FapR regulates HssRS-mediated heme homeostasis in Bacillus anthracis.

Bacillus anthracis , a Gram-positive facultative anaerobe and the causative agent of anthrax, multiplies to extraordinarily high numbers in vertebrate blood, resulting in considerable heme exposure. Heme is an essential nutrient and the preferred iron source for bacteria during vertebrate colonization, but its high redox potential makes it toxic in excess. To regulate heme homeostasis, many Gram-positive bacteria, including B. anthracis , rely on the two-component signaling system HssRS. HssRS comprises the heme sensing histidine kinase HssS, which modulates the activity of the HssR transcription factor to enable bacteria to circumvent heme toxicity. However, the regulation of the HssRS system remains unclear. Here we identify FapR, the transcriptional regulator of fatty acid biosynthesis, as a key factor in HssRS function. FapR plays an important role in maintaining membrane integrity and the localization of the histidine kinase HssS. Specifically, disruption of fapR leads to increased membrane rigidity, which hinders the penetration of HssRS inducers, resulting in the inactivation of HssRS. Furthermore, deletion of fapR affects the loading of HssS onto the cell membrane, compromising its heme sensing function and subsequently reducing endogenous heme biosynthesis. These findings shed light on the molecular mechanisms governing bacterial adaptation to heme stress and provide potential targets for antimicrobial intervention strategies. IMPORTANCE: Understanding the mechanisms by which B. anthracis regulates heme homeostasis is crucial for developing new strategies to combat anthrax, a serious disease affecting both humans and animals. This study uncovers the role of the transcriptional regulator FapR in maintaining membrane integrity and facilitating the proper function of the HssRS two-component signaling system, which is essential for managing heme toxicity in B. anthracis , as well as other Gram-positive pathogens. By elucidating the connection between FapR and HssRS, our findings provide new insights into the molecular adaptation of bacteria to heme stress and expand our knowledge of bacterial physiology and pathogenicity. More importantly, targeting the regulatory pathways involved in heme sensing and homeostasis presents a promising approach for developing novel therapeutics against anthrax and potentially other bacterial infections that rely on similar mechanisms.

Copper ions inhibit pentose phosphate pathway function in Staphylococcus aureus.

To gain a better insight of how Copper (Cu) ions toxify cells, metabolomic analyses were performed in S. aureus strains that lacks the described Cu ion detoxification systems (ΔcopBL ΔcopAZ; cop-). Exposure of the cop- strain to Cu(II) resulted in an increase in the concentrations of metabolites utilized to synthesize phosphoribosyl diphosphate (PRPP). PRPP is created using the enzyme phosphoribosylpyrophosphate synthetase (Prs) which catalyzes the interconversion of ATP and ribose 5-phosphate to PRPP and AMP. Supplementing growth medium with metabolites requiring PRPP for synthesis improved growth in the presence of Cu(II). A suppressor screen revealed that a strain with a lesion in the gene coding adenine phosphoribosyltransferase (apt) was more resistant to Cu. Apt catalyzes the conversion of adenine with PRPP to AMP. The apt mutant had an increased pool of adenine suggesting that the PRPP pool was being redirected. Over-production of apt, or alternate enzymes that utilize PRPP, increased sensitivity to Cu(II). Increasing or decreasing expression of prs resulted in decreased and increased sensitivity to growth in the presence of Cu(II), respectively. We demonstrate that Prs is inhibited by Cu ions in vivo and in vitro and that treatment of cells with Cu(II) results in decreased PRPP levels. Lastly, we establish that S. aureus that lacks the ability to remove Cu ions from the cytosol is defective in colonizing the airway in a murine model of acute pneumonia, as well as the skin. The data presented are consistent with a model wherein Cu ions inhibits pentose phosphate pathway function and are used by the immune system to prevent S. aureus infections.

Elevated transferrin receptor impairs T cell metabolism and function in systemic lupus erythematosus.

