Iron metabolism and resistance to infection by invasive bacteria in the social amoeba Dictyostelium discoideum. - Related Documents




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58601.0000Iron metabolism and resistance to infection by invasive bacteria in the social amoeba Dictyostelium discoideum. Dictyostelium cells are forest soil amoebae, which feed on bacteria and proliferate as solitary cells until bacteria are consumed. Starvation triggers a change in life style, forcing cells to gather into aggregates to form multicellular organisms capable of cell differentiation and morphogenesis. As a soil amoeba and a phagocyte that grazes on bacteria as the obligate source of food, Dictyostelium could be a natural host of pathogenic bacteria. Indeed, many pathogens that occasionally infect humans are hosted for most of their time in protozoa or free-living amoebae, where evolution of their virulence traits occurs. Due to these features and its amenability to genetic manipulation, Dictyostelium has become a valuable model organism for studying strategies of both the host to resist infection and the pathogen to escape the defense mechanisms. Similarly to higher eukaryotes, iron homeostasis is crucial for Dictyostelium resistance to invasive bacteria. Iron is essential for Dictyostelium, as both iron deficiency or overload inhibit cell growth. The Dictyostelium genome shares with mammals many genes regulating iron homeostasis. Iron transporters of the Nramp (Slc11A) family are represented with two genes, encoding Nramp1 and Nramp2. Like the mammalian ortholog, Nramp1 is recruited to phagosomes and macropinosomes, whereas Nramp2 is a membrane protein of the contractile vacuole network, which regulates osmolarity. Nramp1 and Nramp2 localization in distinct compartments suggests that both proteins synergistically regulate iron homeostasis. Rather than by absorption via membrane transporters, iron is likely gained by degradation of ingested bacteria and efflux via Nramp1 from phagosomes to the cytosol. Nramp gene disruption increases Dictyostelium sensitivity to infection, enhancing intracellular growth of Legionella or Mycobacteria. Generation of mutants in other "iron genes" will help identify genes essential for iron homeostasis and resistance to pathogens.201324066281
58710.9996The Nramp (Slc11) proteins regulate development, resistance to pathogenic bacteria and iron homeostasis in Dictyostelium discoideum. The Dictyostelium discoideum genome harbors two genes encoding members of the Nramp superfamily, which is conserved from bacteria (MntH proteins) to humans (Slc11 proteins). Nramps are proton-driven metal ion transporters with a preference for iron and manganese. Acquisition of these metal cations is vital for all cells, as they act as redox cofactors and regulate key cellular processes, such as DNA synthesis, electron transport, energy metabolism and oxidative stress. Dictyostelium Nramp1 (Slc11a1), like its mammalian ortholog, mediates resistance to infection by invasive bacteria. We have extended the analysis to the nramp2 gene, by generating single and double nramp1/nramp2 knockout mutants and cells expressing GFP fusion proteins. In contrast to Nramp1, which is recruited to phagosomes and macropinosomes, the Nramp2 protein is localized exclusively in the membrane of the contractile vacuole, a vesicular tubular network regulating cellular osmolarity. Both proteins colocalize with the V-H(+)-ATPase, which can provide the electrogenic force for vectorial transport. Like nramp1, nramp2 gene disruption affects resistance to Legionella pneumophila. Disrupting both genes additionally leads to defects in development, with strong delay in cell aggregation, formation of large streams and multi-tipped aggregates. Single and double mutants display differential sensitivity to cell growth under conditions of iron overload or depletion. The data favor the hypothesis that Nramp1 and Nramp2, under control of the V-H(+)-ATPase, synergistically regulate iron homeostasis, with the contractile vacuole possibly acting as a store for metal cations.201322992462
835420.9993Are inflammatory phagocytes responsible for resistance to facultative intracellular bacteria? At least 38 deaths from listeriosis in the United States in recent months have drawn attention to how little we know about resistance to facultative intracellular bacteria. Much more is known about resistance to obligate intracellular parasites. Macrophages, activated by T cell-derived macrophage activating factors (MAF), are able to kill obligate intracellular parasites and tumor cells(1-10) including Leishmania, certain trypanosomes, Toxoplasma, the obligate intracellular bacterium Rickettsia, and perhaps bacteria such as mycobacteria and Legionella. However, macrophages, stimulated by MAF, may not be the only host cells which can defend against infection by facultative intracellular bacteria such as Salmonella typhimurium or Listeria monocytogenes. Six different observations made by Priscilla Campbell and colleagues, and by others, suggest that it is not the so-called 'activated' macrophage which is primarily responsible for resistance against facultative intracellular bacteria. Rather, she proposes that an early inflammatory cell recently recruited in response to an inflammatory stimulus - a cell whose presence seems to be under the control of immunologically-specific T cells - plays a critical role in resistance to infection by these organisms.198625289573
