A Critical Role of Zinc Importer AdcABC in Group A Streptococcus-Host Interactions During Infection and Its Implications for Vaccine Development

Manganese uptake by MtsABC contributes to the pathogenesis of human pathogen group A streptococcus by resisting host nutritional immune defenses

The interplay between host nutritional immune mechanisms and bacterial nutrient uptake systems has a major impact on the disease outcome. The host immune factor calprotectin (CP) limits the availability of essential transition metals, such as manganese (Mn) and zinc (Zn), to control the growth of invading pathogens. We previously demonstrated that the competition between CP and the human pathogen group A streptococcus (GAS) for Zn impacts GAS pathogenesis. However, the contribution of Mn sequestration by CP in GAS infection control and the role of GAS Mn acquisition systems in overcoming host-imposed Mn limitation remain unknown. Using a combination of in vitro and in vivo studies, we show that GAS-encoded mtsABC is a Mn uptake system that aids bacterial evasion of CP-imposed Mn scarcity and promotes GAS virulence. Mn deficiency caused by either the inactivation of mtsC or CP also impaired the protective function of GAS-encoded Mn-dependent superoxide dismutase. Our ex vivo studies using human saliva show that saliva is a Mn-scant body fluid, and Mn acquisition by MtsABC is critical for GAS survival in human saliva. Finally, animal infection studies using wild-type (WT) and CP-/- mice showed that MtsABC is critical for GAS virulence in WT mice but dispensable in mice lacking CP, indicating the direct interplay between MtsABC and CP in vivo. Together, our studies elucidate the role of the Mn import system in GAS evasion of host-imposed metal sequestration and underscore the translational potential of MtsABC as a therapeutic or prophylactic target.

ZrgA contributes to zinc acquisition in Vibrio parahaemolyticus

Metals are nutrients essential for almost all lifeforms. Bacteria have evolved several mechanisms to overcome the metal restrictions imposed by the host. Vibrio parahaemolyticus causes severe threats to public health and significant economic losses in shrimp aquaculture. Herein, we report that ZrgA contributes to zinc acquisition in this pathogen. The operon VP_RS01455 to VP_RS01475 of V. parahaemolyticus encodes the putative Zn transporter ZrgABCDE, whose homologs are widely distributed in Vibrionaceae. RNA sequencing analysis revealed that V. parahaemolyticus modulates the transcriptome in response to Zn limitation. Genes in the Zinc uptake regulator (Zur) regulon are upregulated during Zn limitation, including three genes annotated to encode Zn-binding proteins. Significant upregulation of these three genes during Zn limitation was also confirmed by quantitative real-time PCR (qRT-PCR) analysis. However, only the mutants containing a VP_RS01470 (zrgA) deletion exhibited impaired growth under Zn-deficient conditions, indicating that VP_RS01470 plays the predominant role in V. parahaemolyticus Zn acquisition. The VP_RS01470 deletion mutant displayed a false appearance of decreased swimming motility under Zn-deficient conditions, as revealed by the fact that the polar flagellar-related genes were not downregulated in the mutant. Moreover, VP_RS01470 deletion produced no noticeable impact on the swarming motility and virulence in mice. qRT-PCR analysis and β-galactosidase activity assays indicated that Zur negatively regulates VP_RS01470 expression in V. parahaemolyticus. Collectively, our findings suggest that ZrgA is required for Zn acquisition in V. parahaemolyticus and highlight the importance of detecting the expression of flagellar genes during analysis of motility of a mutant deficient in growth.

The adcA and lmb Genes Play an Important Role in Drug Resistance and Full Virulence of Streptococcus suis

Streptococcus suis is an recognized zoonotic pathogen of swine and severely threatens human health. Zinc is the second most abundant transition metal in biological systems. Here, we investigated the contribution of zinc to the drug resistance and pathogenesis of S. suis. We knocked out the genes of AdcACB and Lmb, two Zn-binding lipoproteins. Compared to the wild-type strain, we found that the survival rate of this double-mutant strain (ΔadcAΔlmb) was reduced in Zinc-limited medium, but not in Zinc-supplemented medium. Additionally, phenotypic experiments showed that the ΔadcAΔlmb strain displayed impaired adhesion to and invasion of cells, biofilm formation, and tolerance of cell envelope-targeting antibiotics. In a murine infection model, deletion of the adcA and lmb genes in S. suis resulted in a significant decrease in strain virulence, including survival rate, tissue bacterial load, inflammatory cytokine levels, and histopathological damage. These findings show that AdcA and Lmb are important for biofilm formation, drug resistance, and virulence in S. suis. IMPORTANCE Transition metals are important micronutrients for bacterial growth. Zn is necessary for the catalytic activity and structural integrity of various metalloproteins involved in bacterial pathogenic processes. However, how these invaders adapt to host-imposed metal starvation and overcome nutritional immunity remains unknown. Thus, pathogenic bacteria must acquire Zn during infection in order to successfully survive and multiply. The host uses nutritional immunity to limit the uptake of Zn by the invading bacteria. The bacterium uses a set of high-affinity Zn uptake systems to overcome this host metal restriction. Here, we identified two Zn uptake transporters in S. suis, AdcA and Lmb, by bioinformatics analysis and found that an adcA and lmb double-mutant strain could not grow in Zn-deficient medium and was more sensitive to cell envelope-targeting antibiotics. It is worth noting that the Zn uptake system is essential for biofilm formation, drug resistance, and virulence in S. suis. The Zn uptake system is expected to be a target for the development of novel antimicrobial therapies.

