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Infect Immun. Feb 2009; 77(2): 667–675.
Published online Nov 10, 2008. doi:  10.1128/IAI.01027-08
PMCID: PMC2632022

The Potassium Transporter Trk and External Potassium Modulate Salmonella enterica Protein Secretion and Virulence[down-pointing small open triangle]

Abstract

Potassium (K+) is the most abundant intracellular cation and is essential for many physiological functions of all living organisms; however, its role in the pathogenesis of human pathogens is not well understood. In this study, we characterized the functions of the bacterial Trk K+ transport system and external K+ in the pathogenesis of Salmonella enterica, a major food-borne bacterial pathogen. Here we report that Trk is important for Salmonella to invade and grow inside epithelial cells. It is also necessary for the full virulence of Salmonella in an animal infection model. Analysis of proteins of Salmonella indicated that Trk is involved in the expression and secretion of effector proteins of the type III secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1) that were previously shown to be necessary for Salmonella invasion. In addition to the role of the Trk transporter in the pathogenesis of Salmonella, we discovered that external K+ modulates the pathogenic properties of Salmonella by increasing the expression and secretion of effector proteins of the SPI1-encoded TTSS and by enhancing epithelial cell invasion. Our studies demonstrated that K+ is actively involved in the pathogenesis of Salmonella and indicated that Salmonella may take advantage of the high K+ content inside host cells and in the intestinal fluid during diarrhea to become more virulent.

Potassium (K+) is the most abundant intracellular cation in all living organisms, including bacteria (22). Previous studies on K+ have focused mostly on its role in the physiology of eukaryotic and prokaryotic organisms, while its role, if any, in the pathogenesis of human pathogens has not been well characterized. Human pathogens often need to survive and grow in conditions with a wide range of K+ concentrations, from environmental niches such as rivers, soil, and sewage with low K+ concentrations to inside host cells with high K+ concentrations (100 to 160 mM) (22). Therefore, it is important to understand how pathogens adapt to various K+ concentrations and how K+ affects pathogens.

Bacteria maintain a relatively constant intracellular K+ concentration (300 to 500 mM) for many essential cellular functions, including maintenance of cell turgor and homeostasis, adaptation of cells to osmotic conditions, and activation of cytoplasmic enzymes (8, 15). Since bacteria are exposed to a wide range of external K+ concentrations, they use a number of transporters and efflux pumps to maintain their intracellular K+ concentrations. The best-characterized K+ transport systems include the Trk, Kdp, and Kup K+ transport systems in gram-negative bacteria. The Trk system is a low-affinity, rapid-transport system that is the main K+ transporter at neutral or alkaline pH (7, 15, 43). It is a multiunit protein complex formed by gene products that are constitutively expressed. The Kdp system is a high-affinity K+ transport system that is induced in low-K+ environments (K+ concentration, 5 mM or less) (15, 17). The Kup system has an affinity for K+ similar to that of the Trk system and is believed to be the major K+ transport system under acidic conditions (44, 48). Previous studies showed that Salmonella trkA (sapG), which encodes an essential NAD+ binding subunit of the Trk system, was necessary for resistance to antimicrobial peptides (37). TrkA of Vibrio vulnificus has been reported to be required for serum, protamine, and polymyxin B resistance (9). These results suggest that intracellular K+ is important for the virulence characteristics of bacterial pathogens.

Salmonella enterica is a gram-negative bacterium and a major human pathogen that causes significant mortality and morbidity worldwide. In this study, we analyzed the role of the Trk K+ transport system in the pathogenesis of Salmonella and investigated the influence of external K+ on the virulence characteristics of Salmonella. Our results demonstrate that both intracellular K+ and external K+ modulate the virulence characteristics of Salmonella.

MATERIALS AND METHODS

Reagents.

Growth media for bacteria and HeLa cells were purchased from BD Diagnostics (Sparks, MD) and Invitrogen (Carlsbad, CA), respectively. Chemicals and antibiotics were obtained from Sigma-Aldrich (St. Louis, MO) unless otherwise indicated. Restriction and modifying enzymes for manipulation of DNA were obtained from New England Biolabs (Ipswich, MA). Custom oligonucleotides were purchased from Sigma Genosys (The Woodlands, TX).

Bacterial strains and culture conditions.

