Specific inhibition of Escherichia coli ferrienterochelin uptake by a normal human serum immunoglobulin

Evaluation of the Immunogenic Response of a Novel Enterobactin Conjugate Vaccine in Chickens for the Production of Enterobactin-Specific Egg Yolk Antibodies

Passive immunization with specific egg yolk antibodies (immunoglobulin Y, IgY) is emerging as a promising alternative to antibiotics to control bacterial infections. Recently, we developed a novel conjugate vaccine that could trigger a strong immune response in rabbits directed against enterobactin (Ent), a highly conserved siderophore molecule utilized by different Gram-negative pathogens. However, induction of Ent-specific antibodies appeared to be affected by the choice of animal host and vaccination regimen. It is still unknown if the Ent conjugate vaccine can trigger a specific immune response in layers for the purpose of production of anti-Ent egg yolk IgY. In this study, three chicken vaccination trials with different regimens were performed to determine conditions for efficient production of anti-Ent egg yolk IgY. Purified Ent was conjugated to three carrier proteins, keyhole limpet hemocyanin (KLH), bovine serum albumin (BSA) and CmeC (a subunit vaccine candidate), respectively. Intramuscular immunization of Barred Rock layers with KLH-Ent conjugate four times induced strong immune response against whole conjugate vaccine but the titer of Ent-specific IgY did not change in yolk with only a 4 fold increase detected in serum. In the second trial, three different Ent conjugate vaccines were evaluated in Rhode Island Red pullets with four subcutaneous injections. The KLH-Ent or CmeC-Ent conjugate consistently induced high level of Ent-specific IgY in both serum (up to 2,048 fold) and yolk (up to 1,024 fold) in each individual chicken. However, the Ent-specific immune response was only temporarily and moderately induced using a BSA-Ent vaccination. In the third trial, ten White Leghorn layers were subcutaneously immunized three times with KLH-Ent, leading to consistent and strong immune response against both whole conjugate and the Ent molecule in each chicken; the mean titer of Ent-specific IgY increased approximately 32 and 256 fold in serum and yolk, respectively. Consistent with its potent binding to various Ent derivatives, the Ent-specific egg yolk IgY also inhibited in vitro growth of a representative Escherichia coli strain. Together, this study demonstrated that the novel Ent conjugate vaccine could induce strong, specific, and robust immune response in chickens. The Ent-specific hyperimmune egg yolk IgY has potential for passive immune intervention against Gram-negative infections.

Subversion of nutritional immunity by the pathogenic Neisseriae

The pathogenic Neisseria species, including Neisseria meningitidis and Neisseria gonorrhoeae, are obligate human pathogens that cause significant morbidity and mortality. The success of these pathogens, with regard to causing disease in humans, is inextricably linked to their ability to acquire necessary nutrients in the hostile environment of the host. Humans deploy a significant arsenal of weaponry to defend against bacterial pathogens, not least of which are the metal-sequestering proteins that entrap and withhold transition metals, including iron, zinc and manganese, from invaders. This review will discuss the general strategies that bacteria employ to overcome these metal-sequestering attempts by the host, and then will focus on the relatively uncommon 'metal piracy' approaches utilized by the pathogenic Neisseria for this purpose. Because acquiring metals from the environment is critical to microbial survival, interfering with this process could impede growth and therefore disease initiation or progression. This review will also discuss how interfering with metal uptake by the pathogenic Neisseriae could be deployed in the development of novel or improved preventative or therapeutic measures against these important pathogens.

Regulation of iron transport systems in Enterobacteriaceae in response to oxygen and iron availability

Iron is an essential nutrient for most bacteria. Depending on the oxygen available in the surrounding environment, iron is found in two distinct forms: ferrous (Fe(II)) or ferric (Fe(III)). Bacteria utilize different transport systems for the uptake of the two different forms of iron. In oxic growth conditions, iron is found in its insoluble, ferric form, and in anoxic growth conditions iron is found in its soluble, ferrous form. Enterobacteriaceae have adapted to transporting the two forms of iron by utilizing the global, oxygen-sensing regulators, ArcA and Fnr to regulate iron transport genes in response to oxygen.