Salivary Antimicrobial Peptide Histatin-5 Does Not Display Zn(II)-Dependent or -Independent Activity against Streptococci

Histatin-5 (Hst5) is a member of the histatin superfamily of cationic, His-rich, Zn(II)-binding peptides in human saliva. Hst5 displays antimicrobial activity against fungal and bacterial pathogens, often in a Zn(II)-dependent manner. In contrast, here we showed that under in vitro conditions that are characteristic of human saliva, Hst5 does not kill seven streptococcal species that normally colonize the human oral cavity and oropharynx. We further showed that Zn(II) does not influence this outcome. We then hypothesized that Hst5 exerts more subtle effects on streptococci by modulating Zn(II) availability. We initially proposed that Hst5 contributes to nutritional immunity by limiting nutrient Zn(II) availability and promoting bacterial Zn(II) starvation. By examining the interactions between Hst5 and Streptococcus pyogenes as a model Streptococcus species, we showed that Hst5 does not influence the expression of Zn(II) uptake genes. In addition, Hst5 did not suppress growth of a ΔadcAI mutant strain that is impaired in Zn(II) uptake. These observations establish that Hst5 does not promote Zn(II) starvation. Biochemical examination of purified peptides further confirmed that Hst5 binds Zn(II) with high micromolar affinities and does not compete with the AdcAI high-affinity Zn(II) uptake protein for binding nutrient Zn(II). Instead, we showed that Hst5 weakly limits the availability of excess Zn(II) and suppresses Zn(II) toxicity to a ΔczcD mutant strain that is impaired in Zn(II) efflux. Altogether, our findings led us to reconsider the function of Hst5 as a salivary antimicrobial agent and the role of Zn(II) in Hst5 function.

Metal Homeostasis in Pathogenic Streptococci

Streptococcus spp. are an important genus of Gram-positive bacteria, many of which are opportunistic pathogens that are capable of causing invasive disease in a wide range of populations. Metals, especially transition metal ions, are an essential nutrient for all organisms. Therefore, to survive across dynamic host environments, Streptococci have evolved complex systems to withstand metal stress and maintain metal homeostasis, especially during colonization and infection. There are many different types of transport systems that are used by bacteria to import or export metals that can be highly specific or promiscuous. Focusing on the most well studied transition metals of zinc, manganese, iron, nickel, and copper, this review aims to summarize the current knowledge of metal homeostasis in pathogenic Streptococci, and their role in virulence.

Managing Manganese: The Role of Manganese Homeostasis in Streptococcal Pathogenesis

Pathogenic streptococci require manganese for survival in the host. In response to invading pathogens, the host recruits nutritional immune effectors at infection sites to withhold manganese from the pathogens and control bacterial growth. The manganese scarcity impairs several streptococcal processes including oxidative stress defenses, de novo DNA synthesis, bacterial survival, and virulence. Emerging evidence suggests that pathogens also encounter manganese toxicity during infection and manganese excess impacts streptococcal virulence by manganese mismetallation of non-cognate molecular targets involved in bacterial antioxidant defenses and cell division. To counter host-imposed manganese stress, the streptococcal species employ a sophisticated sensory system that tightly coordinates manganese stress-specific molecular strategies to negate host induced manganese stress and proliferate in the host. Here we review the molecular details of host-streptococcal interactions in the battle for manganese during infection and the significance of streptococcal effectors involved to bacterial pathophysiology.

The AdcR-regulated AdcA and AdcAII contribute additively to zinc acquisition and virulence in Streptococcus suis

Metals are necessary elements for bacteria. Typically, vertebrate hosts restrict invading bacterial pathogens from accessing metals. Therefore, bacteria have evolved high-affinity metal importers to acquire metals. Streptococcus suis is a major swine pathogen and an emerging zoonotic agent that endangers the swine industry and human health worldwide. Herein, we aimed to identify the zinc acquisition systems in S. suis and evaluate their roles in bacterial virulence. Bioinformatic analyses revealed that S. suis encodes homologues of AdcA and AdcAII, two well-characterised Zn-binding lipoproteins in certain streptococci. Quantitative reverse transcription PCR (qRT-PCR) analysis revealed that the expressions of adcA and adcAII were significantly upregulated in response to Zn limitation, with a higher expression level of adcAII than adcA. Gene deletion mutants and complementation strains were constructed; their growth characteristics under Zn-deficient and Zn-replete conditions indicated that AdcA and AdcAII have overlapping functionality in Zn acquisition. A mouse infection model was used to evaluate the roles of AdcA and AdcAII in S. suis virulence. Mice infected with the double mutant ΔadcAΔadcAII exhibited a significantly higher survival rate, decreased bacterial burden, and lower production of inflammatory cytokines compared to those infected with the wild type (WT) strain. Furthermore, ΔadcAΔadcAII showed reduced competitiveness in infection establishment compared with the WT strain. RNA sequencing, qRT-PCR, and electrophoretic mobility shift assays revealed that AdcR negatively regulates the expressions of adcA and adcAII. Collectively, our results demonstrated that AdcA and AdcAII, which are negatively regulated by AdcR, contribute additively to zinc acquisition and virulence in S. suis.

