Siderophore-dependent iron uptake systems as gates for antibiotic Trojan horse strategies against Pseudomonas aeruginosa

Metal-Based Approaches for the Fight against Antimicrobial Resistance: Mechanisms, Opportunities, and Challenges

The rapid emergency and spread of antimicrobial-resistant (AMR) bacteria and the lack of new antibiotics being developed pose serious threats to the global healthcare system. Therefore, the development of more effective therapies to overcome AMR is highly desirable. Metal ions have a long history of serving as antimicrobial agents, and metal-based compounds are now attracting more interest from scientific communities in the fight against AMR owing to their unique mechanism. Moreover, they may also serve as antibiotic adjuvants to enhance the efficacy of clinically used antibiotics. In this perspective, we highlight important showcase studies in the last 10 years on the development of metal-based strategies to overcome the AMR crisis. Specifically, we categorize these metallo-antimicrobials into five classes based on their modes of action (i.e., metallo-enzymes and metal-binding enzyme inhibitors, membrane perturbants, uptake/efflux system inhibitors/regulators, persisters inhibitors, and oxidative stress inducers). The significant advantages of metallo-antimicrobials over traditional antibiotics lie in their multitargeted mechanisms, which render less likelihood to generate resistance. However, we notice that such modes of action of metallo-antimicrobials may also raise concern over their potential side effects owing to the low selectivity toward pathogens and host, which appears to be the biggest obstacle for downstream translational research. We anticipate that combination therapy through repurposing (metallo)drugs with antibiotics and the optimization of their absorption route through formulation to achieve a target-oriented delivery will be a powerful way to combat AMR. Despite significant challenges, metallo-antimicrobials hold great opportunities for the therapeutic intervention of infection by resistant bacteria.

Natural sideromycins and siderophore-conjugated natural products as inspiration for novel antimicrobial agents

The widespread emergence of multidrug-resistant (MDR) Gram-negative pathogens has posed a major challenge to clinical anti-infective therapy, and new effective treatments are urgently needed. A promising "Trojan horse" strategy involves conjugating antibiotics to siderophore molecules; the resulting siderophore-antibiotic conjugates (SACs) deliver antibiotics directly into cells by hijacking the sophisticated iron transport systems of Gram-negative bacteria, bypassing the outer membrane permeability barrier to enhance uptake and antibacterial efficacy. The clinical release of the first siderophore-antibiotic conjugate, cefiderocol, has aroused tremendous interest in the field among researchers and pharmaceutical companies. To date, most of the reported SACs have focused on the conjugation of siderophores to traditional antibacterial drugs. However, these antibacterial agents designed on the basis of the traditional antibiotic skeleton theoretically bear the risk of cross-resistance caused by shared molecular scaffolds. In this case, exploring novel natural product antibacterial conjugate scaffolds to circumvent the risk of early cross-resistance represents a presumably more sustainable approach for the development of SACs. In this review, we systematically summarize the research progress on siderophore-natural product conjugates as novel antimicrobial agents reported since 2010. Additionally, we propose challenges to be overcome and prospects for future development in this field.

New insights into iron uptake in Streptococcus mutans: evidence for a role of siderophore-like molecules

Streptococcus mutans, a gram-positive coccus commonly found in the human oral cavity, is the primary causative agent of dental caries as well as infective endocarditis. Bacteria produce potent iron chelators called siderophores to absorb iron. Because, there are few studies on siderophore-mediated iron transport in S. mutans, the current study investigates the presence of such a mechanism in S. mutans GS-5. Deferration of culture medium and different concentrations of 2, 2'-Bipyridyl has been used to simulate iron-restricted conditions. Iron restriction alters the colony morphology and slows bacterial growth. Cross-feeding conditioned medium into an iron-restricted medium promotes bacterial growth, indicating the presence of siderophore-like molecules. This was further confirmed by Chrome Azurol S (CAS) assay and Modified CAS-agar assay. Cśaky's and Arnow's assays detected the presence of hydroxamate and catecholate-type molecules in optimal and iron-restricted conditions, respectively. Further, the siderophore-like molecules were extracted and purified with thin layer chromatography (TLC). TLC elutes were also found to be positive for iron-chelation in CAS-agar assay and aided growth of S. mutans under iron-restricted conditions. LC-MS analysis of culture supernatants under iron-restricted conditions identified iron-binding small molecules, including a catechol structural motif. Computational analysis utilizing KEGG and BLASTp suggested homologues of siderophore biosynthesis and transport proteins, including genes associated with mutanobactin production. These findings indicate a possible siderophore-mediated iron uptake mechanism in S. mutans GS-5, warranting further molecular studies and advanced spectroscopic characterization of this unidentified siderophore. Once confirmed, this mechanism can be used as a potential drug target to control streptococcal infection.

Bacterial siderophores: diversity, uptake pathways and applications

Iron is an essential nutrient for the growth, survival and virulence of almost all bacteria. To access iron, many bacteria produce siderophores, molecules with a high affinity for iron. Research has highlighted substantial diversity in the chemical structure of siderophores produced by bacteria, as well as remarkable variety in the molecular mechanisms involved in strategies for acquiring iron through these molecules. The metal-chelating properties of siderophores, characterized by their high affinity for iron and ability to chelate numerous other metals (albeit with lower affinity compared with iron), have also generated interest in diverse fields. Siderophores find applications in the environment, such as in bioremediation and agriculture, in which emerging and innovative strategies are being developed to address pollution and enhance nutrient availability for plants. Moreover, in medicine, siderophores could be used as a tool for novel antimicrobial therapies and medical imaging, as well as in haemochromatosis, thalassemia or cancer treatments. This Review offers insights into the diversity of siderophores, highlighting their potential applications in environmental and medical contexts.

