Menaquinone biosynthesis potentiates haem toxicity in Staphylococcus aureus

A Genetic Screen Reveals that Synthesis of 1,4-Dihydroxy-2-Naphthoate (DHNA), but Not Full-Length Menaquinone, Is Required for Listeria monocytogenes Cytosolic Survival

Through unknown mechanisms, the host cytosol restricts bacterial colonization; therefore, only professional cytosolic pathogens are adapted to colonize this host environment. Listeria monocytogenes is a Gram-positive intracellular pathogen that is highly adapted to colonize the cytosol of both phagocytic and nonphagocytic cells. To identify L. monocytogenes determinants of cytosolic survival, we designed and executed a novel screen to isolate L. monocytogenes mutants with cytosolic survival defects. Multiple mutants identified in the screen were defective for synthesis of menaquinone (MK), an essential molecule in the electron transport chain. Analysis of an extensive set of MK biosynthesis and respiratory chain mutants revealed that cellular respiration was not required for cytosolic survival of L. monocytogenes but that, instead, synthesis of 1,4-dihydroxy-2-naphthoate (DHNA), an MK biosynthesis intermediate, was essential. Recent discoveries showed that modulation of the central metabolism of both host and pathogen can influence the outcome of host-pathogen interactions. Our results identify a potentially novel function of the MK biosynthetic intermediate DHNA and specifically highlight how L. monocytogenes metabolic adaptations promote cytosolic survival and evasion of host immunity.

Cytosolic bacterial pathogens, such as Listeria monocytogenes and Francisella tularensis, are exquisitely evolved to colonize the host cytosol in a variety of cell types. Establishing an intracellular niche shields these pathogens from effectors of humoral immunity, grants access to host nutrients, and is essential for pathogenesis. Through yet-to-be-defined mechanisms, the host cytosol restricts replication of non-cytosol-adapted bacteria, likely through a combination of cell autonomous defenses (CADs) and nutritional immunity. Utilizing a novel genetic screen, we identified determinants of L. monocytogenes cytosolic survival and virulence and identified a role for the synthesis of the menaquinone precursor 1,4-dihydroxy-2-naphthoate (DHNA) in cytosolic survival. Together, these data begin to elucidate adaptations that allow cytosolic pathogens to survive in their intracellular niches.

Impaired respiration elicits SrrAB-dependent programmed cell lysis and biofilm formation in Staphylococcus aureus

Biofilms are communities of microorganisms attached to a surface or each other. Biofilm-associated cells are the etiologic agents of recurrent Staphylococcus aureus infections. Infected human tissues are hypoxic or anoxic. S. aureus increases biofilm formation in response to hypoxia, but how this occurs is unknown. In the current study we report that oxygen influences biofilm formation in its capacity as a terminal electron acceptor for cellular respiration. Genetic, physiological, or chemical inhibition of respiratory processes elicited increased biofilm formation. Impaired respiration led to increased cell lysis via divergent regulation of two processes: increased expression of the AtlA murein hydrolase and decreased expression of wall-teichoic acids. The AltA-dependent release of cytosolic DNA contributed to increased biofilm formation. Further, cell lysis and biofilm formation were governed by the SrrAB two-component regulatory system. Data presented support a model wherein SrrAB-dependent biofilm formation occurs in response to the accumulation of reduced menaquinone.

DOI:http://dx.doi.org/10.7554/eLife.23845.001

Millions of bacteria live on the human body. Generally these bacteria co-exist with us peacefully, but sometimes certain bacteria may enter the body and cause infections, such as gum disease or a bone infection called osteomyelitis. Many of these infections are thought to occur when the bacteria become able to form complex communities called biofilms. Bacteria living in a biofilm cooperate and make lifestyle choices as a community, so in this way, they behave like a single organism containing many cells.

A sticky glue-like material called the matrix holds the bacteria in a biofilm together. This matrix protects the bacteria in the biofilm from both the human immune system and antibiotics, allowing infections to develop and making them difficult to treat.

Previous research has shown that the supply and level of oxygen in infected tissues decreases as an infection gets worse. One bacterium that typically lives peacefully on our bodies, called Staphylococcus aureus, can sometimes cause serious biofilm-associated infections. S. aureus forms biofilms more readily when oxygen is in short supply, but it was not known how these biofilms form. Understanding how S. aureus forms biofilms could help scientists develop better treatments for bacterial infections.

