Role of transferrin receptor from a Neisseria meningitidis tbpB isotype II strain in human transferrin binding and virulence

Vertebrate and Invertebrate Animal and New In Vitro Models for Studying Neisseria Biology

The history of Neisseria research has involved the use of a wide variety of vertebrate and invertebrate animal models, from insects to humans. In this review, we itemise these models and describe how they have made significant contributions to understanding the pathophysiology of Neisseria infections and to the development and testing of vaccines and antimicrobials. We also look ahead, briefly, to their potential replacement by complex in vitro cellular models.

Point Mutations in TbpA Abrogate Human Transferrin Binding in Neisseria gonorrhoeae

TonB-dependent transporters (TDTs) are essential proteins for metal acquisition, an important step in the growth and pathogenesis of many pathogens, including Neisseria gonorrhoeae, the causative agent of gonorrhea. There is currently no available vaccine for gonorrhea; TDTs are being investigated as vaccine candidates because they are highly conserved and expressed in vivo. Transferrin binding protein A (TbpA) is an essential virulence factor in the initiation of experimental infection in human males and functions by acquiring iron upon binding to host transferrin (human transferrin [hTf]). The loop 3 helix (L3H) is a helix finger that inserts into the hTf C-lobe and is required for hTf binding and subsequent iron acquisition. This study identified and characterized the first TbpA single-point substitutions resulting in significantly decreased hTf binding and iron acquisition, suggesting that the helix structure is more important than charge for hTf binding and utilization. The tbpA D355P ΔtbpB and tbpA A356P ΔtbpB mutants demonstrated significantly reduced hTf binding and impaired iron uptake from Fe-loaded hTf; however, only the tbpA A356P ΔtbpB mutant was able to grow when hTf was the sole source of iron. The expression of tbpB was able to restore function in all tbpA mutants. These results implicate both D355 and A356 in the key binding, extraction, and uptake functions of gonococcal TbpA.

Stealthy microbes: How Neisseria gonorrhoeae hijacks bulwarked iron during infection

Transition metals are essential for metalloprotein function among all domains of life. Humans utilize nutritional immunity to limit bacterial infections, employing metalloproteins such as hemoglobin, transferrin, and lactoferrin across a variety of physiological niches to sequester iron from invading bacteria. Consequently, some bacteria have evolved mechanisms to pirate the sequestered metals and thrive in these metal-restricted environments. Neisseria gonorrhoeae, the causative agent of the sexually transmitted infection gonorrhea, causes devastating disease worldwide and is an example of a bacterium capable of circumventing human nutritional immunity. Via production of specific outer-membrane metallotransporters, N. gonorrhoeae is capable of extracting iron directly from human innate immunity metalloproteins. This review focuses on the function and expression of each metalloprotein at gonococcal infection sites, as well as what is known about how the gonococcus accesses bound iron.

Multiple bacterial virulence factors focused on adherence and biofilm formation associate with outcomes in cirrhosis

BACKGROUND & AIMS

Altered gut microbiota is associated with poor outcomes in cirrhosis, including infections and hepatic encephalopathy (HE). However, the role of bacterial virulence factors (VFs) is unclear. Aim: Define association of VFs with cirrhosis severity and infections, their linkage with outcomes, and impact of fecal microbiota transplant (FMT).

METHODS

VF abundances were determined using metagenomic analysis in stools from controls and cirrhosis patients (compensated, HE-only, ascites-only, both and infected). Patients were followed for 90-day hospitalizations and 1-year death. Stool samples collected before/after a placebo-controlled FMT trial were also analyzed. Bacterial species and VFs for all species and selected pathogens (Escherichia, Klebsiella, Pseudomonas, Staphylococcus, Streptococcus, and Enterococcus spp) were compared between groups. Multi-variable analyses were performed for clinical biomarkers and VFs for outcome prediction. Changes in VFs pre/post-FMT and post-FMT/placebo were analyzed. Results: We included 233 subjects (40 controls, 43 compensated, 30 HE-only, 20 ascites-only, 70 both, and 30 infected). Decompensated patients, especially those with infections, had higher VFs coding for siderophores, biofilms, and adhesion factors versus the rest. Biofilm and adhesion VFs from Enterobacteriaceae and Enterococcus spp associated with death and hospitalizations independent of clinical factors regardless of when all VFs or selected pathogens were analyzed. FMT was associated with reduced VF post-FMT versus pre-FMT and post-placebo groups.