S. enterica serovar Enteritidis isolate SE2472 (a clinical isolate) was used as the wild-type parental strain in all experiments (Table (Table1)1) (30-32). Escherichia coli DH5α was used for construction of recombinant plasmid DNA (Table (Table11).

TABLE 1.
Bacterial strains and plasmids

All bacteria were routinely cultured in Luria-Bertani (LB) broth at 37°C with shaking at 225 rpm (3). A minimal K+ medium consisting of 6.78 g of Na2HPO4/liter, 2.9587 g of NaH2PO4·H2O/liter, 10 g of NaCl/liter, 1 g of NH4Cl/liter, 4 g of glucose/liter, 2 mM MgSO4, and 0.2 mM CaCl2 (pH 7) was used to examine the role of K+ in bacterial growth. Although no K+ was added to the medium, the medium contained approximately 0.2 mM K+ from K+ contaminants in the salts used, as measured with a K+-selective electrode (Denver Instruments, Denver, CO). Antibiotics were added when necessary.

Growth curves and stress resistance assays.

The growth and survival of bacteria in various media and with various chemicals were determined as previously described (32). The following media and conditions were used to determine the growth and stress resistance of bacteria: Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum, mouse serum, LB broth, LB broth containing 5% sodium dodecyl sulfate (SDS) or 1 M NaCl, and LB broth at pH 4.0. Survival of bacteria at 50°C was determined in LB broth incubated at 50°C without shaking. Potassium chloride was added as a K+ supplement when appropriate. In each assay, bacterial concentrations were determined by plating appropriately diluted aliquots onto LB agar plates.

Invasion of HeLa cells by Salmonella and intracellular growth of Salmonella in HeLa cells.

A HeLa cell invasion assay was performed as described previously (32). Briefly, all Salmonella strains were cultured in LB broth overnight at 37°C without shaking. Antibiotics were added when appropriate. An overnight culture of bacteria was added to HeLa cells at multiplicities of infection of 5:10 to 10:1, and intracellular bacteria were quantified after 1 or 2 h of incubation for invasion, which was followed by incubation in the presence of 50 μg/ml of gentamicin to kill extracellular bacteria. The invasiveness of Salmonella was measured by determining the ratio of intracellular bacteria, which was calculated as follows: (number of intracellular bacteria/number of input bacteria) × 100.

The intracellular growth of Salmonella in HeLa cells was determined by infecting HeLa cells with Salmonella for 1 h. The HeLa cells were then washed with phosphate-buffered saline to remove extracellular bacteria and incubated in fresh medium containing 50 μg/ml of gentamicin for 1 h to kill extracellular bacteria. A set of cells was washed and lysed to quantify the intracellular bacteria and used as the zero-time sample. Additional sets of cells were harvested after an additional 4 or 8 h of incubation to quantify intracellular Salmonella (4- and 8-h samples). The growth of Salmonella inside HeLa cells was measured by determining the increase in the number of intracellular Salmonella cells, which was calculated by dividing the quantity of intracellular bacteria in a 4- or 8-h sample by the quantity of intracellular bacteria in the zero-time sample.

Epitope tagging of Salmonella proteins and analysis of tagged proteins.

Protein tagging was carried out as described by Uzzau et al. (45), except that both six-His and FLAG tags were used. First, plasmid pUC-H1PF1 was constructed as a template. This plasmid was designed to contain a kanamycin resistance cassette (Kanr) and the DNA sequence encoding the six-His and FLAG small epitope tags (45). Kanr in plasmid pKD4 (Table (Table1)1) was amplified by PCR using primers H1PF1(fwd) and H1PF1(rev) (Table (Table2).2). Primer H1PF1(fwd) contained a BamHI linker, the sequence encoding the six-His and FLAG epitopes, and the sequence corresponding to nucleotides 31 to 49 of plasmid pKD4. Primer H1PF1(rev) contained an EcoRI linker and the sequence corresponding to nucleotides 1485 to 1507 of plasmid pKD4. The PCR product was then cloned into plasmid pUC19 (New England Biolabs, Ipswich, MA) between the BamHI and EcoRI sites. The sequence of the resulting plasmid, pUC-H1PF1, was verified, and this plasmid was used as template in PCRs with SipA-, SipC-, or SopB-specific primers listed in Table Table2.2. Each PCR produced a DNA module consisting of the 3′ end of the target gene, FLAG and six-His tag nucleotides, the stop codon, Kanr, and the target gene sequence immediately following the stop codon. The DNA module was introduced into the Salmonella genome, the sequence was verified, and the module was transduced into fresh SE2472 (12). Kanr was subsequently removed using plasmid pCP20 (12), and the sequence of the resulting strain was verified. When necessary, mutations were transduced into strains with epitope tags by general transduction with phage P22, and phage-free colonies were used for analysis of expression (35).