Most bacterial cells have a cell wall to provide them with structural support. Mashruwala et al. found that, when oxygen levels are low, S. aureus decreases the production of a type of sugar that makes up the cell wall. At the same time, the bacteria produce more of an enzyme that breaks down cell walls. Together, these processes cause some of the bacteria cells to break open. The contents of these broken cells, including their DNA, help form the matrix that will hold together and protect the other bacterial cells in the biofilm. The experiments also identified a protein called SrrAB that switches on the process that ruptures the cells when oxygen is low.

The findings of Mashruwala et al. show how bacteria grown in the laboratory form biofilms when they are starved of oxygen. The next steps following on from this work are to find out whether the same thing happens when bacteria infect animals and whether drugs that block the rupturing of bacterial cells could be used to treat infections.

DOI:http://dx.doi.org/10.7554/eLife.23845.002

Intracellular Zn(II) Intoxication Leads to Dysregulation of the PerR Regulon Resulting in Heme Toxicity in Bacillus subtilis

Transition metal ions (Zn(II), Cu(II)/(I), Fe(III)/(II), Mn(II)) are essential for life and participate in a wide range of biological functions. Cellular Zn(II) levels must be high enough to ensure that it can perform its essential roles. Yet, since Zn(II) binds to ligands with high avidity, excess Zn(II) can lead to protein mismetallation. The major targets of mismetallation, and the underlying causes of Zn(II) intoxication, are not well understood. Here, we use a forward genetic selection to identify targets of Zn(II) toxicity. In wild-type cells, in which Zn(II) efflux prevents intoxication of the cytoplasm, extracellular Zn(II) inhibits the electron transport chain due to the inactivation of the major aerobic cytochrome oxidase. This toxicity can be ameliorated by depression of an alternate oxidase or by mutations that restrict access of Zn(II) to the cell surface. Conversely, efflux deficient cells are sensitive to low levels of Zn(II) that do not inhibit the respiratory chain. Under these conditions, intracellular Zn(II) accumulates and leads to heme toxicity. Heme accumulation results from dysregulation of the regulon controlled by PerR, a metal-dependent repressor of peroxide stress genes. When metallated with Fe(II) or Mn(II), PerR represses both heme biosynthesis (hemAXCDBL operon) and the abundant heme protein catalase (katA). Metallation of PerR with Zn(II) disrupts this coordination, resulting in depression of heme biosynthesis but continued repression of catalase. Our results support a model in which excess heme partitions to the membrane and undergoes redox cycling catalyzed by reduced menaquinone thereby resulting in oxidative stress.

Novel Pathways for Ameliorating the Fitness Cost of Gentamicin Resistant Small Colony Variants

Small colony variants (SCVs) of the human pathogen Staphylococcus aureus are associated with persistent infections. Phenotypically, SCVs are characterized by slow growth and they can arise upon interruption of the electron transport chain that consequently reduce membrane potential and thereby limit uptake of aminoglycosides (e.g., gentamicin). In this study, we have examined the pathways by which the fitness cost of SCVs can be ameliorated. Five gentamicin resistant SCVs derived from S. aureus JE2 were independently selected on agar plates supplemented with gentamicin. The SCVs carried mutations in the menaquinone and hemin biosynthesis pathways, which caused a significant reduction in exponential growth rates relative to wild type (WT; 0.59-0.72) and reduced membrane potentials. Fifty independent lineages of the low-fitness, resistant mutants were serially passaged for up to 500 generations with or without sub-lethal concentrations of gentamicin. Amelioration of the fitness cost followed three evolutionary trajectories and was dependent on the initial mutation type (point mutation vs. deletion) and the passage condition (absence or presence of gentamicin). For SCVs evolved in the absence of gentamicin, 12 out of 15 lineages derived from SCVs with point mutations acquired intra-codonic suppressor mutations restoring membrane potential, growth rate, gentamicin susceptibility and colony size to WT levels. For the SCVs carrying deletions, all lineages enhanced fitness independent of membrane potential restoration without alterations in gentamicin resistance levels. By whole genome sequencing, we identified compensatory mutations in genes related to the σ^B stress response (7 out of 10 lineages). Inactivation of rpoF that encode for the alternative sigma factor SigB (σ^B) partially restored fitness of SCVs. For all lineages passaged in the presence of gentamicin, fitness compensation via membrane potential restoration was suppressed, however, selected for secondary mutations in fusA and SAUSA300_0749. This study is the first to describe fitness compensatory events in SCVs with deletion mutations and adaptation of SCVs to continued exposure to gentamicin.