CONCLUSIONS

Virulence factors from multiple species focused on adhesion and biofilms increased with decompensation and infections, associated with death and hospitalizations independent of clinical factors, and were attenuated with FMT. Strategies focused on targeting multiple virulence factors could potentially impact outcomes in cirrhosis.

PRESENTATIONS

Portions of this manuscript were an oral presentation in the virtual International Liver Congress 2021.

ABBREVIATIONS

VF: virulence factors, HE: hepatic encephalopathy, FMT: Fecal microbiota transplant, PPI: proton pump inhibitors, LPS: lipopolysaccharides, VFDB: Virulence factor database, OTU: operational taxonomic units, SBP: spontaneous bacterial peritonitis, UTI: urinary tract infections, MRSA: methicillin resistant Staphylococcus aureus, VRE: vancomycin-resistant Enterococcus, MAAsLin2: Microbiome Multivariable Associations with Linear Models, LPS: lipopolysaccharides, AKI: acute kidney injury.

Iron Acquisition Strategies of Bacterial Pathogens

Iron is an essential micronutrient for both microbes and humans alike. For well over half a century we have known that this element, in particular, plays a pivotal role in health and disease and, most especially, in shaping host-pathogen interactions. Intracellular iron concentrations serve as a critical signal in regulating the expression not only of high-affinity iron acquisition systems in bacteria, but also of toxins and other noted virulence factors produced by some major human pathogens. While we now are aware of many strategies that the host has devised to sequester iron from invading microbes, there are as many if not more sophisticated mechanisms by which successful pathogens overcome nutritional immunity imposed by the host. This review discusses some of the essential components of iron sequestration and scavenging mechanisms of the host, as well as representative Gram-negative and Gram-positive pathogens, and highlights recent advances in the field. Last, we address how the iron acquisition strategies of pathogenic bacteria may be exploited for the development of novel prophylactics or antimicrobials.

Iron acquisition through the bacterial transferrin receptor

Transferrin is one of the sources of iron that is most readily available to colonizing and invading pathogens. In this review, we look at iron uptake by the bacterial transferrin receptor that is found in the families Neisseriaceae, Pasteurellaceae and Moraxellaceae. This bipartite receptor consists of the TonB-dependent transporter, TbpA, and the surface lipoprotein, TbpB. In the past three decades, major advancements have been made in our understanding of the mechanism through which the Tbps take up iron. We summarize these findings and discuss how they relate to the diversity and specificity of the transferrin receptor. We also outline several of the remaining unanswered questions about iron uptake via the bacterial transferrin receptor and suggest directions for future research.

Whole-Genome Sequence Analysis and Genome-Wide Virulence Gene Identification of Riemerella anatipestifer Strain Yb2

Riemerella anatipestifer is a well-described pathogen of waterfowl and other avian species that can cause septicemic and exudative diseases. In this study, we sequenced the complete genome of R. anatipestifer strain Yb2 and analyzed it against the published genomic sequences of R. anatipestifer strains DSM15868, RA-GD, RA-CH-1, and RA-CH-2. The Yb2 genome contains one circular chromosome of 2,184,066 bp with a 35.73% GC content and no plasmid. The genome has 2,021 open reading frames that occupy 90.88% of the genome. A comparative genomic analysis revealed that genome organization is highly conserved among R. anatipestifer strains, except for four inversions of a sequence segment in Yb2. A phylogenetic analysis found that the closest neighbor of Yb2 is RA-GD. Furthermore, we constructed a library of 3,175 mutants by random transposon mutagenesis, and 100 mutants exhibiting more than 100-fold-attenuated virulence were obtained by animal screening experiments. Southern blot analysis and genetic characterization of the mutants led to the identification of 49 virulence genes. Of these, 25 encode cytoplasmic proteins, 6 encode cytoplasmic membrane proteins, 4 encode outer membrane proteins, and the subcellular localization of the remaining 14 gene products is unknown. The functional classification of orthologous-group clusters revealed that 16 genes are associated with metabolism, 6 are associated with cellular processing and signaling, and 4 are associated with information storage and processing. The functions of the other 23 genes are poorly characterized or unknown. This genome-wide study identified genes important to the virulence of R. anatipestifer.