TABLE 2.
Oligonucleotide primers

The levels of the tagged proteins in bacterial lysates and culture supernatant were determined by Western blot analysis. Bacterial lysates were prepared by centrifuging bacterial cultures, resuspending the bacterial pellets in sodium dodecyl sulfate (SDS) sample buffer (50 mM Tris-HCl [pH 6.8], 100 mM dithiothreitol, 2% SDS, 10% glycerol, 0.1% bromophenol blue), and boiling the preparations for 10 min (42). Secreted proteins were prepared as described by Komoriya et al. (23). For Western blot analysis, a monoclonal anti-FLAG antibody (Sigma-Aldrich, St. Louis, MO) was used at a 1:1,000 dilution as the primary antibody, and horseradish peroxidase-conjugated sheep anti-mouse immunoglobulin G (GE Healthcare, Piscataway, NJ) was used at a 1:2,000 dilution as the secondary antibody. The signal was visualized by the enhanced chemiluminescence method (GE Healthcare, Piscataway, NJ). Coomassie blue-stained gels were used for comparison of protein bands.

Mutagenesis of trkA and complementation of the mutant.

A trkA deletion (ΔtrkA) mutant of SE2472 was constructed using the one-step mutagenesis method of Datsenko and Wanner, as described previously (12, 30). Primers TrkA5KO and TrkA3KO were used to replace the trkA sequence between nucleotides 91 and 1270 with Kanr (Table (Table2).2). The deletion mutation was subsequently transduced into fresh SE2472 by general transduction using phage P22 (35), and phage-free colonies were used for further analysis. A nonpolar trkA mutant was also generated as described by Datsenko and Wanner (12).

To complement the ΔtrkA mutant of SE2472, we cloned the wild-type allele of trkA into plasmid pRB3-273C (6). Primers TrkA5H3 and TrkA3H3 were used to amplify trkA from SE2472 (Table (Table2),2), and the PCR product was digested with HindIII and cloned into pRB3-273C at the HindIII site. The sequence of the resulting plasmid, pRB3-trkA, was confirmed, and the plasmid was transformed into the ΔtrkA mutant of SE2472 to complement the deletion mutation (30). Vector pRB3-273C was also transformed into the ΔtrkA mutant of SE2472 as a control for any possible effect of the plasmid transformation alone (30).

Infection of mice with Salmonella.

Salmonella cultured overnight in LB broth at 37°C with shaking at 225 rpm was used in all animal infection experiments. Mice were infected intragastrically or intraperitoneally, and the 50% lethal dose (LD50) was determined by infecting groups of five mice with 10-fold dilutions of bacteria in phosphate-buffered saline (32, 41).

RESULTS

Construction of a Salmonella ΔtrkA mutant and analysis of its growth properties.

To determine the roles of K+ transport systems in the pathogenesis of Salmonella, we constructed a deletion mutant with a mutation in the low-affinity Trk K+ transport system of S. enterica serovar Enteritidis SE2472 (ΔtrkA) (32). In the ΔtrkA mutant of SE2472, the trkA open reading frame, which encodes a subunit essential for the Trk function, was replaced by Kanr (12, 30). The ΔtrkA mutant formed smaller colonies on LB agar but otherwise had the same colony morphology as wild-type strain SE2472 (data not shown).