Heme Synthesis and Acquisition in Bacterial Pathogens

Bacterial pathogens require the iron-containing cofactor heme to cause disease. Heme is essential to the function of hemoproteins, which are involved in energy generation by the electron transport chain, detoxification of host immune effectors, and other processes. During infection, bacterial pathogens must synthesize heme or acquire heme from the host; however, host heme is sequestered in high-affinity hemoproteins. Pathogens have evolved elaborate strategies to acquire heme from host sources, particularly hemoglobin, and both heme acquisition and synthesis are important for pathogenesis. Paradoxically, excess heme is toxic to bacteria and pathogens must rely on heme detoxification strategies. Heme is a key nutrient in the struggle for survival between host and pathogen, and its study has offered significant insight into the molecular mechanisms of bacterial pathogenesis.

Bacterial Nitric Oxide Synthase Is Required for the Staphylococcus aureus Response to Heme Stress

Staphylococcus aureus is a pathogen that causes significant morbidity and mortality worldwide. Within the vertebrate host, S. aureus requires heme as a nutrient iron source and as a cofactor for multiple cellular processes. Although required for pathogenesis, excess heme is toxic. S. aureus employs a two-component system, the heme sensor system (HssRS), to sense and protect against heme toxicity. Upon activation, HssRS induces the expression of the heme-regulated transporter (HrtAB), an efflux pump that alleviates heme toxicity. The ability to sense and respond to heme is critical for the pathogenesis of numerous Gram-positive organisms, yet the mechanism of heme sensing remains unknown. Compound '3981 was identified in a high-throughput screen as an activator of staphylococcal HssRS that triggers HssRS independently of heme accumulation. '3981 is toxic to S. aureus; however, derivatives of '3981 were synthesized that lack toxicity while retaining HssRS activation, enabling the interrogation of the heme stress response without confounding toxic effects of the parent molecule. Using '3981 derivatives as probes of the heme stress response, numerous genes required for '3981-induced activation of HssRS were uncovered. Specifically, multiple genes involved in the production of nitric oxide were identified, including the gene encoding bacterial nitric oxide synthase (bNOS). bNOS protects S. aureus from oxidative stress imposed by heme. Taken together, this work identifies bNOS as crucial for the S. aureus heme stress response, providing evidence that nitric oxide synthesis and heme sensing are intertwined.

A partial metabolic pathway enables group b streptococcus to overcome quinone deficiency in a host bacterial community

Aerobic respiration metabolism in Group B Streptococcus (GBS) is activated by exogenous heme and menaquinone. This capacity enhances resistance of GBS to acid and oxidative stress and improves its survival. In this work, we discovered that GBS is able to respire in the presence of heme and 1,4-dihydroxy-2-naphthoic acid (DHNA). DHNA is a biosynthetic precursor of demethylmenaquinone (DMK) in many bacterial species. A GBS gene (gbs1789) encodes a homolog of the MenA 1,4-dihydroxy-2-naphthoate prenyltransferase enzyme, involved in the synthesis of demethylmenaquinone. In this study, we showed that gbs1789 is involved in the biosynthesis of long-chain demethylmenaquinones (DMK-10). The Δgbs1789 mutant cannot respire in the presence of heme and DHNA, indicating that endogenously synthesized DMKs are cofactors of the GBS respiratory chain. We also found that isoprenoid side chains from GBS DMKs are produced by the protein encoded by the gbs1783 gene, since this gene can complement an Escherichia coli ispB mutant defective for isoprenoids chain synthesis. In the gut or vaginal microbiote, where interspecies metabolite exchanges occur, this partial DMK biosynthetic pathway can be important for GBS respiration and survival in different niches.