Exploring Staphylococcus aureus pathways to disease for vaccine development

Staphylococcus aureus is a commensal of the human skin or nares and a pathogen that frequently causes skin and soft tissue infections as well as bacteremia and sepsis. Recent efforts in understanding the molecular mechanisms of pathogenesis revealed key virulence strategies of S. aureus in host tissues: bacterial scavenging of iron, induction of coagulation pathways to promote staphylococcal agglutination in the vasculature, and suppression of innate and adaptive immune responses. Advances in all three areas have been explored for opportunities in vaccine design in an effort to identify the critical protective antigens of S. aureus. Human clinical trials with specific subunit vaccines have failed, yet provide important insights for the design of future trials that must address the current epidemic of S. aureus infections with drug-resistant isolates (MRSA, methicillin-resistant S. aureus).

Siderophore-based iron acquisition and pathogen control

High-affinity iron acquisition is mediated by siderophore-dependent pathways in the majority of pathogenic and nonpathogenic bacteria and fungi. Considerable progress has been made in characterizing and understanding mechanisms of siderophore synthesis, secretion, iron scavenging, and siderophore-delivered iron uptake and its release. The regulation of siderophore pathways reveals multilayer networks at the transcriptional and posttranscriptional levels. Due to the key role of many siderophores during virulence, coevolution led to sophisticated strategies of siderophore neutralization by mammals and (re)utilization by bacterial pathogens. Surprisingly, hosts also developed essential siderophore-based iron delivery and cell conversion pathways, which are of interest for diagnostic and therapeutic studies. In the last decades, natural and synthetic compounds have gained attention as potential therapeutics for iron-dependent treatment of infections and further diseases. Promising results for pathogen inhibition were obtained with various siderophore-antibiotic conjugates acting as "Trojan horse" toxins and siderophore pathway inhibitors. In this article, general aspects of siderophore-mediated iron acquisition, recent findings regarding iron-related pathogen-host interactions, and current strategies for iron-dependent pathogen control will be reviewed. Further concepts including the inhibition of novel siderophore pathway targets are discussed.

Yersiniabactin and other siderophores produced by clinical isolates of Enterobacter spp. and Citrobacter spp

We analyzed the ability of extraintestinal strains of Enterobacter spp. and Citrobacter spp. to employ different siderophore-mediated strategies of iron acquisition. All strains produced iron-chelating compounds. Cross-feeding assays indicated that most isolates of both Enterobacter spp. and Citrobacter spp. excreted catecholate siderophore enterobactin, less produced aerobactin, and single strains excreted hydroxamates different from aerobactin. Besides, we analyzed if the strains had the ability to produce the siderophore yersiniabactin coded by the Yersinia high-pathogenicity island (HPI). The presence of HPI genes was observed in single isolates of three species: E. cloaceae, E. aerogenes and C. koseri. A detailed polymerase chain reaction analysis revealed differences in the genetic organization of the HPIs; however, in a cross-feeding test we proved that yersiniabactin was produced and the island was functional.

Occurrence of the Yersinia high-pathogenicity island and iron uptake systems in clinical isolates of Klebsiella pneumoniae

The ability to acquire iron is crucial to bacteria during an infection. Thirty-four strains of Klebsiella pneumoniae isolated from clinical specimens were examined for the use of various strategies to obtain iron. The isolates employed several iron uptake mechanisms, including production of enterobactin (100%) and aerobactin (50%). Few isolates (18%) produced yersiniabactin, a siderophore encoded by the Yersinia high-pathogenicity island (HPI) despite genetic diversity of the HPI. Majority of the isolates used human transferrin (74%), lactoferrin (97%), hemoglobin (74%), and hemoglobin-haptoglobin complex (56%) as a sole source of iron. Multiple iron uptake systems may be of benefit to the bacteria during infection.

The Yersinia high-pathogenicity island and iron-uptake systems in clinical isolates of Escherichia coli

The ability to acquire iron is crucial for bacteria during an infection. The capacity of 35 strains of Escherichia coli, isolated from clinical specimens, to use various strategies to obtain iron was analysed. The isolates employed several iron-uptake mechanisms, including production of enterobactin (86 %) and aerobactin (71 %). The majority of the isolates also excreted yersiniabactin, which is encoded by the Yersinia high-pathogenicity island (HPI). However, PCR analysis of the Yersinia HPI revealed diversity in its genetic organization. Use of human transferrin (91 %), lactoferrin (94 %), haemoglobin (80 %) and haemoglobin-haptoglobin complex (63 %) as the sole source of iron was common among E. coli isolates. Multiple iron-uptake systems may be of benefit to bacteria during an infection.