58530.9993Genetic susceptibility to intracellular infections: Nramp1, macrophage function and divalent cations transport. Nramp1 is one of the few host resistance genes that have been characterized at the molecular level. Nramp1 is an integral membrane protein expressed in the lysosomal compartment of macrophages and is recruited to the membrane of bacterial phagosomes where it affects intracellular microbial replication. Nramp1 is part of a very large gene family conserved from bacteria and man that codes for transporters of divalent cations transporters. We propose that Nramp1 affects the intraphagosomal microbial replication by modulating divalent cations content in this organelle. Both mammalian and bacterial transporters may compete for the same substrate in the phagosomal space.200010679418
72740.9993Bacillus subtilis extracytoplasmic function (ECF) sigma factors and defense of the cell envelope. Bacillus subtilis provides a model for investigation of the bacterial cell envelope, the first line of defense against environmental threats. Extracytoplasmic function (ECF) sigma factors activate genes that confer resistance to agents that threaten the integrity of the envelope. Although their individual regulons overlap, σ(W) is most closely associated with membrane-active agents, σ(X) with cationic antimicrobial peptide resistance, and σ(V) with resistance to lysozyme. Here, I highlight the role of the σ(M) regulon, which is strongly induced by conditions that impair peptidoglycan synthesis and includes the core pathways of envelope synthesis and cell division, as well as stress-inducible alternative enzymes. Studies of these cell envelope stress responses provide insights into how bacteria acclimate to the presence of antibiotics.201626901131
69850.9993Genome-wide transcriptional changes induced by phagocytosis or growth on bacteria in Dictyostelium. BACKGROUND: Phagocytosis plays a major role in the defense of higher organisms against microbial infection and provides also the basis for antigen processing in the immune response. Cells of the model organism Dictyostelium are professional phagocytes that exploit phagocytosis of bacteria as the preferred way to ingest food, besides killing pathogens. We have investigated Dictyostelium differential gene expression during phagocytosis of non-pathogenic bacteria, using DNA microarrays, in order to identify molecular functions and novel genes involved in phagocytosis. RESULTS: The gene expression profiles of cells incubated for a brief time with bacteria were compared with cells either incubated in axenic medium or growing on bacteria. Transcriptional changes during exponential growth in axenic medium or on bacteria were also compared. We recognized 443 and 59 genes that are differentially regulated by phagocytosis or by the different growth conditions (growth on bacteria vs. axenic medium), respectively, and 102 genes regulated by both processes. Roughly one third of the genes are up-regulated compared to macropinocytosis and axenic growth. Functional annotation of differentially regulated genes with different tools revealed that phagocytosis induces profound changes in carbohydrate, amino acid and lipid metabolism, and in cytoskeletal components. Genes regulating translation and mitochondrial biogenesis are mostly up-regulated. Genes involved in sterol biosynthesis are selectively up-regulated, suggesting a shift in membrane lipid composition linked to phagocytosis. Very few changes were detected in genes required for vesicle fission/fusion, indicating that the intracellular traffic machinery is mostly in common between phagocytosis and macropinocytosis. A few putative receptors, including GPCR family 3 proteins, scaffolding and adhesion proteins, components of signal transduction and transcription factors have been identified, which could be part of a signalling complex regulating phagocytosis and adaptational downstream responses. CONCLUSION: The results highlight differences between phagocytosis and macropinocytosis, and provide the basis for targeted functional analysis of new candidate genes and for comparison studies with transcriptomes during infection with pathogenic bacteria.200818559084
