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

Carbon dioxide regulates Mycobacterium tuberculosis PhoPR signaling and virulence

The Mycobacterium tuberculosis (Mtb) two-component regulatory system PhoPR is implicated in pH sensing within the macrophage because it is strongly induced by acidic pH both in vitro and the macrophage phagosome. The carbonic anhydrase (CA) inhibitor ethoxzolamide inhibits PhoPR signaling supporting the hypothesis that CO2 may also play a role in regulating PhoPR. Here, we show that increasing CO2 concentration induces PhoPR signaling, at both pH 7.0 and pH 5.7. At acidic pH 5.7, a normally strong inducer of PhoPR signaling, increasing CO2 from 0.5% to 5% further induces the pathway, showing CO2 acts synergistically with acidic pH to induce the PhoPR regulon. Based on these findings, we propose that PhoPR functions as a CO2 sensor. Mtb has three CA (CanA, CanB, and CanC), and using CRISPR interference knockdowns and gene deletion mutants, we assessed which CAs regulate PhoPR signaling and macrophage survival. We first examined if CA played a role in Mtb pathogenesis and observed that CanB was required for survival in macrophages, where the knockdown strain had ~1-log reduction in survival. To further define the interplay of CO2 and Mtb signaling, we conducted transcriptional profiling experiments at varying pH and CO2 concentrations. As hypothesized, we observed that the induction of PhoPR at acidic pH is dependent on CO2 concentration, with a subset of core PhoPR regulon genes dependent on both 5% CO2 and acidic pH for their induction, including expression of the ESX-1 secretion system. Transcriptional profiling also revealed core CO2-responsive genes that were differentially expressed independently of the PhoPR regulon or the acidic pH-inducible regulon. Notably, genes regulated by a second two-component regulatory system, TrcRS, are associated with adaptation to changes in CO2.

Mechanisms of Virulence of Mycobacterium abscessus and Interaction with the Host Immune System

Mycobacterium abscessus is a non-tuberculosis fast-growing mycobacterium that has recently become a serious concern due to its rapidly increasing prevalence worldwide, mainly in individuals with a high susceptibility to pulmonary infections, for example, patients with cystic fibrosis, bronchiectasis, chronic obstructive pulmonary disease, and previous tuberculosis infection. According to present estimations, at least 20% of patients with cystic fibrosis are infected with M. abscessus. This bacterium is extremely resistant to most drugs, leading to a severe and difficult-to-treat infection. That is why M. abscessus, previously classified as a low-virulent opportunistic pathogen, is now reconsidered as a true pathogenic bacterium. There are no effective drugs for successful M. abscessus infection therapy, as well as no vaccines to prevent its spread. This review focuses on the molecular mechanisms ensuring M. abscessus resistance to immune response and its ability to survive in the aggressive intracellular environment of human immune cells, and describes virulence factors that can serve as potential targets for the development of innovative therapeutic approaches to combat the spread of infections caused by M. abscessus.

Arabinogalactan enhances Mycobacterium marinum virulence by suppressing host innate immune responses