Neutrophil-generated oxidative stress and protein damage in Staphylococcus aureus

Staphylococcus aureus is a ubiquitous, versatile and dangerous pathogen. It colonizes over 30% of the human population, and is one of the leading causes of death by an infectious agent. During S. aureus colonization and invasion, leukocytes are recruited to the site of infection. To combat S. aureus, leukocytes generate an arsenal of reactive species including superoxide, hydrogen peroxide, nitric oxide and hypohalous acids that modify and inactivate cellular macromolecules, resulting in growth defects or death. When S. aureus colonization cannot be cleared by the immune system, antibiotic treatment is necessary and can be effective. Yet, this organism quickly gains resistance to each new antibiotic it encounters. Therefore, it is in the interest of human health to acquire a deeper understanding of how S. aureus evades killing by the immune system. Advances in this field will have implications for the design of future S. aureus treatments that complement and assist the host immune response. In that regard, this review focuses on how S. aureus avoids host-generated oxidative stress, and discusses the mechanisms used by S. aureus to survive oxidative damage including antioxidants, direct repair of damaged proteins, sensing oxidant stress and transcriptional changes. This review will elucidate areas for studies to identify and validate future antimicrobial targets.

A Methyl 4-Oxo-4-phenylbut-2-enoate with in Vivo Activity against MRSA that Inhibits MenB in the Bacterial Menaquinone Biosynthesis Pathway

4-Oxo-4-phenyl-but-2-enoates inhibit MenB, the 1,4-dihydroxyl-2-naphthoyl-CoA synthase in the bacterial menaquinone (MK) biosynthesis pathway, through the formation of an adduct with coenzyme A (CoA). Here, we show that the corresponding methyl butenoates have MIC values as low as 0.35-0.75 µg/mL against drug sensitive and resistant strains of Staphylococcus aureus. Mode of action studies on the most potent compound, methyl 4-(4-chlorophenyl)-4-oxobut-2-enoate (1), reveal that 1 is converted into the corresponding CoA adduct in S. aureus cells, and that this adduct binds to the S. aureus MenB (saMenB) with a K_d value of 2 µM. The antibacterial spectrum of 1 is limited to bacteria that utilize MK for respiration, and the activity of 1 can be complemented with exogenous MK or menadione. Finally, treatment of methicillin-resistant S. aureus (MRSA) with 1 results in the small colony variant phenotype and thus 1 phenocopies knockout of the menB gene. Taken together the data indicate that the antibacterial activity of 1 results from a specific effect on MK biosynthesis. We also evaluated the in vivo efficacy of 1 using two mouse models of MRSA infection. Notably, compound 1 increased survival in a systemic infection model and resulted in a dose-dependent decrease in bacterial load in a thigh infection model, validating MenB as a target for the development of new anti-MRSA candidates.

Heme sensing in Bacillus thuringiensis: a supplementary HssRS-regulated heme resistance system

Several Gram-positive pathogens scavenge host-derived heme to satisfy their nutritional iron requirement. However, heme is a toxic molecule capable of damaging the bacterial cell. Gram-positive pathogens within the phylum Firmicutes overcome heme toxicity by sensing heme through HssRS, a two-component system that regulates the heme detoxification transporter HrtAB. Here we show that heme sensing by HssRS and heme detoxification by HrtAB occur in the insect pathogen Bacillus thuringiensis We find that in B. thuringiensis, HssRS directly regulates an operon, hrmXY, encoding hypothetical membrane proteins that are not found in other Firmicutes with characterized HssRS and HrtAB systems. This novel HssRS-regulated operon or its orthologs BMB171_c3178 and BMB171_c3330 are required for maximal heme resistance. Furthermore, the activity of HrmXY is not dependent on expression of HrtAB. These results suggest that B. thuringiensis senses heme through HssRS and induces expression of separate membrane-localized systems capable of overcoming different aspects of heme toxicity.

Adaptation of Staphylococcus xylosus to Nutrients and Osmotic Stress in a Salted Meat Model