76260.9993MerR family transcription activators: similar designs, different specificities. Living organisms use metals for a variety of essential functions, and face the problems of how to acquire and regulate the intracellular levels of those metals they need, differentiate between essential and toxic metals, and remove from the cell or detoxify metals that are toxic. In bacteria, cytoplasmic metal ion responsive transcriptional regulators are important in regulating the expression of genes involved in metal ion homeostasis and efflux systems. The MerR family of transcriptional activators are metal sensing regulators that are found in different bacteria and have a common design, but have evolved to recognize and respond to different metals. In this issue of Molecular Microbiology, work by Checa and colleagues describes for the first time a gold-specific MerR family regulator named GolS from Salmonella enterica serovar Typhimurium that controls the production of an efflux pump and a metal chaperone protein that confer resistance to Au salts.200717302809
76170.9993Copper-responsive gene regulation in bacteria. Copper is an essential cofactor of various enzymes, but free copper is highly toxic to living cells. To maintain cellular metabolism at different ambient copper concentrations, bacteria have evolved specific copper homeostasis systems that mostly act as defence mechanisms. As well as under free-living conditions, copper defence is critical for virulence in pathogenic bacteria. Most bacteria synthesize P-type copper export ATPases as principal defence determinants when copper concentrations exceed favourable levels. In addition, many bacteria utilize resistance-nodulation-cell division (RND)-type efflux systems and multicopper oxidases to cope with excess copper. This review summarizes our current knowledge on copper-sensing transcriptional regulators, which we assign to nine different classes. Widespread one-component regulators are CueR, CopY and CsoR, which were initially identified in Escherichia coli, Enterococcus hirae and Mycobacterium tuberculosis, respectively. CueR activates homeostasis gene expression at elevated copper concentrations, while CopY and CsoR repress their target genes under copper-limiting conditions. Besides these one-component systems, which sense the cytoplasmic copper status, many Gram-negative bacteria utilize two-component systems, which sense periplasmic copper concentrations. In addition to these well-studied transcriptional factors, copper control mechanisms acting at the post-transcriptional and the post-translational levels will be discussed.201222918892
69780.9993Step-wise loss of bacterial flagellar torsion confers progressive phagocytic evasion. Phagocytosis of bacteria by innate immune cells is a primary method of bacterial clearance during infection. However, the mechanisms by which the host cell recognizes bacteria and consequentially initiates phagocytosis are largely unclear. Previous studies of the bacterium Pseudomonas aeruginosa have indicated that bacterial flagella and flagellar motility play an important role in colonization of the host and, importantly, that loss of flagellar motility enables phagocytic evasion. Here we use molecular, cellular, and genetic methods to provide the first formal evidence that phagocytic cells recognize bacterial motility rather than flagella and initiate phagocytosis in response to this motility. We demonstrate that deletion of genes coding for the flagellar stator complex, which results in non-swimming bacteria that retain an initial flagellar structure, confers resistance to phagocytic binding and ingestion in several species of the gamma proteobacterial group of Gram-negative bacteria, indicative of a shared strategy for phagocytic evasion. Furthermore, we show for the first time that susceptibility to phagocytosis in swimming bacteria is proportional to mot gene function and, consequently, flagellar rotation since complementary genetically- and biochemically-modulated incremental decreases in flagellar motility result in corresponding and proportional phagocytic evasion. These findings identify that phagocytic cells respond to flagellar movement, which represents a novel mechanism for non-opsonized phagocytic recognition of pathogenic bacteria.201121949654
831490.9993Interactions between Bacteria and Bile Salts in the Gastrointestinal and Hepatobiliary Tracts. Bile salts and bacteria have intricate relationships. The composition of the intestinal pool of bile salts is shaped by bacterial metabolism. In turn, bile salts play a role in intestinal homeostasis by controlling the size and the composition of the intestinal microbiota. As a consequence, alteration of the microbiome-bile salt homeostasis can play a role in hepatic and gastrointestinal pathological conditions. Intestinal bacteria use bile salts as environmental signals and in certain cases as nutrients and electron acceptors. However, bile salts are antibacterial compounds that disrupt bacterial membranes, denature proteins, chelate iron and calcium, cause oxidative damage to DNA, and control the expression of eukaryotic genes involved in host defense and immunity. Bacterial species adapted to the mammalian gut are able to endure the antibacterial activities of bile salts by multiple physiological adjustments that include remodeling of the cell envelope and activation of efflux systems and stress responses. Resistance to bile salts permits that certain bile-resistant pathogens can colonize the hepatobiliary tract, and an outstanding example is the chronic infection of the gall bladder by Salmonella enterica. A better understanding of the interactions between bacteria and bile salts may inspire novel therapeutic strategies for gastrointestinal and hepatobiliary diseases that involve microbiome alteration, as well as novel schemes against bacterial infections.201729043249