T cells in systemic lupus erythematosus (SLE) exhibit multiple metabolic abnormalities. Excess iron can impair mitochondria and may contribute to SLE. To gain insights into this potential role of iron in SLE, we performed a CRISPR screen of iron handling genes on T cells. Transferrin receptor (CD71) was identified as differentially critical for TH1 and inhibitory for induced regulatory T cells (iTregs). Activated T cells induced CD71 and iron uptake, which was exaggerated in SLE-prone T cells. Cell surface CD71 was enhanced in SLE-prone T cells by increased endosomal recycling. Blocking CD71 reduced intracellular iron and mTORC1 signaling, which inhibited TH1 and TH17 cells yet enhanced iTregs. In vivo treatment reduced kidney pathology and increased CD4 T cell production of IL-10 in SLE-prone mice. Disease severity correlated with CD71 expression on TH17 cells from patients with SLE, and blocking CD71 in vitro enhanced IL-10 secretion. T cell iron uptake via CD71 thus contributes to T cell dysfunction and can be targeted to limit SLE-associated pathology.

Ultraviolet light oxidation of fresh hemoglobin eliminates aggregate formation seen in commercially sourced hemoglobin.

Elevated levels of circulating cell-free hemoglobin (CFH) are an integral feature of several clinical conditions including sickle cell anemia, sepsis, hemodialysis and cardiopulmonary bypass. Oxidized (Fe3+, ferric) hemoglobin contributes to the pathophysiology of these disease states and is therefore widely studied in experimental models, many of which use commercially sourced CFH. In this study, we treated human endothelial cells with commercially sourced ferric hemoglobin and observed the appearance of dense cytoplasmic aggregates (CAgg) over time. These CAgg were intensely autofluorescent, altered intracellular structures (such as mitochondria), formed in multiple cell types and with different media composition, and formed regardless of the presence or absence of cells. An in-depth chemical analysis of these CAgg revealed that they contain inorganic components and are not pure hemoglobin. To oxidize freshly isolated hemoglobin without addition of an oxidizing agent, we developed a novel method to convert ferrous CFH to ferric CFH using ultraviolet light without the need for additional redox agents. Unlike commercial ferric hemoglobin, treatment of cells with the fresh ferric hemoglobin did not lead to CAgg formation. These studies suggest that commercially sourced CFH may contain stabilizers and additives which contribute to CAgg formation.

An Iron Refractory Phenotype in Obese Adipose Tissue Macrophages Leads to Adipocyte Iron Overload.

Adipocyte iron overload is a maladaptation associated with obesity and insulin resistance. The objective of the current study was to determine whether and how adipose tissue macrophages (ATMs) regulate adipocyte iron concentrations and whether this is impacted by obesity. Using bone marrow-derived macrophages (BMDMs) polarized to M0, M1, M2, or metabolically activated (MMe) phenotypes, we showed that MMe BMDMs and ATMs from obese mice have reduced expression of several iron-related proteins. Furthermore, the bioenergetic response to iron in obese ATMs was hampered. ATMs from iron-injected lean mice increased their glycolytic and respiratory capacities, thus maintaining metabolic flexibility, while ATMs from obese mice did not. Using an isotope-based system, we found that iron exchange between BMDMs and adipocytes was regulated by macrophage phenotype. At the end of the co-culture, MMe macrophages transferred and received more iron from adipocytes than M0, M1, and M2 macrophages. This culminated in a decrease in total iron in MMe macrophages and an increase in total iron in adipocytes compared with M2 macrophages. Taken together, in the MMe condition, the redistribution of iron is biased toward macrophage iron deficiency and simultaneous adipocyte iron overload. These data suggest that obesity changes the communication of iron between adipocytes and macrophages and that rectifying this iron communication channel may be a novel therapeutic target to alleviate insulin resistance.

Zn-regulated GTPase metalloprotein activator 1 modulates vertebrate zinc homeostasis.