High-Throughput Discovery of Synthetic Siderophores for Trojan Horse Antibiotics

To cause infection, bacterial pathogens must overcome host immune factors and barriers to nutrient acquisition. Reproducing these aspects of host physiology in vitro has shown great promise for antibacterial drug discovery. When used as a bacterial growth medium, human serum replicates several aspects of the host environment, including innate immunity and iron limitation. We previously reported that a high-throughput chemical screen using serum as the growth medium enabled the discovery of novel growth inhibitors overlooked by conventional screens. Here, we report that a subset of compounds from this high-throughput serum screen display an unexpected growth enhancing phenotype and are enriched for synthetic siderophores. We selected 35 compounds of diverse chemical structure and quantified their ability to enhance bacterial growth in human serum. We show that many of these compounds chelate iron, suggesting they were acting as siderophores and providing iron to the bacteria. For two different pharmacophores represented among these synthetic siderophores, conjugation to the β-lactam antibiotic ampicillin imparted iron-dependent enhancement in antibacterial activity. Conjugation of the most potent growth-enhancing synthetic siderophore with the monobactam aztreonam produced MLEB-22043, a broad-spectrum antibiotic with significantly improved activity against Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Pseudomonas aeruginosa. This synthetic siderophore-monobactam conjugate uses multiple TonB-dependent transporters for uptake into P. aeruginosa. Like aztreonam, MLEB-22043 demonstrated activity against metallo-β-lactamase expressing bacteria, and, when combined with the β-lactamase inhibitor avibactam, was active against clinical strains coexpressing the NDM-1 metallo-β-lactamase and serine β-lactamases. Our work shows that human serum is an effective bacterial growth medium for the high-throughput discovery of synthetic siderophores, enabling the development of novel Trojan Horse antibiotics.

Synthesis and antibacterial properties under blue LED light of conjugates between the siderophore desferrioxamine B (DFOB) and an Iridium(III) complex

The decline of antibiotics efficacy worldwide has recently reached a critical point urging for the development of new strategies to regain upper hand on multidrug resistant bacterial strains. In this context, the raise of photodynamic therapy (PDT), initially based on organic photosensitizers (PS) and more recently on organometallic PS, offers promising perspectives. Many PS exert their biological effects through the generation of reactive oxygen species (ROS) able to freely diffuse into and to kill surrounding bacteria. Hijacking of the bacterial iron-uptake systems with siderophore-PS conjugates would specifically target pathogens. Here, we report the synthesis of unprecedented conjugates between the siderophore desferrioxamine B (DFOB) and an antibacterial iridium(III) PS. Redox properties of the new conjugates have been determined at excited states and compared to that of an antibacterial iridium PS previously reported by our groups. Tested on nosocomial pathogen Pseudomonas aeruginosa and other bacteria, these conjugates demonstrated significant inhibitory activity when activated with blue LED light. Ir(III) conjugate and iridium free DFOB-2,2'-dipyridylamine ligands were crystallized in complex with FoxA, the outer membrane transporter involved in DFOB uptake in P. aeruginosa and revealed details of the binding mode of these unprecedented conjugates.

Hybrids of 3-Hydroxypyridin-4(1H)-ones and Long-Chain 4-Aminoquinolines as Potent Biofilm Inhibitors of Pseudomonas aeruginosa Potentiate Tobramycin and Polymyxin B Activity

The biofilm formation of Pseudomonas aeruginosa involves multiple complex regulatory pathways; thus, blocking a single pathway is unlikely to achieve the desired antibiofilm efficacy. Herein, a series of hybrids of 3-hydroxypyridin-4(1H)-ones and long-chain 4-aminoquinolines were synthesized as biofilm inhibitors against P. aeruginosa based on a multipathway antibiofilm strategy. Comprehensive structure-activity relationship studies identified compound 30b as the most valuable antagonist, which significantly inhibited P. aeruginosa biofilm formation (IC50 = 5.8 μM) and various virulence phenotypes. Mechanistic studies revealed that 30b not only targets the three quorum sensing systems but also strongly induces iron deficiency signals in P. aeruginosa. Furthermore, 30b demonstrated a favorable in vitro and in vivo safety profile. Moreover, 30b specifically enhanced the antibacterial activity of tobramycin and polymyxin B in in vitro and in vivo combination therapy. Overall, these results highlight the potential of 30b as a novel anti-infective candidate for treating P. aeruginosa infections.

The role of FoxA, FiuA, and FpvB in iron acquisition via hydroxamate-type siderophores in Pseudomonas aeruginosa

Siderophores are specialized molecules produced by bacteria and fungi to scavenge iron, a crucial nutrient for growth and metabolism. Catecholate-type siderophores are mainly produced by bacteria, while hydroxamates are mostly from fungi. This study investigates the capacity of nine hydroxamate-type siderophores from fungi and Streptomyces to facilitate iron acquisition by the human pathogen Pseudomonas aeruginosa. Growth assays under iron limitation and 55Fe incorporation tests showed that all nine siderophores promoted bacterial growth and iron transport. The study also aimed to identify the TonB-dependent transporters (TBDTs) involved in iron import by these siderophores. Using mutant strains lacking specific TBDT genes, it was found that iron is imported into P. aeruginosa cells by FpvB for coprogen, triacetylfusarinine, fusigen, ferrirhodin, and ferrirubin. Iron complexed by desferioxamine G is transported by FpvB and FoxA, ferricrocin-Fe and ferrichrycin-Fe by FpvB and FiuA, and rhodotoluric acid-Fe by FpvB, FiuA, and another unidentified TBDT. These findings highlight the effectiveness of hydroxamate-type siderophores in iron transport into P. aeruginosa and provide insights into the complex molecular mechanisms involved, which are important for understanding microbial interactions and ecological balance.

Occurrence of 40 sanitary indicators in French digestates derived from different anaerobic digestion processes and raw organic wastes from agricultural and urban origin

This study investigated the sanitary quality of digestates resulting from the mesophilic anaerobic digestion (AD) of urban and agricultural organic wastes (OWs). 40 sanitary indicators, including pathogenic bacteria, antimicrobial resistance genes, virulence factor genes, and mobile genetic elements were evaluated using real-time PCR and/or droplet digital PCR. 13 polycyclic aromatic hydrocarbons (PAHs) and 13 pharmaceutical products (PHPs) were also measured. We assessed agricultural OWs from three treatment plants to study the effect of different AD processes (feeding mode, number of stages, pH), and used three laboratory-scale reactors to study the effect of different feed-supplies (inputs). The lab-scale reactors included: Lab1 fed with 97% activated sludge (urban waste) and 3% cow manure; Lab2 fed with 85% sludge-manure mixture supplemented with 15% wheat straw (WS); and Lab3 fed with 81% sludge-manure mixture, 15% WS, and 4% zeolite powder. Activated sludge favored the survival of the food-borne pathogens Clostridium perfringens and Bacillus cereus, carrying the toxin-encoding genes cpe and ces, respectively. Globally, the reactors fed with fecal matter supplemented with straw (Lab2) or with straw and zeolite (Lab3) had a higher hygienization efficiency than the reactor fed uniquely with fecal matter (Lab1). Three pathogenic bacteria (Enterococcus faecalis, Enterococcus faecium, and Mycobacterium tuberculosis complex), a beta-lactam resistance gene (bla TEM), and three mobile genetic elements (intI1, intI2, and IS26) were significantly decreased in Lab2 and Lab3. Moreover, the concentrations of 11 PAHs and 11 PHPs were significantly lower in Lab2 and Lab3 samples than in Lab1 samples. The high concentrations of micropollutants, such as triclosan, found in Lab1, could explain the lower hygienization efficiency of this reactor. Furthermore, the batch-fed reactor had a more efficient hygienization effect than the semi-continuous reactors, with complete removal of the ybtA gene, which is involved in the production of the siderophore yersiniabactin, and significant reduction of intI2 and tetO. These data suggest that it is essential to control the level of chemical pollutants in raw OWs to optimize the sanitary quality of digestates, and that adding co-substrate, such as WS, may overcome the harmful effect of pollutants.