To determine if TrkA is necessary for the growth of bacteria, we constructed growth curves for the ΔtrkA mutant and wild-type strain SE2472 grown in LB broth, minimal K+ medium, and minimal K+ medium supplemented with various concentrations of potassium chloride (KCl) as the source of K+. Using a K+-selective electrode (Denver Instruments, Denver, CO), the concentration of K+ in LB broth was determined to be approximately 8 mM (data not shown), a concentration at which Trk is expected to be the main K+ transporter. Minimal K+ medium was prepared without any K+ salt; however, it contained approximately 0.2 mM K+ due to K+ contamination in the salts used for the medium (data not shown). In LB broth, minimal K+ medium, or minimal K+ medium supplemented with less than 10 mM K+, the growth of the ΔtrkA mutant was similar to the growth of wild-type strain SE2472 (data not shown). When more than 10 mM supplemental K+ was used, the ΔtrkA mutant displayed a small but consistent growth delay compared to the wild-type strain between 4 to 8 h of growth (Fig. (Fig.1).1). The ΔtrkA mutant appeared to have the same lag phase as wild-type strain SE2472, but it had a lower growth rate in the log phase of growth (Fig. (Fig.1).1). The growth delay was observed at all K+ concentrations that we tested (10 to 100 mM), and the delay was rescued by transforming the ΔtrkA mutant with plasmid pRB3-trkA, which contained a wild-type allele of trkA in plasmid vector pRB3-273C (Fig. (Fig.1)1) (6). Transformation with the pRB3-273C vector alone had no effect (data not shown). These results indicate that TrkA is involved in optimal growth of Salmonella at relatively high concentrations of K+.

FIG. 1.
Growth of the ΔtrkA mutant of Salmonella with a high concentration of potassium. Wild-type strain SE2472, the ΔtrkA mutant, and the complemented mutant (ΔtrkA-comp) were cultured in minimal K+ medium supplemented with 50 ...

The resistance of the ΔtrkA mutant to general stress conditions was tested to determine the overall fitness of this mutant. The ΔtrkA mutant was exposed to salt (1 M NaCl), detergent (5% SDS), heat (incubation at 50°C), and acidic medium (pH 4). No difference in bacterial growth or survival was observed between the ΔtrkA mutant and wild-type strain SE2472 (data now shown). Since the TrkA protein of V. vulnificus was reported to be necessary for serum resistance (9), we tested the survival and growth of the ΔtrkA mutant of SE2472 in mouse serum, and we detected no defect in the ΔtrkA mutant (data not shown).

TrkA is necessary for the invasion of epithelial cells by Salmonella.

Since epithelial cell invasion is crucial in natural infection by Salmonella, we examined whether TrkA affects the ability of Salmonella to invade epithelial cells. The invasion of HeLa cells by the ΔtrkA mutant and wild-type strain SE2472 was assayed, and the ratio of the number of intracellular bacteria to the number of input bacteria was determined after 1 or 2 h of incubation. The ratio obtained for wild-type strain SE2472 intracellular bacteria at 1 h postinfection was arbitrarily defined as 100%, and ratios of intracellular bacteria for other samples were expressed as relative values (Fig. (Fig.2A).2A). At both 1 and 2 h postinfection, the ratios for the intracellular ΔtrkA mutant were approximately 35 to 40% of those for wild-type strain SE2472 (Fig. (Fig.2A).2A). The invasion defect of the ΔtrkA mutant was rescued by transformation with plasmid pRB3-trkA (Fig. (Fig.2A),2A), while transformation with the vector alone had no effect (data not shown). The invasion defect of the ΔtrkA mutant was not due to a growth defect in the medium used for the assay (DMEM with 10% fetal bovine serum), as the ΔtrkA mutant grew as well as wild-type strain SE2472 in this medium (data not shown). DMEM contains approximately 5 mM K+ as measured with a K+-selective electrode (data not shown); therefore, the normal growth of the ΔtrkA mutant in DMEM is consistent with the previous observation that the ΔtrkA mutant had no growth defect when the K+ concentration was less than 10 mM.

FIG. 2.
Epithelial cell invasion and intracellular growth of the wild-type strain and the ΔtrkA mutant of Salmonella. (A) Invasion of HeLa cells. The ratio of the number of intracellular bacteria to the number of input bacteria was determined for wild-type ...

The ΔtrkA mutant of Salmonella is defective for growth inside HeLa cells.

