Oxidative Reactions Catalyzed by Hydrogen Peroxide Produced by Streptococcus pneumoniae and Other Streptococci Cause the Release and Degradation of Heme from Hemoglobin

Fluorescent antibody-based detection and ultrastructural analysis of Streptococcus pneumoniae in human sputum

BACKGROUND

Pneumococcal pneumonia continues to be a significant global health burden, affecting both children and adults. Traditional diagnostic methods for sputum analysis remain challenging. The objective of this study was twofold: to develop a rapid and easy-to-perform assay for the identification of Streptococcus pneumoniae (Spn) directly in sputum specimens using fluorescence microscopy, and to characterize with high-resolution confocal microscopy the ultrastructure of pneumococci residing in human sputum.

METHODS

We fluorescently labeled antibodies against the pneumococcal capsule (Spn-FLUO). The specificity and sensitivity of Spn-FLUO for detecting Spn was evaluated in vitro and in vivo using mouse models of carriage and disease, human nasopharyngeal specimens, and sputum from patients with pneumococcal pneumonia. Spn was confirmed in the specimens using culture and a species-specific qPCR assay. Spn strains were serotyped by Quellung. Confocal microscopy and Imaris software analysis were utilized to resolve the ultrastructure of pneumococci in human sputum.

RESULTS

Compared with cultures and qPCR, Spn-FLUO demonstrated high sensitivity (78-96%) in nasopharyngeal samples from mice and humans. The limit of detection (LOD) in nasopharyngeal samples was ≥ 1.6 × 10⁴ GenEq/ml. The specificity in human nasopharyngeal specimens was 100%. In lung specimens from mice infected with pneumococci, Spn-FLUO reached 100% sensitivity with a LOD of ≥ 1.39 × 10⁴ GenEq/ml. In human sputum, the sensitivity for detecting Spn was 92.7% with a LOD of 3.6 × 10³ GenEq/ml. Ultrastructural studies revealed that pneumococci are expectorated as large aggregates with a median size of 1336 μm².

CONCLUSIONS

Spn-FLUO is a rapid and sensitive assay for detecting Spn in human sputum within 30 min, encompassing a range of both vaccine and non-vaccine serotypes associated with pneumococcal pneumonia. The study highlights that most pneumococci form aggregates in human sputum.

A Novel Heme-Degrading Enzyme that Regulates Heme and Iron Homeostasis and Promotes Virulence in Enterococcus faecalis

Enterococcus faecalis, a gut commensal, is a leading cause of opportunistic infections. Its virulence is linked to its ability to thrive in hostile environments, which includes host-imposed metal starvation. We recently showed that E. faecalis evades iron starvation using five dedicated transporters that collectively scavenge iron from host tissues and other iron-deprived conditions. Interestingly, heme, the most abundant source of iron in the human body, supported growth of a strain lacking all five iron transporters (Δ5Fe). To release iron from heme, many bacterial pathogens utilize heme oxygenase enzymes to degrade the porphyrin that coordinates the iron ion of heme. Although E. faecalis lacks these enzymes, bioinformatics revealed a potential ortholog of the anaerobic heme-degrading enzyme anaerobilin synthase, found in Escherichia coli and a few other Gram-negative bacteria. Here, we demonstrated that deletion of OG1RF_RS05575 in E. faecalis (ΔRS05575) or in the Δ5Fe background (Δ5FeΔRS05575) led to intracellular heme accumulation and hypersensitivity under anaerobic conditions, suggesting RS05575 encodes an anaerobilin synthase, the first of its kind described in Gram-positive bacteria. Additionally, deletion of RS05575, either alone or in the Δ5Fe background, impaired E. faecalis colonization in the mouse gastrointestinal tract and virulence in mouse peritonitis and rabbit infective endocarditis models. These results reveal that RS05575 is responsible for anaerobic degradation of heme and identify this relatively new enzyme class as a novel factor in bacterial pathogenesis. Findings from this study are likely to have broad implications, as homologues of RS05575 are found in other Gram-positive facultative anaerobes.

Unraveling the full impact of SPD_0739: a key effector in S. pneumoniae iron homeostasis

Streptococcus pneumoniae is a common member of the nasopharynx commensal microflora and the leading etiological agent of bacterial pneumonia in young children and aging adults. SPD_0739, a highly expressed lipoprotein, is the predicted substrate-binding component of an ABC transporter linked to the uptake of nucleosides and heme by independent studies (named PnrA or Spbhp-37, respectively). Here, we demonstrate that SPD_0739 binds heme in vitro and contributes to the bacterial binding to hemoglobin. A ∆spd_0739 strain exhibited growth attenuation that was relieved by the inactivation of the piuBCDA transporter. Knocking out spd_0739 in the wild type, or the ΔpiuBCDA strain resulted in heme accumulation, higher sensitivity to heme toxicity, and a small growth reduction compared to medium supplemented with a nucleoside mixture. In addition, spd_0739 loss results in higher iron- and heme-related gene expression and lower H2O2 production. Altogether, the data are consistent with a role in nucleoside import and show that SPD_0739 does not import heme. Instead, it indirectly influences iron and heme metabolism, linking nucleosides and iron status in S. pneumoniae.