Zinc (Zn) is an essential micronutrient and cofactor for up to 10% of proteins in living organisms. During Zn limitation, specialized enzymes called metallochaperones are predicted to allocate Zn to specific metalloproteins. This function has been putatively assigned to G3E GTPase COG0523 proteins, yet no Zn metallochaperone has been experimentally identified in any organism. Here, we functionally characterize a family of COG0523 proteins that is conserved across vertebrates. We identify Zn metalloprotease methionine aminopeptidase 1 (METAP1) as a COG0523 client, leading to the redesignation of this group of COG0523 proteins as the Zn-regulated GTPase metalloprotein activator (ZNG1) family. Using biochemical, structural, genetic, and pharmacological approaches across evolutionarily divergent models, including zebrafish and mice, we demonstrate a critical role for ZNG1 proteins in regulating cellular Zn homeostasis. Collectively, these data reveal the existence of a family of Zn metallochaperones and assign ZNG1 an important role for intracellular Zn trafficking.

Host Polyunsaturated Fatty Acids Potentiate Aminoglycoside Killing of Staphylococcus aureus.

Aminoglycoside antibiotics rely on the proton motive force to enter the bacterial cell, and facultative anaerobes like Staphylococcus aureus can shift energy generation from respiration to fermentation, becoming tolerant of aminoglycosides. Following this metabolic shift, high concentrations of aminoglycosides are required to eradicate S. aureus infections, which endangers the host due to the toxicity of aminoglycosides. Membrane-disrupting molecules prevent aminoglycoside tolerance in S. aureus by facilitating passive entry of the drug through the membrane. Polyunsaturated fatty acids (PUFAs) increase membrane permeability when incorporated into S. aureus. Here, we report that the abundant host-derived PUFA arachidonic acid increases the susceptibility of S. aureus to aminoglycosides, decreasing the aminoglycoside concentration needed to kill S. aureus. We demonstrate that PUFAs and aminoglycosides synergize to kill multiple strains of S. aureus, including both methicillin-resistant and -susceptible S. aureus. We also present data showing that PUFAs and aminoglycosides effectively kill S. aureus small colony variants, strains that are particularly recalcitrant to killing by many antibiotics. We conclude that cotreatment with PUFAs, which are molecules with low host toxicity, and aminoglycosides decreases the aminoglycoside concentration necessary to kill S. aureus, lowering the toxic side effects to the host associated with prolonged aminoglycoside exposure. IMPORTANCE Staphylococcus aureus infects every niche of the human host, and these infections are the leading cause of Gram-positive sepsis. Aminoglycoside antibiotics are inexpensive, stable, and effective against many bacterial infections. However, S. aureus can shift its metabolism to become tolerant of aminoglycosides, requiring increased concentrations and/or longer courses of treatment, which can cause severe host toxicity. Here, we report that polyunsaturated fatty acids (PUFAs), which have low host toxicity, disrupt the S. aureus membrane, making the pathogen susceptible to aminoglycosides. Additionally, cotreatment with aminoglycosides is effective at killing S. aureus small colony variants, strains that are difficult to treat with antibiotics. Taken together, the data presented herein show the promise of PUFA cotreatment to increase the efficacy of aminoglycosides against S. aureus infections and decrease the risk to the human host of antibiotic-induced toxicity.

Increased Dietary Manganese Impairs Neutrophil Extracellular Trap Formation Rendering Neutrophils Ineffective at Combating Staphylococcus aureus.

Dietary metals can modify the risk to infection. Previously, we demonstrated that heightened dietary manganese (Mn) during systemic Staphylococcus aureus infection increases S. aureus virulence. However, immune cells also operate in these same environments and the effect of dietary Mn on neutrophil function in vivo has not been assessed. This study reveals that increased concentrations of Mn impairs mitochondrial respiration and superoxide production in neutrophils responding to S. aureus. As a result, high Mn accelerates primary degranulation, while impairing suicidal neutrophil extracellular trap (NET) formation, which decreases bactericidal activity. In vivo, elevated dietary Mn accumulated extracellularly in the heart, indicating that excess Mn may be more bioavailable in the heart. Coinciding with this phenotype, neutrophil function in the heart was most impacted by a high Mn diet, as neutrophils produced lower levels of mitochondrial superoxide and underwent less suicidal NET formation. Consistent with an ineffective neutrophil response when mice are on a high Mn diet, S. aureus burdens were increased in the heart and mice were more susceptible to systemic infection. Therefore, elevated dietary Mn not only affects S. aureus but also renders neutrophils less capable of restricting staphylococcal infection.

