Dual role of the MgtC virulence factor in host and non-host environments

The Mobilome-Enriched Genome of the Competence-Deficient Streptococcus pneumoniae BM6001, the Original Host of Integrative Conjugative Element Tn5253, Is Phylogenetically Distinct from Historical Pneumococcal Genomes

Streptococcus pneumoniae is an important human pathogen causing both mild and severe diseases. In this work, we determined the complete genome sequence of the S. pneumoniae clinical isolate BM6001, which is the original host of the ICE Tn5253. The BM6001 genome is organized in one circular chromosome of 2,293,748 base pairs (bp) in length, with an average GC content of 39.54%; the genome harbors a type 19F capsule locus, two tandem copies of pspC, the comC1-comD1 alleles and the type I restriction modification system SpnIII. The BM6001 mobilome accounts for 15.54% (356,521 bp) of the whole genome and includes (i) the ICE Tn5253 composite; (ii) the novel IME Tn7089; (iii) the novel transposon Tn7090; (iv) 3 prophages and 2 satellite prophages; (v) 5 genomic islands (GIs); (vi) 72 insertion sequences (ISs); (vii) 69 RUPs; (viii) 153 BOX elements; and (ix) 31 SPRITEs. All MGEs, except for the GIs, produce excised circular forms and attB site restoration. Tn7089 is 9089 bp long and contains 11 ORFs, of which 6 were annotated and code for three functions: integration/excision, mobilization and adaptation. Tn7090 is 9053 bp in size, flanked by two copies of ISSpn7, and contains seven ORFs organized as a single transcriptional unit, with genes encoding for proteins likely involved in the uptake and binding of Mg²⁺ cations in the adhesion to host cells and intracellular survival. BM6001 GIs, except for GI-BM6001.4, are variants of the pneumococcal TIGR4 RD5 region of diversity, pathogenicity island PPI1, R6 Cluster 4 and PTS island. Overall, prophages and satellite prophages contain genes predicted to encode proteins involved in DNA replication and lysogeny, in addition to genes encoding phage structural proteins and lytic enzymes carried only by prophages. ΦBM6001.3 has a mosaic structure that shares sequences with prophages IPP69 and MM1 and disrupts the competent comGC/cglC gene after chromosomal integration. Treatment with mitomycin C results in a 10-fold increase in the frequency of ΦBM6001.3 excised forms and comGC/cglC coding sequence restoration but does not restore competence for genetic transformation. In addition, phylogenetic analysis showed that BM6001 clusters in a small lineage with five other historical strains, but it is distantly related to the lineage due to its unique mobilome, suggesting that BM6001 has progressively accumulated many MGEs while losing competence for genetic transformation.

Virulence Mechanisms of Mycobacterium abscessus: Current Knowledge and Implications for Vaccine Design

Mycobacterium abscessus is a member of the non-tuberculous mycobacteria (NTM) group, responsible for chronic infections in individuals with cystic fibrosis (CF) or those otherwise immunocompromised. While viewed traditionally as an opportunistic pathogen, increasing research into M. abscessus in recent years has highlighted its continued evolution into a true pathogen. This is demonstrated through an extensive collection of virulence factors (VFs) possessed by this organism which facilitate survival within the host, particularly in the harsh environment of the CF lung. These include VFs resembling those of other Mycobacteria, and non-mycobacterial VFs, both of which make a notable contribution in shaping M. abscessus interaction with the host. Mycobacterium abscessus continued acquisition of VFs is cause for concern and highlights the need for novel vaccination strategies to combat this pathogen. An effective M. abscessus vaccine must be suitably designed for target populations (i.e., individuals with CF) and incorporate current knowledge on immune correlates of protection against M. abscessus infection. Vaccination strategies must also build upon lessons learned from ongoing efforts to develop novel vaccines for other pathogens, particularly Mycobacterium tuberculosis (M. tb); decades of research into M. tb has provided insight into unconventional and innovative vaccine approaches that may be applied to M. abscessus. Continued research into M. abscessus pathogenesis will be critical for the future development of safe and effective vaccines and therapeutics to reduce global incidence of this emerging pathogen.

Limitation of phosphate assimilation maintains cytoplasmic magnesium homeostasis

Phosphorus (P) is essential for life. As the fifth-most-abundant element in living cells, P is required for the synthesis of an array of biological molecules including (d)NTPs, nucleic acids, and membranes. Organisms typically acquire environmental P as inorganic phosphate (Pi). While essential for growth and viability, excess intracellular Pi is toxic for both bacteria and eukaryotes. Using the bacterium Salmonella enterica serovar Typhimurium as a model, we establish that Pi cytotoxicity is manifested following its assimilation into adenosine triphosphate (ATP), which acts as a chelating agent for Mg²⁺ and other cations. Our findings identify physiological processes disrupted by excessive Pi and how bacteria tune P assimilation to cytoplasmic Mg²⁺ levels.

Phosphorus (P) is an essential component of core biological molecules. In bacteria, P is acquired mainly as inorganic orthophosphate (Pi) and assimilated into adenosine triphosphate (ATP) in the cytoplasm. Although P is essential, excess cytosolic Pi hinders growth. We now report that bacteria limit Pi uptake to avoid disruption of Mg²⁺-dependent processes that result, in part, from Mg²⁺ chelation by ATP. We establish that the MgtC protein inhibits uptake of the ATP precursor Pi when Salmonella enterica serovar Typhimurium experiences cytoplasmic Mg²⁺ starvation. This response prevents ATP accumulation and overproduction of ribosomal RNA that together ultimately hinder bacterial growth and result in loss of viability. Even when cytoplasmic Mg²⁺ is not limiting, excessive Pi uptake increases ATP synthesis, depletes free cytoplasmic Mg²⁺, inhibits protein synthesis, and hinders growth. Our results provide a framework to understand the molecular basis for Pi toxicity. Furthermore, they suggest a regulatory logic that governs P assimilation based on its intimate connection to cytoplasmic Mg²⁺ homeostasis.

The intracellular phase of extracellular respiratory tract bacterial pathogens and its role on pathogen-host interactions during infection

PURPOSE OF REVIEW

An initial intracellular phase of usually extracellular bacterial pathogens displays an important strategy to hide from the host's immune system and antibiotics therapy. It helps the bacteria, including bacterial pathogens of airway diseases, to persist and eventually switch to a typical extracellular infection. Several infectious diseases of the lung are life-threatening and their control is impeded by intracellular persistence of pathogens. Thus, molecular adaptations of the pathogens to this niche but also the host's response and potential targets to interfere are of relevance. Here we discuss examples of historically considered extracellular pathogens of the respiratory airway where the intracellular survival and proliferation is well documented, including infections by Staphylococcus aureus, Bordetella pertussis, Haemophilus influenzae, Pseudomonas aeruginosa, and others.