In addition to determining the ability of the ΔtrkA mutant of SE2472 to invade, we also examined the growth of the ΔtrkA mutant inside HeLa cells and compared it to the growth of wild-type strain SE2472. Since the ΔtrkA mutant was less invasive than wild-type strain SE2472 (Fig. (Fig.2A),2A), the ΔtrkA mutant and wild-type strain SE2472 were used to infect HeLa cells at multiplicities of infection of 10:1 and 2:1, respectively, in order to obtain similar numbers of intracellular bacteria immediately after invasion. The infected HeLa cells were incubated for up to 8 h in the presence of gentamicin to stop further bacterial invasion, and the intracellular bacteria were quantified to determine the growth of the ΔtrkA mutant and wild-type strain SE2472 inside HeLa cells. Compared to the initial levels of intracellular bacteria at zero time, the levels of wild-type strain SE2472 were 13.4- ± 2.2- and 16.4- ± 3.1-fold higher at 4 and 8 h postinfection, respectively, while the levels of the ΔtrkA mutant were only 4.3- ± 0.8- and 6.0- ± 1.4-fold higher, respectively (Fig. (Fig.2B).2B). Complementation with plasmid pRB3-trkA partially rescued the growth defect of the ΔtrkA mutant inside HeLa cells (Fig. (Fig.2B).2B). The assay was terminated at 8 h, when the number of bacteria started to decrease, possibly because HeLa cells were being damaged by intracellular Salmonella and Salmonella was exposed to the gentamicin in the medium (data not shown). The defective intracellular growth of the ΔtrkA mutant was not due to increased susceptibility to the gentamicin used in the assay, as the ΔtrkA mutant had the same susceptibility as the wild-type Salmonella strain to all of the antibiotics that we tested, including ampicillin, chloramphenicol, gentamicin, and tetracycline (data not shown). The observed intracellular growth defect is consistent with the delayed growth the ΔtrkA mutant observed at K+ concentrations higher than 10 mM, since host cells have high concentrations of K+.

The ΔtrkA mutant of Salmonella is less virulent in infection of mice.

To determine if TrkA is important for the virulence of Salmonella, we infected mice with the ΔtrkA mutant and wild-type strain SE2472 and determined the LD50 for intragastric and intraperitoneal infections. For both infection routes, the ΔtrkA mutant had a higher LD50 and was less virulent than wild-type strain SE2472 (P < 0.05, Student's t test) (Table (Table3).3). The virulence defect was rescued in the complemented mutant by plasmid pRB3-trkA, and the difference in LD50 between the complemented ΔtrkA mutant and wild-type strain SE2472 was statistically insignificant (P > 0.4, Student's t test) (Table (Table3),3), suggesting that deletion of trkA was responsible for the decreased virulence of the ΔtrkA Salmonella mutant.

TABLE 3.
LD50 of Salmonella wild-type strain SE2472 and the ΔtrkA mutant in intragastric and intraperitoneal infections of mice

TrkA is important for expression and secretion of selected effector proteins of the SPI1-encoded TTSS of Salmonella.

We next investigated how Trk contributes to the epithelial cell invasion by and virulence of Salmonella. Since invasion of epithelial cells by Salmonella is dependent on the effector proteins of the type III secretion system (TTSS) encoded by Salmonella pathogenicity island 1 (SPI1), we reasoned that the expression and secretion of the effector proteins of the SPI1-encoded TTSS could be altered in the ΔtrkA mutant. We selected SipA, SipC, and SopB for this analysis because these proteins have previously been shown to have important functions in host cell invasion and in the modulation of the host cytoskeleton to facilitate Salmonella infection (20, 21, 40). To monitor the levels of these proteins, we introduced into the chromosome a tandem tag consisting of six histidines and a FLAG tag fused in frame to the C terminus of each protein (45) to generate tagged strains SipA(HF), SipC(HF), and SopB(HF) (Table (Table1).1). No difference between the tagged strains and parental strain SE2472 was observed for growth in vitro and for virulence in mouse infections, suggesting that tagging did not have deleterious effects (data not shown). The ΔtrkA mutation was transduced into each tagged strain, and the mutant strains were complemented by transformation with plasmid pRB3-trkA. Equal quantities of proteins from both whole-cell lysate and culture supernatant were used for each strain. Since less protein was recovered from the culture supernatant of the ΔtrkA mutant, proteins obtained from larger volumes of cultures were used to ensure that the total quantities of proteins from each strain were equivalent (data not shown). The levels of each tagged protein in the whole bacterial lysates and culture supernatants of the corresponding set of tagged strains with the wild-type, mutant, or complemented trkA allele were analyzed by Western hybridization using an anti-FLAG antibody (Fig. (Fig.33).