Patterns of structural and sequence variation within isotype lineages of the Neisseria meningitidis transferrin receptor system

Neisseria meningitidis inhabits the human upper respiratory tract and is an important cause of sepsis and meningitis. A surface receptor comprised of transferrin-binding proteins A and B (TbpA and TbpB), is responsible for acquiring iron from host transferrin. Sequence and immunological diversity divides TbpBs into two distinct lineages; isotype I and isotype II. Two representative isotype I and II strains, B16B6 and M982, differ in their dependence on TbpB for in vitro growth on exogenous transferrin. The crystal structure of TbpB and a structural model for TbpA from the representative isotype I N. meningitidis strain B16B6 were obtained. The structures were integrated with a comprehensive analysis of the sequence diversity of these proteins to probe for potential functional differences. A distinct isotype I TbpA was identified that co-varied with TbpB and lacked sequence in the region for the loop 3 α-helix that is proposed to be involved in iron removal from transferrin. The tightly associated isotype I TbpBs had a distinct anchor peptide region, a distinct, smaller linker region between the lobes and lacked the large loops in the isotype II C-lobe. Sequences of the intact TbpB, the TbpB N-lobe, the TbpB C-lobe, and TbpA were subjected to phylogenetic analyses. The phylogenetic clustering of TbpA and the TbpB C-lobe were similar with two main branches comprising the isotype 1 and isotype 2 TbpBs, possibly suggesting an association between TbpA and the TbpB C-lobe. The intact TbpB and TbpB N-lobe had 4 main branches, one consisting of the isotype 1 TbpBs. One isotype 2 TbpB cluster appeared to consist of isotype 1 N-lobe sequences and isotype 2 C-lobe sequences, indicating the swapping of N-lobes and C-lobes. Our findings should inform future studies on the interaction between TbpB and TbpA and the process of iron acquisition.

Role of transition metal exporters in virulence: the example of Neisseria meningitidis

Transition metals such as iron, manganese, and zinc are essential micronutrients for bacteria. However, at high concentration, they can generate non-functional proteins or toxic compounds. Metal metabolism is therefore regulated to prevent shortage or overload, both of which can impair cell survival. In addition, equilibrium among these metals has to be tightly controlled to avoid molecular replacement in the active site of enzymes. Bacteria must actively maintain intracellular metal concentrations to meet physiological needs within the context of the local environment. When intracellular buffering capacity is reached, they rely primarily on membrane-localized exporters to maintain metal homeostasis. Recently, several groups have characterized new export systems and emphasized their importance in the virulence of several pathogens. This article discusses the role of export systems as general virulence determinants. Furthermore, it highlights the contribution of these exporters in pathogens emergence with emphasis on the human nasopharyngeal colonizer Neisseria meningitidis.

Bacterial receptors for host transferrin and lactoferrin: molecular mechanisms and role in host–microbe interactions

Iron homeostasis in the mammalian host limits the availability of iron to invading pathogens and is thought to restrict iron availability for microbes inhabiting mucosal surfaces. The presence of surface receptors for the host iron-binding glycoproteins transferrin (Tf) and lactoferrin (Lf) in globally important Gram-negative bacterial pathogens of humans and food production animals suggests that Tf and Lf are important sources of iron in the upper respiratory or genitourinary tracts, where they exclusively reside. Lf receptors have the additional function of protecting against host cationic antimicrobial peptides, suggesting that the bacteria expressing these receptors reside in a niche where exposure is likely. In this review we compare Tf and Lf receptors with respect to their structural and functional features, their role in colonization and infection, and their distribution among pathogenic and commensal bacteria.

Shared and distinct mechanisms of iron acquisition by bacterial and fungal pathogens of humans

Iron is the most abundant transition metal in the human body and its bioavailability is stringently controlled. In particular, iron is tightly bound to host proteins such as transferrin to maintain homeostasis, to limit potential damage caused by iron toxicity under physiological conditions and to restrict access by pathogens. Therefore, iron acquisition during infection of a human host is a challenge that must be surmounted by every successful pathogenic microorganism. Iron is essential for bacterial and fungal physiological processes such as DNA replication, transcription, metabolism, and energy generation via respiration. Hence, pathogenic bacteria and fungi have developed sophisticated strategies to gain access to iron from host sources. Indeed, siderophore production and transport, iron acquisition from heme and host iron-containing proteins such as hemoglobin and transferrin, and reduction of ferric to ferrous iron with subsequent transport are all strategies found in bacterial and fungal pathogens of humans. This review focuses on a comparison of these strategies between bacterial and fungal pathogens in the context of virulence and the iron limitation that occurs in the human body as a mechanism of innate nutritional defense.