S100A12 promotes Mn(II) binding to pneumococcal PsaA and staphylococcal MntC by Zn(II) sequestration

Human S100A12 (calgranulin C, EN-RAGE) is a Zn(II)-sequestering host-defense protein that contributes to the metal-withholding innate immune response against microbial pathogens. S100A12 coordinates Zn(II) ions at two His₃Asp sites with high affinity. A similar His₃Asp site found in calprotectin (S100A8/S100A9, calgranulin A/B), a closely related human S100 protein, can sequester divalent metal ions from the solute-binding proteins (SBPs) pneumococcal PsaA (pneumococcal surface protein A) and staphylococcal MntC (manganese transport protein C). Both SBPs are components of Mn(II) transporters and capture extracellular Mn(II) ions for subsequent delivery into the bacterial cytosol. Nevertheless, PsaA and MntC exhibit a thermodynamic preference for Zn(II) over Mn(II), and Zn(II) binding can interfere with Mn(II) acquisition. In this work, we have used a biotinylated variant of S100A12 to show that S100A12 can sequester Zn(II) ions from PsaA and MntC. Moreover, electron paramagnetic resonance (EPR) spectroscopy indicates that by sequestering Zn(II) from Zn(II)-bound PsaA and MntC, S100A12 promotes Mn(II) binding to the SBPs. These results inform the function of S100A12 in Zn(II) sequestration, and further suggest that Zn(II)-sequestering S100 proteins may inadvertently protect bacterial pathogens during infection.

The AdcACB/AdcAII system is essential for zinc homeostasis and an important contributor of Enterococcus faecalis virulence

Bacterial pathogens require a variety of micronutrients for growth, including trace metals such as iron, manganese, and zinc (Zn). Despite their relative abundance in host environments, access to these metals is severely restricted during infection due to host-mediated defense mechanisms collectively known as nutritional immunity. Despite a growing appreciation of the importance of Zn in host-pathogen interactions, the mechanisms of Zn homeostasis and the significance of Zn to the pathophysiology of E. faecalis, a major pathogen of nosocomial and community-associated infections, have not been thoroughly investigated. Here, we show that E. faecalis encoded ABC-type transporter AdcACB and an orphan substrate-binding lipoprotein AdcAII that work cooperatively to maintain Zn homeostasis. Simultaneous inactivation of adcA and adcAII or the entire adcACB operon led to a significant reduction in intracellular Zn under Zn-restricted conditions and heightened sensitivity to Zn-chelating agents including human calprotectin, aberrant cell morphology, and impaired fitness in serum ex vivo. Additionally, inactivation of adcACB and adcAII significantly reduced bacterial tolerance toward cell envelope-targeting antibiotics. Finally, we showed that the AdcACB/AdcAII system contributes to E. faecalis virulence in a Galleria mellonella invertebrate infection model and in two catheter-associated mouse infection models that recapitulate many of the host conditions associated with enterococcal human infections. Collectively, this report reveals that high-affinity Zn import is important for the pathogenesis of E. faecalis establishing the surface-associated AdcA and AdcAII lipoproteins as potential therapeutic targets.

Zinc sequestration by human calprotectin facilitates manganese binding to the bacterial solute-binding proteins PsaA and MntC

Zinc is an essential transition metal nutrient for bacterial survival and growth but may become toxic when present at elevated levels. The Gram-positive bacterial pathogen Streptococcus pneumoniae is sensitive to zinc poisoning, which results in growth inhibition and lower resistance to oxidative stress. Streptococcus pneumoniae has a relatively high manganese requirement, and zinc toxicity in this pathogen has been attributed to the coordination of Zn(II) at the Mn(II) site of the solute-binding protein (SBP) PsaA, which prevents Mn(II) uptake by the PsaABC transport system. In this work, we investigate the Zn(II)-binding properties of pneumococcal PsaA and staphylococcal MntC, a related SBP expressed by another Gram-positive bacterial pathogen, Staphylococcus aureus, which contributes to Mn(II) uptake. X-ray absorption spectroscopic studies demonstrate that both SBPs harbor Zn(II) sites best described as five-coordinate, and metal-binding studies in solution show that both SBPs bind Zn(II) reversibly with sub-nanomolar affinities. Moreover, both SBPs exhibit a strong thermodynamic preference for Zn(II) ions, which readily displace bound Mn(II) ions from these proteins. We also evaluate the Zn(II) competition between these SBPs and the human S100 protein calprotectin (CP, S100A8/S100A9 oligomer), an abundant host-defense protein that is involved in the metal-withholding innate immune response. CP can sequester Zn(II) from PsaA and MntC, which facilitates Mn(II) binding to the SBPs. These results demonstrate that CP can inhibit Zn(II) poisoning of the SBPs and provide molecular insight into how S100 proteins may inadvertently benefit bacterial pathogens rather than the host.

