In vitro and in silico studies of enterobactin-inspired Ciprofloxacin and Fosfomycin first generation conjugates on the antibiotic resistant E. coli OQ866153

BACKGROUND

The emergence of antimicrobial resistance in bacterial pathogens is a growing concern worldwide due to its impact on the treatment of bacterial infections. The "Trojan Horse" strategy has been proposed as a potential solution to overcome drug resistance caused by permeability issues.

OBJECTIVE

The objective of our research was to investigate the bactericidal activity and mechanism of action of the "Trojan Horse" strategy using enterobactin conjugated with Ciprofloxacin and Fosfomycin against the antibiotic-resistant Escherichia coli strain OQ866153.

METHODOLOGY

Enterobactin, a mixed ligand of E. coli OQ866153, was conjugated with Ciprofloxacin and Fosfomycin individually to aid active absorption via specific enterobactin binding proteins (FepABCDG). The effectiveness of the conjugates was assessed by measuring their bactericidal activity against E. coli OQ866153, as well as their ability to inhibit DNA gyrase enzyme and biofilm formation.

RESULTS

The Fe+3-enterobactin-Ciprofloxacin conjugate effectively inhibited the DNA gyrase enzyme (Docking score = -8.597 kcal/mol) and resulted in a lower concentration (25 μg/ml) required to eliminate supercoiled DNA plasmids compared to the parent drug (35 μg/ml; Docking score = -6.264 kcal/mol). The Fe+3-Enterobactin-Fosfomycin conjugate showed a higher inhibition percentage (100%) of biofilm formation compared to Fosfomycin (21.58%) at a concentration of 2 mg/ml, with docking scores of -5.481 and -3.756 kcal/mol against UDP-N acetylglucosamine 1-carboxyvinyltransferase MurA.

CONCLUSION

The findings of this study suggest that the "Trojan Horse" strategy using enterobactin conjugated with Ciprofloxacin and Fosfomycin can effectively overcome permeability issues caused by efflux proteins and enhance the bactericidal activity of these drugs against antibiotic-resistant strains of E. coli.

Conjugation to Native and Nonnative Triscatecholate Siderophores Enhances Delivery and Antibacterial Activity of a β-Lactam to Gram-Negative Bacterial Pathogens

Developing new antibiotics and delivery strategies is of critical importance for treating infections caused by Gram-negative bacterial pathogens. Hijacking bacterial iron uptake machinery, such as that of the siderophore enterobactin (Ent), represents one promising approach toward these goals. Here, we report a novel Ent-inspired siderophore-antibiotic conjugate (SAC) employing an alternative siderophore moiety as the delivery vector and demonstrate the potency of our SACs harboring the β-lactam antibiotic ampicillin (Amp) against multiple pathogenic Gram-negative bacterial strains. We establish the ability of N,N',N''-(nitrilotris(ethane-2,1-diyl))tris(2,3-dihydroxybenzamide) (TRENCAM, hereafter TC), a synthetic mimic of Ent, to facilitate drug delivery across the outer membrane (OM) of Gram-negative pathogens. Conjugation of Amp to a new monofunctionalized TC scaffold affords TC-Amp, which displays markedly enhanced antibacterial activity against the gastrointestinal pathogen Salmonella enterica serovar Typhimurium (STm) compared with unmodified Amp. Bacterial uptake, antibiotic susceptibility, and microscopy studies with STm show that the TC moiety facilitates TC-Amp uptake by the OM receptors FepA and IroN and that the Amp warhead inhibits penicillin-binding proteins. Moreover, TC-Amp achieves targeted activity, selectively killing STm in the presence of a commensal lactobacillus. Remarkably, we uncover that TC-Amp and its Ent-based predecessor Ent-Amp achieve enhanced antibacterial activity against diverse Gram-negative ESKAPE pathogens that express Ent uptake machinery, including strains that possess intrinsic β-lactam resistance. TC-Amp and Ent-Amp exhibit potency comparable to that of the FDA-approved SAC cefiderocol against Gram-negative pathogens. These results demonstrate the effective application of native and appropriately designed nonnative siderophores as vectors for drug delivery across the OM of multiple Gram-negative bacterial pathogens.