Adaptation and multiplication of bacteria in host tissues

The multiplication of bacteria in the largely undefined and changing environment of host tissues is an essential feature of any infection. Bacterial behaviour is determined both by genetic structures and also by the environment. Little is known about the effect that host factors may have on invading bacteria nor about the way in which alterations in bacterial properties aid proliferationin vivo. Recently our understanding of one feature of this environm ent and of the way in which pathogenic bacteria adapt to it has increased considerably. We now know that the amount of iron that might be readily available to bacteria in body fluids is extremely small. This iron-restricted environment induces phenotypic changes both in the metabolism and in the composition of the outer membrane of bacteria growingin vivo. These and other host-induced changes are now providing a fresh insight into the capability of bacteria to multiplyin vivoduring infection.

Iron uptake mechanisms of pathogenic bacteria

Most of the iron in a mammalian body is complexed with various proteins. Moreover, in response to infection, iron availability is reduced in both extracellular and intracellular compartments. Bacteria need iron for growth and successful bacterial pathogens have therefore evolved to compete successfully for iron in the highly iron-stressed environment of the host's tissues and body fluids. Several strategies have been identified among pathogenic bacteria, including reduction of ferric to ferrous iron, occupation of intracellular niches, utilisation of host iron compounds, and production of siderophores. While direct evidence that high affinity mechanisms for iron acquisition function as bacterial virulence determinants has been provided in only a small number of cases, it is likely that many if not all such systems play a central role in the pathogenesis of infection.

Activity and specificity of a mouse monoclonal antibody to ferric aerobactin

We isolated a monoclonal antibody directed against the ferric complex of aerobactin purified from Escherichia coli KH576. This antibody, which we designated MAb AERO1, was identified as an immunoglobulin G, subtype 2. A competitive enzyme-linked immunosorbent assay with MAb AERO1 had a limit of 10 nM for the detection of purified ferric aerobactin and allowed detection of the crude aerobactin produced by various members of the family Enterobacteriaceae isolated from cancer patients with bacteremia. The only two other structurally related siderophores recognized by MAb AERO1 were ferric arthrobactin and ferrioxamine B. These results suggest that the epitope recognized by MAb AERO1 was the lysyl moiety of ferric aerobactin. We also showed that MAb AERO1 reduced the growth of an aerobactin-producing strain of E. coli in newborn calf serum, which indicates that it might be effective in reducing the severity of infections caused by bacteria for which the production of aerobactin is an important virulence factor.

Relative availability of transferrin-bound iron and cell-derived iron to aerobactin-producing and enterochelin-producing strains of Escherichia coli and to other microorganisms

A method is described for determination of the relative availability of transferrin-bound iron and cell-derived iron to microbial iron-scavenging mechanisms. This involved incubation of parallel cultures of microorganisms in dialysis tubes placed in RPMI 1640 tissue culture medium containing 30%-iron-saturated transferrin and K562 erythroleukemia cells. In one culture the transferrin was labelled with 59Fe and in the other the cells were labelled, and the relative uptake of radioiron by the microorganisms determined. The results showed that Staphylococcus epidermidis and Staphylococcus aureus acquired iron predominantly from cells, while Candida albicans and the enteropathogenic Escherichia coli NCTC 8623 tended to acquire iron from transferrin. E. coli K-12 strains W3110 and LG1705, which (like NCTC 8623) produce the siderophore enterochelin but not aerobactin, acquired predominantly transferrin-bound iron, whereas the related E. coli strains LG1315 and LG1628, which produce aerobactin but not enterochelin, showed a preference for cell-derived iron. When the cells were incubated in the presence of 59Fe-labelled transferrin and 55Fe-labelled ferritin, no difference in relative availability of iron to E. coli was observed, suggesting that differences in the ability of aerobactin and enterochelin to remove iron from intracellular ferritin were not responsible for this preference. These results may help to explain why production of aerobactin, despite its relatively low affinity for iron, is more closely associated with invasiveness in E. coli than is enterochelin production. Reduced availability of cell-bound iron during inflammation may contribute to antimicrobial defenses.