Arabinogalactan (AG) participates in forming the cell wall core of mycobacteria, a structure known as the mAGP complex. Few studies have reported the virulence of inartificial AG or its interaction with the host immune system. Using clustered regularly interspaced short palindromic repeats interference gene editing technology, conditional Mycobacterium marinum mutants were constructed with a low expression of embA or glfT2 (EmbA_KD or GlfT2_KD), which are separately involved in the biosynthesis of AG arabinose and galactose domains. High-performance gel permeation chromatography and high-performance liquid chromatography assays confirmed that the EmbA_KD strain showed a remarkable decrease in AG content with fragmentary arabinose chains, and the GlfT2_KD strain displayed less reduction in content with cut-down galactose chains. Based on transmission and scanning electron microscopy observations, the cell walls of the two mutants were found to be dramatically thickened, and the boundaries of different layers were more distinct. Phenotypes including the over-secretion of extracellular substances and enhanced spreading motility with a concomitant decreased resistance to ethambutol appeared in the EmbA_KD strain. The EmbA_KD and GlfT2_KD strains displayed limited intracellular proliferation after infecting murine J774A.1 macrophages. The disease progression infected with the EmbA_KD or GlfT2_KD strain significantly slowed down in zebrafish/murine tail infection models as well. Through transcriptome profiling, macrophages infected by EmbA_KD/GlfT2_KD strains showed enhanced oxidative metabolism. The cell survival measured using the CCK8 assay of macrophages exposed to the EmbA_KD strain was upregulated and consistent with the pathway enrichment analysis of differentially expressed genes in terms of cell cycle/apoptosis. The overexpression of C/EBPβ and the increasing secretion of proinflammatory cytokines were validated in the macrophages infected by the EmbA_KD mutant. In conclusion, the AG of Mycobacterium appears to restrain the host innate immune responses to enhance intracellular proliferation by interfering with oxidative metabolism and causing macrophage death. The arabinose chains of AG influence the Mycobacterium virulence and pathogenicity to a greater extent.

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.

A multi-biomarker study on Atlantic salmon (Salmo salar L.) affected by the emerging Red Skin Disease in the Baltic Sea

For half a decade, the Atlantic salmon in the Baltic Sea has been facing severe health issues. Clinical signs like haemorrhage, erosions and ulcerative/necrotic skin conditions in returning adults have been reported from different Swedish rivers. These primary disease signs precede a secondary, terminal fungal infection. As initial investigations of the disease did not provide conclusive answers regarding the pathogenesis, this study was initiated to gain insight into a possible link between this so-called Red Skin Disease and anthropogenic influences. Therefore, returning salmon were caught in rivers along the Swedish coast and different tissues were sampled. The focus was put on the measurements of a battery of biomarkers as well as biochemical and haematological parameters, which were analysed using multivariate statistics. The main findings were a severe osmotic haemodilution, an immune response and an alteration of the carbohydrate metabolism in diseased fish. Furthermore, oxidative stress does not seem to be a likely factor in the pathogenesis. Concluding, certain changes in physiological parameters were shown to be indicative for the disease patterns, while others were ruled out as significant factors. Thus, this study contributes to the understanding of the Red Skin Disease and may act as a hypothesis generator for future studies.

One Size Fits All? Not in In Vivo Modeling of Tuberculosis Chemotherapeutics

Tuberculosis (TB) remains a global health problem despite almost universal efforts to provide patients with highly effective chemotherapy, in part, because many infected individuals are not diagnosed and treated, others do not complete treatment, and a small proportion harbor Mycobacterium tuberculosis (Mtb) strains that have become resistant to drugs in the standard regimen. Development and approval of new drugs for TB have accelerated in the last 10 years, but more drugs are needed due to both Mtb's development of resistance and the desire to shorten therapy to 4 months or less. The drug development process needs predictive animal models that recapitulate the complex pathology and bacterial burden distribution of human disease. The human host response to pulmonary infection with Mtb is granulomatous inflammation usually resulting in contained lesions and limited bacterial replication. In those who develop progressive or active disease, regions of necrosis and cavitation can develop leading to lasting lung damage and possible death. This review describes the major vertebrate animal models used in evaluating compound activity against Mtb and the disease presentation that develops. Each of the models, including the zebrafish, various mice, guinea pigs, rabbits, and non-human primates provides data on number of Mtb bacteria and pathology resolution. The models where individual lesions can be dissected from the tissue or sampled can also provide data on lesion-specific bacterial loads and lesion-specific drug concentrations. With the inclusion of medical imaging, a compound's effect on resolution of pathology within individual lesions and animals can also be determined over time. Incorporation of measurement of drug exposure and drug distribution within animals and their tissues is important for choosing the best compounds to push toward the clinic and to the development of better regimens. We review the practical aspects of each model and the advantages and limitations of each in order to promote choosing a rational combination of them for a compound's development.