IMPORTANCE

S. pneumoniae obtains growth essential iron from hemoglobin and other host hemoproteins. Still, the bacterial mechanisms involved are only partially understood, and there are inconsistent reports regarding the function of several transporters implicated in iron uptake. In this study, we clarified the role of PnrA/Spbhp-37, a ligand-binding protein previously linked to nucleoside or heme by different studies. We present data supporting a role in nucleoside scavenging rather than heme import and reveal that PnrA/Spbhp-37 modulates iron and heme uptake, likely by influencing the nucleoside cellular pool. Hence, this work provides a new understanding of a process critical to the pathophysiology of a significant human pathogen. Moreover, PnrA/Spbhp-37 is an abundant and immunogenic surface protein that is highly conserved. Hence, this study also clarifies the function of a promising vaccine target.

Heme utilization by the enterococci

Heme consists of a tetrapyrrole ring ligating an iron ion and has important roles in biological systems. While well-known as the oxygen-binding molecule within hemoglobin of mammals, heme is also cofactor for several enzymes and a major iron source for bacteria within the host. The enterococci are a diverse group of Gram-positive bacteria that exist primarily within the gastrointestinal tract of animals. However, some species within this genus can transform into formidable opportunistic pathogens, largely owing to their extraordinary adaptability to hostile environments. Although enterococci cannot synthesize heme nor depend on heme to grow, several species within the genus encode proteins that utilize heme as a cofactor, which appears to increase their fitness and ability to thrive in challenging environments. This includes more efficient energy generation via aerobic respiration and protection from reactive oxygen species. Here, we review the significance of heme to enterococci, primarily the major human pathogen Enterococcus faecalis, use bioinformatics to assess the prevalence of hemoproteins throughout the genus, and highlight recent studies that underscore the central role of the heme-E. faecalis relationship in host-pathogen dynamics and interspecies bacterial interactions.

Oxidation of hemoproteins by Streptococcus pneumoniae collapses the cell cytoskeleton and disrupts mitochondrial respiration leading to the cytotoxicity of human lung cells

IMPORTANCE

Streptococcus pneumoniae (Spn) colonizes the lungs, killing millions every year. During its metabolism, Spn produces abundant amounts of hydrogen peroxide. When produced in the lung parenchyma, Spn-hydrogen peroxide (H2O2) causes the death of lung cells, and details of the mechanism are studied here. We found that Spn-H2O2 targets intracellular proteins, resulting in the contraction of the cell cytoskeleton and disruption of mitochondrial function, ultimately contributing to cell death. Intracellular proteins targeted by Spn-H2O2 included cytochrome c and, surprisingly, a protein of the cell cytoskeleton, beta-tubulin. To study the details of oxidative reactions, we used, as a surrogate model, the oxidation of another hemoprotein, hemoglobin. Using the surrogate model, we specifically identified a highly reactive radical whose creation was catalyzed by Spn-H2O2. In sum, we demonstrated that the oxidation of intracellular targets by Spn-H2O2 plays an important role in the cytotoxicity caused by Spn, thus providing new targets for interventions.

Endogenously produced H2O2 is intimately involved in iron metabolism in Streptococcus pneumoniae

IMPORTANCE

Heme degradation provides pathogens with growth essential iron, leveraging on the host heme reservoir. Bacteria typically import and degrade heme enzymatically, and here, we demonstrated a significant deviation from this dogma. We found that Streptococcus pneumoniae liberates iron from met-hemoglobin extracellularly, in a hydrogen peroxide (H2O2)- and cell-dependent manner; this activity serves as a major iron acquisition mechanism for S. pneumoniae. Inhabiting oxygen-rich environments is a major part of pneumococcal biology, and hence, H2O2-mediated heme degradation likely supplies iron during infection. Moreover, H2O2 reaction with ferrous hemoglobin but not with met-hemoglobin is known to result in heme breakdown. Therefore, the ability of pneumococci to degrade heme from met-hemoglobin is a new paradigm. Lastly, this study will inform other research as it demonstrates that extracellular degradation must be considered in the interpretations of experiments in which H2O2-producing bacteria are given heme or hemoproteins as an iron source.