Staphylococcus xylosus is commonly used as starter culture for meat fermentation. Its technological properties are mainly characterized in vitro, but the molecular mechanisms for its adaptation to meat remain unknown. A global transcriptomic approach was used to determine these mechanisms. S. xylosus modulated the expression of about 40-50% of the total genes during its growth and survival in the meat model. The expression of many genes involved in DNA machinery and cell division, but also in cell lysis, was up-regulated. Considering that the S. xylosus population remained almost stable between 24 and 72 h of incubation, our results suggest a balance between cell division and cell lysis in the meat model. The expression of many genes encoding enzymes involved in glucose and lactate catabolism was up-regulated and revealed that glucose and lactate were used simultaneously. S. xylosus seemed to adapt to anaerobic conditions as revealed by the overexpression of two regulatory systems and several genes encoding cofactors required for respiration. In parallel, genes encoding transport of peptides and peptidases that could furnish amino acids were up-regulated and thus concomitantly a lot of genes involved in amino acid synthesis were down-regulated. Several genes involved in glutamate homeostasis were up-regulated. Finally, S. xylosus responded to the osmotic stress generated by salt added to the meat model by overexpressing genes involved in transport and synthesis of osmoprotectants, and Na(+) and H(+) extrusion.

Mechanism of MenE inhibition by acyl-adenylate analogues and discovery of novel antibacterial agents

MenE is an o-succinylbenzoyl-CoA (OSB-CoA) synthetase in the bacterial menaquinone biosynthesis pathway and is a promising target for the development of novel antibacterial agents. The enzyme catalyzes CoA ligation via an acyl-adenylate intermediate, and we have previously reported tight-binding inhibitors of MenE based on stable acyl-sulfonyladenosine analogues of this intermediate, including OSB-AMS (1), which has an IC50 value of ≤25 nM for Escherichia coli MenE. Herein, we show that OSB-AMS reduces menaquinone levels in Staphylococcus aureus, consistent with its proposed mechanism of action, despite the observation that the antibacterial activity of OSB-AMS is ∼1000-fold lower than the IC50 for enzyme inhibition. To inform the synthesis of MenE inhibitors with improved antibacterial activity, we have undertaken a structure-activity relationship (SAR) study stimulated by the knowledge that OSB-AMS can adopt two isomeric forms in which the OSB side chain exists either as an open-chain keto acid or a cyclic lactol. These studies revealed that negatively charged analogues of the keto acid form bind, while neutral analogues do not, consistent with the hypothesis that the negatively charged keto acid form of OSB-AMS is the active isomer. X-ray crystallography and site-directed mutagenesis confirm the importance of a conserved arginine for binding the OSB carboxylate. Although most lactol isomers tested were inactive, a novel difluoroindanediol inhibitor (11) with improved antibacterial activity was discovered, providing a pathway toward the development of optimized MenE inhibitors in the future.

Phenotypic and Genotypic Characteristics of Small Colony Variants and Their Role in Chronic Infection

Small colony variant (SCV) bacteria arise spontaneously within apparently homogeneous microbial populations, largely in response to environmental stresses, such as antimicrobial treatment. They display unique phenotypic characteristics conferred in part by heritable genetic changes. Characteristically slow growing, SCVs comprise a minor proportion of the population from which they arise but persist by virtue of their inherent resilience and host adaptability. Consequently, SCVs are problematic in chronic infection, where antimicrobial treatment is administered during the acute phase of infection but fails to eradicate SCVs, which remain within the host causing recurrent or chronic infection. This review discusses some of the phenotypic and genotypic changes that enable SCVs to successfully proliferate within the host environment as potential pathogens and strategies that could ameliorate the resolution of infection where SCVs are present.

Inter- and intraspecies metabolite exchange promotes virulence of antibiotic-resistant Staphylococcus aureus

Adaptations that enable antimicrobial resistance often pose a fitness cost to the microorganism. Resistant pathogens must therefore overcome such fitness decreases to persist within their hosts. Here we demonstrate that the reduced fitness associated with one resistance-conferring mutation can be offset by community interactions with microorganisms harboring alternative mutations or via interactions with the human microbiota. Mutations that confer antibiotic resistance in the human pathogen Staphylococcus aureus led to decreased fitness, whereas coculture or coinfection of two distinct mutants resulted in collective recovery of fitness comparable to that of wild-type. Such fitness enhancements result from the exchange of metabolites between distinct mutants, leading to enhanced growth, virulence factor production, and pathogenicity. Interspecies fitness enhancements were also identified, as members of the human microbiota can promote growth of antibiotic-resistant S. aureus. Thus, inter- and intraspecies community interactions offset fitness costs and enable S. aureus to develop antibiotic resistance without loss of virulence.