Teleological cooption of Mycobacterium tuberculosis PE/PPE proteins as porins: Role in molecular immigration and emigration

Permeation through bacterial cells for exchange or uptake of biomolecules and ions invariably depend upon the existence of pore-forming proteins (porins) in their outer membrane. Mycobacterium tuberculosis (M. tb) harbours one of the most rigid cell envelopes across bacterial genera and is devoid of the classical porins for solute transport across the cell membrane. Though canonical porins are incompatible with the evolution of permeability barrier, porin like activity has been reported from membrane preparations of pathogenic mycobacteria. This suggests a sophisticated transport mechanism that has been elusive until now, along with the protein family responsible for it. Recent evidence suggests that these slow-growing mycobacteria have co-opted some of PE/PPE family proteins as molecular transport channels, in place of porins, to facilitate uptake of nutrients required to thrive in the restrictive host environment. These reports advocate that PE/PPE proteins, due to their structural ability, have a potential role in importing small molecules to the cell's interior. This mechanism unveils how a successful pathogen overcomes its restrictive membrane's transport limitations for selective uptake of nutrients. If extrapolated to have a role in drug transport, these channels could help understand the emergence of drug resistance. Further, as these proteins are associated with the export of virulence factors, they can be exploited as novel drug targets. There remains, however, an interesting question that as the PE/PPE proteins can allow the 'import' of molecules from outside the cell, is the reverse transport also possible across the M. tb membrane. In this review, we have discussed recent evidence supporting PE/PPE's role as a specific transport channel for selective uptake of small molecule nutrients and, as possible molecular export machinery of M. tb. This newly discovered role as transmembrane channels demands further research on this enigmatic family of proteins to comprehend the pathomechanism of this very smart pathogen.

The Diverse Roles of Phagocytes During Bacterial and Fungal Infections and Sterile Inflammation: Lessons From Zebrafish

The immediate and natural reaction to both infectious challenges and sterile insults (wounds, tissue trauma or crystal deposition) is an acute inflammatory response. This inflammatory response is mediated by activation of the innate immune system largely comprising professional phagocytes (neutrophils and macrophages). Zebrafish (danio rerio) larvae possess many advantages as a model organism, including their genetic tractability and highly conserved innate immune system. Exploiting these attributes and the live imaging potential of optically transparent zebrafish larvae has greatly contributed to our understanding of how neutrophils and macrophages orchestrate the initiation and resolution phases of inflammatory responses. Numerous bacterial and fungal infection models have been successfully established using zebrafish as an animal model and studies investigating neutrophil and macrophage behavior to sterile insults have also provided unique insights. In this review we highlight how examining the larval zebrafish response to specific bacterial and fungal pathogens has uncovered cellular and molecular mechanisms behind a variety of phagocyte responses, from those that protect the host to those that are detrimental. We also describe how modeling sterile inflammation in larval zebrafish has provided an opportunity to dissect signaling pathways that control the recruitment, and fate, of phagocytes at inflammatory sites. Finally, we briefly discuss some current limitations, and opportunities to improve, the zebrafish model system for studying phagocyte biology.

PE/PPE proteins mediate nutrient transport across the outer membrane of Mycobacterium tuberculosis

Mycobacterium tuberculosis has an unusual outer membrane that lacks canonical porin proteins for the transport of small solutes to the periplasm. We discovered that 3,3-bis-di(methylsulfonyl)propionamide (3bMP1) inhibits the growth of M. tuberculosis, and resistance to this compound is conferred by mutation within a member of the proline-proline-glutamate (PPE) family, PPE51. Deletion of PPE51 rendered M. tuberculosis cells unable to replicate on propionamide, glucose, or glycerol. Growth was restored upon loss of the mycobacterial cell wall component phthiocerol dimycocerosate. Mutants in other proline-glutamate (PE)/PPE clusters, responsive to magnesium and phosphate, also showed a phthiocerol dimycocerosate-dependent growth compromise upon limitation of the corresponding substrate. Phthiocerol dimycocerosate determined the low permeability of the mycobacterial outer membrane, and the PE/PPE proteins apparently act as solute-specific channels.