Mitochondrial Calcium Uniporter Affects Neutrophil Bactericidal Activity during Staphylococcus aureus Infection.

Neutrophils simultaneously restrict Staphylococcus aureus dissemination and facilitate bactericidal activity during infection through the formation of neutrophil extracellular traps (NETs). Neutrophils that produce higher levels of mitochondrial superoxide undergo enhanced terminal NET formation (suicidal NETosis) in response to S. aureus; however, mechanisms regulating mitochondrial homeostasis upstream of neutrophil antibacterial processes are not fully resolved. Here, we demonstrate that mitochondrial calcium uptake 1 (MICU1)-deficient (MICU1-/-) neutrophils accumulate higher levels of calcium and iron within the mitochondria in a mitochondrial calcium uniporter (MCU)-dependent manner. Corresponding with increased ion flux through the MCU, mitochondrial superoxide production is elevated, thereby increasing the propensity for MICU1-/- neutrophils to undergo suicidal NETosis rather than primary degranulation in response to S. aureus. Increased NET formation augments macrophage killing of bacterial pathogens. Similarly, MICU1-/- neutrophils alone are not more antibacterial toward S. aureus, but rather, enhanced suicidal NETosis by MICU1-/- neutrophils facilitates increased bactericidal activity in the presence of macrophages. Similarly, mice with a deficiency in MICU1 restricted to cells expressing LysM exhibit lower bacterial burdens in the heart with increased survival during systemic S. aureus infection. Coinciding with the decrease in S. aureus burdens, MICU1-/- neutrophils in the heart produce higher levels of mitochondrial superoxide and undergo enhanced suicidal NETosis. These results demonstrate that ion flux by the MCU affects the antibacterial function of neutrophils during S. aureus infection.

Clostridioides difficile strain-dependent and strain-independent adaptations to a microaerobic environment.

Clostridioides difficile (formerly Clostridium difficile) colonizes the gastrointestinal tract following disruption of the microbiota and can initiate a spectrum of clinical manifestations ranging from asymptomatic to life-threatening colitis. Following antibiotic treatment, luminal oxygen concentrations increase, exposing gut microbes to potentially toxic reactive oxygen species. Though typically regarded as a strict anaerobe, C. difficile can grow at low oxygen concentrations. How this bacterium adapts to a microaerobic environment and whether those responses to oxygen are conserved amongst strains is not entirely understood. Here, two C. difficile strains (630 and CD196) were cultured in 1.5% oxygen and the transcriptional response to long-term oxygen exposure was evaluated via RNA-sequencing. During growth in a microaerobic environment, several genes predicted to protect against oxidative stress were upregulated, including those for rubrerythrins and rubredoxins. Transcription of genes involved in metal homeostasis was also positively correlated with increased oxygen levels and these genes were amongst the most differentially transcribed. To directly compare the transcriptional landscape between C. difficile strains, a 'consensus-genome' was generated. On the basis of the identified conserved genes, basal transcriptional differences as well as variations in the response to oxygen were evaluated. While several responses were similar between the strains, there were significant differences in the abundance of transcripts involved in amino acid and carbohydrate metabolism. Furthermore, intracellular metal concentrations significantly varied both in an oxygen-dependent and oxygen-independent manner. Overall, these results indicate that C. difficile adapts to grow in a low oxygen environment through transcriptional changes, though the specific strategy employed varies between strains.

Clostridioides difficile infection induces a rapid influx of bile acids into the gut during colonization of the host.