Staphylococcus aureus adapts to oxidative stress by producing H2O2-resistant small-colony variants via the SOS response

The development of chronic and recurrent Staphylococcus aureus infections is associated with the emergence of slow-growing mutants known as small-colony variants (SCVs), which are highly tolerant of antibiotics and can survive inside host cells. However, the host and bacterial factors which underpin SCV emergence during infection are poorly understood. Here, we demonstrate that exposure of S. aureus to sublethal concentrations of H₂O₂ leads to a specific, dose-dependent increase in the population frequency of gentamicin-resistant SCVs. Time course analyses revealed that H₂O₂ exposure caused bacteriostasis in wild-type cells during which time SCVs appeared spontaneously within the S. aureus population. This occurred via a mutagenic DNA repair pathway that included DNA double-strand break repair proteins RexAB, recombinase A, and polymerase V. In addition to triggering SCV emergence by increasing the mutation rate, H₂O₂ also selected for the SCV phenotype, leading to increased phenotypic stability and further enhancing the size of the SCV subpopulation by reducing the rate of SCV reversion to the wild type. Subsequent analyses revealed that SCVs were significantly more resistant to the toxic effects of H₂O₂ than wild-type bacteria. With the exception of heme auxotrophs, gentamicin-resistant SCVs displayed greater catalase activity than wild-type bacteria, which contributed to their resistance to H₂O₂. Taken together, these data reveal a mechanism by which S. aureus adapts to oxidative stress via the production of a subpopulation of H₂O₂-resistant SCVs with enhanced catalase production.

The crimson conundrum: heme toxicity and tolerance in GAS

The massive erythrocyte lysis caused by the Group A Streptococcus (GAS) suggests that the β-hemolytic pathogen is likely to encounter free heme during the course of infection. In this study, we investigated GAS mechanisms for heme sensing and tolerance. We compared the minimal inhibitory concentration of heme among several isolates and established that excess heme is bacteriostatic and exposure to sub-lethal concentrations of heme resulted in noticeable damage to membrane lipids and proteins. Pre-exposure of the bacteria to 0.1 μM heme shortened the extended lag period that is otherwise observed when naive cells are inoculated into heme-containing medium, implying that GAS is able to adapt. The global response to heme exposure was determined using microarray analysis revealing a significant transcriptome shift that included 79 up regulated and 84 down regulated genes. Among other changes, the induction of stress-related chaperones and proteases, including groEL/ES (8x), the stress regulators spxA2 (5x) and ctsR (3x), as well as redox active enzymes were prominent. The heme stimulon also encompassed a number of regulatory proteins and two-component systems that are important for virulence. A three-gene cluster that is homologous to the pefRCD system of the Group B Streptococcus was also induced by heme. PefR, a MarR-like regulator, specifically binds heme with stoichiometry of 1:2 and protoporphyrin IX (PPIX) with stoichiometry of 1:1, implicating it is one of the GAS mediators to heme response. In summary, here we provide evidence that heme induces a broad stress response in GAS, and that its success as a pathogen relies on mechanisms for heme sensing, detoxification, and repair.

Evidence for function overlapping of CymA and the cytochrome bc1 complex in the Shewanella oneidensis nitrate and nitrite respiration

Shewanella oneidensis is an important model organism for its versatility of anaerobic respiration. CymA, a cytoplasmic membrane-bound tetraheme c-type cytochrome, plays a central role in anaerobic respiration by transferring electrons from the quinone pool to a variety of terminal reductases. Although loss of CymA results in defect in respiration of many electron acceptors (EAs), a significant share of the capacity remains in general. In this study, we adopted a transposon random mutagenesis method in a cymA null mutant to identify substituent(s) of CymA with respect to nitrite and nitrate respiration. A total of 87 insertion mutants, whose ability to reduce nitrite was further impaired, were obtained. Among the interrupted genes, the petABC operon appeared to be the most likely candidate given the involvement of the cytochrome bc1 complex that it encodes in electron transport. Subsequent analyses not only confirmed that the complex and CymA were indeed functionally overlapping in nitrate/nitrite respiration but also revealed that both proteins were able to draw electrons from ubiquinone and menaquinone. Furthermore, we found that expression of the bc1 complex was affected by oxygen but not nitrate or nitrite and by global regulators ArcA and Crp in an indirect manner.