Immunosuppression of the host and delivery of iron to the pathogen: a possible dual role of siderophores in the pathogenesis of microbial infections?

Iron is an essential growth factor for procaryotes as well as for eucaryotes. Microorganisms have developed specific iron-uptake systems by producing low-molecular weight iron-chelating compounds (siderophores). We have examined the effect of the siderophores desferrioxamine (DFO), desferrichrome (DFC), desferriaerobactin (DFAB) and desferrienterobactin (DFEA) on the mitogen-stimulated activation and proliferation of mouse T cells. The hydroxamate siderophores DFO, DFC and DFAB cause an immunosuppressive effect on T cells which is related to the iron complexation constant of the siderophores and can be reversed by equimolar ferric iron. In contrast, the catecholate siderophore DFEB and its ferrated derivative turned out to be cytotoxic for T cells. These results suggest a dual role of siderophores in the infectious process, i.e., growth enhancement of the invading pathogen and inhibition of the host immune defense.

Aerobactin production as a virulence factor: a reevaluation

Iron starvation is one of the major barriers that virulent bacteria must overcome in order to proliferate in the host. Virtually all microorganisms possess high affinity iron (III) transport systems mediated by low molecular weight iron specific chelators called siderophores, the synthesis of which is activated under iron-limiting conditions. Siderophore aerobactin is frequently produced by enterobacteria which cause various types of infections in humans and animals. The status of aerobactin production as a virulence factor is evaluated both from data derived from experimental infection systems and the actual presence of this siderophore in clinical isolates. Aerobactin appears to be an important contributor to extracellular pathogenesis (mostly, that of Escherichia coli strains causing septicaemia and urinary tract infections) and to the extracellular stages of growth of intracellular pathogens like Shigella. When invasive bacteria actually enter target cells, acquisition of iron seems to occur independently of siderophore production. The feasibility of an antimicrobial therapy aimed at interfering with siderophore functioning is discussed.

Iron and virulence in the family Enterobacteriaceae

The ability of bacterial pathogens to acquire iron in the host is an essential component of the disease process. Pathogenic Enterobacteriaceae spp. may either scavenge host iron sources such as heme or induce high-affinity iron-transport systems to remove iron from host proteins. The ease with which iron is acquired from the host will be at least partially determined by the iron status of the host at the time of infection. In response to infection, mammalian hosts reduce serum iron levels and withhold iron from the invading microorganisms. Thus the competition for iron is an active process which influences the outcome of a host-bacterial interaction.

Aerobactin genes in clinical isolates of Escherichia coli

The location of the aerobactin gene complex on either the chromosome or plasmid was determined in eight aerobactin-positive clinical isolates of Escherichia coli by Southern hybridization analysis, using as probes the cloned aerobactin genes from the ColV-K30 plasmid. The aerobactin genes were in two cases detected on large plasmids, whereas in the other strains the aerobactin genes are most likely located on the chromosome. Restriction mapping revealed only slight variations in the structural genes and an at least 3.4-kilobase-long upstream region conserved in all three plasmid-coded systems. A 7.7-kilobase HindIII fragment upstream and adjacent to the 16.3-kilobase HindIII fragment carrying the complete aerobactin system was cloned from the ColV-K30 plasmid. Fine-structure restriction mapping identified the left insertion sequence in the upstream region as IS1, in inverted orientation to the IS1 element downstream from the aerobactin operon. The upstream and downstream sequences of IS1 appear to have perfect homology, as indicated by S1 nuclease resistance of a 760-base-pair DNA duplex formed by both IS1 elements.

[Bioassay for the detection of hydroxamate-iron-chelators (Aerobactin)]

Different Salmonella strains were tested for aerobactin production in a hydroxamate-bioassay with the aerobactin indicator strain E. coli LG 1522. The majority of hospital strains of Salmonella typhimurium produce hydroxamate siderophore. On the other hand S. typhimurium strains belonging to phage type n. c. 1/72/n. c., biochemical type b from human and animal sources, were unable to produce this siderophore. Serotypes other than S. typhimurium for example the multiresistent S. wien hospital strains, which were isolated in western europe and in the GDR, can excreate hydroxamate siderophore. Plasmids pIE 528 and pIE 5 234 isolated from Salmonella hospital strains produce hydroxamate siderophore in the enterobactin negative Salmonella typhimurium-strain enb-7. Thus, the hydroxamate bioassay may be a useful supplementary test for epidemiological strain characterization.