The transferrin-iron import system from pathogenic Neisseria species

Two pathogenic species within the genus Neisseria cause the diseases gonorrhoea and meningitis. While vaccines are available to protect against four N. meningitidis serogroups, there is currently no commercial vaccine to protect against serogroup B or against N. gonorrhoeae. Moreover, the available vaccines have significant limitations and with antibiotic resistance becoming an alarming issue, the search for effective vaccine targets to elicit long-lasting protection against Neisseria species is becoming more urgent. One strategy for vaccine development has targeted the neisserial iron import systems. Without iron, the Neisseriae cannot survive and, therefore, these iron import systems tend to be relatively well conserved and are promising vaccine targets, having the potential to offer broad protection against both gonococcal and meningococcal infections. These efforts have been boosted by recent reports of the crystal structures of the neisserial receptor proteins TbpA and TbpB, each solved in complex with human transferrin, an iron binding protein normally responsible for delivering iron to human cells. Here, we review the recent structural reports and put them into perspective with available functional studies in order to derive the mechanism(s) for how the pathogenic Neisseriae are able to hijack human iron transport systems for their own survival and pathogenesis.

Steric and allosteric factors prevent simultaneous binding of transferrin-binding proteins A and B to transferrin

The ability to acquire iron directly from host Tf (transferrin) is an adaptation common to important bacterial pathogens belonging to the Pasteurellaceae, Moraxellaceae and Neisseriaceae families. A surface receptor comprising an integral outer membrane protein, TbpA (Tf-binding protein A), and a surface-exposed lipoprotein, TbpB (Tf-binding protein B), mediates the iron acquisition process. TbpB is thought to extend from the cell surface for capture of Tf to initiate the process and deliver Tf to TbpA. TbpA functions as a gated channel for the passage of iron into the periplasm. In the present study we have mapped the effect of TbpA from Actinobacillus pleuropneumoniae on pTf (porcine Tf) using H/DX-MS (hydrogen/deuterium exchange coupled to MS) and compare it with a previously determined binding site for TbpB. The proposed TbpA footprint is adjacent to and potentially overlapping the TbpB-binding site, and induces a structural instability in the TbpB site. This suggests that simultaneous binding to pTf by both receptors would be hindered. We demonstrate that a recombinant TbpB lacking a portion of its anchor peptide is unable to form a stable ternary TbpA–pTf–TbpB complex. This truncated TbpB does not bind to a preformed Tf–TbpA complex, and TbpA removes pTf from a preformed Tf–TbpB complex. Thus the results of the present study support a model whereby TbpB ‘hands-off’ pTf to TbpA, which completes the iron removal and transport process.

A novel metal transporter mediating manganese export (MntX) regulates the Mn to Fe intracellular ratio and Neisseria meningitidis virulence

Neisseria meningitidis (Nm) and N. gonorrhoeae (Ng) are adapted to different environments within their human host. If the basis of this difference has not yet been fully understood, previous studies (including our own data) have reported that, unlike Ng, Nm tolerates high manganese concentrations. As transition metals are essential regulators of cell growth and host pathogen interactions, we aimed to address mechanisms of Nm Mn²⁺ tolerance and its pathogenic consequences. Using bioinformatics, gene deletion and heterologous expression we identified a conserved bacterial manganese resistance factor MntX (formerly YebN). The predicted structure suggests that MntX represents a new family of transporters exporting Mn. In the Neisseria genus, this exporter is present and functional in all Nm isolates but it is mutated in a majority of Ng strains and commonly absent in nonpathogenic species. In Nm, Mn²⁺ export via MntX regulates the intracellular Mn/Fe ratio and protects against manganese toxicity that is exacerbated in low iron conditions. MntX is also important for N. meningitidis to resist killing by human serum and for survival in mice blood during septicemia. The present work thus points to new clues about Mn homeostasis, its interplay with Fe metabolism and the influence on N. meningitidis physiology and pathogenicity.

