation when grown in human urine.⁷¹ As long as bacteria use the Pho regulon to regulate attachment to surfaces and aggregate to form biofilms, formation of these sessile communities and their inherent resistance to antimicrobial agents are at the root of many persistent and chronic bacterial infections.⁶⁶

The lower susceptibility of biofilm-grown bacteria to antimicrobial agents (antibiotics, biocides) has been extensively investigated.^72-74 Vilain et al.⁷⁵ showed that Pi deprivation of E. coli in gel-entrapped conditions (like bacteria in natural biofilms), is associated with a high β-lactam antibiotic resistance (latamoxef). Furthermore, it is known that Pi excess in the medium suppresses the biosynthesis of many secondary metabolites such as antibiotics.^76,77 As mentioned above, cross-talk activation of PhoB comes from VanS, the HK partner of the VanR–VanS TCS that orchestrates vancomycin resistance in Enterococcus faecium.²⁰ These findings imply a role for Pi homeostasis and PhoB in a complex regulation of antibiotic stress resistance.

Oxidative stress resistance

Several studies reported the activation of oxidative stress resistance in conditions of Pi starvation or when Pst is abolished, and that both situations activate the Pho regulon. These reports concerned different bacterial species: Sinorhizobium meliloti, P. aeruginosa and A. tumefaciens,⁷⁸ V. cholerae biotype El Tor,⁷⁹ avian pathogenic E. coli,⁸⁰ and C. rodentium.⁶¹ Taken together, these studies indicate that Pho regulon activation leads to increased gene expression of catalases that protect bacteria by degrading hydrogen peroxide. There was also greater gene expression of genes encoding DNA protection proteins and superoxide dismutase. However, since the major oxidative stress response regulators, OxyR and SoxRS seem not to be under control of the Pho regulon induction, the pathway linking the reduction of oxidative stress functions to Pi management remains unknown.

Bacterial secretion systems

Gram-negative bacteria use a variety of secretion systems to target proteins to both prokaryotic and eukaryotic cells. Secretion systems such as the type III, IV, and VI secretion (T3SS, T4SS, and T6SS) pathways, translocate substrates directly into recipient cells in a contact-dependent manner, and constitute important virulence factors. Recent observations link the regulation of Pi homeostasis with Sec-independent secretion systems in some bacterial species. A well-studied example occurs in the pathogen Edwardsiella tarda.

E. tarda colonizes a broad range of aquatic species in addition to fish.⁸¹ It can also be found in the intestinal tract of birds, reptiles, and mammals including humans.⁸² The T3SS and T6SS were determined as the two most important virulence mechanisms in E. tarda.^83,84 In response to both, Pi and iron availabilities, Chakraborty et al.⁸⁵ investigated the effects of PhoB and Fur regulators on T3SS and T6SS in this organism. The authors demonstrated that EsrC (a positive regulator of both T3SS and T6SS), and PhoB, both bind to the evpA promoter (E. tarda virulence protein) to regulate directly and positively gene transcription of T6SS. In addition, PhoB interacts with PhoU to activate esrC. Fur however, senses high iron and inhibits EsrC-binding to evpA. It was then concluded that PhoB and Fur negatively and indirectly cross-talk via unidentified factors to regulate the T3SS and T6SS in E. tarda. As such, the regulation of virulence genes through the PhoB/Pi pathway becomes more complex through interactions with other master regulators.

Pho regulon expression during infection and immune system hijack

Depletion of extracellular Pi after a surgical injury via phosphatonin induction of phosphaturia resulted in a reduction of Pi in serum.⁸⁶ Under these Pi-limiting conditions, it has been shown that in Bacteroides fragilis PhoB was necessary for successful infection and survival in peritoneal abscesses.⁸⁷ In this study, activation of the Pho regulon induced a shift of B. fragilis from gut symbiosis to pathogenicity. Recently, it has been shown that Mycobacterium tuberculosis requires the Pst system to negatively regulate activity of RegX3 (a PhoB analog) in response to in vivo available Pi.⁸⁸ The authors showed that the M. tuberculosis Pst system was essential for virulence in mice and to counteract IFN-γ-dependent host immunity. Using selective capture of transcribed sequences (SCOTS), we previously showed that APEC O78 strain χ7122 can express PhoB in vivo in an infectious context, in infected chickens.⁸⁹ However, the pst deletion increases susceptibility of APEC O78 to rabbit serum, while this strain was not killed by chicken serum.⁹⁰ This suggests the presence of species specific differences in host innate immune defenses and complement-mediated killing.

Summary

All bacterial species including pathogens and commensals are equipped to cope and adapt to nutritional limitation. In most cases, the same input signal such as Pi availability is directly controlled by Pst and the PhoB/R TCS, allowing a Pi homeostasis response. Depending on the level of Pi signal, pathogenic bacteria regulate different downstream systems such as quorum sensing regulatory networks and production of c-di-GMP, adhesins, or cytotoxins. This results in virulence variation and different adaptive changes including biofilm formation, motility, and antimicrobial and oxidative stress resistance.

Disclosure of Potential Conflicts of Interest

The authors declare that they have no competing interests.

Acknowledgments

We thank Judith Kashul for editing services.

Funding

We thank the Natural Sciences and Engineering Research Council of Canada (RGPIN 250129-07) for financial support to C.M.D. and (RGPIN SD-25120-09) to J.H. and Fonds de la recherche du Québec en nature et technologies to J.H. (FRQNT PT165375) and a studentship to S.M.C. from Fonds CRIPA (FRQNT Regroupements stratégiques 111946).