Effect of iron on antibacterial immunity in vaccinated mice

The effect of iron on resistance to Salmonella typhimurium was investigated in mice inoculated with vaccines prepared from live and avirulent (SL3770) or killed and virulent (SR11 or LT2) bacteria. It has been found that mice vaccinated with SL3770 vaccine develop an immunity which can be neutralized with iron. Iron promoted the development of lethal infections by serving as a growth-essential nutrilite for infecting bacteria and by neutralizing the acquired immunity. The titration of this dual effect of iron showed that more iron was needed to neutralize the immunity in vaccinated animals than to promote bacterial growth in normal animals. In the presence of a sufficient amount of exogenous iron, as few as 10 bacteria caused lethal infections in normal and immune mice with the same effectiveness. This iron-sensitive immunity could be changed to iron-resistant immunity by the immunological stimulation of SL3770-vaccinated mice with a sonicated vaccine prepared from heat-killed SR11 or LT2 bacteria. In distinction to iron-sensitive immunity, iron-resistant immunity could be transferred from SR11- or LT2-stimulated to normal mice with serum. Although effective in the transfer of antibacterial immunity, sera of SR11- or LT2-stimulated mice supported the growth of virulent bacteria as well as did sera of normal mice. The absorption of immune serum with either SR11 or LT2 bacteria removed its protective quality, but the sensitized bacteria remained as infectious as untreated bacteria for iron-treated normal mice. Only in SL3770-vaccinated mice were the immune serum-sensitized bacteria not able to cause the infection in spite of daily treatment with iron. These results suggest that iron-resistant immunity is due to the synergistic action of specific antibody and phagocytes of immunologically stimulated animals.

Vibrio cholerae expresses iron-regulated outer membrane proteins in vivo

A comparison was made, using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, of the outer membrane proteins of four strains of Vibrio cholerae grown in vivo in infant rabbits and in vitro in low-iron and iron-supplemented defined media. In vivo-grown V. cholerae expressed novel outer membrane-associated proteins which, in part, were similar to those observed on V. cholerae grown in vitro under conditions of iron deprivation.

Relative contribution of ColV plasmid and K1 antigen to the pathogenicity of Escherichia coli

To study the relevance of the ColV plasmid and the capsular K1 antigen in the pathogenicity of Escherichia coli, isogenic strains that differ only in these characteristics were constructed. Studies with these variants demonstrated that the presence of the ColV plasmid increased the serum resistance of E. coli. This increase did not depend on the expression of the K1 antigen. This work also demonstrated that the presence of the K1 antigen protects E. coli from the bactericidal activity of serum. Studies using mouse peritoneal macrophages in the presence of normal serum indicated that the presence of K1 antigen protects E. coli from phagocytosis. Similar experiments with the K1(+) strains performed in the presence of anti-K1 antibodies demonstrated that these antibodies opsonized these bacteria very efficiently in the absence of complement. The K1(-)E. coli variants were efficiently phagocytized in the presence of normal human serum and absorbed human serum, indicating that they are able to be opsonized by complement deposited by activation of the alternative pathway of complement. Work using fluorescence microscopy confirmed that the K1(-) strains are able to fix complement in the absence of antibody. It was also found that the presence of the ColV plasmid may interfere with phagocytosis of the E. coli K1 strains and deposition of complement on these cells. To test the relevance of the results of the in vitro experiments for disease, the pathogenicity of the strains was tested in mice. The results showed that the K1 antigen is the main determinant of pathogenicity of these strains and that the presence of ColV can modify the pathogenic potential of the E. coli K1 strains through a mechanism that does not depend on the production of colicin V.

Synergism between iron chelators and complement for bactericidal activity

Iron-binding agents such as the plasma protein transferrin or the siderophore desferal from Streptomyces pilosus inhibit the growth of pathogenic bacteria, supposedly by interfering with iron uptake by those bacteria. This study shows that anti-Escherichia coli activity exerted by desferal and transferrin can be increased in a synergistic way by complement and anti-E. coli antibodies of normal serum.