RECENT FINDINGS

Current studies focus on bacterial factors contributing to adhesion, iron acquisition, and intracellular survival as well as ways to target them for combatting the bacterial infections.

SUMMARY

The investigation of common and specific mechanisms of pathogenesis and persistence of these bacteria in the host may contribute to future investigations and identifications of relevant factors and/or bacterial mechanisms to be blocked to treat or improve prevention strategies.

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

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

Whole-Genome Comparative and Pathogenicity Analysis of Salmonella enterica subsp. enterica Serovar Rissen

Salmonella are a type of bacteria known to cause food-borne illness. Their host range varies widely, and their susceptibility to the host determines its pathogenicity. Salmonella enterica serovar Rissen (S. Rissen) is a widely distributed serotype; however, its virulence and pathogenicity are poorly understood. In this study, the pathogenicity and antibiotic resistance of a representative S. Rissen isolate were investigated. The cell model results showed that S. Rissen preferred to replicate in human macrophage cells U937 compared to murine macrophage cells RAW264.7, suggesting that it has a level of host adaptability. Genome sequencing and comparison analysis revealed that the distribution and nonsynonymous single nucleotide polymorphisms of virulence factors in S. Rissen were similar to those in S. Typhi rather than to those in S. Typhimurium. Taken together, our results suggest that although S. Rissen is a common serotype distributed in swine herds, pork and chicken products, it has strong ability to infect humans.

Mg2+ homeostasis and transport in cyanobacteria - at the crossroads of bacterial and chloroplast Mg2+ import

Magnesium cation (Mg2+) is the most abundant divalent cation in living cells, where it is required for various intracellular functions. In chloroplasts and cyanobacteria, established photosynthetic model systems, Mg2+ is the central ion in chlorophylls, and Mg2+ flux across the thylakoid membrane is required for counterbalancing the light-induced generation of a ΔpH across the thylakoid membrane. Yet, not much is known about Mg2+ homoeostasis, transport and distribution within cyanobacteria. However, Mg2+ transport across membranes has been studied in non-photosynthetic bacteria, and first observations and findings are reported for chloroplasts. Cyanobacterial cytoplasmic membranes appear to contain the well-characterized Mg2+ channels CorA and/or MgtE, which both facilitate transmembrane Mg2+ flux down the electrochemical gradient. Both Mg2+ channels are typical for non-photosynthetic bacteria. Furthermore, Mg2+ transporters of the MgtA/B family are also present in the cytoplasmic membrane to mediate active Mg2+ import into the bacterial cell. While the cytoplasmic membrane of cyanobacteria resembles a 'classical' bacterial membrane, essentially nothing is known about Mg2+ channels and/or transporters in thylakoid membranes of cyanobacteria or chloroplasts. As discussed here, at least one Mg2+ channelling protein must be localized within thylakoid membranes. Thus, either one of the 'typical' bacterial Mg2+ channels has a dual localization in the cytoplasmic plus the thylakoid membrane, or another, yet unidentified channel is present in cyanobacterial thylakoid membranes.

The mgtCBR mRNA Leader Secures Growth of Salmonella in Both Host and Non-host Environments

Upon intracellular cues, bacterial mRNA leaders often form secondary structures that determine expression of a downstream protein-coding region(s), thereby providing bacteria with a mechanism to control the amounts of necessary proteins in the right locales. Here we describe a polycistronic mRNA leader that secures bacterial growth by preventing dysregulated expression of the protein-coding regions. In Salmonella, the mgtCBR mRNA encodes the virulence protein MgtC and the Mg²⁺ transporter MgtB. A mutant designed to produce leaderless mgtCBR mRNA induced MgtC and MgtB in conditions that promote mgtC transcription. The dysregulated expression of MgtC and MgtB impaired bacterial growth under all such non-host environments. While MgtC, but not MgtB, normally reduces ATP levels in a process requiring the F₁F₀ ATP synthase, dysregulated MgtC and MgtB reduced ATP levels independently of the F₁F₀ ATP synthase, which correlated with the mutant’s growth defect. The mutant showed dysregulated MgtC expression and attenuated survival inside macrophages. While MgtB normally does not affect the phenotype, MgtB impaired intramacrophage survival of the mutant in the presence of MgtC. We provide an example showing that a polycistronic mRNA leader prevents the dysregulated function of protein-coding regions to allow bacteria to proliferate across complex niches.

Betaproteobacteria are predominant in drinking water: are there reasons for concern?

Betaproteobacteria include some of the most abundant and ubiquitous bacterial genera that can be found in drinking water, including mineral water. The combination of physiology and ecology traits place some Betaproteobacteria in the list of potential, yet sometimes neglected, opportunistic pathogens that can be transmitted by water or aqueous solutions. Indeed, some drinking water Betaproteobacteria with intrinsic and sometimes acquired antibiotic resistance, harbouring virulence factors and often found in biofilm structures, can persist after water disinfection and reach the consumer. This literature review summarises and discusses the current knowledge about the occurrence and implications of Betaproteobacteria in drinking water. Although the sparse knowledge on the ecology and physiology of Betaproteobacteria thriving in tap or bottled natural mineral/spring drinking water (DW) is an evidence of this review, it is demonstrated that DW holds a high diversity of Betaproteobacteria, whose presence may not be innocuous. Frequently belonging to genera also found in humans, DW Betaproteobacteria are ubiquitous in different habitats, have the potential to resist antibiotics either due to intrinsic or acquired mechanisms, and hold different virulence factors. The combination of these factors places DW Betaproteobacteria in the list of candidates of emerging opportunistic pathogens. Improved bacterial identification of clinical isolates associated with opportunistic infections and additional genomic and physiological studies may contribute to elucidate the potential impact of these bacteria.

Synthetic hydrophobic peptides derived from MgtR weaken Salmonella pathogenicity and work with a different mode of action than endogenously produced peptides

Due to the antibiotic resistance crisis, novel therapeutic strategies need to be developed against bacterial pathogens. Hydrophobic bacterial peptides (small proteins under 50 amino acids) have emerged as regulatory molecules that can interact with bacterial membrane proteins to modulate their activity and/or stability. Among them, the Salmonella MgtR peptide promotes the degradation of MgtC, a virulence factor involved in Salmonella intramacrophage replication, thus providing the basis for an antivirulence strategy. We demonstrate here that endogenous overproduction of MgtR reduced Salmonella replication inside macrophages and lowered MgtC protein level, whereas a peptide variant of MgtR (MgtR-S17I), which does not interact with MgtC, had no effect. We then used synthetic peptides to evaluate their action upon exogenous addition. Unexpectedly, upon addition of synthetic peptides, both MgtR and its variant MgtR-S17I reduced Salmonella intramacrophage replication and lowered MgtC and MgtB protein levels, suggesting a different mechanism of action of exogenously added peptides versus endogenously produced peptides. The synthetic peptides did not act by reducing bacterial viability. We next tested their effect on various recombinant proteins produced in Escherichia coli and showed that the level of several inner membrane proteins was strongly reduced upon addition of both peptides, whereas cytoplasmic or outer membrane proteins remained unaffected. Moreover, the α-helical structure of synthetic MgtR is important for its biological activity, whereas helix-helix interacting motif is dispensable. Cumulatively, these results provide perspectives for new antivirulence strategies with the use of peptides that act by reducing the level of inner membrane proteins, including virulence factors.