Neisseria meningitidis is an obligate resident of the human nasopharynx but can also be responsible for septicemia and meningitis. During our efforts to understand the specific selective pressure underwent by N. meningitidis to survive in its human niche, we have brought to light a new family of bacterial manganese-exporters (MntX) strongly conserved in N. meningitidis but often inactivated or absent in other Neisseria species. As iron, manganese is an essential metallo-nutrient for bacteria. Thus, the N. meningitidis need for a manganese-exporter seemed rather surprising. In fact, we were able to show that MntX is an important player in the regulation of the manganese/iron equilibrium and that this regulation via MntX is critical to survive in presence of manganese in particular when iron is rare. It is expected that excessive iron replacement by manganese into the active site of enzymes would handicap bacteria. Accordingly, MntX is required for full virulence of N. meningitidis in a mice model of septicemia or to resist killing by human serum. More generally, this equilibrium may be tightly regulated in other respiratory tract pathogens such as S. pneumoniae and therefore, interferences with this balance may be a promising strategy for infectious diseases therapy.

NafA negatively controls Neisseria meningitidis piliation

Bacterial auto-aggregation is a critical step during adhesion of N. meningitidis to host cells. The precise mechanisms and functions of bacterial auto-aggregation still remain to be fully elucidated. In this work, we characterize the role of a meningococcal hypothetical protein, NMB0995/NMC0982, and show that this protein, here denoted NafA, acts as an anti-aggregation factor. NafA was confirmed to be surface exposed and was found to be induced at a late stage of bacterial adherence to epithelial cells. A NafA deficient mutant was hyperpiliated and formed bundles of pili. Further, the mutant displayed increased adherence to epithelial cells when compared to the wild-type strain. In the absence of host cells, the NafA deficient mutant was more aggregative than the wild-type strain. The in vivo role of NafA in sepsis was studied in a murine model of meningococcal disease. Challenge with the NafA deficient mutant resulted in lower bacteremia levels and mortality when compared to the wild-type strain. The present study reveals that meningococcal NafA is an anti-aggregation factor with strong impact on the disease outcome. These data also suggest that appropriate bacterial auto-aggregation is controlled by both aggregation and anti-aggregation factors during Neisseria infection in vivo.

Conserved interaction between transferrin and transferrin-binding proteins from porcine pathogens

Gram-negative porcine pathogens from the Pasteurellaceae family possess a surface receptor complex capable of acquiring iron from porcine transferrin (pTf). This receptor consists of transferrin-binding protein A (TbpA), a transmembrane iron transporter, and TbpB, a surface-exposed lipoprotein. Questions remain as to how the receptor complex engages pTf in such a way that iron is positioned for release, and whether divergent strains present distinct recognition sites on Tf. In this study, the TbpB-pTf interface was mapped using a combination of mass shift analysis and molecular docking simulations, localizing binding uniquely to the pTf C lobe for multiple divergent strains of Actinobacillus plueropneumoniae and suis. The interface was further characterized and validated with site-directed mutagenesis. Although targeting a common lobe, variants differ in preference for the two sublobes comprising the iron coordination site. Sublobes C1 and C2 participate in high affinity binding, but sublobe C1 contributes in a minor fashion to the overall affinity. Further, the TbpB-pTf complex does not release iron independent of other mediators, based on competitive iron binding studies. Together, our findings support a model whereby TbpB efficiently captures and presents iron-loaded pTf to other elements of the uptake pathway, even under low iron conditions.

Distribution of transferrin binding protein B gene (tbpB) variants among Neisseria species

BACKGROUND

Transferrin binding protein B (tbpB), an outer membrane lipoprotein, is required for the acquisition of iron from human transferrin. Two tbpB families have been documented in Neisseria meningitidis: an isotype I tbpB gene of 1.8 kb and an isotype II tbpB gene of 2.1 kb, the former expressed by meningococci in the disease-associated ST-11 clonal complex and the latter found among meningococci belonging to the hyper-invasive clonal complexes including ST-8, ST-18, ST-32, ST-41/44 as well as N. gonorrhoeae isolates. The origin of the isotype I tbpB gene is unknown, however several features in common with non-pathogenic Neisseria and the ST-11 clonal complex N. meningitidis isolate FAM18 have been documented leading to the hypothesis that the isotype I tbpB gene may also be shared between non-pathogenic Neisseria and ST-11 meningococci. As a result, the diversity of the tbpB gene was investigated in a defined collection of Neisseria species.

RESULTS

Two families of isotype I tbpB were identified: family A containing conserved genes belonging to ST-11 meningococci, N. polysaccharea and N. lactamica isolates and family B including more diverse isotype I tbpB genes from N. sicca, N. mucosa, N. flava, N. subflava as well as N. cinerea, N. flavescens and N. polysaccharea isolates. Three isotype II tbpB families were identified with: family C containing diverse tbpB genes belonging to N. polysaccharea, N. lactamica, N. gonorrhoeae and N. meningitidis isolates, family D including another subset of isotype II tbpB genes from N. lactamica isolates and family E solely composed of N. gonorrhoeae tbpB genes.