Biosynthesis of a clickable pyoverdine via in vivo enzyme engineering of an adenylation domain

The engineering of non ribosomal peptide synthetases (NRPS) for new substrate specificity is a potent strategy to incorporate non-canonical amino acids into peptide sequences, thereby creating peptide diversity and broadening applications. The non-ribosomal peptide pyoverdine is the primary siderophore produced by Pseudomonas aeruginosa and holds biomedical promise in diagnosis, bio-imaging and antibiotic vectorization. We engineered the adenylation domain of PvdD, the terminal NRPS in pyoverdine biosynthesis, to accept a functionalized amino acid. Guided by molecular modeling, we rationally designed mutants of P. aeruginosa with mutations at two positions in the active site. A single amino acid change results in the successful incorporation of an azido-L-homoalanine leading to the synthesis of a new pyoverdine analog, functionalized with an azide function. We further demonstrated that copper free click chemistry is efficient on the functionalized pyoverdine and that the conjugated siderophore retains the iron chelation properties and its capacity to be recognized and transported by P. aeruginosa. The production of clickable pyoverdine holds substantial biotechnological significance, paving the way for numerous downstream applications.

4-Trifluoromethyl bithiazoles as broad-spectrum antimicrobial agents for virus-related bacterial infections or co-infections

Respiratory tract infections involving a variety of microorganisms such as viruses, bacteria, and fungi are a prominent cause of morbidity and mortality globally, exacerbating various pre-existing respiratory and non-respiratory conditions. Moreover, the ability of bacteria and viruses to coexist might impact the development and severity of lung infections, promoting bacterial colonization and subsequent disease exacerbation. Secondary bacterial infections following viral infections represent a complex challenge to be overcome from a therapeutic point of view. We report herein our efforts in the development of new bithiazole derivatives showing broad-spectrum antimicrobial activity against both viruses and bacteria. A series of 4-trifluoromethyl bithiazole analogues was synthesized and screened against selected viruses (hRVA16, EVD68, and ZIKV) and a panel of Gram-positive and Gram-negative bacteria. Among them, two promising broad-spectrum antimicrobial compounds (8a and 8j) have been identified: both compounds showed low micromolar activity against all tested viruses, 8a showed synergistic activity against E. coli and A. baumannii in the presence of a subinhibitory concentration of colistin, while 8j showed a broader spectrum of activity against Gram-positive and Gram-negative bacteria. Activity against antibiotic-resistant clinical isolates is also reported. Given the ever-increasing need to adequately address viral and bacterial infections or co-infections, this study paves the way for the development of new agents with broad antimicrobial properties and synergistic activity with common antivirals and antibacterials.

Searching for Conjugates as New Structures for Antifungal Therapies

The progressive increase in fungal infections and the decrease in the effectiveness of current therapy explain research on new drugs. The synthesis of compounds with proven antifungal activity, favorable physicochemical and pharmacokinetic properties affecting their pharmaceutical availability and bioavailability, and limiting or eliminating side effects has become the goal of many studies. The publication describes the directions of searching for new compounds with antifungal activity, focusing on conjugates. The described modifications include, among others, azoles or amphotericin B in combination with fatty acids, polysaccharides, proteins, and synthetic polymers. The benefits of these combinations in terms of activity, mechanism of action, and bioavailability were indicated. The possibilities of creating or using nanoparticles, "umbrella" conjugates, siderophores (iron-chelating compounds), and monoclonal antibodies were also presented. Taking into account the role of vaccinations in prevention, the scope of research related to developing a vaccine protecting against fungal infections was also indicated.

Benzothiazole DNA gyrase inhibitors and their conjugates with siderophore mimics: design, synthesis and evaluation

Benzothiazole-based bacterial DNA gyrase and topoisomerase IV inhibitors are promising new antibacterial agents with potent activity against Gram-positive and Gram-negative bacterial strains. The aim of this study was to improve the uptake of these inhibitors into the cytoplasm of Gram-negative bacteria by conjugating them to the small siderophore mimics. The best conjugate 18b displayed potent Escherichia coli DNA gyrase and topoisomerase IV inhibition. The interaction analysis of molecular dynamics simulation trajectory showed the important contribution of the siderophore mimic moiety to binding affinity. By NMR spectroscopy, we demonstrated that the hydroxypyridinone moiety alone was responsible for the chelation of iron(iii). Moreover, 18b showed an enhancement of antibacterial activity against E. coli JW5503 in an iron-depleted medium, clearly indicating an increased uptake of 18b in this bacterial strain.

Tackling the outer membrane: facilitating compound entry into Gram-negative bacterial pathogens

Emerging resistance to all available antibiotics highlights the need to develop new antibiotics with novel mechanisms of action. Most of the currently used antibiotics target Gram-positive bacteria while Gram-negative bacteria easily bypass the action of most drug molecules because of their unique outer membrane. This additional layer acts as a potent barrier restricting the entry of compounds into the cell. In this scenario, several approaches have been elucidated to increase the accumulation of compounds into Gram-negative bacteria. This review includes a brief description of the physicochemical properties that can aid compounds to enter and accumulate in Gram-negative bacteria and covers different strategies to target or bypass the outer membrane-mediated barrier in Gram-negative bacterial pathogens.