The Salmonella virulence protein MgtC promotes phosphate uptake inside macrophages

The MgtC virulence protein from the intracellular pathogen Salmonella enterica is required for its intramacrophage survival and virulence in mice and this requirement of MgtC is conserved in several intracellular pathogens including Mycobacterium tuberculosis. Despite its critical role in survival within macrophages, only a few molecular targets of the MgtC protein have been identified. Here, we report that MgtC targets PhoR histidine kinase and activates phosphate transport independently of the available phosphate concentration. A single amino acid substitution in PhoR prevents its binding to MgtC, thus abrogating MgtC-mediated phosphate transport. Surprisingly, the removal of MgtC’s effect on the ability to transport phosphate renders Salmonella hypervirulent and decreases a non-replicating population inside macrophages, indicating that MgtC-mediated phosphate transport is required for normal Salmonella pathogenesis. This provides an example of a virulence protein directly activating a pathogen’s phosphate transport inside host.

The virulence factor MgtC is essential for intracellular macrophage survival of Salmonella enterica. Here, the authors show that MgtC targets the PhoB/PhoR regulatory system leading to phosphate uptake inside macrophages and that both phoR mutation and phoB deletion renders Salmonella hypervirulent in mice.

Killing from the inside: Intracellular role of T3SS in the fate of Pseudomonas aeruginosa within macrophages revealed by mgtC and oprF mutants

While considered solely an extracellular pathogen, increasing evidence indicates that Pseudomonas aeruginosa encounters intracellular environment in diverse mammalian cell types, including macrophages. In the present study, we have deciphered the intramacrophage fate of wild-type P. aeruginosa PAO1 strain by live and electron microscopy. P. aeruginosa first resided in phagosomal vacuoles and subsequently could be detected in the cytoplasm, indicating phagosomal escape of the pathogen, a finding also supported by vacuolar rupture assay. The intracellular bacteria could eventually induce cell lysis, both in a macrophage cell line and primary human macrophages. Two bacterial factors, MgtC and OprF, recently identified to be important for survival of P. aeruginosa in macrophages, were found to be involved in bacterial escape from the phagosome as well as in cell lysis caused by intracellular bacteria. Strikingly, type III secretion system (T3SS) genes of P. aeruginosa were down-regulated within macrophages in both mgtC and oprF mutants. Concordantly, cyclic di-GMP (c-di-GMP) level was increased in both mutants, providing a clue for negative regulation of T3SS inside macrophages. Consistent with the phenotypes and gene expression pattern of mgtC and oprF mutants, a T3SS mutant (ΔpscN) exhibited defect in phagosomal escape and macrophage lysis driven by internalized bacteria. Importantly, these effects appeared to be largely dependent on the ExoS effector, in contrast with the known T3SS-dependent, but ExoS independent, cytotoxicity caused by extracellular P. aeruginosa towards macrophages. Moreover, this macrophage damage caused by intracellular P. aeruginosa was found to be dependent on GTPase Activating Protein (GAP) domain of ExoS. Hence, our work highlights T3SS and ExoS, whose expression is modulated by MgtC and OprF, as key players in the intramacrophage life of P. aeruginosa which allow internalized bacteria to lyse macrophages.

The ability of professional phagocytes to ingest and kill microorganisms is central to host defense and Pseudomonas aeruginosa has developed mechanisms to avoid being killed by phagocytes. While considered an extracellular pathogen, P. aeruginosa has been reported to be engulfed by macrophages in animal models. Here, we visualized the fate of P. aeruginosa within cultured macrophages, revealing macrophage lysis driven by intracellular P. aeruginosa. Two bacterial factors, MgtC and OprF, recently discovered to be involved in the intramacrophage survival of P. aeruginosa, appeared to play a role in this cytotoxicity caused by intracellular bacteria. We provided evidence that type III secretion system (T3SS) gene expression is lowered intracellularly in mgtC and oprF mutants. We further showed that intramacrophage P. aeruginosa uses its T3SS, specifically the ExoS effector, to promote phagosomal escape and cell lysis. We thus describe a transient intramacrophage stage of P. aeruginosa that could contribute to bacterial dissemination.

Activity of a Synthetic Peptide Targeting MgtC on Pseudomonas aeruginosa Intramacrophage Survival and Biofilm Formation

Antivirulence strategies aim to target pathogenicity factors while bypassing the pressure on the bacterium to develop resistance. The MgtC membrane protein has been proposed as an attractive target that is involved in the ability of several major bacterial pathogens, including Pseudomonas aeruginosa, to survive inside macrophages. In liquid culture, P. aeruginosa MgtC acts negatively on biofilm formation. However, a putative link between these two functions of MgtC in P. aeruginosa has not been experimentally addressed. In the present study, we first investigated the contribution of exopolysaccharides (EPS) in the intramacrophage survival defect and biofilm increase of mgtC mutant. Within infected macrophages, expression of EPS genes psl and alg was increased in a P. aeruginosa mgtC mutant strain comparatively to wild-type strain. However, the intramacrophage survival defect of mgtC mutant was not rescued upon introduction of psl or alg mutation, suggesting that MgtC intramacrophage role is unrelated to EPS production, whereas the increased biofilm formation of mgtC mutant was partially suppressed by introduction of psl mutation. We aimed to develop an antivirulence strategy targeting MgtC, by taking advantage of a natural antagonistic peptide, MgtR. Heterologous expression of mgtR in P. aeruginosa PAO1 was shown to reduce its ability to survive within macrophages. We investigated for the first time the biological effect of a synthetic MgtR peptide on P. aeruginosa. Exogenously added synthetic MgtR peptide lowered the intramacrophage survival of wild-type P. aeruginosa PAO1, thus mimicking the phenotype of an mgtC mutant as well as the effect of endogenously produced MgtR peptide. In correlation with this finding, addition of MgtR peptide to bacterial culture strongly reduced MgtC protein level, without reducing bacterial growth or viability, thus differing from classical antimicrobial peptides. On the other hand, the addition of exogenous MgtR peptide did not affect significantly biofilm formation, indicating an action toward EPS-independent phenotype rather than EPS-related phenotype. Cumulatively, our results show an antivirulence action of synthetic MgtR peptide, which may be more potent against acute infection, and provide a proof of concept for further exploitation of anti-Pseudomonas strategies.