Non-tuberculous mycobacteria and the rise of Mycobacterium abscessus

Infections caused by non-tuberculous mycobacteria (NTM) are increasing globally and are notoriously difficult to treat due to intrinsic resistance of these bacteria to many common antibiotics. NTM are diverse and ubiquitous in the environment, with only a few species causing serious and often opportunistic infections in humans, including Mycobacterium abscessus. This rapidly growing mycobacterium is one of the most commonly identified NTM species responsible for severe respiratory, skin and mucosal infections in humans. It is often regarded as one of the most antibiotic-resistant mycobacteria, leaving us with few therapeutic options. In this Review, we cover the proposed infection process of M. abscessus, its virulence factors and host interactions and highlight the commonalities and differences of M. abscessus with other NTM species. Finally, we discuss drug resistance mechanisms and future therapeutic options. Taken together, this knowledge is essential to further our understanding of this overlooked and neglected global threat.

In Vivo Methods to Study Protein-Protein Interactions as Key Players in Mycobacterium Tuberculosis Virulence

Studies on protein–protein interactions (PPI) can be helpful for the annotation of unknown protein functions and for the understanding of cellular processes, such as specific virulence mechanisms developed by bacterial pathogens. In that context, several methods have been extensively used in recent years for the characterization of Mycobacterium tuberculosis PPI to further decipher tuberculosis (TB) pathogenesis. This review aims at compiling the most striking results based on in vivo methods (yeast and bacterial two-hybrid systems, protein complementation assays) for the specific study of PPI in mycobacteria. Moreover, newly developed methods, such as in-cell native mass resonance and proximity-dependent biotinylation identification, will have a deep impact on future mycobacterial research, as they are able to perform dynamic (transient interactions) and integrative (multiprotein complexes) analyses.

Teleost contributions to the understanding of mycobacterial diseases

Few pathogens have shaped human medicine as the mycobacteria. From understanding biological phenomena driving disease spread, to mechanisms of host-pathogen interactions and antibiotic resistance, the Mycobacterium genus continues to challenge and offer insights into the basis of health and disease. Teleost fish models of mycobacterial infections have progressed significantly over the past three decades, now supplying a range of unique tools and new opportunities to define the strategies employed by these Gram-positive bacteria to overcome host defenses, as well as those host antimicrobial pathways that can be used to limit its growth and spread. Herein, we take a comparative perspective and provide an update on the contributions of teleost models to our understanding of mycobacterial diseases.

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.

Mycobacterium abscessus-Induced Granuloma Formation Is Strictly Dependent on TNF Signaling and Neutrophil Trafficking

Mycobacterium abscessus is considered the most common respiratory pathogen among the rapidly growing non-tuberculous mycobacteria. Infections with M. abscessus are increasingly found in patients with chronic lung diseases, especially cystic fibrosis, and are often refractory to antibiotic therapy. M. abscessus has two morphotypes with distinct effects on host cells and biological responses. The smooth (S) variant is recognized as the initial airway colonizer while the rough (R) is known to be a potent inflammatory inducer associated with invasive disease, but the underlying immunopathological mechanisms of the infection remain unsolved. We conducted a comparative stepwise dissection of the inflammatory response in S and R pathogenesis by monitoring infected transparent zebrafish embryos. Loss of TNFR1 function resulted in increased mortality with both variants, and was associated with unrestricted intramacrophage bacterial growth and decreased bactericidal activity. The use of transgenic zebrafish lines harboring fluorescent macrophages and neutrophils revealed that neutrophils, like macrophages, interact with M. abscessus at the initial infection sites. Impaired TNF signaling disrupted the IL8-dependent neutrophil mobilization, and the defect in neutrophil trafficking led to the formation of aberrant granulomas, extensive mycobacterial cording, unrestricted bacterial growth and subsequent larval death. Our findings emphasize the central role of neutrophils for the establishment and maintenance of the protective M. abscessus granulomas. These results also suggest that the TNF/IL8 inflammatory axis is necessary for protective immunity against M. abscessus and may be of clinical relevance to explain why immunosuppressive TNF therapy leads to the exacerbation of M. abscessus infections.