730100.9993How intracellular bacteria survive: surface modifications that promote resistance to host innate immune responses. Bacterial pathogens regulate the expression of virulence factors in response to environmental signals. In the case of salmonellae, many virulence factors are regulated via PhoP/PhoQ, a two-component signal transduction system that is repressed by magnesium and calcium in vitro. PhoP/PhoQ-activated genes promote intracellular survival within macrophages, whereas PhoP-repressed genes promote entrance into epithelial cells and macrophages by macropinocytosis and stimulate epithelial cell cytokine production. PhoP-activated genes include those that alter the cell envelope through structural alterations of lipopolysaccharide and lipid A, the bioactive component of lipopolysaccharide. PhoP-activated changes in the bacterial envelope likely promote intracellular survival by increasing resistance to host cationic antimicrobial peptides and decreasing host cell cytokine production.199910081503
686110.9993SigB-dependent general stress response in Bacillus subtilis and related gram-positive bacteria. One of the strongest and most noticeable responses of Bacillus subtilis cells to a range of stress and starvation stimuli is the dramatic induction of about 150 SigB-dependent general stress genes. The activity of SigB itself is tightly regulated by a complex signal transduction cascade with at least three main signaling pathways that respond to environmental stress, energy depletion, or low temperature. The SigB-dependent response is conserved in related gram-positive bacteria but is missing in strictly anaerobic or in some facultatively anaerobic gram-positive bacteria. It covers functions from nonspecific and multiple stress resistance to the control of virulence in pathogenic bacteria. A comprehensive understanding of this crucial stress response is essential not only for bacterial physiology but also for applied microbiology, including pathogenicity and pathogen control.200718035607
9338120.9992Polyamines in bacteria: pleiotropic effects yet specific mechanisms. Extensive data in a wide range of organisms point to the importance of polyamine homeostasis for growth. The two most common polyamines found in bacteria are putrescine and spermidine. The investigation of polyamine function in bacteria has revealed that they are involved in a number of functions other than growth, which include incorporation into the cell wall and biosynthesis of siderophores. They are also important in acid resistance and can act as a free radical ion scavenger. More recently it has been suggested that polyamines play a potential role in signaling cellular differentiation in Proteus mirabilis. Polyamines have also been shown to be essential in biofilm formation in Yersinia pestis. The pleiotropic nature of polyamines has made their investigation difficult, particularly in discerning any specific effect from more global growth effects. Here we describe key developments in the investigation of the function of polyamines in bacteria that have revealed new roles for polyamines distinct from growth. We describe the bacterial genes necessary for biosynthesis and transport, with a focus on Y. pestis. Finally we review a novel role for polyamines in the regulation of biofilm development in Y. pestis and provide evidence that the investigation of polyamines in Y. pestis may provide a model for understanding the mechanism through which polyamines regulate biofilm formation.200717966408
8283130.9992Stress responses as determinants of antimicrobial resistance in Gram-negative bacteria. Bacteria encounter a myriad of potentially growth-compromising conditions in nature and in hosts of pathogenic bacteria. These 'stresses' typically elicit protective and/or adaptive responses that serve to enhance bacterial survivability. Because they impact upon many of the same cellular components and processes that are targeted by antimicrobials, adaptive stress responses can influence antimicrobial susceptibility. In targeting and interfering with key cellular processes, antimicrobials themselves are 'stressors' to which protective stress responses have also evolved. Cellular responses to nutrient limitation (nutrient stress), oxidative and nitrosative stress, cell envelope damage (envelope stress), antimicrobial exposure and other growth-compromising stresses, have all been linked to the development of antimicrobial resistance in Gram-negative bacteria - resulting from the stimulation of protective changes to cell physiology, activation of resistance mechanisms, promotion of resistant lifestyles (biofilms), and induction of resistance mutations.201222424589