A rule governing the FtsH-mediated proteolysis of the MgtC virulence protein from Salmonella enterica serovar Typhimurium

A tightly controlled turnover of membrane proteins is required for lipid bilayer stability, cell metabolism, and cell viability. Among the energy-dependent AAA⁺ proteases in Salmonella, FtsH is the only membrane-bound protease that contributes to the quality control of membrane proteins. FtsH preferentially degrades the C-terminus or N-terminus of misfolded, misassembled, or damaged proteins to maintain physiological functions. We found that FtsH hydrolyzes the Salmonella MgtC virulence protein when we substitute the MgtC 226^th Trp, which is well conserved in other intracellular pathogens and normally protects MgtC from the FtsH-mediated proteolysis. Here we investigate a rule determining the FtsH-mediated proteolysis of the MgtC protein at Trp226 residue. Substitution of MgtC tryptophan 226^th residue to alanine, glycine, or tyrosine leads to MgtC proteolysis in a manner dependent on the FtsH protease whereas substitution to phenylalanine, methionine, isoleucine, leucine, or valine resists MgtC degradation by FtsH. These data indicate that a large and hydrophobic side chain at 226^th residue is required for protection from the FtsH-mediated MgtC proteolysis.

A protein that controls the onset of a Salmonella virulence program

The mechanism of action and contribution to pathogenesis of many virulence genes are understood. By contrast, little is known about anti-virulence genes, which contribute to the start, progression, and outcome of an infection. We now report how an anti-virulence factor in Salmonella enterica serovar Typhimurium dictates the onset of a genetic program that governs metabolic adaptations and pathogen survival in host tissues. Specifically, we establish that the anti-virulence protein CigR directly restrains the virulence protein MgtC, thereby hindering intramacrophage survival, inhibition of ATP synthesis, stabilization of cytoplasmic pH, and gene transcription by the master virulence regulator PhoP. We determine that, like MgtC, CigR localizes to the bacterial inner membrane and that its C-terminal domain is critical for inhibition of MgtC. As in many toxin/anti-toxin genes implicated in antibiotic tolerance, the mgtC and cigR genes are part of the same mRNA. However, cigR is also transcribed from a constitutive promoter, thereby creating a threshold of CigR protein that the inducible MgtC protein must overcome to initiate a virulence program critical for pathogen persistence in host tissues.

Chlortetracycline and florfenicol induce expression of genes associated with pathogenicity in multidrug-resistant Salmonella enterica serovar Typhimurium

Background

Multidrug-resistant (MDR) Salmonella enterica serovar Typhimurium (S. Typhimurium) is a serious public health threat as infections caused by these strains are more difficult and expensive to treat. Livestock serve as a reservoir for MDR Salmonella, and the antibiotics chlortetracycline and florfenicol are frequently administrated to food-producing animals to treat and prevent various diseases. Therefore, we evaluated the response of MDR S. Typhimurium after exposure to these two antibiotics.

Results

We exposed four MDR S. Typhimurium isolates to sub-inhibitory concentrations of chlortetracycline (16 and 32 µg/ml) or florfenicol (16 µg/ml) for 30 min during early-log phase. Differentially expressed genes following antibiotic treatment were identified using RNA-seq, and genes associated with attachment and those located within the Salmonella pathogenicity islands were significantly up-regulated following exposure to either antibiotic. The effect of antibiotic exposure on cellular invasion and motility was also assessed. Swimming and swarming motility were decreased due to antibiotic exposure. However, we observed chlortetracycline enhanced cellular invasion in two strains and florfenicol enhanced invasion in a third isolate.

Conclusions

Chlortetracycline and florfenicol exposure during early-log growth altered the expression of nearly half of the genes in the S. Typhimurium genome, including a large number of genes associated with virulence and pathogenesis; this transcriptional alteration was not due to the SOS response. The results suggest that exposure to either of these two antibiotics may lead to the expression of virulence genes that are typically only transcribed in vivo, as well as only during late-log or stationary phase in vitro.

Electronic supplementary material

The online version of this article (10.1186/s13099-018-0236-y) contains supplementary material, which is available to authorized users.

MgtC as a Host-Induced Factor and Vaccine Candidate against Mycobacterium abscessus Infection

Mycobacterium abscessus is an emerging pathogenic mycobacterium involved in pulmonary and mucocutaneous infections, presenting a serious threat for patients with cystic fibrosis (CF). The lack of an efficient treatment regimen and the emergence of multidrug resistance in clinical isolates require the development of new therapeutic strategies against this pathogen. Reverse genetics has revealed genes that are present in M. abscessus but absent from saprophytic mycobacteria and that are potentially involved in pathogenicity. Among them, MAB_3593 encodes MgtC, a known virulence factor involved in intramacrophage survival and adaptation to Mg(2+) deprivation in several major bacterial pathogens. Here, we demonstrated a strong induction of M. abscessus MgtC at both the transcriptional and translational levels when bacteria reside inside macrophages or upon Mg(2+) deprivation. Moreover, we showed that M. abscessus MgtC was recognized by sera from M. abscessus-infected CF patients. The intramacrophage growth (J774 or THP1 cells) of a M. abscessus knockout mgtC mutant was, however, not significantly impeded. Importantly, our results indicated that inhibition of MgtC in vivo through immunization with M. abscessus mgtC DNA, formulated with a tetrafunctional amphiphilic block copolymer, exerted a protective effect against an aerosolized M. abscessus challenge in CF (ΔF508 FVB) mice. The formulated DNA immunization was likely associated with the production of specific MgtC antibodies, which may stimulate a protective effect by counteracting MgtC activity during M. abscessus infection. These results emphasize the importance of M. abscessus MgtC in vivo and provide a basis for the development of novel therapeutic tools against pulmonary M. abscessus infections in CF patients.

Enhanced succinic acid productivity by expression of mgtCB gene in Escherichia coli mutant

In this study, a novel engineering Escherichia coli strain (CBMG111) with the expression of mgtCB gene was constructed for the enhanced fermentative production of succinic acid by utilizing the synergetic effect of mgtC gene to improve the growth of strains at the environment of low Mg2+ concentration and mgtB to enhance the transport of Mg2+ into cells. After the effect of the expression of the individual genes (mgtA, mgtB, mgtC) on the growth of E. coli was clarified, the fermentative production of succinic acid by CBMG111 was studied with the low-price mixture of Mg(OH)2 and NH3·H2O as the alkaline neutralizer and the biomass hydrolysates as the carbon sources, which demonstrated that the expression of mgtCB gene can significantly increase the productivity of succinic acid (2.97 g L−1 h−1) compared with that by using the engineering strain with the overexpression of mgtA gene.