Enterobactin, an iron chelating bacterial siderophore, arrests cancer cell proliferation

Iron is essential for many biological functions, including being a cofactor for enzymes involved in cell proliferation. In line, it has been shown that cancer cells can perturb their iron metabolism towards retaining an abundant iron supply for growth and survival. Accordingly, it has been suggested that iron deprivation through the use of iron chelators could attenuate cancer progression. While they have exhibited anti-tumor properties in vitro, the current therapeutic iron chelators are inadequate due to their low efficacy. Therefore, we investigated whether the bacterial catecholate-type siderophore, enterobactin (Ent), could be used as a potent anti-cancer agent given its strong iron chelation property. We demonstrated that iron-free Ent can exert cytotoxic effects specifically towards monocyte-related tumor cell lines (RAW264.7 and J774A.1), but not primary cells, i.e. bone marrow-derived macrophages (BMDMs), through two mechanisms. First, we observed that RAW264.7 and J774A.1 cells preserve a bountiful intracellular labile iron pool (LIP), whose homeostasis can be disrupted by Ent. This may be due, in part, to the lower levels of lipocalin 2 (Lcn2; an Ent-binding protein) in these cell lines, whereas the higher levels of Lcn2 in BMDMs could prevent Ent from hindering their LIP. Secondly, we observed that Ent could dose-dependently impede reactive oxygen species (ROS) generation in the mitochondria. Such disruption in LIP balance and mitochondrial function may in turn promote cancer cell apoptosis. Collectively, our study highlights Ent as an anti-cancer siderophore, which can be exploited as an unique agent for cancer therapy.

Neutrophil gelatinase-associated lipocalin increases HLA-G(+)/FoxP3(+) T-regulatory cell population in an in vitro model of PBMC

Background

Neutrophil gelatinase-associated lipocalin (NGAL) is emerging as a mediator of various biological and pathological states. However, the specific biological role of this molecule remains unclear, as it serves as a biomarker for many conditions. The high sensitivity of NGAL as a biomarker coupled with relatively low specificity may hide important biological roles. Data point toward an acute compensatory, protective role for NGAL in response to adverse cellular stresses, including inflammatory and oxidative stress. The aim of this study was to understand whether NGAL modulates the T-cell response through regulation of the human leukocyte antigen G (HLA-G) complex, which is a mediator of tolerance.

Methodology/Principal Findings

Peripheral blood mononuclear cells (PBMCs) were obtained from eight healthy donors and isolated by centrifugation on a Ficoll gradient. All donors gave informed consent. PBMCs were treated with four different concentrations of NGAL (40–320 ng/ml) in an iron-loaded or iron-free form. Changes in cell phenotype were analyzed by flow cytometry. NGAL stimulated expression of HLA-G on CD4+ T cells in a dose- and iron-dependent manner. Iron deficiency prevented NGAL-mediated effects, such that HLA-G expression was unaltered. Furthermore, NGAL treatment affected stimulation of regulatory T cells and in vitro expansion of CD4⁺ CD25⁺ FoxP3⁺ cells. An NGAL neutralizing antibody limited HLA-G expression and significantly decreased the percentage of CD4⁺ CD25⁺ FoxP3⁺ cells.

Conclusions/Significance

We provide in vitro evidence that NGAL is involved in cellular immunity. The potential role of NGAL as an immunomodulatory molecule is based on its ability to induce immune tolerance by upregulating HLA-G expression and expansion of T-regulatory cells in healthy donors. Future studies should further evaluate the role of NGAL in immunology and immunomodulation and its possible relationship to immunosuppressive therapy efficacy, tolerance induction in transplant patients, and other immunological disorders.

Mucosal lipocalin 2 has pro-inflammatory and iron-sequestering effects in response to bacterial enterobactin

Nasal colonization by both gram-positive and gram-negative pathogens induces expression of the innate immune protein lipocalin 2 (Lcn2). Lcn2 binds and sequesters the iron-scavenging siderophore enterobactin (Ent), preventing bacterial iron acquisition. In addition, Lcn2 bound to Ent induces release of IL-8 from cultured respiratory cells. As a countermeasure, pathogens of the Enterobacteriaceae family such as Klebsiella pneumoniae produce additional siderophores such as yersiniabactin (Ybt) and contain the iroA locus encoding an Ent glycosylase that prevents Lcn2 binding. Whereas the ability of Lcn2 to sequester iron is well described, the ability of Lcn2 to induce inflammation during infection is unknown. To study each potential effect of Lcn2 on colonization, we exploited K. pneumoniae mutants that are predicted to be susceptible to Lcn2-mediated iron sequestration (iroA ybtS mutant) or inflammation (iroA mutant), or to not interact with Lcn2 (entB mutant). During murine nasal colonization, the iroA ybtS double mutant was inhibited in an Lcn2-dependent manner, indicating that the iroA locus protects against Lcn2-mediated growth inhibition. Since the iroA single mutant was not inhibited, production of Ybt circumvents the iron sequestration effect of Lcn2 binding to Ent. However, colonization with the iroA mutant induced an increased influx of neutrophils compared to the entB mutant. This enhanced neutrophil response to Ent-producing K. pneumoniae was Lcn2-dependent. These findings suggest that Lcn2 has both pro-inflammatory and iron-sequestering effects along the respiratory mucosa in response to bacterial Ent. Therefore, Lcn2 may represent a novel mechanism of sensing microbial metabolism to modulate the host response appropriately.