3-Hydroxypyridin-4(1H)-one Derivatives as pqs Quorum Sensing Inhibitors Attenuate Virulence and Reduce Antibiotic Resistance in Pseudomonas aeruginosa

The development of quorum sensing inhibitors capable of decreasing the production of virulence factors is an effective strategy to overcome resistance in Pseudomonas aeruginosa due to the less selective pressure exerted on bacteria. In this study, a series of 3-hydroxypyridin-4(1H)-one derivatives bearing a 4-aminomethyl-1,2,3-triazole linker were designed and synthesized as antivirulence agents against P. aeruginosa. The most potent derivative 16e was identified as a selective inhibitor of the pqs system (IC50 = 3.7 μM) and its related virulence factor pyocyanin (IC50 = 2.7 μM). In addition, 16e exhibited moderate biofilm inhibition and significant inhibition of P. aeruginosa motility phenotypes with low cytotoxicity. Compound 16e showed an obvious antibacterial synergistic effect in combination with antibiotics such as ciprofloxacin and tobramycin in in vitro and in vivo Caenorhabditis elegans infection models. Overall, the excellent antivirulence properties of compound 16e make it a potential antibiotic adjuvant for the treatment of P. aeruginosa infections that may be advanced into preclinical development in the future.

In vivo efficacy of pyochelin-mediated delivery of zingerone in Pseudomonas aeruginosa-induced peritonitis

Aim: The efficacy of a pyochelin-zingerone conjugate (PZC) against Pseudomonas aeruginosa in vivo in a mouse model of peritonitis, as well as mode of action in vitro, were investigated. Methods & results: Intraperitoneal administration of PZC (220 mg kg-1 b.wt.) resulted in a significant reduction in bacterial count in liver tissue by 2 log10 on the 4th day post infection. This was supported by reduced levels of inflammatory markers, liver function, inflammatory cytokines and improved histopathology. PZC showed its ability to disrupt the cellular membrane, increase permeability of the membrane and leakage of intracellular contents of P. aeruginosa, resulting in its death. Conclusion: The present study reports the hepatoprotective potential of PZC in an experimental model of P. aeruginosa-induced peritonitis.

Strategies for the Discovery of Oxazolidinone Antibacterial Agents: Development and Future Perspectives

Oxazolidinones represent a significant class of synthetic bacterial protein synthesis inhibitors that are primarily effective against Gram-positive bacteria. The commercial success of linezolid, the first FDA-approved oxazolidinone antibiotic, has motivated researchers to develop more potent oxazolidinones by employing various drug development strategies to fight against antimicrobial resistance, some of which have shown promising results. Thus, this Perspective aims to discuss the strategies employed in constructing oxazolidinone-based antibacterial agents and summarize recent advances in discovering oxazolidinone antibiotics to provide valuable insights for potentially developing next-generation oxazolidinone antibacterial agents or other pharmaceuticals.

Mobilization of iron stored in bacterioferritin, a new target for perturbing iron homeostasis and developing antibacterial and antibiofilm molecules

Antibiotic resistance is a global public health threat. The care of chronic infections is complicated by bacterial biofilms. Biofilm embedded cells can be up to 1000-fold more tolerant to antibiotic treatment than planktonic cells. Antibiotic tolerance is a condition which does not involve mutation and enables bacteria to survive in the presence of antibiotics. The antibiotic tolerance of biofilm-cells often renders antibiotics ineffective, even against strains that do not carry resistance-impairing mutations. This review discusses bacterial iron homeostasis and the strategies being developed to target this bacterial vulnerability, with emphasis on a recently proposed approach which aims at targeting the iron storage protein bacterioferritin (Bfr) and its physiological partner, the ferredoxin Bfd. Bfr regulates cytosolic iron concentrations by oxidizing Fe2+ and storing Fe3+ in its internal cavity, and by forming a complex with Bfd to reduce Fe3+ in the internal cavity and release Fe2+ to the cytosol. Blocking the Bfr-Bfd complex in P. aeruginosa cells causes an irreversible accumulation of Fe3+ in BfrB and simultaneous cytosolic iron depletion, which leads to impaired biofilm maintenance and biofilm cell death. Recently discovered small molecule inhibitors of the Bfr-Bfd complex, which bind Bfr at the Bfd binding site, inhibit iron mobilization, and elicit biofilm cell death.

TonB-Dependent Transport Across the Bacterial Outer Membrane

TonB-dependent transporters (TBDTs) are present in all gram-negative bacteria and mediate energy-dependent uptake of molecules that are too scarce or large to be taken up efficiently by outer membrane (OM) diffusion channels. This process requires energy that is derived from the proton motive force and delivered to TBDTs by the TonB-ExbBD motor complex in the inner membrane. Together with the need to preserve the OM permeability barrier, this has led to an extremely complex and fascinating transport mechanism for which the fundamentals, despite decades of research, are still unclear. In this review, we describe our current understanding of the transport mechanism of TBDTs, their potential role in the delivery of novel antibiotics, and the important contributions made by TBDT-associated (lipo)proteins.

Vectorization via Siderophores Increases Antibacterial Activity of K(RW)3 Peptides against Pseudomonas aeruginosa

A series of new conjugates comprised from a small synthetic antimicrobial peptide (AMP) and a siderophore-type vector component was designed and tested for activity on P. aeruginosa PAO1 and several genetically modified strains. As AMP, the well-established arginine-tryptophane combination K(RW)3 (P1) was chosen with an added lysine for siderophore attachment. This peptide is easy to prepare, modify, and possesses good anti-bacterial activity. On the vector part, we examined several moieties: (i) the natural siderophore deferoxamine (DFO); (ii) bidentate iron chelators based on the hydroxamate building block (4 a-c) ; (iii) the non-siderophore chelators deferasirox (DFX) and deferiprone-carboxylate (DFP-COOH). All conjugates were prepared by solid phase synthesis techniques and fully characterized by HPLC and mass spectrometry (including HR-MS). 55 Fe uptake assays indicate a receptor-mediated uptake for 4 a-c, DFP-COOH and DFO, which is dependent on the outer membrane transporter FoxA in the case of DFO. All conjugates showed increased antibacterial activity against P. aeruginosa compared to the parent peptide P1 alone when investigated in iron-depleted medium. MIC values were as low as 2 μM (for P1-DFP) on wild type P. aeruginosa. The activity of P1-DFO and P1-DFP was even better on genetically mutated strains unable to produce siderophores (down to 0.5 μM). Although the DFX vector on its own was not able to transport iron inside the bacterial cell as shown by 55 Fe uptake studies, the P1-DFX conjugate had excellent antibacterial activity compared to P1 (2 μM, and as low as 0.25 μM on a receptor-deficient strain unable to produce siderophores), suggesting that the conjugates were indeed recognized and internalized by an (unknown) transporter. Control experiments with an equimolar mixture of P1 and DFX confirm that the observed activity is intrinsic to vectorization. This work thus demonstrates the power of linking small AMPs covalently to siderophores for a new class of Trojan Horse antibiotics, with P1-DFP and P1-DFX being the most potent conjugates.