Use of the Salmonella MgtR peptide as an antagonist of the Mycobacterium MgtC virulence factor

Background: The MgtC virulence factor has been proposed as an attractive target for antivirulence strategies because it is shared by several important bacterial pathogens, including Salmonella enterica and Mycobacterium tuberculosis (Mtb). Aim: A natural antagonistic peptide, MgtR, which interacts with MgtC and modulates its stability, has been identified in Salmonella, and we investigated its efficiency to target MgtC in another pathogen. Materials & methods: We evaluated the interaction between Salmonella MgtR peptide and the Mtb MgtC protein using an in vivo bacterial two-hybrid system and we addressed the effect of exogenously added synthetic MgtR and endogenously expressed peptide. Results: MgtR peptide strongly interacted with Mtb MgtC protein and exogenously added synthetic MgtR peptide-reduced Mtb MgtC level and interfered with the dimerization of Mtb MgtC. Importantly, heterologous expression of MgtR in Mycobacterium bovis BCG resulted in increased phagocytosis and reduced intramacrophage survival. Conclusion: MgtR peptide can target Mtb MgtC protein and reduce mycobacterial macrophage resistance, thus providing a promising new scaffold for the development of antivirulence compounds.

Magnesium Transport and Magnesium Homeostasis

This review reviews the properties and regulation of the Salmonella enterica serovar Typhimurium and Escherichia coli transporters that mediate Mg2+ influx: CorA and the Mgt P-type ATPases. In addition, potential Mg2+ regulation of transcription and translation, largely via the PhoPQ two component system, is discussed. CorA proteins are a unique class of transporters and are widespread in the Bacteria and Archaea, with rather distant but functional homologs in eukaryotes. The Mgt transporters are highly homologous to other P-type ATPases but are more closely related to the eukaryotic H+ and Ca2+ ATPases than to most prokaryotic ATPases. Hundreds of homologs of CorA are currently known from genomic sequencing. In contrast, only when extracellular and possibly intracellular Mg2+ levels fall significantly is the expression of mgtA and mgtB induced. Topology studies using blaM and lacZ fusions initially indicated that the Salmonella serovar Typhimurium CorA contained three transmembrane (TM) segments; however, subsequent data obtained using a variety of approaches showed that the CorA superfamily of proteins have only two TMs at the extreme C terminus. PhoP-PhoQ is a two-component system consisting of PhoQ, the sensor/receptor histidine kinase, and PhoP, the response regulator/transcriptional activator. The expression of both mgtA and mgtCB in either E. coli or Salmonella serovar Typhimurium is markedly induced in a PhoPQ-dependent manner by low concentrations of Mg2+ in the medium. phoP and phoQ form an operon with two promoters in both E. coli and Salmonella serovar Typhimurium.

A Macrophage Subversion Factor Is Shared by Intracellular and Extracellular Pathogens

Pathogenic bacteria have developed strategies to adapt to host environment and resist host immune response. Several intracellular bacterial pathogens, including Salmonella enterica and Mycobacterium tuberculosis, share the horizontally-acquired MgtC virulence factor that is important for multiplication inside macrophages. MgtC is also found in pathogenic Pseudomonas species. Here we investigate for the first time the role of MgtC in the virulence of an extracellular pathogen, Pseudomonas aeruginosa. A P. aeruginosa mgtC mutant is attenuated in the systemic infection model of zebrafish embryos, and strikingly, the attenuated phenotype is dependent on the presence of macrophages. In ex vivo experiments, the P. aeruginosa mgtC mutant is more sensitive to macrophage killing than the wild-type strain. However, wild-type and mutant strains behave similarly toward macrophage killing when macrophages are treated with an inhibitor of the vacuolar proton ATPase. Importantly, P. aeruginosa mgtC gene expression is strongly induced within macrophages and phagosome acidification contributes to an optimal expression of the gene. Thus, our results support the implication of a macrophage intracellular stage during P. aeruginosa acute infection and suggest that Pseudomonas MgtC requires phagosome acidification to play its intracellular role. Moreover, we demonstrate that P. aeruginosa MgtC is required for optimal growth in Mg²⁺ deprived medium, a property shared by MgtC factors from intracellular pathogens and, under Mg²⁺ limitation, P. aeruginosa MgtC prevents biofilm formation. We propose that MgtC shares a similar function in intracellular and extracellular pathogens, which contributes to macrophage resistance and fine-tune adaptation to host immune response in relation to the different bacterial lifestyles. In addition, the phenotypes observed with the mgtC mutant in infection models can be mimicked in wild-type P. aeruginosa strain by producing a MgtC antagonistic peptide, thus highlighting MgtC as a promising new target for anti-virulence strategies.

Pathogenic bacteria have to resist host immune response and MgtC is used by several intracellular pathogens to promote bacterial multiplication inside macrophages. Here we investigated MgtC’s role in the virulence of an extracellular pathogen, Pseudomonas aeruginosa. A P. aeruginosa mgtC mutant is attenuated in zebrafish embryos, but only in the presence of macrophages. Moreover, this mutant is more rapidly killed by macrophages than the wild-type strain. Both phenotypes can be mimicked upon production of a MgtC antagonistic peptide in wild-type Pseudomonas strain. MgtC thus provides a singular example of a virulence determinant that promotes strategies to subvert the antimicrobial behavior of macrophages, in both intracellular and extracellular pathogens and our results support an intramacrophage stage during in P. aeruginosa acute infection, as well as an interplay between MgtC role and phagosome acidification. In addition, P. aeruginosa MgtC is required for growth in Mg²⁺ deprived medium, a property shared by MgtC factors from intracellular pathogens, and limits biofilm formation. MgtC may share a similar function in intracellular and extracellular pathogens, with an outcome adapted to the different bacterial lifestyles

Salmonella promotes virulence by repressing cellulose production

Cellulose is the most abundant organic polymer on Earth. In bacteria, cellulose confers protection against environmental insults and is a constituent of biofilms typically formed on abiotic surfaces. We report that, surprisingly, Salmonella enterica serovar Typhimurium makes cellulose when inside macrophages. We determine that preventing cellulose synthesis increases virulence, whereas stimulation of cellulose synthesis inside macrophages decreases virulence. An attenuated mutant lacking the mgtC gene exhibited increased cellulose levels due to increased expression of the cellulose synthase gene bcsA and of cyclic diguanylate, the allosteric activator of the BcsA protein. Inactivation of bcsA restored wild-type virulence to the Salmonella mgtC mutant, but not to other attenuated mutants displaying a wild-type phenotype regarding cellulose. Our findings indicate that a virulence determinant can promote pathogenicity by repressing a pathogen's antivirulence trait. Moreover, they suggest that controlling antivirulence traits increases long-term pathogen fitness by mediating a trade-off between acute virulence and transmission.