Bacterial pathogens such as Klebsiella pneumoniae require iron and use secreted molecules called siderophores to strip iron from mammalian proteins. When bacteria colonize the upper respiratory tract, the mucosa secretes the protein lipocalin 2 (Lcn2) that binds to the siderophore enterobactin (Ent) and disrupts bacterial iron acquisition. In addition, Lcn2 bound to Ent stimulates release of the neutrophil-recruitment signal IL-8 from cultured respiratory cells. Some pathogens avoid Lcn2 binding by attaching glucose to Ent (to make Gly-Ent) or by making alternative siderophores. To determine the effect of Lcn2 on bacterial colonization, we colonized mice that express or lack Lcn2 with K. pneumoniae mutants that express or lack Ent, Gly-Ent and the alternative siderophore Yersiniabactin (Ybt). Our results indicate that mucosal Lcn2 inhibits colonization through iron sequestration and increases the influx of neutrophils in response to K. pneumoniae producing Ent. Therefore, Lcn2 acts as a barrier to colonization that pathogens must overcome to persist in the upper respiratory tract.

FepA- and TonB-dependent bacteriophage H8: receptor binding and genomic sequence

H8 is derived from a collection of Salmonella enterica serotype Enteritidis bacteriophage. Its morphology and genomic structure closely resemble those of bacteriophage T5 in the family Siphoviridae. H8 infected S. enterica serotypes Enteritidis and Typhimurium and Escherichia coli by initial adsorption to the outer membrane protein FepA. Ferric enterobactin inhibited H8 binding to E. coli FepA (50% inhibition concentration, 98 nM), and other ferric catecholate receptors (Fiu, Cir, and IroN) did not participate in phage adsorption. H8 infection was TonB dependent, but exbB mutations in Salmonella or E. coli did not prevent infection; only exbB tolQ or exbB tolR double mutants were resistant to H8. Experiments with deletion and substitution mutants showed that the receptor-phage interaction first involves residues distributed over the protein's outer surface and then narrows to the same charged (R316) or aromatic (Y260) residues that participate in the binding and transport of ferric enterobactin and colicins B and D. These data rationalize the multifunctionality of FepA: toxic ligands like bacteriocins and phage penetrate the outer membrane by parasitizing residues in FepA that are adapted to the transport of the natural ligand, ferric enterobactin. DNA sequence determinations revealed the complete H8 genome of 104.4 kb. A total of 120 of its 143 predicted open reading frames (ORFS) were homologous to ORFS in T5, at a level of 84% identity and 89% similarity. As in T5, the H8 structural genes clustered on the chromosome according to their function in the phage life cycle. The T5 genome contains a large section of DNA that can be deleted and that is absent in H8: compared to T5, H8 contains a 9,000-bp deletion in the early region of its chromosome, and nine potentially unique gene products. Sequence analyses of the tail proteins of phages in the same family showed that relative to pb5 (Oad) of T5 and Hrs of BF23, the FepA-binding protein (Rbp) of H8 contains unique acidic and aromatic residues. These side chains may promote binding to basic and aromatic residues in FepA that normally function in the adsorption of ferric enterobactin. Furthermore, a predicted H8 tail protein showed extensive identity and similarity to pb2 of T5, suggesting that it also functions in pore formation through the cell envelope. The variable region of this protein contains a potential TonB box, intimating that it participates in the TonB-dependent stage of the phage infection process.

Interleukin-8 secretion in response to aferric enterobactin is potentiated by siderocalin

Siderophores are low-molecular-weight iron chelators secreted by microbes to obtain iron under deprivation. We hypothesized that the catecholate siderophore enterobactin, produced by Enterobacteriaceae, serves as a proinflammatory signal for respiratory epithelial cells. Respiratory tract responses were explored, since at this site siderocalin, an enterobactin-binding mammalian gene product, is expressed inducibly at high levels and enterobactin-secreting respiratory flora is rare, suggesting selection against a dependence on enterobactin. Addition of aferric, but not iron-saturated, enterobactin elicits a dose-dependent increase in secretion of the proinflammatory chemokine interleukin-8 by human respiratory epithelial cells in culture. This response to purified enterobactin is potentiated by recombinant siderocalin at physiologically relevant concentrations. Conditioned media from genetically modified Escherichia coli strains expressing various levels of enterobactin induce an enterobactin-mediated proinflammatory response. Siderocalin has been shown to deliver enterobactin to other mammalian cell types, exogenously supplied siderocalin can be detected within epithelial cells, and siderocalin increases delivery of enterobactin to the intracellular compartment. Although many siderophores perturb labile cellular iron pools, only enterobactin elicits interleukin-8 secretion, suggesting that iron chelation is necessary but not sufficient. Thus, aferric enterobactin may be a proinflammatory signal for respiratory epithelial cells, permitting detection of microbial communities that have disturbed local iron homeostasis, and siderocalin expression by the host amplifies this signal. This may be a novel mechanism for the mucosa to respond to metabolic signals of expanding microbial communities.