Genome-wide high-throughput screening to investigate essential genes involved in methicillin-resistant Staphylococcus aureus Sequence Type 398 survival

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) Sequence Type 398 (ST398) is an opportunistic pathogen that is able to colonize and cause disease in several animal species including humans. To better understand the adaptation, evolution, transmission and pathogenic capacity, further investigations into the importance of the different genes harboured by LA-MRSA ST398 are required. In this study we generated a genome-wide transposon mutant library in an LA-MRSA ST398 isolate to evaluate genes important for bacterial survival in laboratory and host-specific environments. The transposon mutant library consisted of approximately 1 million mutants with around 140,000 unique insertion sites and an average number of unique inserts per gene of 44.8. We identified LA-MRSA ST398 essential genes comparable to other high-throughput S. aureus essential gene studies. As ST398 is the most common MRSA isolated from pigs, the transposon mutant library was screened in whole porcine blood. Twenty-four genes were specifically identified as important for bacterial survival in porcine blood. Mutations in 23 of these genes resulted in attenuated bacterial fitness. Seven of the 23 genes were of unknown function, whereas 16 genes were annotated with functions predominantly related to carbon metabolism, pH shock and a variety of regulations and only indirectly to virulence factors. Mutations in one gene of unknown function resulted in a hypercompetitive mutant. Further evaluation of these genes is required to determine their specific relevance in blood survival.

Differential activation of Staphylococcus aureus heme detoxification machinery by heme analogues

The reactive nature of heme enables its use as an enzymatic cofactor while rendering excess heme toxic. The importance of heme detoxification machinery is highlighted by the presence of various types of these homeostatic systems in Gram-positive and Gram-negative microorganisms. A number of pathogens possess orthologs of the HssRS/HrtAB heme detoxification system, underscoring a potential role this system plays in the survival of bacteria in heme-rich environments such as the vertebrate host. In this work, we sought to determine the role of this system in protection against metalloporphyrin heme analogues identified by previous studies as antimicrobial agents. Our findings demonstrate that only toxic metalloporphyrins maximally activate expression of the Staphylococcus aureus heme detoxification system, suggesting that the sensing mechanism of HssRS might require a component of the associated toxicity rather than or in addition to the metalloporphyrin itself. We further establish that only a subset of toxic metalloporphyrins elicit the oxidative damage previously shown to be a significant component of heme toxicity whereas all toxic noniron metalloporphyrins inhibit bacterial respiration. Finally, we demonstrate that, despite the fact that toxic metalloporphyrin treatment induces expression of S. aureus heme detoxification machinery, the HrtAB heme export pump is unable to detoxify most of these molecules. The ineffectiveness of HrtAB against toxic heme analogues provides an explanation for their increased antimicrobial activity relative to heme. Additionally, these studies define the specificity of HssRS/HrtAB, which may provide future insight into the biochemical mechanisms of these systems.

Iron in infection and immunity

Iron is an essential nutrient for both humans and pathogenic microbes. Because of its ability to exist in one of two oxidation states, iron is an ideal redox catalyst for diverse cellular processes including respiration and DNA replication. However, the redox potential of iron also contributes to its toxicity; thus, iron concentration and distribution must be carefully controlled. Given the absolute requirement for iron by virtually all human pathogens, an important facet of the innate immune system is to limit iron availability to invading microbes in a process termed nutritional immunity. Successful human pathogens must therefore possess mechanisms to circumvent nutritional immunity in order to cause disease. In this review, we discuss regulation of iron metabolism in the setting of infection and delineate strategies used by human pathogens to overcome iron-withholding defenses.

Menaquinone analogs inhibit growth of bacterial pathogens

Gram-positive bacteria cause serious human illnesses through combinations of cell surface and secreted virulence factors. We initiated studies with four of these organisms to develop novel topical antibacterial agents that interfere with growth and exotoxin production, focusing on menaquinone analogs. Menadione, 1,4-naphthoquinone, and coenzymes Q1 to Q3 but not menaquinone, phylloquinone, or coenzyme Q10 inhibited the growth and to a greater extent exotoxin production of Staphylococcus aureus, Bacillus anthracis, Streptococcus pyogenes, and Streptococcus agalactiae at concentrations of 10 to 200 μg/ml. Coenzyme Q1 reduced the ability of S. aureus to cause toxic shock syndrome in a rabbit model, inhibited the growth of four Gram-negative bacteria, and synergized with another antimicrobial agent, glycerol monolaurate, to inhibit S. aureus growth. The staphylococcal two-component system SrrA/B was shown to be an antibacterial target of coenzyme Q1. We hypothesize that menaquinone analogs both induce toxic reactive oxygen species and affect bacterial plasma membranes and biosynthetic machinery to interfere with two-component systems, respiration, and macromolecular synthesis. These compounds represent a novel class of potential topical therapeutic agents.