FIG. 3.
Expression and secretion of selected effector proteins of the SPI1-encoded TTSS by the wild-type strain and ΔtrkA mutant of Salmonella. Epitope-tagged strains SipA(HF), SipC(HF), and SopB(HF) with the wild-type (WT) or ΔtrkA mutant allele ...

We analyzed SipA, SipC, and SopB, and SipA was detected in both the whole bacterial lysate and the culture supernatant, while SipC and SopB were detected only in the culture supernatant (Fig. (Fig.3).3). Compared to the results for the parental strains with the wild-type trkA allele, the level of SipA was lower in the whole bacterial lysate of the ΔtrkA mutant (Fig. (Fig.3A)3A) and the levels of SipA, SipC, and SopB were lower in the culture supernatant of the ΔtrkA mutant (Fig. (Fig.3B).3B). In each complemented ΔtrkA mutant strain, the level of SipA, SipC, or SopB was restored to the level in the parental strain (Fig. (Fig.33).

Potassium and sodium enhance the invasion of epithelial cells by Salmonella.

We showed in the experiments described above that the Trk K+ transport system is necessary for Salmonella to invade and grow in epithelial cells and for expression and secretion of effector proteins of the SPI1-encoded TTSS. Next we examined the effect of external K+ on Salmonella and whether external K+ modulated the invasion of Salmonella and the expression of effector proteins of the TTSS. The invasion of HeLa cells by bacteria cultured with various concentrations of KCl was analyzed, and the results were compared with the results for organisms cultured in LB broth alone. We chose moderate K+ concentrations (50 and 100 mM) that are in the physiological range that Salmonella may encounter in its hosts. When cultured in the presence of either concentration of supplemental KCl, both the wild type and the ΔtrkA mutant showed enhanced invasion of HeLa cells (Fig. (Fig.4A).4A). The level of invasion of HeLa cells by the ΔtrkA mutant cultured with supplemental KCl increased to the level of invasion by wild-type strain SE2472 cultured without the supplement, although the ΔtrkA mutant was consistently less invasive than wild-type strain SE2472 cultured in the same medium (Fig. (Fig.4A).4A). To determine if the enhanced invasion of HeLa cells by Salmonella could also be modulated by other cations that have physiological relevance, we determined the effect of supplemental NaCl on the invasiveness of Salmonella. Na+ is the most abundant extracellular cation, and the plasma level of this cation in humans is 135 to 145 mM. As shown in Fig. Fig.4B,4B, the presence of supplemental NaCl increased the invasiveness of both wild-type SE2472 and the ΔtrkA mutant of SE2472, although the increase in invasion after addition of supplemental NaCl was not as pronounced as the increase in invasion after addition of KCl (Fig. (Fig.4A4A and and4B).4B). This suggests that Salmonella responds to physiological concentrations of both Na+ and K+ and that K+ is likely to have a greater effect on the invasion of HeLa cells by Salmonella.

FIG. 4.
Effects of external potassium and sodium chloride on the invasion of HeLa cells by the wild-type strain and ΔtrkA mutant of Salmonella. Salmonella wild-type strain SE2472 (WT) or the ΔtrkA mutant was cultured overnight in LB broth or LB ...

Potassium and sodium chloride have overlapping yet distinct effects on effector proteins of the SPI1-encoded TTSS in the ΔtrkA mutant of Salmonella.

Since Na+ and K+ enhanced the invasiveness of Salmonella in the HeLa cell invasion assay, we examined the effects of Na+ and K+ on the expression and secretion of effector proteins of the SPI1-encoded TTSS. The wild-type and ΔtrkA mutant SE2472 strains were cultured in LB broth supplemented with 50 or 100 mM KCl or NaCl, and the levels of SipA, SipC, and SopB in the bacterial lysates and secreted proteins were analyzed as described above (Fig. (Fig.55).

FIG. 5.
Comparison of the effects of external potassium and sodium chloride on the expression and secretion of selected effector proteins of the SPI1-encoded TTSS by the wild-type strain (WT) and the ΔtrkA mutant of Salmonella. Epitope-tagged strains ...