CONCLUSION

This study reveals another instance of similarity between meningococci of the ST-11 clonal complex and non-pathogenic Neisseria with the origin of the isotype I tbpB gene resulting from a horizontal genetic transfer event occurring between these two populations.

The role of the synergistic phosphate anion in iron transport by the periplasmic iron-binding protein from Haemophilus influenzae

The acquisition of iron from transferrin by Gram-negative bacterial pathogens is dependent on a periplasmic ferric-ion-binding protein, FbpA. FbpA shuttles iron from the outer membrane to an inner membrane transport complex. A bound phosphate anion completes the iron co-ordination shell of FbpA and kinetic studies demonstrate that the anion plays a critical role in iron binding and release in vitro. The present study was initiated to directly address the hypothesis that the synergistic anion is required for transport of iron in intact cells. A series of site-directed mutants in the anion-binding amino acids of the Haemophilus influenzae FbpA (Gln-58, Asn-175 and Asn-193) were prepared to provide proteins defective in binding of the phosphate anion. Crystal structures of various mutants have revealed that alteration of the C-terminal domain ligands (Asn-175 or Asn-193) but not the N-terminal domain ligand (Gln-58) abrogated binding of the phosphate anion. The mutant proteins were introduced into H. influenzae to evaluate their ability to mediate iron transport. All of the single site-directed mutants (Q58L, N175L and N193L) were capable of mediating iron acquisition from transferrin and from limiting concentrations of ferric citrate. The results suggest that the transport of iron by FbpA is not dependent on binding of phosphate in the synergistic anion-binding site.

Determination of human transferrin concentrations in mouse models of neisserial infection

Transferrin constitutes the major protein involved in the transport of iron from the sites of absorption to the sites of storage and utilization. Despite the high affinity of transferrin for iron, most bacterial pathogens, such as the human restricted Neisseria meningitidis, have developed iron acquisition mechanisms. Several animal models of bacterial infection that include the exogenous supply of human transferrin have been implemented, and tests using transgenic mouse models are underway. Here we describe an ELISA sandwich procedure based on two monoclonal antibodies with negligible cross-reactivity to murine transferrin, to estimate human transferrin concentrations in mouse sera. The assay can detect as little as 10 ng/ml of human transferrin with coefficients of variation ranging from 1.6% to 4.4% (intra-assay) and 3.8% to 5% (inter-assay). The recovery values range from 90% to 110% in the assay working range (25-400 ng/ml). Human transferrin concentrations estimated in sera from 41 human transferrin transgenic mice ranged from 2 to 14 microg/ml. Further estimations of human transferrin levels in mouse sera of a previously described mouse model of N. meningitidis were also carried out. The intraperitoneal injection of 8 mg of human transferrin achieved a sustained value of human transferrin in mouse sera in the range of 1-2mg/ml over the first 24h, indicating that bacteria reaching the blood stream during this time would be exposed to levels of hTf found in normal human serum.

Iron acquisition from transferrin by Candida albicans depends on the reductive pathway

Host-pathogen interactions that alter virulence are influenced by critical nutrients such as iron. In humans, free iron is unavailable, being present only in high-affinity iron binding proteins such as transferrin. The fungal pathogen Candida albicans grows as a saprophyte on mucosal surfaces. Occasionally it invades systemically, and in this circumstance it will encounter transferrin iron. Here we report that C. albicans is able to acquire iron from transferrin. Iron-loaded transferrin restored growth to cultures arrested by iron deprivation, whereas apotransferrin was unable to promote growth. By using congenic strains, we have been able to show that iron uptake by C. albicans from transferrin was mediated by the reductive pathway (via FTR1). The genetically separate siderophore and heme uptake systems were not involved. FRE10 was required for a surface reductase activity and for efficient transferrin iron uptake activity in unbuffered medium. Other reductase genes were apparently up-regulated in medium buffered at pH 6.3 to 6.4, and the fre10(-/-) mutant had no effect under these conditions. Experiments in which transferrin was sequestered in a dialysis bag demonstrated that cell contact with the substrate was required for iron reduction and release. The requirement of FTR1 for virulence in a systemic infection model and its role in transferrin iron uptake raise the possibility that transferrin is a source of iron during systemic C. albicans infections.