Clostridioides difficile is the leading cause of nosocomial intestinal infections in the United States. Ingested C. difficile spores encounter host bile acids and other cues that are necessary for germinating into toxin-producing vegetative cells. While gut microbiota disruption (often by antibiotics) is a prerequisite for C. difficile infection (CDI), the mechanisms C. difficile employs for colonization remain unclear. Here, we pioneered the application of imaging mass spectrometry to study how enteric infection changes gut metabolites. We find that CDI induces an influx of bile acids into the gut within 24 h of the host ingesting spores. In response, the host reduces bile acid biosynthesis gene expression. These bile acids drive C. difficile outgrowth, as mice receiving the bile acid sequestrant cholestyramine display delayed colonization and reduced germination. Our findings indicate that C. difficile may facilitate germination upon infection and suggest that altering flux through bile acid pathways can modulate C. difficile outgrowth in CDI-prone patients.

Staphylococcus aureus Peptide Methionine Sulfoxide Reductases Protect from Human Whole-Blood Killing.

The generation of oxidative stress is a host strategy used to control Staphylococcus aureus infections. Sulfur-containing amino acids, cysteine and methionine, are particularly susceptible to oxidation because of the inherent reactivity of sulfur. Due to the constant threat of protein oxidation, many systems evolved to protect S. aureus from protein oxidation or to repair protein oxidation after it occurs. The S. aureus peptide methionine sulfoxide reductase (Msr) system reduces methionine sulfoxide to methionine. Staphylococci have four Msr enzymes, which all perform this reaction. Deleting all four msr genes in USA300 LAC (Δmsr) sensitizes S. aureus to hypochlorous acid (HOCl) killing; however, the Δmsr strain does not exhibit increased sensitivity to H2O2 stress or superoxide anion stress generated by paraquat or pyocyanin. Consistent with increased susceptibility to HOCl killing, the Δmsr strain is slower to recover following coculture with both murine and human neutrophils than USA300 wild type. The Δmsr strain is attenuated for dissemination to the spleen following murine intraperitoneal infection and exhibits reduced bacterial burdens in a murine skin infection model. Notably, no differences in bacterial burdens were observed in any organ following murine intravenous infection. Consistent with these observations, USA300 wild-type and Δmsr strains have similar survival phenotypes when incubated with murine whole blood. However, the Δmsr strain is killed more efficiently by human whole blood. These findings indicate that species-specific immune cell composition of the blood may influence the importance of Msr enzymes during S. aureus infection of the human host.

Lipocalin Blc is a potential heme-binding protein.

Lipocalins are a superfamily of functionally diverse proteins defined by a well-conserved tertiary structure despite variation in sequence. Lipocalins bind and transport small hydrophobic molecules in organisms of all kingdoms. However, there is still uncertainty regarding the function of some members of the family, including bacterial lipocalin Blc from Escherichia coli. Here, we present evidence that lipocalin Blc may be involved in heme binding, trans-periplasmic transport, or heme storage. This conclusion is supported by a cocrystal structure, mass-spectrometric data, absorption titration, and in silico analysis. Binding of heme is observed at low micromolar range with one-to-one ligand-to-protein stoichiometry. However, the absence of classical coordination to the iron atom leaves the possibility that the primary ligand of Blc is another tetrapyrrole.

Staphylococcus aureus lacking a functional MntABC manganese import system has increased resistance to copper.

S. aureus USA300 isolates utilize the copBL and copAZ gene products to prevent Cu intoxication. We created and examined a ΔcopAZ ΔcopBL mutant strain (cop-). The cop- strain was sensitive to Cu and accumulated intracellular Cu. We screened a transposon (Tn) mutant library in the cop- background and isolated strains with Tn insertions in the mntABC operon that permitted growth in the presence of Cu. The mutations were in mntA and they were recessive. Under the growth conditions utilized, MntABC functioned in manganese (Mn) import. When cultured with Cu, strains containing a mntA::Tn accumulated less Cu than the parent strain. Mn(II) supplementation improved growth when cop- was cultured with Cu and this phenotype was dependent upon the presence of MntR, which is a repressor of mntABC transcription. A ΔmntR strain had an increased Cu load and decreased growth in the presence of Cu, which was abrogated by the introduction of mntA::Tn. Over-expression of mntABC increased cellular Cu load and sensitivity to Cu. The presence of a mntA::Tn mutation protected iron-sulfur (FeS) enzymes from inactivation by Cu. The data presented are consistent with a model wherein defective MntABC results in decreased cellular Cu accumulation and protection to FeS enzymes from Cu poisoning.