Differentiating the roles of Mycobacterium tuberculosis substrate binding proteins, FecB and FecB2, in iron uptake

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, poses a great threat to human health. With the emergence of drug resistant Mtb strains, new therapeutics are desperately needed. As iron is critical to the growth and survival of Mtb, mechanisms through which Mtb acquires host iron represent attractive therapeutic targets. Mtb scavenges host iron via Mtb siderophore-dependent and heme iron uptake pathways. While multiple studies describe the import of heme and ferric-siderophores and the export of apo-siderophores across the inner membrane, little is known about their transport across the periplasm and cell-wall environments. Mtb FecB and FecB2 are predicted periplasmic binding proteins implicated in host iron acquisition; however, their precise roles are not well understood. This study sought to differentiate the roles FecB and FecB2 play in Mtb iron acquisition. The crystallographic structures of Mtb FecB and FecB2 were determined to 2.0 Å and 2.2 Å resolution, respectively, and show distinct ligand binding pockets. In vitro ligand binding experiments for FecB and FecB2 were performed with heme and bacterial siderophores from Mtb and other species, revealing that both FecB and FecB2 bind heme, while only FecB binds the Mtb sideophore ferric-carboxymycobactin (Fe-cMB). Subsequent structure-guided mutagenesis of FecB identified a single glutamate residue-Glu339-that significantly contributes to Fe-cMB binding. A role for FecB in the Mtb siderophore-mediated iron acquisition pathway was corroborated by Mycobacterium smegmatis and Mtb pull-down assays, which revealed interactions between FecB and members of the mycobacterial siderophore export and import machinery. Similarly, pull-down assays with FecB2 confirms its role in heme uptake revealing interactions with a potential inner membrane heme importer. Due to ligand preference and protein partners, our data suggest that Mtb FecB plays a role in siderophore-dependent iron and heme acquisition pathways; in addition, we confirm that Mtb FecB2 is involved in heme uptake.

Metallo-sideromycin as a dual functional complex for combating antimicrobial resistance

The rapid emergence of antimicrobial resistance (AMR) pathogens highlights the urgent need to approach this global burden with alternative strategies. Cefiderocol (Fetroja®) is a clinically-used sideromycin, that is utilized for the treatment of severe drug-resistant infections, caused by Gram-negative bacteria; there is evidence of cefiderocol-resistance occurring in bacterial strains however. To increase the efficacy and extend the life-span of sideromycins, we demonstrate strong synergisms between cefiderocol and metallodrugs (e.g., colloidal bismuth citrate (CBS)), against Pseudomonas aeruginosa and Burkholderia cepacia. Moreover, CBS enhances cefiderocol efficacy against biofilm formation, suppresses the resistance development in P. aeruginosa and resensitizes clinically isolated resistant P. aeruginosa to cefiderocol. Notably, the co-therapy of CBS and cefiderocol significantly increases the survival rate of mice and decreases bacterial loads in the lung in a murine acute pneumonia model. The observed phenomena are partially attributable to the competitive binding of Bi3+ to cefiderocol with Fe3+, leading to enhanced uptake of Bi3+ and reduced levels of Fe3+ in cells. Our studies provide insight into the antimicrobial potential of metallo-sideromycins.

Comparative metabolomic profiling of Cupriavidus necator B-4383 revealed production of cupriachelin siderophores, one with activity against Cryptococcus neoformans

Cupriavidus necator H16 is known to be a rich source of linear lipopeptide siderophores when grown under iron-depleted conditions; prior literature termed these compounds cupriachelins. These small molecules bear β-hydroxyaspartate moieties that contribute to a photoreduction of iron when bound as ferric cupriachelin. Here, we present structural assignment of cupriachelins from C. necator B-4383 grown under iron limitation. The characterization of B-4383 cupriachelins is based on MS/MS fragmentation analysis, which was confirmed by 1D- and 2D-NMR for the most abundant analog (1). The cupriachelin congeners distinguish these two strains with differences in the preferred lipid tail; however, our rigorous metabolomic investigation also revealed minor analogs with changes in the peptide core, hinting at a potential mechanism by which these siderophores may reduce biologically unavailable ferric iron (4-6). Antifungal screening of the C. necator B-4383 supernatant extract and the isolated cupriachelin analog (1) revealed inhibitory activity against Cryptococcus neoformans, with IC50 values of 16.6 and 3.2 μg/mL, respectively. This antifungal activity could be explained by the critical role of the iron acquisition pathway in the growth and pathogenesis of the C. neoformans fungal pathogen.

Engineering Siderophore Biosynthesis and Regulation Pathways to Increase Diversity and Availability

Siderophores are small metal chelators synthesized by numerous organisms to access iron. These secondary metabolites are ubiquitously present on Earth, and because their production represents the main strategy to assimilate iron, they play an important role in both positive and negative interactions between organisms. In addition, siderophores are used in biotechnology for diverse applications in medicine, agriculture and the environment. The generation of non-natural siderophore analogs provides a new opportunity to create new-to-nature chelating biomolecules that can offer new properties to expand applications. This review summarizes the main strategies of combinatorial biosynthesis that have been used to generate siderophore analogs. We first provide a brief overview of siderophore biosynthesis, followed by a description of the strategies, namely, precursor-directed biosynthesis, the design of synthetic or heterologous pathways and enzyme engineering, used in siderophore biosynthetic pathways to create diversity. In addition, this review highlights the engineering strategies that have been used to improve the production of siderophores by cells to facilitate their downstream utilization.

Design and Synthesis of Novel Antimicrobial Agents

The necessity for the discovery of innovative antimicrobials to treat life-threatening diseases has increased as multidrug-resistant bacteria has spread. Due to antibiotics' availability over the counter in many nations, antibiotic resistance is linked to overuse, abuse, and misuse of these drugs. The World Health Organization (WHO) recognized 12 families of bacteria that present the greatest harm to human health, where options of antibiotic therapy are extremely limited. Therefore, this paper reviews possible new ways for the development of novel classes of antibiotics for which there is no pre-existing resistance in human bacterial pathogens. By utilizing research and technology such as nanotechnology and computational methods (such as in silico and Fragment-based drug design (FBDD)), there has been an improvement in antimicrobial actions and selectivity with target sites. Moreover, there are antibiotic alternatives, such as antimicrobial peptides, essential oils, anti-Quorum sensing agents, darobactins, vitamin B6, bacteriophages, odilorhabdins, 18β-glycyrrhetinic acid, and cannabinoids. Additionally, drug repurposing (such as with ticagrelor, mitomycin C, auranofin, pentamidine, and zidovudine) and synthesis of novel antibacterial agents (including lactones, piperidinol, sugar-based bactericides, isoxazole, carbazole, pyrimidine, and pyrazole derivatives) represent novel approaches to treating infectious diseases. Nonetheless, prodrugs (e.g., siderophores) have recently shown to be an excellent platform to design a new generation of antimicrobial agents with better efficacy against multidrug-resistant bacteria. Ultimately, to combat resistant bacteria and to stop the spread of resistant illnesses, regulations and public education regarding the use of antibiotics in hospitals and the agricultural sector should be combined with research and technological advancements.