Mycobacterium marinum MgtC plays a role in phagocytosis but is dispensable for intracellular multiplication

MgtC is a virulence factor involved in intramacrophage growth that has been reported in several intracellular pathogens, including Mycobacterium tuberculosis and Salmonella enterica serovar Typhimurium. MgtC participates also in adaptation to Mg2+ deprivation. Herein, we have constructed a mgtC mutant in Mycobacterium marinum to further investigate the role of MgtC in mycobacteria. We show that the M. marinum mgtC gene (Mma mgtC) is strongly induced upon Mg2+ deprivation and is required for optimal growth in Mg2+-deprived medium. The behaviour of the Mma mgtC mutant has been investigated in the Danio rerio infection model using a transgenic reporter zebrafish line that specifically labels neutrophils. Although the mgtC mutant is not attenuated in the zebrafish embryo model based on survival curves, our results indicate that phagocytosis by neutrophils is enhanced with the mgtC mutant compared to the wild-type strain following subcutaneous injection. Increased phagocytosis of the mutant strain is also observed ex vivo with the murine J774 macrophage cell line. On the other hand, no difference was found between the mgtC mutant and the wild-type strain in bacterial adhesion to macrophages and in the internalization into epithelial cells. Unlike the role reported for MgtC in other intracellular pathogens, Mma MgtC does not contribute significantly to intramacrophage replication. Taken together, these results indicate an unanticipated function of Mma MgtC at early step of infection within phagocytic cells. Hence, our results indicate that although the MgtC function is conserved among pathogens regarding adaptation to Mg2+ deprivation, its role towards phagocytic cells can differ, possibly in relation with the specific pathogen's lifestyles.

Binding of MgtR, a Salmonella transmembrane regulatory peptide, to MgtC, a Mycobacterium tuberculosis virulence factor: a structural study

MgtR, a highly hydrophobic peptide expressed in Salmonella enterica serovar Typhimurium, inhibits growth in macrophages through binding to the membrane protein MgtC that has been identified as essential for replication in macrophages. While the Mycobacterium tuberculosis MgtC is highly homologous to its S. Typhi analogue, there does not appear to be an Mtb homologue for MgtR, raising significant pharmacological interest in this system. Here, solid-state NMR and EPR spectroscopy in lipid bilayer preparations were used to demonstrate the formation of a heterodimer between S. Typhi MgtR and the transmembrane helix 4 of Mtb MgtC. Based on the experimental restraints, a structural model of this heterodimer was developed using computational techniques. The result is that MgtR appears to be ideally situated in the membrane to influence the functionality of MgtC.

Salmonella enterica serovar Typhimurium skills to succeed in the host: virulence and regulation

Salmonella enterica serovar Typhimurium is a primary enteric pathogen infecting both humans and animals. Infection begins with the ingestion of contaminated food or water so that salmonellae reach the intestinal epithelium and trigger gastrointestinal disease. In some patients the infection spreads upon invasion of the intestinal epithelium, internalization within phagocytes, and subsequent dissemination. In that case, antimicrobial therapy, based on fluoroquinolones and expanded-spectrum cephalosporins as the current drugs of choice, is indicated. To accomplish the pathogenic process, the Salmonella chromosome comprises several virulence mechanisms. The most important virulence genes are those located within the so-called Salmonella pathogenicity islands (SPIs). Thus far, five SPIs have been reported to have a major contribution to pathogenesis. Nonetheless, further virulence traits, such as the pSLT virulence plasmid, adhesins, flagella, and biofilm-related proteins, also contribute to success within the host. Several regulatory mechanisms which synchronize all these elements in order to guarantee bacterial survival have been described. These mechanisms govern the transitions from the different pathogenic stages and drive the pathogen to achieve maximal efficiency inside the host. This review focuses primarily on the virulence armamentarium of this pathogen and the extremely complicated regulatory network controlling its success.

A bacterial virulence protein promotes pathogenicity by inhibiting the bacterium's own F1Fo ATP synthase

Several intracellular pathogens, including Salmonella enterica and Mycobacterium tuberculosis, require the virulence protein MgtC to survive within macrophages and to cause a lethal infection in mice. We now report that, unlike secreted virulence factors that target the host vacuolar ATPase to withstand phagosomal acidity, the MgtC protein acts on Salmonella's own F1Fo ATP synthase. This complex couples proton translocation to ATP synthesis/hydrolysis and is required for virulence. We establish that MgtC interacts with the a subunit of the F1Fo ATP synthase, hindering ATP-driven proton translocation and NADH-driven ATP synthesis in inverted vesicles. An mgtC null mutant displays heightened ATP levels and an acidic cytoplasm, whereas mgtC overexpression decreases ATP levels. A single amino acid substitution in MgtC that prevents binding to the F1Fo ATP synthase abolishes control of ATP levels and attenuates pathogenicity. MgtC provides a singular example of a virulence protein that promotes pathogenicity by interfering with another virulence protein.

The C-terminal domain of the virulence factor MgtC is a divergent ACT domain

MgtC is a virulence factor of unknown function important for survival inside macrophages in several intracellular bacterial pathogens, including Mycobacterium tuberculosis. It is also involved in adaptation to Mg(2+) deprivation, but previous work suggested that MgtC is not a Mg(2+) transporter. In this study, we demonstrated that the amount of the M. tuberculosis MgtC protein is not significantly increased by Mg(2+) deprivation. Members of the MgtC protein family share a conserved membrane N-terminal domain and a more divergent cytoplasmic C-terminal domain. To get insights into MgtC functional and structural organization, we have determined the nuclear magnetic resonance (NMR) structure of the C-terminal domain of M. tuberculosis MgtC. This structure is not affected by the Mg(2+) concentration, indicating that it does not bind Mg(2+). The structure of the C-terminal domain forms a βαββαβ fold found in small molecule binding domains called ACT domains. However, the M. tuberculosis MgtC ACT domain differs from canonical ACT domains because it appears to lack the ability to dimerize and to bind small molecules. We have shown, using a bacterial two-hybrid system, that the M. tuberculosis MgtC protein can dimerize and that the C-terminal domain somehow facilitates this dimerization. Taken together, these results indicate that M. tuberculosis MgtC does not have an intrinsic function related to Mg(2+) uptake or binding but could act as a regulatory factor based on protein-protein interaction that could be facilitated by its ACT domain.