A Small-Molecule Modulator of Metal Homeostasis in Gram-Positive Pathogens.

Metals are essential nutrients that all living organisms acquire from their environment. While metals are necessary for life, excess metal uptake can be toxic; therefore, intracellular metal levels are tightly regulated in bacterial cells. Staphylococcus aureus, a Gram-positive bacterium, relies on metal uptake and metabolism to colonize vertebrates. Thus, we hypothesized that an expanded understanding of metal homeostasis in S. aureus will lead to the discovery of pathways that can be targeted with future antimicrobials. We sought to identify small molecules that inhibit S. aureus growth in a metal-dependent manner as a strategy to uncover pathways that maintain metal homeostasis. Here, we demonstrate that VU0026921 kills S. aureus through disruption of metal homeostasis. VU0026921 activity was characterized through cell culture assays, transcriptional sequencing, compound structure-activity relationship, reactive oxygen species (ROS) generation assays, metal binding assays, and metal level analyses. VU0026921 disrupts metal homeostasis in S. aureus, increasing intracellular accumulation of metals and leading to toxicity through mismetalation of enzymes, generation of reactive oxygen species, or disruption of other cellular processes. Antioxidants partially protect S. aureus from VU0026921 killing, emphasizing the role of reactive oxygen species in the mechanism of killing, but VU0026921 also kills S. aureus anaerobically, indicating that the observed toxicity is not solely oxygen dependent. VU0026921 disrupts metal homeostasis in multiple Gram-positive bacteria, leading to increased reactive oxygen species and cell death, demonstrating the broad applicability of these findings. Further, this study validates VU0026921 as a probe to further decipher mechanisms required to maintain metal homeostasis in Gram-positive bacteria.IMPORTANCEStaphylococcus aureus is a leading agent of antibiotic-resistant bacterial infections in the world. S. aureus tightly controls metal homeostasis during infection, and disruption of metal uptake systems impairs staphylococcal virulence. We identified small molecules that interfere with metal handling in S. aureus to develop chemical probes to investigate metallobiology in this organism. Compound VU0026921 was identified as a small molecule that kills S. aureus both aerobically and anaerobically. The activity of VU0026921 is modulated by metal supplementation, is enhanced by genetic inactivation of Mn homeostasis genes, and correlates with increased cellular reactive oxygen species. Treatment with VU0026921 causes accumulation of multiple metals within S. aureus cells and concomitant upregulation of genes involved in metal detoxification. This work defines a small-molecule probe for further defining the role of metal toxicity in S. aureus and validates future antibiotic development targeting metal toxicity pathways.

Clostridioides difficile Senses and Hijacks Host Heme for Incorporation into an Oxidative Stress Defense System.

Clostridioides difficile infection of the colon leads to severe inflammation and damage to the gastrointestinal epithelium due to the production of potent toxins. This inflammatory tissue damage causes the liberation of high concentrations of host heme at infection sites. Here, we identify the C. difficile heme-sensing membrane protein system (HsmRA) and show that this operon induces a protective response that repurposes heme to counteract antimicrobial oxidative stress responses. HsmR senses vertebrate heme, leading to increased expression of the hsmRA operon and subsequent deployment of HsmA to capture heme and reduce redox damage caused by inflammatory mediators of protection and antibiotic therapy. Strains with inactivated hsmR or hsmA have increased sensitivity to redox-active compounds and reduced colonization persistence in a murine model of relapse C. difficile infection. These results define a mechanism exploited by C. difficile to repurpose toxic heme within the inflamed gut as a shield against antimicrobial compounds.