Mechanisms of iron homeostasis in Pseudomonas aeruginosa and emerging therapeutics directed to disrupt this vital process

Pseudomonas aeruginosa is an opportunistic pathogen able to infect any human tissue. One of the reasons for its high adaptability and colonization of host tissues is its capacity of maintaining iron homeostasis through a wide array of iron acquisition and removal mechanisms. Due to their ability to cause life-threatening acute and chronic infections, especially among cystic fibrosis and immunocompromised patients, and their propensity to acquire resistance to many antibiotics, the World Health Organization (WHO) has encouraged the scientific community to find new strategies to eradicate this pathogen. Several recent strategies to battle P. aeruginosa focus on targeting iron homeostasis mechanisms, turning its greatest advantage into an exploitable weak point. In this review, we discuss the different mechanisms used by P. aeruginosa to maintain iron homeostasis and the strategies being developed to fight this pathogen by blocking these mechanisms. Among others, the use of iron chelators and mimics, as well as disruption of siderophore production and uptake, have shown promising results in reducing viability and/or virulence of this pathogen. The so-called 'Trojan-horse' strategy taking advantage of the siderophore uptake systems is emerging as an efficient method to improve delivery of antibiotics into the bacterial cells. Moreover, siderophore transporters are considered promising targets for the developing of P. aeruginosa vaccines.

Pseudomonas aeruginosa virulence attenuation by inhibiting siderophore functions

Pseudmonas aeruginosa is a Gram-negative bacterium known to be ubiquitous and recognized as one of the leading causes of infections such as respiratory, urinary tract, burns, cystic fibrosis, and in immunocompromised individuals. Failure of antimicrobial therapy has been documented to be attributable due to the development of various resistance mechanisms, with a proclivity to develop additional resistance mechanisms rapidly. P. aeruginosa virulence attenuation is an alternate technique for disrupting pathogenesis without impacting growth. The iron-scavenging siderophores (pyoverdine and pyochelin) generated by P. aeruginosa have various properties like scavenging iron, biofilm formation, quorum sensing, increasing virulence, and toxicity to the host. As a result, developing an antivirulence strategy, specifically inhibiting the P. aeruginosa siderophore, has been a promising therapeutic option to limit their infection. Several natural, synthetic compounds and nanoparticles have been identified as potent inhibitors of siderophore production/biosynthesis, function, and transport system. The current review discussed pyoverdine and pyochelin's synthesis and transport system in P. aeruginosa. Furthermore, it is also focused on the role of several natural and synthetic compounds in reducing P. aeruginosa virulence by inhibiting siderophore synthesis, function, and transport. The underlying mechanism involved in inhibiting the siderophore by natural and synthetic compounds has also been explained. KEY POINTS: • Pseudomonas aeruginosa is an opportunistic pathogen linked to chronic respiratory, urinary tract, and burns infections, as well as cystic fibrosis and immunocompromised patients. • P. aeruginosa produces two virulent siderophores forms: pyoverdine and pyochelin, which help it to survive in iron-deficient environments. • The inhibition of siderophore production, transport, and activity using natural and synthesized drugs has been described as a potential strategy for controlling P. aeruginosa infection.

Rapid identification of pyoverdines of fluorescent Pseudomonas spp. by UHPLC-IM-MS

Siderophores are iron-chelating molecules produced by bacteria and other microbes. They are involved with virulence in infections and play key roles in bacterial community assembly and as plant protectants due to their pathogen control properties. Although assays exist to screen whether newly isolated bacteria can produce siderophores, the chemical structures of many of these bio-active molecules remain unidentified due to the lack of rapid analytical procedures. An important group of siderophores are pyoverdines. They consist of a structurally diverse group of chromopeptides, whose amino acid sequence is characteristic for the fluorescent Pseudomonas species that secrets them. Although over 60 pyoverdine structures have been described so far, their characterization is cumbersome and several methods (isoelectrofocusing, iron uptake measurement, mass determination) are typically combined as ambiguous results are often achieved by a single method. Those additional experiments consume valuable time and resources and prevent high-throughput analysis. In this work, we present a new pyoverdine characterisation option by recording their collision cross sections (CCS) using trapped ion mobility spectrometry. This can be done simultaneously in combination with UHPLC and high-resolution MS resulting in a rapid identification of pyoverdines. The high specificity of CCS values is presented for 17 pyoverdines secreted by different Pseudomonas strains. The pyoverdine mass determination by full scan MS was supported by fragments obtained from broadband collision induced dissociation (bbCID). As iron contaminations in laboratories are not uncommon, CCS values of ferripyoverdines were also evaluated. Thereby, unusual and highly characteristic ion mobility patterns were obtained that are suitable as an alternative identification marker.

Rejuvenating the Activity of Usual Antibiotics on Resistant Gram-Negative Bacteria: Recent Issues and Perspectives

Antibiotic resistance continues to evolve and spread beyond all boundaries, resulting in an increase in morbidity and mortality for non-curable infectious diseases. Due to the failure of conventional antimicrobial therapy and the lack of introduction of a novel class of antibiotics, novel strategies have recently emerged to combat these multidrug-resistant infectious microorganisms. In this review, we highlight the development of effective antibiotic combinations and of antibiotics with non-antibiotic activity-enhancing compounds to address the widespread emergence of antibiotic-resistant strains.