The incidence of non-tuberculous mycobacterial infections has recently increased and has even surpassed tuberculosis as a public health concern in many developed countries. These infections require long treatment regimens that are often unsuccessful. Among these, Mycobacterium abscessus has emerged as perhaps the most difficult-to-manage pathogen, especially in cystic fibrosis patients. Unfortunately, very little is known regarding the contributions of the pro-inflammatory and innate immune responses during M. abscessus infection. Here, we exploited the transparency of zebrafish embryos to study, at high resolution, the interactions of M. abscessus with macrophages and neutrophils, and found that both cell types are required to control the infection. We also describe the dramatic consequences of impaired TNF/IL8 immunity on the outcome of the infection. Most importantly, by tracking the dynamics of neutrophil mobilization, we demonstrated the crucial role of these cells in the formation and integrity of protective granulomas. Together, our data provide a significant advance in deciphering the immunopathology of M. abscessus infection, which is particularly relevant for understanding the exquisite vulnerability of cystic fibrosis patients to this bacterium.

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.

Efferocytosis and extrusion of leukocytes determine the progression of early mycobacterial pathogenesis

Macrophages and neutrophils are the first responders to invading pathogens and contribute strongly to the host defense against intracellular pathogens. The collective interplay and dynamic interactions between these leukocytes are to a large extent not understood. In the present study, we have investigated their role using a combination of confocal laser-scanning and electron microscopy in a zebrafish model for tuberculosis, a local Mycobacterium marinum infection in the tissue of the larval tail fin. Our results show that neutrophils are efficient in phagocytosis of mycobacteria and that they contribute largely to their dissemination. Macrophages appear to play a major role in efferocytosis, phagocytosis of dead cells that contain bacterial content. Phagocytic cells with large bacterial aggregates are formed that can be extruded out of the tissue after cell death. Alternatively, these excessively infected cells can undergo necrosis leading to immediate recruitment of surrounding leukocytes and subsequent phagocytosis of released bacteria. Our data show that these necrotic burst events result in progression of the infection, whereas extrusion abates the infection.

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.

Protection and pathology in TB: learning from the zebrafish model

Zebrafish has earned its place among animal models of tuberculosis. Its natural pathogen, Mycobacterium marinum, shares major virulence factors with the human pathogen Mycobacterium tuberculosis. In adult zebrafish, which possess recombination-activated adaptive immunity, it can cause acute infection or a chronic progressive disease with containment of mycobacteria in well-structured, caseating granulomas. In addition, a low-dose model that closely mimics human latent infection has recently been developed. These models are used alongside infection of optically transparent zebrafish embryos and larvae that rely on innate immunity and permit non-invasive visualization of the early stages of developing granulomas that are inaccessible in other animal models. By microinjecting mycobacteria intravenously or into different tissues, systemic and localized infections can be induced, each useful for studying particular aspects of early pathogenesis, such as phagocyte recruitment, granuloma expansion and maintenance, vascularization of granulomas, and the phagocyte-mediated dissemination of mycobacteria. This has contributed to new insights into the mycobacteria-driven mechanisms that promote granuloma formation, the double-edged role of inflammation, the mechanisms of macrophage cell death that favor disease progression, and the host-protective role of autophagy. As a result, zebrafish models are now increasingly used to explore strategies for adjunctive therapy of tuberculosis with host-directed drugs.

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