COG0523 proteins: a functionally diverse family of transition metal-regulated G3E P-loop GTP hydrolases from bacteria to man

Transition metal homeostasis ensures that cells and organisms obtain sufficient metal to meet cellular demand while dispensing with any excess so as to avoid toxicity. In bacteria, zinc restriction induces the expression of one or more Zur (zinc-uptake repressor)-regulated Cluster of Orthologous Groups (COG) COG0523 proteins. COG0523 proteins encompass a poorly understood sub-family of G3E P-loop small GTPases, others of which are known to function as metallochaperones in the maturation of cobalamin (CoII) and NiII cofactor-containing metalloenzymes. Here, we use genomic enzymology tools to functionally analyse over 80 000 sequences that are evolutionarily related to Acinetobacter baumannii ZigA (Zur-inducible GTPase), a COG0523 protein and candidate zinc metallochaperone. These sequences segregate into distinct sequence similarity network (SSN) clusters, exemplified by the ZnII-Zur-regulated and FeIII-nitrile hydratase activator CxCC (C, Cys; X, any amino acid)-containing COG0523 proteins (SSN cluster 1), NiII-UreG (clusters 2, 8), CoII-CobW (cluster 4), and NiII-HypB (cluster 5). A total of five large clusters that comprise ≈ 25% of all sequences, including cluster 3 which harbors the only structurally characterized COG0523 protein, Escherichia coli YjiA, and many uncharacterized eukaryotic COG0523 proteins. We also establish that mycobacterial-specific protein Y (Mpy) recruitment factor (Mrf), which promotes ribosome hibernation in actinomycetes under conditions of ZnII starvation, segregates into a fifth SSN cluster (cluster 17). Mrf is a COG0523 paralog that lacks all GTP-binding determinants as well as the ZnII-coordinating Cys found in CxCC-containing COG0523 proteins. On the basis of this analysis, we discuss new perspectives on the COG0523 proteins as cellular reporters of widespread nutrient stress induced by ZnII limitation.

Potential factors involved in the early pathogenesis of Streptococcus uberis mastitis: a review

Bovine mastitis is an inflammation of the mammary gland, which could be the result of allergy, physical trauma, or invasion by pathogens as Streptococcus uberis. This pathogen is an environmental pathogen associated with subclinical and clinical intramammary infection (IMI) in both lactating and non-lactating cows, which can persist in the udder and cause a chronic infection in the mammary gland. In spite of the important economic losses and increased prevalence caused by S. uberis mastitis, virulence factors involved in bacterial colonization of mammary glands and the pathogenic mechanisms are not yet clear. In the last 30 years, several studies have defined adherence and internalization of S. uberis as the early stages in IMI. S. uberis adheres to and invades into mammary gland cells, and this ability has been observed in in vitro assays. Until now, these abilities have not been determined in vivo challenges since they have been difficult to study. Bacterial surface proteins are able to bind to extracellular matrix protein components such as fibronectin, collagen and laminin, as well as proteins in milk. These proteins play a role in adhesion to host cells and have been denominated microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). This article aims to summarize our current knowledge on the most relevant properties of the potential factors involved in the early pathogenesis of S. uberis mastitis.

Zn2+ Intoxication of Mycobacterium marinum during Dictyostelium discoideum Infection Is Counteracted by Induction of the Pathogen Zn2+ Exporter CtpC

Microelements are essential for the function of the innate immune system. A deficiency in zinc or copper results in an increased susceptibility to bacterial infections.

Macrophages use diverse strategies to restrict intracellular pathogens, including either depriving the bacteria of (micro)nutrients such as transition metals or intoxicating them via metal accumulation. Little is known about the chemical warfare between Mycobacterium marinum, a close relative of Mycobacterium tuberculosis (Mtb), and its hosts. We use the professional phagocyte Dictyostelium discoideum to investigate the role of Zn²⁺ during M. marinum infection. We show that M. marinum senses toxic levels of Zn²⁺ and responds by upregulating one of its isoforms of the Zn²⁺ efflux transporter CtpC. Deletion of ctpC (MMAR_1271) leads to growth inhibition in broth supplemented with Zn²⁺ as well as reduced intracellular growth. Both phenotypes were fully rescued by constitutive ectopic expression of the Mtb CtpC orthologue demonstrating that MMAR_1271 is the functional CtpC Zn²⁺ efflux transporter in M. marinum. Infection leads to the accumulation of Zn²⁺ inside the Mycobacterium-containing vacuole (MCV), achieved by the induction and recruitment of the D. discoideum Zn²⁺ efflux pumps ZntA and ZntB. In cells lacking ZntA, there is further attenuation of M. marinum growth, presumably due to a compensatory efflux of Zn²⁺ into the MCV, carried out by ZntB, the main Zn²⁺ transporter in endosomes and phagosomes. Counterintuitively, bacterial growth is also impaired in zntB KO cells, in which MCVs appear to accumulate less Zn²⁺ than in wild-type cells, suggesting restriction by other Zn²⁺-mediated mechanisms. Absence of CtpC further epistatically attenuates the intracellular proliferation of M. marinum in zntA and zntB KO cells, confirming that mycobacteria face noxious levels of Zn²⁺.