STM2209-STM2208 (opvAB): a phase variation locus of Salmonella enterica involved in control of O-antigen chain length

STM2209 and STM2208 are contiguous loci annotated as putative protein-coding genes in the chromosome of Salmonella enterica. Lack of homologs in related Enterobacteria and low G+C content suggest that S. enterica may have acquired STM2209-STM2208 by horizontal transfer. STM2209 and STM2208 are co-transcribed from a promoter upstream STM2209, and their products are inner (cytoplasmic) membrane proteins. Analysis with the bacterial adenylate cyclase two-hybrid system suggests that STM2209 and STM2208 may interact. Expression of STM2209-STM2208 is subjected to phase variation in wild type Salmonella enterica serovar Typhimurium. Switching frequencies in LB medium are 6.1×10⁻⁵ (OFF→ON) and 3.7×10⁻² (ON→OFF) per cell and generation. Lack of DNA adenine methylation locks STM2209-STM2208 in the ON state, and lack of the LysR-type factor OxyR locks STM2209-STM2208 in the OFF state. OxyR-dependent activation of STM2209-STM2208 expression is independent of the oxidation state of OxyR. Salmonella cultures locked in the ON state show alteration of O-antigen length in the lipopolysaccharide, reduced absorption of bacteriophage P22, impaired resistance to serum, and reduced proliferation in macrophages. Phenotypic heterogeneity generated by STM2209-STM2208 phase variation may thus provide defense against phages. In turn, formation of a subpopulation unable to proliferate in macrophages may restrain Salmonella spread in animal organs, potentially contributing to successful infection.

Phylogenetic and functional analysis of Aspergillus fumigatus MGTC, a fungal protein homologous to a bacterial virulence factor

MgtC is important for the survival of several bacterial pathogens in macrophages and for growth under magnesium limitation. Among eukaryotes, a gene homologous to mgtC was found only in the pathogenic fungus Aspergillus fumigatus. Our data show that the A. fumigatus MgtC (AfuMgtC) protein does not have the same function as the bacterial MgtC proteins.

Contribution of the PhoP/Q regulon to survival and replication of Salmonella enterica serovar Typhimurium in macrophages

The ability of serovars of Salmonella enterica to cause systemic disease is dependent upon their survival and replication within macrophages. To do this, bacteria must withstand or surmount bacteriostatic and bactericidal responses by the host cell, including the delivery of hydrolytic enzymes from lysosomes to the phagosome. The bacterial two-component regulatory system PhoP/Q has been implicated in avoidance of phagolysosomal fusion by S. enterica serovar Typhimurium (S. Typhimurium) in murine macrophages. In this study, the involvement of PhoP/Q-activated genes in avoidance of phagolysosomal fusion was analysed: of all the S. Typhimurium mutant strains tested, only an mgtC mutant strain partially reproduced the phenotype of the phoP mutant strain. As this gene is required for bacterial growth in magnesium-depleted conditions in vitro, the contributions of PhoP/Q to intramacrophage replication and survival were reappraised. Although PhoP/Q was required for both replication and survival of S. Typhimurium within murine macrophages, subsequent analysis of the kinetics of phagolysosomal fusion, taking account of differences in the replication rates of wild-type and phoP mutant strains, provided no evidence for a PhoP/Q-dependent role in this process. PhoP/Q appeared to act subsequent to the process of phagolysosomal avoidance and to promote replication of those bacteria that had already escaped a phagolysosomal fate. Therefore, we conclude that the PhoP/Q regulon enables S. Typhimurium to adapt to intramacrophage stresses other than phagolysosomal fusion.

Magnesium depletion triggers production of an immune modulating diterpenoid in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the causative agent of the human disease Tuberculosis, and remains a worldwide health threat responsible for ∼1.7 million deaths annually. During infection, Mtb prevents acidification of the engulfing phagosome, thus blocking endocytic progression and eventually leading to stable residence. The diterpenoid metabolite isotuberculosinol (isoTb) exhibits biological activity indicative of a role in this early arrest of phagosome maturation. Presumably, isoTb production should be induced by phagosomal entry. However, the relevant enzymatic genes are not transcriptionally upregulated during engulfment. Previous examination of the initial biosynthetic enzyme (Rv3377c/MtHPS) involved in isoTb biosynthesis revealed striking inhibition by its Mg(2+) cofactor, leading to the hypothesis that the depletion of Mg(2+) observed upon phagosomal engulfment may act to trigger isoTb biosynthesis. While Mtb is typically grown in relatively high levels of Mg(2+) (0.43 mM), shifting Mtb to media with phagosomal levels (0.1 mM) led to a significant (∼10-fold) increase in accumulation of the MtHPS product, halimadienyl diphosphate, as well as easily detectable amounts of the derived bioactive isoTb. These results demonstrate isoTb production by Mtb specifically under conditions that mimic phagosomal cation concentrations, and further support a role for isoTb in the Mtb infection process.

A decade of Burkholderia cenocepacia virulence determinant research

The Burkholderia cepacia complex (Bcc) is a group of genetically related environmental bacteria that can cause chronic opportunistic infections in patients with cystic fibrosis (CF) and other underlying diseases. These infections are difficult to treat due to the inherent resistance of the bacteria to antibiotics. Bacteria can spread between CF patients through social contact and sometimes cause cepacia syndrome, a fatal pneumonia accompanied by septicemia. Burkholderia cenocepacia has been the focus of attention because initially it was the most common Bcc species isolated from patients with CF in North America and Europe. Today, B. cenocepacia, along with Burkholderia multivorans, is the most prevalent Bcc species in patients with CF. Given the progress that has been made in our understanding of B. cenocepacia over the past decade, we thought that it was an appropriate time to review our knowledge of the pathogenesis of B. cenocepacia, paying particular attention to the characterization of virulence determinants and the new tools that have been developed to study them. A common theme emerging from these studies is that B. cenocepacia establishes chronic infections in immunocompromised patients, which depend more on determinants mediating host niche adaptation than those involved directly in host cells and tissue damage.

Salmonella transcriptional signature in Tetrahymena phagosomes and role of acid tolerance in passage through the protist

Salmonella enterica Typhimurium remains undigested in the food vacuoles of the common protist, Tetrahymena. Contrary to its interaction with Acanthamoeba spp., S. Typhimurium is not cytotoxic to Tetrahymena and is egested as viable cells in its fecal pellets. Through microarray gene expression profiling we investigated the factors in S. Typhimurium that are involved in its resistance to digestion by Tetrahymena. The transcriptome of S. Typhimurium in Tetrahymena phagosomes showed that 989 and 1282 genes were altered in expression compared with that in water and in LB culture medium, respectively. A great proportion of the upregulated genes have a role in anaerobic metabolism and the use of alternate electron acceptors. Many genes required for survival and replication within macrophages and human epithelial cells also had increased expression in Tetrahymena, including mgtC, one of the most highly induced genes in all three cells types. A ΔmgtC mutant of S. Typhimurium did not show decreased viability in Tetrahymena, but paradoxically, was egested at a higher cell density than the wild type. The expression of adiA and adiY, which are involved in arginine-dependent acid resistance, also was increased in the protozoan phagosome. A ΔadiAY mutant had lower viability after passage through Tetrahymena, and a higher proportion of S. Typhimurium wild-type cells within pellets remained viable after exposure to pH 3.4 as compared with uningested cells. Our results provide evidence that acid resistance has a role in the resistance of Salmonella to digestion by Tetrahymena and that passage through the protist confers physiological advantages relevant to its contamination cycle.