Ferrichrome, a fungal-type siderophore, confers high ammonium tolerance to fission yeast

Microorganisms and plants produce siderophores, which function to transport environmental iron into cells as well as participate in cellular iron use and deposition. Their biological functions are diverse although their role in primary metabolism is poorly understood. Ferrichrome is a fungal-type siderophore synthesized by nonribosomal peptide synthetase (NRPS). Herein we show that ferrichrome induces adaptive growth of fission yeast on high ammonium media. Ammonium is a preferred nitrogen source as it suppresses uptake and catabolism of less preferred nitrogen sources such as leucine through a mechanism called nitrogen catabolite repression (NCR). Therefore, the growth of fission yeast mutant cells with leucine auxotrophy is suppressed in the presence of high concentrations of ammonium. This growth suppression was canceled by ferrichrome in a manner dependent on the amino acid transporter Cat1. Additionally, growth retardation of wild-type cells by excess ammonium was exacerbated by deleting the NRPS gene sib1, which is responsible for the biosynthesis of ferrichrome, suggesting that intrinsically produced ferrichrome functions in suppressing the metabolic action of ammonium. Furthermore, ferrichrome facilitated the growth of both wild-type and sib1-deficient cells under low glucose conditions. These results suggest that intracellular iron regulates primary metabolism, including NCR, which is mediated by siderophores.

Strategies and progresses for enhancing targeted antibiotic delivery

Antibiotic resistance is a global health issue and a potential risk for society. Antibiotics administered through conventional formulations are devoid of targeting effect and often spread to various undesired body sites, leading to sub-lethal concentrations at the site of action and thus resulting in emergence of resistance, as well as side effects. Moreover, we have a very slim antibiotic pipeline. Drug-delivery systems have been designed to control the rate, time, and site of drug release, and innovative approaches for antibiotic delivery provide a glint of hope for addressing these issues. This review elaborates different delivery strategies and approaches employed to overcome the limitations of conventional antibiotic therapy. These include antibiotic conjugates, prodrugs, and nanocarriers for local and targeted antibiotic release. In addition, a wide range of stimuli-responsive nanocarriers and biological carriers for targeted antibiotic delivery are discussed. The potential advantages and limitations of targeted antibiotic delivery strategies are described along with possible solutions to avoid these limitations. A number of antibiotics successfully delivered through these approaches with attained outcomes and potentials are reviewed.

Novel Insights on Pyoverdine: From Biosynthesis to Biotechnological Application

Pyoverdines (PVDs) are a class of siderophores produced mostly by members of the genus Pseudomonas. Their primary function is to accumulate, mobilize, and transport iron necessary for cell metabolism. Moreover, PVDs also play a crucial role in microbes’ survival by mediating biofilm formation and virulence. In this review, we reorganize the information produced in recent years regarding PVDs biosynthesis and pathogenic mechanisms, since PVDs are extremely valuable compounds. Additionally, we summarize the therapeutic applications deriving from the PVDs’ use and focus on their role as therapeutic target themselves. We assess the current biotechnological applications of different sectors and evaluate the state-of-the-art technology relating to the use of synthetic biology tools for pathway engineering. Finally, we review the most recent methods and techniques capable of identifying such molecules in complex matrices for drug-discovery purposes.

Hijacking of the Enterobactin Pathway by a Synthetic Catechol Vector Designed for Oxazolidinone Antibiotic Delivery in Pseudomonas aeruginosa

Enterobactin (ENT) is a tris-catechol siderophore used to acquire iron by multiple bacterial species. These ENT-dependent iron uptake systems have often been considered as potential gates in the bacterial envelope through which one can shuttle antibiotics (Trojan horse strategy). In practice, siderophore analogues containing catechol moieties have shown promise as vectors to which antibiotics may be attached. Bis- and tris-catechol vectors (BCVs and TCVs, respectively) were shown using structural biology and molecular modeling to mimic ENT binding to the outer membrane transporter PfeA in Pseudomonas aeruginosa. TCV but not BCV appears to cross the outer membrane via PfeA when linked to an antibiotic (linezolid). TCV is therefore a promising vector for Trojan horse strategies against P. aeruginosa, confirming the ENT-dependent iron uptake system as a gate to transport antibiotics into P. aeruginosa cells.

Identification of Key Functions Required for Production and Utilization of the Siderophore Piscibactin Encoded by the High-Pathogenicity Island irp-HPI in Vibrionaceae

Piscibactin is a widespread siderophore system present in many different bacteria, especially within the Vibrionaceae family. Previous works showed that most functions required for biosynthesis and transport of this siderophore are encoded by the high-pathogenicity island irp-HPI. In the present work, using Vibrio anguillarum as a model, we could identify additional key functions encoded by irp-HPI that are necessary for piscibactin production and transport and that have remained unknown. Allelic exchange mutagenesis, combined with cross-feeding bioassays and LC-MS analysis, were used to demonstrate that Irp4 protein is an essential component for piscibactin synthesis since it is the thioesterase required for nascent piscibactin be released from the NRPS Irp1. We also show that Irp8 is a MFS-type protein essential for piscibactin secretion. In addition, after passage through the outer membrane transporter FrpA, the completion of ferri-piscibactin internalization through the inner membrane would be achieved by the ABC-type transporter FrpBC. The expression of this transporter is coordinated with the expression of FrpA and with the genes encoding biosynthetic functions. Since piscibactin is a major virulence factor of some pathogenic vibrios, the elements of biosynthesis and transport described here could be additional interesting targets for the design of novel antimicrobials against these bacterial pathogens.

Engineering siderophore production in Pseudomonas to improve asbestos weathering

Iron plays a key role in microbial metabolism and bacteria have developed multiple siderophore-driven mechanisms due to its poor bioavailability for organisms in the environment. Iron-bearing minerals generally serve as a nutrient source to sustain bacterial growth after bioweathering. Siderophores are high-affinity ferric iron chelators, of which the biosynthesis is tightly regulated by the presence of iron. Pyoverdine-producing Pseudomonas have shown their ability to extract iron and magnesium from asbestos waste as nutrients. However, such bioweathering is rapidly limited due to repression of the pyoverdine pathway and the low bacterial requirement for iron. We developed a metabolically engineered strain of Pseudomonas aeruginosa for which pyoverdine production was no longer repressed by iron as a proof of concept. We compared siderophore-promoted dissolution of flocking asbestos waste by this optimized strain to that by the wild-type strain. Interestingly, pyoverdine production by the optimized strain was seven times higher in the presence of asbestos waste and the dissolution of magnesium and iron from the chrysotile fibres contained in flocking asbestos waste was significantly enhanced. This innovative mineral weathering process contributes to remove toxic iron from the asbestos fibres and may contribute to the development of an eco-friendly method to manage asbestos waste.