The chlorite dismutase (HemQ) from Staphylococcus aureus has a redox-sensitive heme and is associated with the small colony variant phenotype

The chlorite dismutases (C-family proteins) are a widespread family of heme-binding proteins for which chemical and biological roles remain unclear. An association of the gene with heme biosynthesis in Gram-positive bacteria was previously demonstrated by experiments involving introduction of genes from two Gram-positive species into heme biosynthesis mutant strains of Escherichia coli, leading to the gene being renamed hemQ. To assess the gene product's biological role more directly, a Staphylococcus aureus strain with an inactivated hemQ gene was generated and shown to be a slow growing small colony variant under aerobic but not anaerobic conditions. The small colony variant phenotype is rescued by the addition of exogenous heme despite an otherwise wild type heme biosynthetic pathway. The ΔhemQ mutant accumulates coproporphyrin specifically under aerobic conditions. Although its sequence is highly similar to functional chlorite dismutases, the HemQ protein has no steady state reactivity with chlorite, very modest reactivity with H2O2 or peracetic acid, and no observable transient intermediates. HemQ's equilibrium affinity for heme is in the low micromolar range. Holo-HemQ reconstituted with heme exhibits heme lysis after <50 turnovers with peroxide and <10 turnovers with chlorite. The heme-free apoprotein aggregates or unfolds over time. IsdG-like proteins and antibiotic biosynthesis monooxygenases are close sequence and structural relatives of HemQ that use heme or porphyrin-like organic molecules as substrates. The genetic and biochemical data suggest a similar substrate role for heme or porphyrin, with possible sensor-regulator functions for the protein. HemQ heme could serve as the means by which S. aureus reversibly adopts an SCV phenotype in response to redox stress.

Two heme-dependent terminal oxidases power Staphylococcus aureus organ-specific colonization of the vertebrate host

Staphylococcus aureus is a significant cause of infections worldwide and is able to utilize aerobic respiration, anaerobic respiration, or fermentation as the means by which it generates the energy needed for proliferation. Aerobic respiration is supported by heme-dependent terminal oxidases that catalyze the final step of aerobic respiration, the reduction of O2 to H2O. An inability to respire forces bacteria to generate energy via fermentation, resulting in reduced growth. Elucidating the roles of these energy-generating pathways during colonization of the host could uncover attractive therapeutic targets. Consistent with this idea, we report that inhibiting aerobic respiration by inactivating heme biosynthesis significantly impairs the ability of S. aureus to colonize the host. Two heme-dependent terminal oxidases support aerobic respiration of S. aureus, implying that the staphylococcal respiratory chain is branched. Systemic infection with S. aureus mutants limited to a single terminal oxidase results in an organ-specific colonization defect, resulting in reduced bacterial burdens in either the liver or the heart. Finally, inhibition of aerobic respiration can be achieved by exposing S. aureus to noniron heme analogues. These data provide evidence that aerobic respiration plays a major role in S. aureus colonization of the host and that this energy-generating process is a viable therapeutic target.

IMPORTANCE

Staphylococcus aureus poses a significant threat to public health as antibiotic-resistant isolates of this pathogen continue to emerge. Our understanding of the energy-generating processes that allow S. aureus to proliferate within the host is incomplete. Host-derived heme is the preferred source of nutrient iron during infection; however, S. aureus can synthesize heme de novo and use it to facilitate aerobic respiration. We demonstrate that S. aureus heme biosynthesis powers a branched aerobic respiratory chain composed of two terminal oxidases. The importance of having two terminal oxidases is demonstrated by the finding that each plays an essential role in colonizing distinct organs during systemic infection. Additionally, this process can be targeted by small-molecule heme analogues called noniron protoporphyrins. This study serves to demonstrate that heme biosynthesis supports two terminal oxidases that are required for aerobic respiration and are also essential for S. aureus pathogenesis.