Bromoenterobactins as Potent Inhibitors of a Pathogen-Associated, Siderophore-Modifying C-Glycosyltransferase

IroB is a C-glycosyltransferase encoded in the iroA cluster. C-Glucosylation of the bacterial siderophore enterobactin by IroB is a strategy some pathogenic bacteria use to evade the host's innate immunity mediated by lipocalin 2 (Lcn2). Without this modification, enterobactin can be tightly bound by host Lcn2, rendering it ineffective as a siderophore. Therefore, IroB inhibitors could be potential antibiotics against iroA-harboring pathogenic bacteria. We used enterobactin analogues to probe the properties of the active site of IroB and found that enterobactin analogues brominated at the C5 positions of the 2,3-dihydroxybenzoyl rings are potent inhibitors of IroB. This finding could lead to the discovery of effective antibiotics targeting iroA-containing bacteria.

The Pseudomonas aeruginosa pirA gene encodes a second receptor for ferrienterobactin and synthetic catecholate analogues

Actively secreted iron chelating agents termed siderophores play an important role in the virulence and rhizosphere competence of fluorescent pseudomonads, including Pseudomonas aeruginosa which secretes a high affinity siderophore, pyoverdine, and the low affinity siderophore, pyochelin. Uptake of the iron-siderophore complexes is an active process that requires specific outer membrane located receptors, which are dependent of the inner membrane-associated protein TonB and two other inner membrane proteins, ExbB and ExbC. P. aeruginosa is also capable of using a remarkable variety of heterologous siderophores as sources of iron, apparently by expressing their cognate receptors. Illustrative of this feature are the 32 (of which 28 putative) siderophore receptor genes observed in the P. aeruginosa PAO1 genome. However, except for a few (pyoverdine, pyochelin, enterobactin), the vast majority of P. aeruginosa siderophore receptor genes still remain to be characterized. Ten synthetic iron chelators of catecholate type stimulated growth of a pyoverdine/pyochelin deficient P. aeruginosa PAO1 mutant under condition of severe iron limitation. Null mutants of the 32 putative TonB-dependent siderophore receptor encoding genes engineered in the same genetic background were screened for obvious deficiencies in uptake of the synthetic siderophores, but none showed decreased growth stimulation in the presence of the different siderophores. However, a double knock-out mutant of ferrienterobactin receptor encoding gene pfeA (PA 2688) and pirA (PA0931) failed to be stimulated by 4 of the tested synthetic catecholate siderophores whose chemical structures resemble enterobactin. Ferric-enterobactin also failed to stimulate growth of the double pfeA-pirA mutant although, like its synthetic analogues, it stimulated growth of the corresponding single mutants. Hence, we confirmed that pirA represents a second P. aeruginosa ferric-enterobactin receptor. The example of these two enterobactin receptors probably illustrates a more general phenomenon of siderophore receptor redundancy in P. aeruginosa.

Linkage between catecholate siderophores and the multicopper oxidase CueO in Escherichia coli

The multicopper oxidase CueO had previously been demonstrated to exhibit phenoloxidase activity and was implicated in intrinsic copper resistance in Escherichia coli. Catecholates can potentially reduce Cu(II) to the prooxidant Cu(I). In this report we provide evidence that CueO protects E. coli cells by oxidizing enterobactin, the catechol iron siderophore of E. coli, in the presence of copper. In vitro, a mixture of enterobactin and copper was toxic for E. coli cells, but the addition of purified CueO led to their survival. Deletion of fur resulted in copper hypersensitivity that was alleviated by additional deletion of entC, preventing synthesis of enterobactin. In addition, copper added together with 2,3-dihydroxybenzoic acid or enterobactin was able to induce a Phi(cueO-lacZ) operon fusion more efficiently than copper alone. The reaction product of the 2,3-dihydroxybenzoic acid oxidation by CueO that can complex Cu(II) ions was determined by gas chromatography-mass spectroscopy and identified as 2-carboxymuconate.

Synthesis and electrochemical study of a new chiral tris-catecholamide analogue of enterobactin

The comparison of siderophore complex redox potentials with those of physiological reductants may aid in the clarification of the mechanism of iron metabolism. In this paper, a new chiral tris-catecholamide compound N,N',N''-tris-(2,3-dihydroxybenzoyl)-1,1,1-tris-(L-methioninemethyl++ +)-ethane or H6L (11) has been synthesised in nine steps, and may mimic the release of iron from enterobactin to the agents which are directly involved in cell metabolism. The choice of methionine as a constituent of the siderophore incorporates divalent sulphur which leads to the increase of the reduction potential of the siderophore, and consequently facilitates the iron release [Fe(III)/Fe(II) redox potential E(1/2)=-0.749 V vs (SCE)].