Group A Streptococcus AdcR Regulon Participates in Bacterial Defense against Host-Mediated Zinc Sequestration and Contributes to Virulence

Colonization by pathogenic bacteria depends on their ability to overcome host nutritional defenses and acquire nutrients. The human pathogen group A streptococcus (GAS) encounters the host defense factor calprotectin (CP) during infection. CP inhibits GAS growth in vitro by imposing zinc (Zn) limitation. However, GAS counterstrategies to combat CP-mediated Zn limitation and the in vivo relevance of CP-GAS interactions to bacterial pathogenesis remain unknown. Here, we report that GAS upregulates the AdcR regulon in response to CP-mediated Zn limitation. The AdcR regulon includes genes encoding Zn import (adcABC), Zn sparing (rpsN.2), and Zn scavenging systems (adcAII, phtD, and phtY). Each gene in the AdcR regulon contributes to GAS Zn acquisition and CP resistance. The ΔadcC and ΔrpsN.2 mutant strains were the most susceptible to CP, whereas the ΔadcA, ΔadcAII, and ΔphtD mutant strains displayed less CP sensitivity during growth in vitro However, the ΔphtY mutant strain did not display an increased CP sensitivity. The varied sensitivity of the mutant strains to CP-mediated Zn limitation suggests distinct roles for individual AdcR regulon genes in GAS Zn acquisition. GAS upregulates the AdcR regulon during necrotizing fasciitis infection in WT mice but not in S100a9-/- mice lacking CP. This suggests that CP induces Zn deficiency in the host. Finally, consistent with the in vitro results, several of the AdcR regulon genes are critical for GAS virulence in WT mice, whereas they are dispensable for virulence in S100a9-/- mice, indicating the direct competition for Zn between CP and proteins encoded by the GAS AdcR regulon during infection.

Disruption of Phosphate Homeostasis Sensitizes Staphylococcus aureus to Nutritional Immunity

To control infection, mammals actively withhold essential nutrients, including the transition metal manganese, by a process termed nutritional immunity. A critical component of this host response is the manganese-chelating protein calprotectin. While many bacterial mechanisms for overcoming nutritional immunity have been identified, the intersection between metal starvation and other essential inorganic nutrients has not been investigated. Here, we report that overexpression of an operon encoding a highly conserved inorganic phosphate importer, PstSCAB, increases the sensitivity of Staphylococcus aureus to calprotectin-mediated manganese sequestration. Further analysis revealed that overexpression of pstSCAB does not disrupt manganese acquisition or result in overaccumulation of phosphate by S. aureus However, it does reduce the ability of S. aureus to grow in phosphate-replete defined medium. Overexpression of pstSCAB does not aberrantly activate the phosphate-responsive two-component system PhoPR, nor was this two-component system required for sensitivity to manganese starvation. In a mouse model of systemic staphylococcal disease, a pstSCAB-overexpressing strain is significantly attenuated compared to wild-type S. aureus This defect is partially reversed in a calprotectin-deficient mouse, in which manganese is more readily available. Given that expression of pstSCAB is regulated by PhoPR, these findings suggest that overactivation of PhoPR would diminish the ability of S. aureus to resist nutritional immunity and cause infection. As PhoPR is also necessary for bacterial virulence, these findings imply that phosphate homeostasis represents a critical regulatory node whose activity must be precisely controlled in order for S. aureus and other pathogens to cause infection.

Availability of Zinc Impacts Interactions between Streptococcus sanguinis and Pseudomonas aeruginosa in Coculture

Airway infections associated with cystic fibrosis (CF) are polymicrobial. We reported previously that clinical isolates of Pseudomonas aeruginosa promote the growth of a variety of streptococcal species. To explore the mechanistic basis of this interaction, we performed a genetic screen to identify mutants of Streptococcus sanginuis SK36 whose growth was no longer enhanced by P. aeruginosa PAO1. Mutations in the zinc uptake systems of S. sanguinis SK36 reduced growth of these strains by 1 to 3 logs compared to that of wild-type S. sanguinis SK36 when grown in coculture with P. aeruginosa PAO1, and exogenous zinc (0.1 to 10 μM) rescued the coculture defect of zinc uptake mutants of S. sanguinis SK36. Zinc uptake mutants of S. sanguinis SK36 had no obvious growth defect in monoculture. Consistent with competition for zinc driving coculture dynamics, S. sanguinis SK36 grown in coculture with P. aeruginosa showed increased expression of zinc uptake genes compared to that of S. sanguinis grown alone. Strains of P. aeruginosa PAO1 defective in zinc transport also supported ∼2-fold more growth by S. sanguinis compared to that in coculture with wild-type P. aeruginosa PAO1. An analysis of 118 CF sputum samples revealed that total zinc levels varied from ∼5 to 145 μM. At relatively low zinc levels, Pseudomonas and Streptococcus spp. were found in approximately equal abundance; at higher zinc levels, we observed a decline in relative abundance of Streptococcus spp., perhaps as a result of increasing zinc toxicity. Together, our data indicate that the relative abundances of these microbes in the CF airway may be impacted by zinc levels.IMPORTANCE Polymicrobial infections in CF cases likely impact patient health, but the mechanism(s) underlying such interactions is poorly understood. Here, we show using an in vitro model system that interactions between Pseudomonas and Streptococcus are modulated by zinc availability, and clinical data are consistent with this model. Together with previous studies, our work supports a role for metal homeostasis as a key factor driving microbial interactions.

Metal sensing and regulation of adaptive responses to manganese limitation by MtsR is critical for group A streptococcus virulence

Pathogenic bacteria encounter host-imposed manganese (Mn) limitation during infection. Herein we report that in the human pathogen Streptococcus pyogenes, the adaptive response to Mn limitation is controlled by a DtxR family metalloregulator, MtsR. Genes upregulated by MtsR during Mn limitation include Mn (mtsABC) and Fe acquisition systems (sia operon), and a metal-independent DNA synthesis enzyme (nrdFEI.2). To elucidate the mechanism of metal sensing and gene regulation by MtsR, we determined the crystal structure of MtsR. MtsR employs two Mn-sensing sites to monitor metal availability, and metal occupancy at each site influences MtsR regulatory activity. The site 1 acts as the primary Mn sensing site, and loss of metal at site 1 causes robust upregulation of mtsABC. The vacant site 2 causes partial induction of mtsABC, indicating that site 2 functions as secondary Mn sensing site. Furthermore, we show that the C-terminal FeoA domains of adjacent dimers participate in the oligomerization of MtsR on DNA, and multimerization is critical for MtsR regulatory activity. Finally, the mtsR mutant strains defective in metal sensing and oligomerization are attenuated for virulence in a mouse model of invasive infection, indicating that Mn sensing and gene regulation by MtsR are critical processes during S. pyogenes infection.