Both KCl and NaCl induced expression and secretion of SipA, SipC, and SopB in wild-type strain SE2472 (Fig. (Fig.5A).5A). In this strain addition of potassium chloride appeared to result in stronger induction of expression of SipA and secretion of SopB than addition of NaCl, and KCl and NaCl had similar effects on secretion of SipA and SipC. In the ΔtrkA mutant, however, addition of both 50 and 100 mM KCl resulted in stronger induction of all proteins. The expression of SipA and secretion of SopB were induced only by KCl and not by NaCl, while the secretion of SipA and SipC was induced less by NaCl than by KCl, especially at a concentration of 100 mM. This demonstrated that both KCl and NaCl modulate the expression and secretion of effector proteins of the SPI1-encoded TTSS; however, the effects of KCl and NaCl were not identical, suggesting that the alteration of the expression and secretion of TTSS effector proteins was not simply due to a change in the osmotic pressure, especially in the ΔtrkA mutant of Salmonella.

DISCUSSION

The importance of K+ for bacterial physiology has been studied by several investigators (for a review, see reference 15); however, the function of intracellular and external K+ in the pathogenicity of bacteria has not been well characterized. In this report, we provide direct evidence that maintaining a proper intracellular K+ level is important for the pathogenicity of Salmonella and that external K+ regulates the virulence characteristics of Salmonella. Salmonella lacking the low-affinity K+ transporter Trk invaded epithelial cells less, was less virulent in mouse infections, and expressed and secreted smaller amounts of effector proteins of the SPI1-encoded TTSS (Fig. (Fig.22 and and3,3, and Table Table3),3), while addition of K+ improved epithelial cell invasion and the expression and secretion of the same set of the effector proteins (Fig. (Fig.44 and and5).5). To our knowledge, this is the first study demonstrating that both intracellular K+ and external K+ influence the virulence characteristics of a pathogenic bacterium. The only other example of the effect of K+ on human pathogens is a recent report by Kumar et al., who demonstrated that exposure of Plasmodium sporozoites to the intracellular concentration of K+ enhances infectivity and reduces cell passage activity (24).

The effect of K+ on human pathogens is not unexpected, since K+ is the most abundant cation inside human cells, where the concentrations are more than 100 mM, while the extracellular concentrations are usually less than 10 mM (22). The K+ concentration inside Salmonella-containing vacuoles has not been reported; however, the K+ concentration inside Mycobacterium-containing vacuoles inside macrophages is approximately 20 to 50 mM and is in the range of K+ concentrations that we used in this analysis (47). In the human small intestine where Salmonella infection occurs, the K+ concentration has been reported to be approximately 6 mM (16). However, during diarrhea, the K+ concentration is likely to be much higher due to electrolytes released from damaged intestinal mucosa. Although results of in vitro assays may not be directly applicable to natural infections, our results indicate that it is possible that Salmonella benefits from the increased K+ concentration in the intestinal fluid when diarrhea occurs and becomes more invasive. In addition to the results for K+, we have also shown that Na+, the most abundant extracellular cation, increased the invasion of epithelial cells by Salmonella and the expression and secretion of effector proteins of the SPI1-encoded TTSS (Fig. (Fig.4B4B and and5).5). The concentrations of Na+ that we used are within the range of the normal plasma concentration of Na+ (135 mM), suggesting that Na+ may modulate the infectivity of Salmonella during systemic infection. It has been reported previously that invasion by Salmonella is modulated by oxygen, acids (acetic and butyric acids), bile, and formate in the distal ileum (19, 27, 39, 46). Taken together, these results suggest that Salmonella responds to multiple signals in vivo in order to regulate its invasion and pathogenesis in different organs, and the ability to respond to a wide spectrum of external signals is likely a factor that contributes to Salmonella's success as a human pathogen.