Protonmotive force, ExbB and ligand-bound FepA drive conformational changes in TonB

TonB couples the cytoplasmic membrane protonmotive force (pmf) to active transport across the outer membrane, potentially through a series of conformational changes. Previous studies of a TonB transmembrane domain mutant (TonB-delta V17) and its phenotypical suppressor (ExbB-A39E) suggested that TonB is conformationally sensitive. Here, two new mutations of the conserved TonB transmembrane domain SHLS motif were isolated, TonB-S16L and -H20Y, as were two new suppressors, ExbB-V35E and -V36D. Each suppressor ExbB restored at least partial function to the TonB mutants, although TonB-delta V17, for which both the conserved motif and the register of the predicted transmembrane domain alpha-helix are affected, was the most refractory. As demonstrated previously, TonB can undergo at least one conformational change, provided both ExbB and a functional TonB transmembrane domain are present. Here, we show that this conformational change reflects the ability of TonB to respond to the cytoplasmic membrane proton gradient, and occurs in proportion to the level of TonB activity attained by mutant-suppressor pairs. The phenotype of TonB-delta V17 was more complex than the -S16L and -H20Y mutations, in that, beyond the inability to be energized efficiently, it was also conditionally unstable. This second defect was evident only after suppression by the ExbB mutants, which allow transmembrane domain mutants to be energized, and presented as the rapid turnover of TonB-delta V17. Importantly, this degradation was dependent upon the presence of a TonB-dependent ligand, suggesting that TonB conformation also changes following the energy transduction event. Together, these observations support a dynamic model of energy transduction in which TonB cycles through a set of conformations that differ in potential energy, with a transition to a higher energy state driven by pmf and a transition to a lower energy state accompanying release of stored potential energy to an outer membrane receptor.

Two iron-regulated putative ferric siderophore receptor genes in Bordetella bronchiseptica and Bordetella pertussis

Two iron-regulated genes with deduced homology to TonB-dependent ferric siderophore receptors were cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants. bfrB and bfrC were iron-repressed in B. bronchiseptica by a Fur-dependent mechanism, and were expressed from promoters overlapped by potential Fur-binding sites. Both genes were highly conserved among Bordetella species and were also iron-regulated in Bordetella pertussis. bfrB and bfrC mutants of both species and a bfrB-bfrC double mutant of B. bronchiseptica had no discernible defects in utilization of known iron sources for Bordetella.

Ligand-specific opening of a gated-porin channel in the outer membrane of living bacteria

Ligand-gated membrane channels selectively facilitate the entry of iron into prokaryotic cells. The essential role of iron in metabolism makes its acquisition a determinant of bacterial pathogenesis and a target for therapeutic strategies. In Gram-negative bacteria, TonB-dependent outer membrane proteins form energized, gated pores that bind iron chelates (siderophores) and internalize them. The time-resolved operation of the Escherichia coli ferric enterobactin receptor FepA was observed in vivo with electron spin resonance spectroscopy by monitoring the mobility of covalently bound nitroxide spin labels. A ligand-binding surface loop of FepA, which normally closes its transmembrane channel, exhibited energy-dependent structural changes during iron and toxin (colicin) transport. These changes were not merely associated with ligand binding, but occurred during ligand uptake through the outer membrane bilayer. The results demonstrate by a physical method that gated-porin channels open and close during membrane transport in vivo.

The activity of microcin E492 from Klebsiella pneumoniae is regulated by a microcin antagonist

Microcin E492 is a polypeptide antibiotic that is produced and excreted by Klebsiella pneumoniae. Different growth conditions of the producer strain affect microcin activity. The production of a microcin antagonist is responsible for the changes in microcin activity. The microcin antagonist is induced when cells are iron-deprived, resulting in a low microcin activity. The microcin antagonist was purified using a procedure developed for the isolation of a catechol-type siderophore, and its activity was titrated using purified microcin. The inhibitory effect of the microcin antagonist is not observed when this compound is forming a complex with iron. The same inhibitory effect on microcin activity was obtained using purified enterochelin from Escherichia coli. The microcin antagonist was identified as enterochelin through thin-layer chromatography.