Nutrient Zinc at the Host-Pathogen Interface

Zinc is an essential cofactor required for life and, as such, mechanisms exist for its homeostatic maintenance in biological systems. Despite the evolutionary distance between vertebrates and microbial life, there are parallel mechanisms to balance the essentiality of zinc with its inherent toxicity. Vertebrates regulate zinc homeostasis through a complex network of metal transporters and buffering systems that respond to changes in nutritional zinc availability or inflammation. Fine-tuning of this network becomes crucial during infections, where host nutritional immunity attempts to limit zinc availability to pathogens. However, accumulating evidence demonstrates that pathogens have evolved mechanisms to subvert host-mediated zinc withholding, and these metal homeostasis systems are important for survival within the host. We discuss here the mechanisms of vertebrate and bacterial zinc homeostasis and mobilization, as well as recent developments in our understanding of microbial zinc acquisition.

The scfCDE Operon Encodes a Predicted ABC Importer Required for Fitness and Virulence during Group A Streptococcus Invasive Infection

As a strict human pathogen, Streptococcus pyogenes (group A Streptococcus, or GAS) causes a wide range of infections, from superficial to life-threatening diseases, upon dissemination. Thus, it is necessary to gain a better understanding of how GAS successfully overcomes host-mediated challenges and infects various host niches. We previously identified subcutaneous fitness (scf) genes in the clinically relevant wild-type (WT) GAS M1T1 5448 strain that are critical for fitness during murine soft-tissue infection at both 24 h and 48 h postinfection. The uncharacterized locus scfCDE was transcribed as an operon and is predicted to encode an ABC importer for nutrient uptake (e.g., amino acids). Individual scfCDE deletion mutants grew comparably to WT 5448 in rich medium but exhibited reduced fitness during competitive growth in murine soft tissue and in nutrient-limiting chemically defined medium (CDM). A deletion of the permease gene scfD resulted in a monoculture growth defect in CDM that could be rescued by addition of excess peptides, suggesting a role as an amino acid importer. Interestingly, the ΔscfC substrate-binding and ΔscfD permease mutants, but not the ΔscfE ATPase mutant, were highly attenuated in murine soft tissue. Moreover, all three genes were required for GAS survival in human blood, indicating their impact is not limited to superficial infections. As such, scfCDE plays an integral role in enhancing GAS adaptation during localized infection as well as dissemination to deeper host environments. Since scfCDE is conserved throughout Firmicutes, this work may contribute to the development of therapeutic strategies against GAS and other Gram-positive pathogens.

Antimicrobial action of calprotectin that does not involve metal withholding

Calprotectin is a potent antimicrobial that inhibits the growth of pathogens by tightly binding transition metals such as Mn and Zn, thereby preventing their uptake and utilization by invading microbes. At sites of infection, calprotectin is abundantly released from neutrophils, but calprotectin is also present in non-neutrophil cell types that may be relevant to infections. We show here that in patients infected with the Lyme disease pathogen Borreliella (Borrelia) burgdorferi, calprotectin is produced in neutrophil-free regions of the skin, in both epidermal keratinocytes and in immune cells infiltrating the dermis, including CD68 positive macrophages. In culture, B. burgdorferi's growth is inhibited by calprotectin, but surprisingly, the mechanism does not involve the classical withholding of metal nutrients. B. burgdorferi cells exposed to calprotectin cease growth with no reduction in intracellular Mn and no loss in activity of Mn enzymes including the SodA superoxide dismutase. Additionally, there is no obvious loss in intracellular Zn. Rather than metal depletion, we find that calprotectin inhibits B. burgdorferi growth through a mechanism that requires physical association of calprotectin with the bacteria. By comparison, calprotectin inhibited E. coli growth without physically interacting with the microbe, and calprotectin effectively depleted E. coli of intracellular Mn and Zn. Our studies with B. burgdorferi demonstrate that the antimicrobial capacity of calprotectin is complex and extends well beyond simple withholding of metal micronutrients.

Microfluidic triple-gradient generator for efficient screening of chemical space

Generation of a combinatorial gradient for multiple chemicals is essential for studies of biochemical stimuli, chemoattraction, protein crystallization and others. While currently available platforms require complex design/settings to obtain a double-gradient chemical matrix, we herein report for the first time a simple triple-gradient matrix (TGM) device for efficient screening of chemical space. The TGM device is composed of two glass slides and works following the concept of SlipChip. The device utilizes XYZ space to distribute three chemicals and establishes a chemical gradient matrix within 5 min. The established matrix contains 24 or 104 screening conditions depending on the device used, which covers a concentration range of [0.117-1, 0.117-1 and 0.686-1] and [0.0830-1, 0.0830-1, 0.686-1] respectively for the three chemicals. With the triple gradients built simultaneously, this TGM device provides order-of-magnitude improvement in screening efficiency over existing single- or double-gradient generators. As a proof of concept, we applied the device to screen the crystallization conditions for two model proteins of lysozyme and trypsin and confirmed the crystal structures using X-ray diffraction. Furthermore, we successfully obtained the crystallization condition of adhesin competence repressor, a protein that senses the alterations in intracellular zinc concentrations. We expect the TGM system to be widely used as an analytical platform for material synthesis and chemical screening beyond for protein crystallization.