Clostridioides difficile proline fermentation in response to commensal clostridia.

Clostridioides difficile colonizes the intestines of susceptible individuals and releases toxins that mediate disease. To replicate and expand in the intestines, C. difficile ferments proline, and this activity is influenced by the availability of proline and trace nutrients. C. difficile must also compete with the commensal microbiota for these limited nutrients. The specific microbes present in the intestines that may shape the ability of C. difficile to benefit from proline fermentation are unknown. In this study we developed a panel of commensal Clostridia to test the hypothesis that the microbiota influences C. difficile growth through proline fermentation. The experimental panel of Clostridia was composed of murine and human isolates that ranged in their capacity to ferment proline in different media. Competition between wild type C. difficile and a mutant strain unable to ferment proline (prdB:CT) in the presence of these Clostridia revealed that bacteria closely related to Paraclostridium benzoelyticum and Paeniclostridium spp. decreased the benefit to C. difficile provided by proline fermentation. Conversely, Clostridium xylanolyticum drove C. difficile towards an increased reliance on proline fermentation for growth. Overall, the ability of C. difficile to benefit from proline fermentation is contextual and in part dependent on the microbiota.

ZupT Facilitates Clostridioides difficile Resistance to Host-Mediated Nutritional Immunity.

Clostridioides difficile is a spore-forming bacterium that causes severe colitis and is a major public health threat. During infection, C. difficile toxin production results in damage to the epithelium and a hyperinflammatory response. The immune response to CDI leads to robust neutrophil infiltration at the sight of infection and the deployment of numerous antimicrobials. One of the most abundant host immune factors associated with CDI is calprotectin, a metal-chelating protein with potent antimicrobial activity. Calprotectin is essential to the innate immune response to C. difficile and increasing levels of calprotectin correlate with disease severity in both adults and children with CDI. The fact that C. difficile persists in the presence of high levels of calprotectin suggests that this organism may deploy strategies to compete with this potent antimicrobial factor for essential nutrient metals during infection. In this report, we demonstrate that a putative zinc (Zn) transporter, ZupT, is employed by C. difficile to survive calprotectin-mediated metal limitation. ZupT is highly expressed in the presence of calprotectin and is required to protect C. difficile against calprotectin-dependent growth inhibition. When competing against wild-type C. difficile, zupT mutants show a defect in colonization and persistence in a murine model of infection. Together these data demonstrate that C. difficile utilizes a metal import system to combat nutritional immunity during CDI and suggest that strategies targeting nutrient acquisition in C. difficile may have therapeutic potential.IMPORTANCE During infection, pathogenic organisms must acquire essential transition metals from the host environment. Through the process of nutritional immunity, the host employs numerous strategies to restrict these key nutrients from invading pathogens. In this study, we describe a mechanism by which the important human pathogen Clostridioides difficile resists transition-metal limitation by the host. We report that C. difficile utilizes a zinc transporter, ZupT, to compete with the host protein calprotectin for nutrient zinc. Inactivation of this transporter in C. difficile renders this important pathogen sensitive to host-mediated metal restriction and confers a fitness disadvantage during infection. Our study demonstrates that targeting nutrient metal transport proteins in C. difficile is a potential avenue for therapeutic development.

The Immune Protein Calprotectin Impacts Clostridioides difficile Metabolism through Zinc Limitation.