An antisense RNA that governs the expression kinetics of a multifunctional virulence gene

Genome-wide transcriptome analyses of several bacterial species have recently uncovered a hitherto unappreciated amount of antisense transcription. However, the physiological role, regulation and significance of such antisense transcripts are presently unclear. We now report the identification of a cis-encoded 1.2 kb long antisense RNA - termed AmgR - that is complementary to the mgtC portion of the mgtCBR polycistronic message from Salmonella enterica. The mgtCBR mRNA specifies the MgtC protein, which is necessary for survival within macrophages, virulence in mice and growth in low Mg(2+); the Mg(2+) transporter MgtB with no apparent role in virulence; and the membrane peptide MgtR mediating MgtC degradation. Expression of AmgR diminished both MgtC and MgtB protein levels in a process requiring RNase E but independent of RNase III, the RNA chaperone Hfq, and the regulatory peptide MgtR. Inactivation of the chromosomal amgR promoter increased MgtC and MgtB protein levels and enhanced Salmonella virulence. Surprisingly, AmgR transcription is governed by the regulatory protein PhoP, which also directs transcription of the sense mgtCBR mRNA. AmgR may function as a timing device that alters MgtC and MgtB levels after the onset of PhoP-inducing conditions.

Characterization of MgtC, a virulence factor of Salmonella enterica Serovar Typhi

The MgtC is a virulence factor in Salmonella Typhimurium that is required for growth at low-Mg2+ concentrations and intramacrophage survival. This gene is codified in a conserved region of the Salmonella pathogenicity island 3 (SPI-3), and is also present in the chromosome of other Salmonella serovars. In this study we characterized the MgtC factor in S. Typhi, a human specific pathogen, by using mgtC and SPI-3 mutant strains. We found that MgtC is the most important factor codified in the SPI-3 of S. Typhi for growth in low-Mg2+ media and survival within human cells. In addition, by using reporter genes we determined that the low-Mg2+ concentration, acidic media and PhoP regulator induce mgtC expression in S. Typhi. We suggest that MgtC is the most important virulence factor codified in the SPI-3 of S. Typhi.

A comparative analysis of metal transportomes from metabolically versatile Pseudomonas

BACKGROUND

The availability of complete genome sequences of versatile Pseudomonas occupying remarkably diverse ecological niches enabled to gain insights into their adaptative assets. The objective of this study was to analyze the complete genetic repertoires of metal transporters (metal transportomes) from four representative Pseudomonas species and to identify metal transporters with "Genomic Island" associated features.

METHODS

A comparative metal transporter inventory was built for the following four Pseudomonas species: P.putida (Ppu) KT2440, P.aeruginosa (Pae) PA01, P.fluorescens (Pfl) Pf-5 and P.syringae (Psy)pv.tomato DC3000 using TIGR-CMR and Transport DB. Genomic analysis of essential and toxic metal ion transporters was accomplished from the above inventory. Metal transporters with "Genomic Island" associated features were identified using Islandpath analysis.

RESULTS

Dataset cataloguing has been executed for 262 metal transporters from the four spp. Additional metal ion transporters belonging to NiCoT, Ca P-type ATPase, Cu P-type ATPases, ZIP and MgtC families were identified. In Psy DC3000, 48% of metal transporters showed strong GI features while it was 45% in Ppu KT2440. In Pfl Pf-5 and Pae PA01 only 26% of their metal transporters exhibited GI features.

CONCLUSION

Our comparative inventory of 262 metal transporters from four versatile Pseudomonas spp is the complete suite of metal transportomes analysed till date in a prokaryotic genus. This study identified differences in the basic composition of metal transportomes from Pseudomonas occupying diverse ecological niches and also elucidated their novel features. Based on this inventory we analysed the role of horizontal gene transfer in expansion and variability of metal transporter families.

Interplay between MgtC and PagC in Salmonella enterica serovar Typhimurium

In Salmonella enterica serovar Typhimurium, MgtC and PagC are positively regulated by the PhoP-PhoQ two-component system, which is activated under magnesium deprivation. Both MgtC and PagC are of unknown function but have been involved in intramacrophage survival. We have found that the amount of PagC is lowered in a DeltamgtC mutant strain grown in magnesium depleted medium. However, the effect of MgtC on PagC does not account for the growth defect of a DeltamgtC mutant in macrophages since, in contrast to previous reports, our results indicate that PagC does not contribute to intramacrophage survival. In addition, a pagC null mutant is only poorly attenuated in Nramp1-negative or Nramp1-positive mice. On the other hand, a mgtC null mutant is significantly more attenuated with Nramp1-positive than Nramp1-negative mice, suggesting that a functional Nramp1 (Slc11a1) further limits the multiplication of this mutant within the host.

Peptide-assisted degradation of the Salmonella MgtC virulence factor

MgtC is a virulence factor common to several intracellular pathogens that is required for intramacrophage survival and growth in magnesium-depleted medium. In Salmonella enterica, MgtC is coexpressed with the MgtB magnesium transporter and transcription of the mgtCB operon is induced by magnesium deprivation. Despite the high level of mgtCB transcriptional induction in magnesium-depleted medium, the MgtC protein is hardly detected in a wild-type Salmonella strain. Here, we show that downregulation of MgtC expression is dependent on a hydrophobic peptide, MgtR, which is encoded by the mgtCB operon. Our results suggest that MgtR promotes MgtC degradation by the FtsH protease, providing a negative regulatory feedback. Bacterial two-hybrid assays demonstrate that MgtR interacts with the inner-membrane MgtC protein. We identified mutant derivatives of MgtR and MgtC that prevent both regulation and interaction between the two partners. In macrophages, overexpression of the MgtR peptide led to a decrease of the replication rate of Salmonella. This study highlights the role of peptides in bacterial regulatory mechanisms and provides a natural antagonist of the MgtC virulence factor.

MgtC: a key player in intramacrophage survival

Several bacterial pathogens have evolved strategies to survive in macrophages and create a replicative niche within phagosomes. The bacterial factor MgtC is a key player in intramacrophage survival, being important for virulence in diverse intracellular pathogens. MgtC is also required for growth under magnesium limitation. Recent studies provide new clues on the role of MgtC in macrophages, which seems to be unlinked to adaptation to a low Mg(2+) microenvironment. In addition, we discuss the unexpected finding that MgtC modulates host P-type ATPase activity.