Biophysical and structural characterization of a zinc-responsive repressor of the MarR superfamily

The uptake of zinc, which is vital in trace amounts, is tightly controlled in bacteria. For this control, bacteria of the Streptococcaceae group use a Zn(II)-binding repressor named ZitR in lactococci and AdcR in streptococci, while other bacteria use a Zur protein of the Ferric uptake regulator (Fur) superfamily. ZitR and AdcR proteins, characterized by a winged helix-turn-helix DNA-binding domain, belong to the multiple antibiotic resistance (MarR) superfamily, where they form a specific group of metallo-regulators. Here, one such Zn(II)-responsive repressor, ZitR of Lactococcus lactis subspecies cremoris strain MG1363, is characterized. Size Exclusion Chromatography-coupled to Multi Angle Light Scattering, Circular Dichroism and Isothermal Titration Calorimetry show that purified ZitR is a stable dimer complexed to Zn(II), which is able to bind its two palindromic operator sites on DNA fragments. The crystal structure of ZitR holo-form (Zn(II)4-ZitR2), has been determined at 2.8 Å resolution. ZitR is the fourth member of the MarR metallo-regulator subgroup whose structure has been determined. The folding of ZitR/AdcR metallo-proteins is highly conserved between both subspecies (cremoris or lactis) in the Lactococcus lactis species and between species (Lactococcus lactis and Streptococcus pneumoniae or pyogenes) in the Streptococcaceae group. It is also similar to the folding of other MarR members, especially in the DNA-binding domain. Our study contributes to better understand the biochemical and structural properties of metallo-regulators in the MarR superfamily.

Gene fitness landscape of group A streptococcus during necrotizing myositis

Necrotizing fasciitis and myositis are devastating infections characterized by high mortality. Group A streptococcus (GAS) is a common cause of these infections, but the molecular pathogenesis is poorly understood. We report a genome-wide analysis using serotype M1 and M28 strains that identified GAS genes contributing to necrotizing myositis in nonhuman primates (NHP), a clinically relevant model. Using transposon-directed insertion-site sequencing (TraDIS), we identified 126 and 116 GAS genes required for infection by serotype M1 and M28 organisms, respectively. For both M1 and M28 strains, more than 25% of the GAS genes required for necrotizing myositis encode known or putative transporters. Thirteen GAS transporters contributed to both M1 and M28 strain fitness in NHP myositis, including putative importers for amino acids, carbohydrates, and vitamins and exporters for toxins, quorum-sensing peptides, and uncharacterized molecules. Targeted deletion of genes encoding 5 transporters confirmed that each isogenic mutant strain was significantly (P < 0.05) impaired in causing necrotizing myositis in NHPs. Quantitative reverse-transcriptase PCR (qRT-PCR) analysis showed that these 5 genes are expressed in infected NHP and human skeletal muscle. Certain substrate-binding lipoproteins of these transporters, such as Spy0271 and Spy1728, were previously documented to be surface exposed, suggesting that our findings have translational research implications.

New Insights into the Role of Zinc Acquisition and Zinc Tolerance in Group A Streptococcal Infection

Zinc plays an important role in host innate immune function. However, the innate immune system also utilizes zinc starvation ("nutritional immunity") to combat infections. Here, we investigate the role of zinc import and export in the protection of Streptococcus pyogenes (group A Streptococcus; GAS), a Gram-positive bacterial pathogen responsible for a wide spectrum of human diseases, against challenge from host innate immune defense. In order to determine the role of GAS zinc import and export during infection, we utilized zinc import (ΔadcA ΔadcAII) and export (ΔczcD) deletion mutants in competition with the wild type in both in vitro and in vivo virulence models. We demonstrate that nutritional immunity is deployed extracellularly, while zinc toxicity is utilized upon phagocytosis of GAS by neutrophils. We also show that lysosomes and azurophilic granules in neutrophils contain zinc stores for use against intracellular pathogens.

Iron and Zinc Regulate Expression of a Putative ABC Metal Transporter in Corynebacterium diphtheriae