The intestines house a diverse microbiota that must compete for nutrients to survive, but the specific limiting nutrients that control pathogen colonization are not clearly defined. Clostridioides difficile colonization typically requires prior disruption of the microbiota, suggesting that outcompeting commensals for resources is critical to establishing C. difficile infection (CDI). The immune protein calprotectin (CP) is released into the gut lumen during CDI to chelate zinc (Zn) and other essential nutrient metals. Yet, the impact of Zn limitation on C. difficile colonization is unknown. To define C. difficile responses to Zn limitation, we performed RNA sequencing on C. difficile exposed to CP. In medium containing CP, C. difficile upregulated genes involved in metal homeostasis and amino acid metabolism. To identify CP-responsive genes important during infection, we measured the abundance of select C. difficile transcripts in a mouse CDI model relative to expression in vitro Gene transcripts involved in selenium (Se)-dependent proline fermentation increased during infection and in response to CP. Increased proline fermentation gene transcription was dependent on CP Zn binding and proline availability, yet proline fermentation was only enhanced when Se was supplemented. CP-deficient mice could not restrain C. difficile proline fermentation-dependent growth, suggesting that CP-mediated Zn sequestration along with limited Se restricts C. difficile proline fermentation. Overall, these results highlight how C. difficile colonization depends on the availability of multiple nutrients whose abundances are dynamically influenced by the host response.IMPORTANCEClostridioides difficile infection (CDI) is the leading cause of postantibiotic nosocomial infection. Antibiotic therapy can be successful, yet up to one-third of individuals suffer from recurrent infections. Understanding the mechanisms controlling C. difficile colonization is paramount in designing novel treatments for primary and recurrent CDI. Here, we found that limiting nutrients control C. difficile metabolism during CDI and influence overall pathogen fitness. Specifically, the immune protein CP limits Zn availability and increases transcription of C. difficile genes necessary for proline fermentation. Paradoxically, this leads to reduced C. difficile proline fermentation. This reduced fermentation is due to limited availability of another nutrient required for proline fermentation, Se. Therefore, CP-mediated Zn limitation combined with low Se levels overall reduce C. difficile fitness in the intestines. These results emphasize the complexities of how nutrient availability influences C. difficile colonization and provide insight into critical metabolic processes that drive the pathogen's growth.

Arachidonic Acid Kills Staphylococcus aureus through a Lipid Peroxidation Mechanism.

Staphylococcus aureus infects every niche of the human host. In response to microbial infection, vertebrates have an arsenal of antimicrobial compounds that inhibit bacterial growth or kill bacterial cells. One class of antimicrobial compounds consists of polyunsaturated fatty acids, which are highly abundant in eukaryotes and encountered by S. aureus at the host-pathogen interface. Arachidonic acid (AA) is one of the most abundant polyunsaturated fatty acids in vertebrates and is released in large amounts during the oxidative burst. Most of the released AA is converted to bioactive signaling molecules, but, independently of its role in inflammatory signaling, AA is toxic to S. aureus Here, we report that AA kills S. aureus through a lipid peroxidation mechanism whereby AA is oxidized to reactive electrophiles that modify S. aureus macromolecules, eliciting toxicity. This process is rescued by cotreatment with antioxidants as well as in a S. aureus strain genetically inactivated for lcpA (USA300 ΔlcpA mutant) that produces lower levels of reactive oxygen species. However, resistance to AA stress in the USA300 ΔlcpA mutant comes at a cost, making the mutant more susceptible to ß-lactam antibiotics and attenuated for pathogenesis in a murine infection model compared to the parental methicillin-resistant S. aureus (MRSA) strain, indicating that resistance to AA toxicity increases susceptibility to other stressors encountered during infection. This report defines the mechanism by which AA is toxic to S. aureus and identifies lipid peroxidation as a pathway that can be modulated for the development of future therapeutics to treat S. aureus infections.IMPORTANCE Despite the ability of the human immune system to generate a plethora of molecules to control Staphylococcus aureus infections, S. aureus is among the pathogens with the greatest impact on human health. One class of host molecules toxic to S. aureus consists of polyunsaturated fatty acids. Here, we investigated the antibacterial properties of arachidonic acid, one of the most abundant polyunsaturated fatty acids in humans, and discovered that the mechanism of toxicity against S. aureus proceeds through lipid peroxidation. A better understanding of the molecular mechanisms by which the immune system kills S. aureus, and by which S. aureus avoids host killing, will enable the optimal design of therapeutics that complement the ability of the vertebrate immune response to eliminate S. aureus infections.