[Microbiological consequences of chelation of bivalent metal cations by nitroxoline]

The present work deals with the physico-chemical and microbiological investigations made in order to explain the role of divalent cations in the mechanism of the action of nitroxoline (NIT), an antibiotic used in the treatment of uncomplicated urinary tract infections. Preliminary studies reported that bacteriostatic and bactericidal activities of NIT on Escherichia coli strains are decreased in the presence of Mg2+ and Mn2+ but not with Ca2+. A spectrophotometric study, conducted in order to elucidate the interaction between metal ions and NIT, showed the formation of drug-metal ion complexes. In addition, we examined the relationships between the metal ions-chelating property of NIT and its effects on the decrease of the mannose sensitive hemagglutination titer and the production of siderophores from E. coli. The results suggested that these antimicrobial activities of NIT would also be due to the complexation of metal ions by this molecule.

Iron supply to Escherichia coli by synthetic analogs of enterochelin

Synthetic analogs of enterochelin (enterobactin) were tested for their ability to support the growth of Escherichia coli K-12 under iron-limiting conditions. The cyclic compound MECAM [1,3,5-N.N'; N"-tris-(2,3-dihydroxybenzoyl)-triamino-methylbenzene] and its N-methyl derivative Me3MECAM promoted growth, whereas the 2,3-dihydroxy-5-sulfonyl derivatives MECAMS and Me3MECAMS were inactive. The same results were obtained with TRIMCAM [1,3,5-tris(2,3-dihydroxybenzoylcarbamido)-benzene] and TRIMCAMS (the 2,3-dihydroxy-5-sulfonyl derivative of TRIMCAM). However, the sulfonic acid-containing linear compound LICAMS [1,5,10-N,N', N"-tris(5-sulfo-2,3-dihydroxybenzoyl)-triaza-decane] supported growth. In contrast, LIMCAMC, in which the sulfonyl groups at the five position of LICAMS are replaced by carboxyl groups at the four position, was inactive. The uptake of the active analogs required the functions specified by the fepB, fesB, and tonB genes. Surprisingly, growth promotion of mutants lacking the enterochelin receptor protein in the outer membrane was observed. Only MECAM protected cells against colicin B (which kills cells after entering at the enterochelin uptake sites) and transported Fe3+ at about half the enterochelin rate.

Antibacterial effect of the scandium and indium complexes of enterochelin on Escherichia coli

Enterochelin, the iron chelator produced by a number of pathogenic enterobacteria, appears to be an essential metabolite for multiplication within the host, where it transports iron from the host iron-binding proteins to the bacteria. Previous work showed that complexes of enterochelin containing either scandium (Sc3+) or indium (In3+) exerted a bacteriostatic effect on Klebsiella pneumoniae in serum, whilst the Sc3+ complex exerted a significant therapeutic effect on mice infected with K. pneumoniae. These observations have now been extended to a number of pathogenic serotypes of Escherichia coli including those carrying either the K1 antigen or the ColV plasmid. The Sc3+ and In3+ complexes each exert a bacteriostatic effect on these organisms growing in either whole serum or media containing an iron-binding protein. Evidence is presented that the Sc3+ complex may act as a competitive inhibitor of the Fe3+ complex. In contrast to their effects on K. pneumoniae, sideramines other than enterochelin fail to reverse the bacteriostatic effect of the Sc3+ complex of enterochelin in E. coli, suggesting that the complex produces a more profound derangement of metabolism in this organism. The Sc3+ complex exerts a significant therapeutic effect on E. coli infections in mice although the In3+ complex is less active.

Kinetics of biosynthesis of iron-regulated membrane proteins in Escherichia coli

Using biological iron chelators to control specifically iron availability to Escherichia coli K-12 in conjunction with radioactive pulse-labels, we examined the biosynthesis of six iron-regulated membrane proteins. Iron deprivation induced the synthesis of five proteins, which had molecular weights of 83,000 (83K), 81K (Fep), 78K (TonA), 74K (Cir), and 25K. The kinetics of induction were the same in entA and entA(+) strains, but were affected by the initial iron availability in the media. Iron-poor cells induced rapidly (half-time, 10 min), whereas iron-rich cells began induction after a lag and showed a slower induction half-time (30 min). Within this general pattern of induction after iron deprivation, several different kinetic patterns were apparent. The 83K, 81K, and 74K proteins were coordinately controlled under all of the conditions examined. The 78K and 25K proteins were regulated differently. The synthesis of a previously unrecognized 90K inner membrane protein was inhibited by iron deprivation and stimulated by iron repletion. Both ferrichrome and ferric enterobactin completely repressed 81K and 74K synthesis when the siderophores were supplied at concentrations of 5 muM in vivo (half-time, 2.5 min). At concentrations less than 5 muM, however, both siderophores repressed synthesis only temporarily; the duration of repression was proportional to the amount of ferric siderophore added. The half-lives of the 81K and 74K mRNAs, as measured by rifampin treatment, were 1.2 and 1.6 min, respectively. The results of this study suggest that enteric bacteria are capable of instantaneously detecting and reacting to fluctuations in the extracellular iron concentration and that they store iron during periods of iron repletion for utilization during periods of iron stress. Neither iron storage nor iron regulation of envelope protein synthesis is dependent on the ability of the bacteria to form heme.