Corynebacterium diphtheriae, a Gram-positive, aerobic bacterium, is the causative agent of diphtheria and cutaneous infections. While mechanisms required for heme iron acquisition are well known in C. diphtheriae, systems involved in the acquisition of other metals such as zinc and manganese remain poorly characterized. In this study, we identified a genetic region that encodes an ABC-type transporter (iutBCD) and that is flanked by two genes (iutA and iutE) encoding putative substrate binding proteins of the cluster 9 family, a related group of transporters associated primarily with the import of Mn and Zn. We showed that IutA and IutE are both membrane proteins with comparable Mn and Zn binding abilities. We demonstrated that the iutABCD genes are cotranscribed and repressed in response to iron by the iron-responsive repressor DtxR. Transcription of iutE was positively regulated in response to iron availability in a DtxR-dependent manner and was repressed in response to Zn by the Zn-dependent repressor Zur. Electrophoretic mobility shift assays showed that DtxR does not bind to the iutE upstream region, which indicates that DtxR regulation of iutE is indirect and that other regulatory factors controlled by DtxR are likely responsible for the iron-responsive regulation. Analysis of the iutE promoter region identified a 50-bp sequence at the 3' end of the iutD gene that is required for the DtxR-dependent and iron-responsive activation of the iutE gene. These findings indicate that transcription of iutE is controlled by a complex mechanism that involves multiple regulatory factors whose activity is impacted by both Zn and Fe.IMPORTANCE Vaccination against diphtheria prevents toxin-related symptoms but does not inhibit bacterial colonization of the human host by the bacterium. Thus, Corynebacterium diphtheriae remains an important human pathogen that poses a significant health risk to unvaccinated individuals. The ability to acquire iron, zinc, and manganese is critical to the pathogenesis of many disease-causing organisms. Here, we describe a gene cluster in C. diphtheriae that encodes a metal importer that is homologous to broadly distributed metal transport systems, some with important roles in virulence in other bacterial pathogens. Two metal binding components of the gene cluster encode surface exposed proteins, and studies of such proteins may guide the development of second-generation vaccines for C. diphtheriae.

Biochemical and Spectroscopic Observation of Mn(II) Sequestration from Bacterial Mn(II) Transport Machinery by Calprotectin

Human calprotectin (CP, S100A8/S100A9 oligomer) is a metal-sequestering host-defense protein that prevents bacterial acquisition of Mn(II). In this work, we investigate Mn(II) competition between CP and two solute-binding proteins that Staphylococcus aureus and Streptococcus pneumoniae, Gram-positive bacterial pathogens of significant clinical concern, use to obtain Mn(II) when infecting a host. Biochemical and electron paramagnetic resonance (EPR) spectroscopic analyses demonstrate that CP outcompetes staphylococcal MntC and streptococcal PsaA for Mn(II). This behavior requires the presence of excess Ca(II) ions, which enhance the Mn(II) affinity of CP. This report presents new spectroscopic evaluation of two Mn(II) proteins important for bacterial pathogenesis, direct observation of Mn(II) sequestration from bacterial Mn(II) acquisition proteins by CP, and molecular insight into the extracellular battle for metal nutrients that occurs during infection.

Biochemical and Functional Evaluation of the Intramolecular Disulfide Bonds in the Zinc-Chelating Antimicrobial Protein Human S100A7 (Psoriasin)

Human S100A7 (psoriasin) is a metal-chelating protein expressed by epithelial cells. It is a 22-kDa homodimer with two EF-hand domains per subunit and two transition-metal-binding His3Asp sites at the dimer interface. Each subunit contains two cysteine residues that can exist as free thiols (S100A7red) or as an intramolecular disulfide bond (S100A7ox). Herein, we examine the disulfide bond redox behavior, the Zn(II) binding properties, and the antibacterial activity of S100A7, as well as the effect of Ca(II) ions on these properties. In agreement with prior work [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044], we show that apo S100A7ox is a substrate for the mammalian thioredoxin system; however, negligible reduction of the disulfide bond is observed for Ca(II)- and Zn(II)-bound S100A7ox. Furthermore, metal binding depresses the midpoint potential of the disulfide bond. S100A7ox and S100A7red each coordinate 2 equiv of Zn(II) with subnanomolar affinity in the absence and presence of Ca(II) ions, and the cysteine thiolates in S100A7red do not form a third high-affinity Zn(II) site. These results refute a prior model implicating the Cys thiolates of S100A7red in high-affinity Zn(II) binding [Hein, K. Z., et al. (2013) Proc. Natl. Acad. Sci. U. S. A. 112, 13039-13044]. S100A7ox and the disulfide-null variants show comparable Zn(II)-depletion profiles; however, only S100A7ox exhibits antibacterial activity against select bacterial species. Metal substitution experiments suggest that the disulfide bonds in S100A7 may enhance metal sequestration by the His3Asp sites and thereby confer growth inhibitory properties to S100A7ox.

Zinc'ing it out: zinc homeostasis mechanisms and their impact on the pathogenesis of human pathogen group A streptococcus

Group A Streptococccus (GAS) is a major human pathogen that causes significant morbidity and mortality. Zinc is an essential trace element required for GAS growth, however, zinc can be toxic at excess concentrations. The bacterial strategies to maintain zinc sufficiency without incurring zinc toxicity play a crucial role in host-GAS interactions and have a significant impact on GAS pathogenesis. The host deploys nutritional immune mechanisms to retard GAS growth by causing either zinc deprivation or zinc poisoning. However, GAS overcomes the zinc-dependent host defenses and survives in the hostile environment by employing complex adaptive strategies. In this review, we describe the different host immune strategies that employ either zinc limitation or zinc toxicity in different host environments to control GAS infection. We also discuss the molecular mechanisms and machineries used by GAS to evade host nutritional defenses and establish successful infection. Emerging evidence suggests that the metal transporters are major GAS virulence factors as they compete against host nutritional immune mechanisms to acquire or expel metals and promote bacterial survival in the host. Thus, identification of GAS molecules and elucidation of the mechanisms by which GAS combats host-mediated alterations in zinc availability may lead to novel interference strategies targeting GAS metal acquisition systems.