8309140.9992The expression of virulence genes increases membrane permeability and sensitivity to envelope stress in Salmonella Typhimurium. Virulence gene expression can represent a substantial fitness cost to pathogenic bacteria. In the model entero-pathogen Salmonella Typhimurium (S.Tm), such cost favors emergence of attenuated variants during infections that harbor mutations in transcriptional activators of virulence genes (e.g., hilD and hilC). Therefore, understanding the cost of virulence and how it relates to virulence regulation could allow the identification and modulation of ecological factors to drive the evolution of S.Tm toward attenuation. In this study, investigations of membrane status and stress resistance demonstrate that the wild-type (WT) expression level of virulence factors embedded in the envelope increases membrane permeability and sensitizes S.Tm to membrane stress. This is independent from a previously described growth defect associated with virulence gene expression in S.Tm. Pretreating the bacteria with sublethal stress inhibited virulence expression and increased stress resistance. This trade-off between virulence and stress resistance could explain the repression of virulence expression in response to harsh environments in S.Tm. Moreover, we show that virulence-associated stress sensitivity is a burden during infection in mice, contributing to the inherent instability of S.Tm virulence. As most bacterial pathogens critically rely on deploying virulence factors in their membrane, our findings could have a broad impact toward the development of antivirulence strategies.202235389980
711150.9992Non-specific, general and multiple stress resistance of growth-restricted Bacillus subtilis cells by the expression of the sigmaB regulon. Bacillus subtilis cells respond almost immediately to different stress conditions by increasing the production of general stress proteins (GSPs). The genes encoding the majority of the GSPs that are induced by heat, ethanol, salt stress or by starvation for glucose, oxygen or phosphate belong to the sigmaB-dependent general stress regulon. Despite a good understanding of the complex regulation of the activity of sigmaB and knowledge of a very large number of general stress genes controlled by sigmaB, first insights into the physiological role of this nonspecific stress response have been obtained only very recently. To explore the physiological role of this reguIon, we and others identified sigmaB-dependent general stress genes and compared the stress tolerance of wild-type cells with mutants lacking sigmaB or general stress proteins. The proteins encoded by sigmaB-dependent general stress genes can be divided into at least five functional groups that most probably provide growth-restricted B. subtilis cells with a multiple stress resistance in anticipation of future stress. In particular, sigB mutants are impaired in non-specific resistance to oxidative stress, which requires the sigmaB-dependent dps gene encoding a DNA-protecting protein. Protection against oxidative damage of membranes, proteins or DNA could be the most essential component of sigmaB mediated general stress resistance in growth-arrested aerobic gram-positive bacteria. Other general stress genes have both a sigmaB-dependent induction pathway and a second sigmaB-independent mechanism of stress induction, thereby partially compensating for a sigmaB deficiency in a sigB mutant. In contrast to sigB mutants, null mutations in genes encoding those proteins, such as cIpP or cIpC, cause extreme sensitivity to salt or heat.19989767581
8315160.9992The Induction and Modulation of Plant Defense Responses by Bacterial Lipopolysaccharides. Lipopolysaccharides (LPSs) are ubiquitous, indispensable components of the cell surface of Gram-negative bacteria that apparently have diverse roles in bacterial pathogenesis of plants. As an outer membrane component, LPS may contribute to the exclusion of plant-derived antimicrobial compounds promoting the ability of a bacterial plant pathogen to infect plants. In contrast, LPS can be recognized by plants to directly trigger some plant defense-related responses. LPS can also alter the response of plants to subsequent bacterial inoculation; these delayed effects include alterations in the expression patterns of genes coding for some pathogenesis-related (PR) proteins, promotion of the synthesis of antimicrobial hydroxycinnamoyl-tyramine conjugates, and prevention of the hypersensitive reaction caused by avirulent bacteria. Prevention of the response may allow expression of resistance in the absence of catastrophic tissue damage. Recognition of LPS (and other nonspecific determinants) may initiate responses in plants that restrict the growth of nonpathogenic bacteria, whereas plant pathogens may possess hrp gene-dependent mechanisms to suppress such responses.200011701843