In gram-negative bacteria, the intracellular concentration of K+ is regulated by multiple K+ transport systems, including the Trk and Kup low-affinity systems and the Kdp high-affinity system (for a review, see reference 15). Since the ΔtrkA mutant has normal growth properties except at high concentrations of K+, we believe that the ΔtrkA mutant has relatively normal physiology and that other K+ transport systems can compensate for the lack of Trk. We do not know yet which K+ transport system(s) compensates for the function of Trk in the ΔtrkA mutant of Salmonella or the extent of the compensation. At high concentrations of K+ (10 mM or higher), Kdp is repressed, which probably explains why the ΔtrkA mutant grew more slowly in the log phase (2, 17, 25, 34) (Fig. (Fig.1).1). Both Kup and Kdp likely contribute to maintaining the K+ homeostasis in the absence of Trk at lower K+ concentrations. In E. coli and Salmonella, the kdp operon is normally repressed at a K+ concentration of 5 mM or higher; however, this operon can be expressed at K+ concentrations around 10 mM in the absence of Trk (2, 17, 25, 34). Our preliminary analysis of the Kup transport system indicated that a Δkup ΔtrkA double mutant was more defective in growth, protein secretion, and pathogenesis than either single mutant (unpublished results). A thorough analysis of mutants with deletions in multiple K+ transport systems should provide insight into the specific functions of each K+ transport system.

The SPI1-encoded TTSS is regulated by multiple signals in a highly complex regulatory network (for a review, see reference 14). Factors that have been reported to directly regulate the SPI-encoded TTSS are HilA (1, 4, 13, 28, 29) and InvF (10, 11, 13). Factors that indirectly regulate SPI1 through HilA and InvF include PhoQ/PhoP, HilE, HilC, HilD, RtsA, FimZY, PhoR/PhoB, Fur, BarA/SirA, and EnvZ/OmpR (for a review, see reference 14). Perhaps more relevant for the pathogenesis of Salmonella are the environmental conditions that regulate genes of the SPI1-encoded TTSS, including pH, oxygen, osmolarity, growth phase, and bile (5, 18, 26-28, 33, 38, 39), some of which have been shown to directly affect the invasiveness of Salmonella. For example, low oxygen tension and acetic acid were shown to enhance Salmonella invasion, while bile and butyric acid reduce invasion (27, 39, 46). Our direct comparison of the expression and secretion of the SipA, SipC, and SopB proteins in the presence of KCl and NaCl and the invasiveness of Salmonella cultured with supplemental KCl or NaCl indicated that both KCl and NaCl regulated the effector proteins of the SPI1-encoded TTSS; however, the effects were not identical (Fig. (Fig.44 and and5).5). Addition of potassium chloride resulted in stronger induction of the proteins overall, especially in the ΔtrkA mutant, whereas addition of NaCl had little or no effect on the expression of SipA and the secretion of SipC and SopB (Fig. (Fig.44 and and5).5). This suggests that the signaling pathways involved were not identical for KCl and NaCl, and the induction of the effector proteins of the SPI1-encoded TTSS by NaCl was at least partially dependent on Trk. However, the molecular mechanisms that determine the different effects that KCl and NaCl have on the TTSS of Salmonella lacking the Trk system have not been characterized yet. One hypothesis to explain the difference between supplemental KCl and supplemental NaCl is that they may affect the alternative K+ transporter(s) that Salmonella uses in the absence of the major K+ transporter Trk. Without Trk, the high-affinity Kdp transporter and the low-affinity Kup transporter likely substitute for Trk, and K+ downregulates Kdp but not Kup. As a result, Kup may be more active with supplemental K+ than with supplemental Na+, leading to a state more similar to that of the wild-type bacteria.

After we performed the experiments reported here, Mizusaki et al. reported that the secretion of Sips was induced by sucrose and several salts, including NaCl and KCl, and that NaCl-induced secretion of Sips was due to induction of hilA expression mediated by BarA/SirA but not by EnvZ (36). We do not know yet how the Trk system affects the regulation of SPI1 and why the expression and secretion of effector proteins of the SPI1-encoded TTSS were defective in the ΔtrkA mutant of Salmonella. The defect did not occur because the ΔtrkA mutant expressed less hilA and invF, which encode direct regulators of SPI1 expression. In fact, we found that the expression of both hilA and invF was elevated in the ΔtrkA mutant (unpublished results). An expression profile analysis of the wild-type strain and ΔtrkA mutant of Salmonella and an analysis of physiological changes caused by mutations in K+ transporters and external K+ should provide more insight into how potassium transporters and external K+ regulate the growth and pathogenicity of Salmonella.

Acknowledgments

We thank Hiroshi Nikaido for insightful discussions and C. S. Shashikant for critical comments on the manuscript.

This work was supported by USDA grant CALR-2005-01892 to S.L.

Notes

Editor: A. J. Bäumler

Footnotes

[down-pointing small open triangle]Published ahead of print on 10 November 2008.

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