Regulation of enterochelin synthetase in Escherichia coli K-12

Enterochelin synthetase activity is controlled by both repression and feed-back inhibition mechanisms. Inclusion of iron in growth media results in synthesis of all four (D, E, F and G) components of enterochelin synthetase being repressed. The specific inhibition of L-serine activation (partial reaction catalyzed by the F component) by the end products, ferric-enterochelin and 2,3-dihydroxybenzoylserine, is shown to inhibit overall enterochelin synthetase activity.

[Enterobacterial sensitivity to colicins in the presence of enterochelin]

The correlation between the colicine resistance of the reference and isolated strains of enterobacteria and their capacity for the biosynthesis of enterocheline, as well as the influence of exogenous enterocheline on the colicine sensitivity of enterobacteria were studied. In the wild strains of enterobacteria sensitivity to colicines was shown to have no correlation with capacity for the accumulation of catechol-type sideriphores. In some cases exogenous enterocheline prevents the lethal effect of colicines on the cultures of microorganisms capable of the biosynthesis of enterocheline.

Antibacterial effect of scandium and indium complexes of enterochelin on Klebsiella pneumoniae

A number of studies point to the conclusion that enterochelin, the iron chelator produced by a number of pathogenic enterobacteria, may be an essential metabolite for bacterial multiplication within the host. The compound removes iron from complexes with the host iron-binding proteins transferrin and lactoferrin, and the resulting ferric enterochelin is assimilated by the bacterial cell. It was reasoned that complexes of enterochelin with ions other than Fe3+ might act as antimetabolites and inhibit bacterial multiplication by interfering with the assimilation of ferric enterochelin. Enterochelin forms complexes with a number of group III and transition metal ions. The complex containing scandium exerts a bacteriostatic effect on Klebsiella pneumoniae in serum, whereas the indium complex induces a large increase in the generation time. The Fe3+ complexes of other microbial iron-transporting compounds are capable of reversing the bacteriostatic effect of the Sc3+ complex of enterochelin, suggesting that the compound acts solely by interfering with the enterochelin system of iron transport. Preliminary experiments show that the Sc3+ complex probably acts as a competitive inhibitor of ferric enterochelin. The Sc3+ complex of enterochelin exerts a therapeutic effect on intraperitoneal K. pneumoniae infections in mice similar to that obtained with kanamycin sulfate.

Involvement of inner and outer membrane components in the transport of iron and in colicin B action in Escherichia coli

Spheroplasts of Escherichia coli mutants were used to investigate the roles of the inner and outer membranes in the transport of iron. tonA mutants, known to be defective in an outer membrane component of the ferrichrome transport system, regained the ability to transport ferrichrome when converted to spheroplasts. On the other hand, the tonB mutant was unable to transport ferric enterochelin in either whole cells or spheroplasts. This implies that an element of the inner membrane is affected. fep mutants were also unable to transport ferric enterochelin, and fell into two classes, fepA and fepB. Spheroplasts of the former class transported ferric enterochelin, and those of the latter did not. This implies that the fepA mutants are defective in ferric enterochelin transport across the outer membrane, and that fepB mutants probably lack the facility to transport ferric enterochelin across the inner membrane. Colicin B action on fepA mutants was found to differ from that on fepB mutants.

Bacteriostatic effect of human milk on Escherichia coli: the role of IgA

Previous work showed that lactoferrin was involved in the bacteriostatic effect of human milk on E. coli O111. Further experiments on the general nature of this effect have been carried out together with an examination of the role of IgA. Milk samples from different individuals differed in their ability to produce bacteriostasis of three pathogenic serotypes of E. coli. The bacteriostatic effect was stable to heating at 60 degrees for 35 min. As in the case of serum, the bacterial iron transporting compound, enterochelin, abolished the bacteriostatic effect of human milk. IgA was isolated from the milk samples in two forms which appeared to differ in molecular weight. When mixed with lactoferrin, some of these fractions induced bacteriostasis which could be reversed by Fe3+. Since the fractions were devoid of bactericidal activity in the presence of complement it appeared that IgA was involved in the induction of bacteriostasis. It was also concluded that the mechanism of bacteriostasis was identical in serum and milk. These results are discussed in relation to both the protective effect of feeding colostrum and milk and also the resistance of the adult gastrointestinal tract to infection.