8310170.9992Dynamic heterogeneity in an E. coli stress response regulon mediates gene activation and antimicrobial peptide tolerance. The bacterial stress response is an intricately regulated system that plays a critical role in cellular resistance to drug treatment. The complexity of this response is further complicated by cell-to-cell heterogeneity in the expression of bacterial stress response genes. These genes are often organized into networks comprising one or more transcriptional regulators that control expression of a suite of downstream genes. While the expression heterogeneity of many of these upstream regulators has been characterized, the way in which this variability affects the larger downstream stress response remains hard to predict, prompting two key questions. First, how does heterogeneity and expression noise in stress response regulators propagate to the diverse downstream genes in their regulons. Second, when expression levels vary, how do multiple downstream genes act together to protect cells from stress. To address these questions, we focus on the transcription factor PhoP, a critical virulence regulator which coordinates pathogenicity in several gram-negative species. We use optogenetic stimulation to precisely control PhoP expression levels and examine how variations in PhoP affect the downstream activation of genes in the PhoP regulon. We find that these downstream genes exhibit differences both in mean expression level and sensitivity to increasing levels of PhoP. These response functions can also vary between individual cells, increasing heterogeneity in the population. We tie these variations to cell survival when bacteria are exposed to a clinically-relevant antimicrobial peptide, showing that high expression of the PhoP-regulon gene pmrD provides a protective effect against Polymyxin B. Overall, we demonstrate that even subtle heterogeneity in expression of a stress response regulator can have clear consequences for enabling bacteria to survive stress.202439677761
8210180.9992Bacterial sensing of antimicrobial peptides. Antimicrobial peptides (AMPs) form a crucial part of human innate host defense, especially in neutrophil phagosomes and on epithelial surfaces. Bacteria have a variety of efficient resistance mechanisms to human AMPs, such as efflux pumps, secreted proteases, and alterations of the bacterial cell surface that are aimed to minimize attraction of the typically cationic AMPs. In addition, bacteria have specific sensors that activate AMP resistance mechanisms when AMPs are present. The prototypical Gram-negative PhoP/PhoQ and the Gram-positive Aps AMP-sensing systems were first described and investigated in Salmonella typhimurium and Staphylococcus epidermidis, respectively. Both include a classical bacterial two-component sensor/regulator system, but show many structural, mechanistic, and functional differences. The PhoP/PhoQ regulon controls a variety of genes not necessarily limited to AMP resistance mechanisms, but apparently aimed to combat innate host defense on a broad scale. In contrast, the staphylococcal Aps system predominantly upregulates AMP resistance mechanisms, namely the D-alanylation of teichoic acids, inclusion of lysyl-phosphati-dylglycerol in the cytoplasmic membrane, and expression of the putative VraFG AMP efflux pump. Notably, both systems are crucial for virulence and represent possible targets for antimicrobial therapy.200919494583
702190.9992Cutting edge: the toll pathway is required for resistance to gram-positive bacterial infections in Drosophila. In Drosophila, the response against various microorganisms involves different recognition and signaling pathways, as well as distinct antimicrobial effectors. On the one hand, the immune deficiency pathway regulates the expression of antimicrobial peptides that are active against Gram-negative bacteria. On the other hand, the Toll pathway is involved in the defense against filamentous fungi and controls the expression of antifungal peptide genes. The gene coding for the only known peptide with high activity against Gram-positive bacteria, Defensin, is regulated by both pathways. So far, survival experiments to Gram-positive bacteria have been performed with Micrococcus luteus and have failed to reveal the involvement of one or the other pathway in host defense against such infections. In this study, we report that the Toll pathway, but not that of immune deficiency, is required for resistance to other Gram-positive bacteria and that this response does not involve Defensin.200211823479