Redox modulating small molecules having antimalarial efficacy

The search for effective antimalarial agents remains a critical priority because malaria is widely spread and drug-resistant strains are becoming more prevalent. In this review, a variety of small molecules capable of modulating redox processes were showcased for their potential as antimalarial agents. The compounds were designed to target the redox balance of Plasmodium parasites, which has a pivotal function in their ability to survive and multiply within the host organism. A thorough screening method was utilized to assess the effectiveness of these compounds against both drug-sensitive and drug-resistant strains of Plasmodium falciparum, the malaria-causing parasite. The results revealed that several of the tested compounds exhibited significant effectiveness against malaria, displaying IC50 values at a low micromolar range. Furthermore, these compounds displayed promising selectivity for the parasite, as they exhibited low cytotoxicity towards mammalian cells. Thorough mechanistic studies were undertaken to clarify how the active compounds exert their mode of action. The findings revealed that these compounds disrupted the parasites' redox balance, causing oxidative stress and interfering with essential cellular functions. Additionally, the compounds showed synergistic effects when combined with existing antimalarial drugs, suggesting their potential for combination therapies to combat drug resistance. Overall, this study highlights the potential of redox-modulating small molecules as effective antimalarial agents. The identified compounds demonstrate promising antimalarial activity, and their mechanism of action offers insights into targeting the redox balance of Plasmodium parasites. Further optimization and preclinical studies are warranted to determine their efficacy, safety, and potential for clinical development as novel antimalarial therapeutics.

Oxidation of hemoglobin in the lung parenchyma facilitates the differentiation of pneumococci into encapsulated bacteria

Pneumococcal pneumonia causes cytotoxicity in the lung parenchyma but the underlying mechanism involves multiple factors contributing to cell death. Here, we discovered that hydrogen peroxide produced by Streptococcus pneumoniae (Spn-H 2 O 2 ) plays a pivotal role by oxidizing hemoglobin, leading to its polymerization and subsequent release of labile heme. At physiologically relevant levels, heme selected a population of encapsulated pneumococci. In the absence of capsule and Spn-H 2 O 2 , host intracellular heme exhibited toxicity towards pneumococci, thus acting as an antibacterial mechanism. Further investigation revealed that heme-mediated toxicity required the ABC transporter GlnPQ. In vivo experiments demonstrated that pneumococci release H 2 O 2 to cause cytotoxicity in bronchi and alveoli through the non-proteolytic degradation of intracellular proteins such as actin, tubulin and GAPDH. Overall, our findings uncover a mechanism of lung toxicity mediated by oxidative stress that favor the growth of encapsulated pneumococci suggesting a therapeutic potential by targeting oxidative reactions.

Graphical abstract

Highlights

Oxidation of hemoglobin by Streptococcus pneumoniae facilitates differentiation to encapsulated pneumococci in vivo Differentiated S. pneumoniae produces capsule and hydrogen peroxide (Spn-H 2 O 2 ) as defense mechanism against host heme-mediated toxicity. Spn-H 2 O 2 -induced lung toxicity causes the oxidation and non-proteolytic degradation of intracellular proteins tubulin, actin, and GAPDH. The ABC transporter GlnPQ is a heme-binding complex that makes Spn susceptible to heme toxicity.

Oxidation of hemoproteins by Streptococcus pneumoniae collapses the cell cytoskeleton and disrupts mitochondrial respiration leading to cytotoxicity of human lung cells

Streptococcus pneumoniae (Spn) causes pneumonia that kills millions through acute toxicity and invasion of the lung parenchyma. During aerobic respiration, Spn releases hydrogen peroxide (Spn-H 2 O 2 ), as a by-product of enzymes SpxB and LctO, and causes cell death with signs of both apoptosis and pyroptosis by oxidizing unknown cell targets. Hemoproteins are molecules essential for life and prone to oxidation by H 2 O 2 . We recently demonstrated that during infection-mimicking conditions, Spn-H 2 O 2 oxidizes the hemoprotein hemoglobin (Hb), releasing toxic heme. In this study, we investigated details of the molecular mechanism(s) by which the oxidation of hemoproteins by Spn-H 2 O 2 causes human lung cell death. Spn strains, but not H 2 O 2 -deficient SpnΔ spxB Δ lctO strains caused time-dependent cell cytotoxicity characterized by the rearrangement of the actin, the loss of the microtubule cytoskeleton and nuclear contraction. Disruption of the cell cytoskeleton correlated with the presence of invasive pneumococci and an increase of intracellular reactive oxygen species. In cell culture, the oxidation of Hb or cytochrome c (Cyt c ) caused DNA degradation and mitochondrial dysfunction from inhibition of complex I-driven respiration, which was cytotoxic to human alveolar cells. Oxidation of hemoproteins resulted in the creation of a radical, which was identified as a protein derived side chain tyrosyl radical by using electron paramagnetic resonance (EPR). Thus, we demonstrate that Spn invades lung cells, releasing H 2 O 2 that oxidizes hemoproteins, including Cyt c , catalyzing the formation of a tyrosyl side chain radical on Hb and causing mitochondrial disruption, that ultimately leads to the collapse of the cell cytoskeleton.