Involvement of outer membrane proteins in enterochelin-mediated iron uptake in Escherichia coli

Escherichia coli K-12 grown in iron-deficient media contained a large amount of outer membrane proteins O-2a, O-2b, and O-3, while cells grown in iron-supplemented media contained far smaller amounts of these proteins. The iron uptake by the iron-deficient cells was significantly stimulated in the presence of enterochelin, while that by the iron-rich cells was not. The outer membrane isolated from cells grown in the iron-deficient media showed enterochelin-stimulated binding of iron, while the outer membrane from iron-rich cells and cytoplasmic membranes from both types of cells did not show such binding activity. The amount of iron bound by the outer membrane was almost equivalent to the amount of O-2a, O2b, or O-3, irrespective of the amount of these proteins in the outer membrane, which is controlled by the amount of iron in the medium. Small particles rich in these proteins were prepared from cells by EDTA extraction. The particles were active in enterochelin-mediated iron binding and the amount of iron bound was equivalent to the amount of each of these proteins in the particles. Although the outer membrane of E. coli B was as active in iron binding as that of E. coli K-12, it did not possess an appreciable amount of O-2a. Gel electrophoretic analysis revealed that 9-2b and 9-3 were identical with the proteins missing mutants feuB and feuA, respectively.

Characterization of group B colicin-resistant mutants of Escherichia coli K-12: colicin resistance and the role of enterochelin

Nine classes of group B colicin-resistant mutants were examined to study the role of enterochelin in colicin resistance. Four of the mutants studied (cbt, exbC, exbB, and tonB) hypersecreted enterochelin. Enterochelin hypersecretion was apparently responsible for resistance of the exbC mutant to colicins G and H and for resistance of the exbB mutant to colicins G, H, Ia, Ib, S1, and V. All four mutants scored as colicin B tolerant, even in the absence of enterochelin synthesis. The mutants produced substantially increased amounts of two high-molecular-weight outer membrane polypeptides when grown under limiting iron conditions. The presence of these polypeptides was correlated with increased colicin B-neutralizing activity in the outer membrane preparations.

Siderophore protection against colicins M, B, V, and Ia in Escherichia coli

A variety of natural and synthetic siderophores capable of supporting the growth of Escherichia coli K-12 on iron-limited media also protect strain RW193+ (tonA+ ent-) from the killing action of colicins B, V, and Ia. Protective activity falls into two categories. The first, characteristic of enterobactin protection against colicin B and ferrichrome protection against colicin M, has properties of a specific receptor competition between the siderophore and the colicin. Thus, enterobactin specifically protects against colicin B in fes- mutants (able to accumulate but unable to utilize enterobactin) as predicted by our proposal that the colicin B receptor functions in the specific binding for uptake of enterobactin (Wayne and Neilands, 1975). Similarly ferrichrome specifically protects against colicin M in SidA mutants (defective in hydroxamate siderophore utilization). The second category of protective response, characteristic of the more general siderophore inhibition of colicins B, V, and Ia, requires the availability or metabolism of siderophore iron. Thus, enterobactin protects against colicins V and Ia, but only when the colicin indicator strain is fes+, and hydroxamate siderophores inhibit colicins B, V, and Ia, but only when the colicin indicator strain is SidA+. Moreover, ferrichrome inhibits colicins B, V, and Ia, yet chromium (III) deferriferrichrome is inactive, and ferrichrome itself does not prevent adsorption of colicin Ia receptor material in vitro. Although the nonspecific protection against colicins B, V, and Ia requires iron, the availability of siderophore iron for cell growth is not sufficient to bring about protection. None of the siderophores tested protect cells against the killing action of colicin E1 or K, or against the energy poisons azide, 2, 4-dinitrophenol, and carbonylcyanide m-chlorophenylhydrazone. We suggest that nonspecific siderophore protection against colicins B, V, and Ia may be due either to an induction of membrane alterations in response to siderophore iron metabolism or to a direct interference by siderophore iron with some unknown step in colicin action subsequent to adsorption.

Effects of enterocin E1A on the ultrastructure of Streptococcus faecium

Streptococcus faecium 158 cells were examined by electron microscopy at sequential intervals after addition of enterocin E1A, a bacteriocin produced by Streptococcus faecium E1. After addition of enterocin E1A, the nuclear material began to concentrate into distinct areas at the center of the bacteria. In a later stage, extensive condensation of the nuclear filaments left a small cluster of dense granules within a cytoplasmic vacuole, and 10-20% of the cells underwent a complete lysis.

Genetics of resistance to colicins in Escherichia coli K-12: cross-resistance among colicins of group B

This paper describes the isolation of mutants resistant to colicins of group B (i.e., colicins B, D, G, H, Ia, Ib, M, Q, S1, and V). The 145 mutants studied in detail can be divided into nine phentopyic classes, based on their colicin-resistance patterns. They include the previously isolated tonA, tonB exbB and cir mutants. Each of the different phenotypic classes of mutants has been partially characterized, and some approximately mapped. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis has revealed substantial changes in the composition of the outer membrane proteins of exbB and tonB mutants.