Design, Synthesis, and Quorum Quenching Potential of Novel Catechol–Zingerone Conjugate to Find an Elixir to Tackle Pseudomonas aeruginosa Through the Trojan Horse Strategy

To address the issue of multidrug resistance in Pseudomonas aeruginosa, a novel catechol–zingerone conjugate (1) linked via a non-hydrolyzable 1,2,3-triazole linker was synthesized and subjected to biological evaluation based on the Trojan horse strategy. To enhance the efficacy, catechol, a xenosiderophore, utilized by P. aeruginosa for iron assimilation, and the dietary phytochemical zingerone, known for its anti-virulent activity against Pseudomonas aeruginosa, were exploited in the present study. Theoretical validation of conjugate (1) was conducted by in silico molecular docking analysis to determine the interaction with outer membrane transport receptor PirA and quorum sensing signal receptors. In addition, nine-fold binding affinity of Conjugate (1) toward PirA (5FP2) in comparison to its natural ligand catechol with D-score −1.13 Å authenticated the designed Trojan horse drug. Conjugate (1) showed stronger anti-virulent activity than zingerone; hence, it exhibited a promising anti-biofilm efficacy as assessed by crystal violet assay and visualized by FESEM toward P. aeruginosa. Encouraging results against P. aeruginosa in terms of quorum sensing regulated virulence factors, motility phenotypes, and biofilm formation with no cell cytotoxicity and could help open hitherto unexplored possibilities of establishing Trojan horse drugs as a successful approach against multidrug resistance in P. aeruginosa.

Uptake Mechanisms and Regulatory Responses to MECAM- and DOTAM-Based Artificial Siderophores and Their Antibiotic Conjugates in Pseudomonas aeruginosa

The development of new antibiotics against Gram-negative bacteria has to deal with the low permeability of the outer membrane. This obstacle can be overcome by utilizing siderophore-dependent iron uptake pathways as entrance routes for antibiotic uptake. Iron-chelating siderophores are actively imported by bacteria, and their conjugation to antibiotics allows smuggling the latter into bacterial cells. Synthetic siderophore mimetics based on MECAM (1,3,5-N,N',N″-tris-(2,3-dihydroxybenzoyl)-triaminomethylbenzene) and DOTAM (1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane) cores, both chelating iron via catechol groups, have been recently applied as versatile carriers of functional cargo. In the present study, we show that MECAM and the MECAM-ampicillin conjugate 3 transport iron into Pseudomonas aeruginosa cells via the catechol-type outer membrane transporters PfeA and PirA and DOTAM solely via PirA. Differential proteomics and quantitative real-time polymerase chain reaction (qRT-PCR) showed that MECAM import induced the expression of pfeA, whereas 3 led to an increase in the expression of pfeA and ampc, a gene conferring ampicillin resistance. The presence of DOTAM did not induce the expression of pirA but upregulated the expression of two zinc transporters (cntO and PA0781), pointing out that bacteria become zinc starved in the presence of this compound. Iron uptake experiments with radioactive ⁵⁵Fe demonstrated that import of this nutrient by MECAM and DOTAM was as efficient as with the natural siderophore enterobactin. The study provides a functional validation for DOTAM- and MECAM-based artificial siderophore mimetics as vehicles for the delivery of cargo into Gram-negative bacteria.

Multilayer dextran derivative based capsules fighting bacteria resistant to Antibiotic: Case of Kanamycin-Resistant Escherichia coli

Bacteria resistance to antibiotics has emerged as a major health problem. Developing new antibacterial systems is then of major interest. In this sense, we present biocapsules presenting inherent antibacterial capacity. The self-assembly of charged biopolymer, namely diethylaminoethyl-dextran hydrochloride (dex+) and dextran sulfate (dex-), were done on calcium carbonate microparticles, used as a template. Zeta potential measurements have shown the successful alternate adsorption of these biopolymers and related charge reversal upon the multilayer film construction onto the particles surface. The shape of the capsules was characterized by scanning electron microscopy (SEM). These particles were tested against bacteria resistant to antibiotics, namely kanamycin-resistant Escherichia coli. An inhibitory effect of the particles was observed during bacterial growth in liquid medium, i.e. in the range of 10 % for (dex+/dex-)n coated CaCO3 materials and of 50% for (dex+/dex-)n capsules. These findings evidence the high potential of capsules to act as antimicrobial agents in future and in treatments against infections.

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Differential Biosynthesis and Roles of Two Ferrichrome-Type Siderophores, ASP2397/AS2488053 and Ferricrocin, in Acremonium persicinum

Ferrichromes are a family of fungal siderophores with cyclic hexapeptide structures. Most fungi produce one or two ferrichrome-type siderophores. Acremonium persicinum MF-347833 produces ferrichrome-like potent Trojan horse antifungal antibiotics ASP2397 and AS2488053, the aluminum- and iron-chelating forms of AS2488059, respectively. Here, we show by gene sequencing followed by gene deletion experiments that A. persicinum MF-347833 possesses two nonribosomal peptide synthetase genes responsible for AS2488059 and ferricrocin assembly. AS2488059 was produced under iron starvation conditions and excreted into the media to serve as a defense metabolite and probably an iron courier. In contrast, ferricrocin was produced under iron-replete conditions and retained inside the cells, likely serving as an iron-sequestering molecule. Notably, the phylogenetic analyses suggest the different evolutionary origin of AS2488059 from that of conventional ferrichrome-type siderophores. Harnessing two ferrichrome-type siderophores with distinct biological properties may give A. persicinum a competitive advantage for surviving the natural environment.

How the Presence of Hemin Affects the Expression of the Different Iron Uptake Pathways in Pseudomonas aeruginosa Cells

Iron is an essential nutriment for almost all organisms, but this metal is poorly bioavailable. During infection, bacteria access iron from the host by importing either iron or heme. Pseudomonas aeruginosa, a gram-negative pathogen, secretes two siderophores, pyoverdine (PVD) and pyochelin (PCH), to access iron and is also able to use many siderophores produced by other microorganisms (called xenosiderophores). To access heme, P. aeruginosa uses three distinct uptake pathways, named Has, Phu, and Hxu. We previously showed that P. aeruginosa expresses the Has and Phu heme uptake systems and the PVD- and PCH-dependent iron uptake pathways in iron-restricted growth conditions, using proteomic and RT-qPCR approaches. Here, using the same approaches, we show that physiological concentrations of hemin in the bacterial growth medium result in the repression of the expression of the proteins of the PVD- and PCH-dependent iron uptake pathways, leading to less production of these two siderophores. This indicates that the pathogen adapts its phenotype to use hemin as an iron source rather than produce PVD and PCH to access iron. Moreover, the presence of both hemin and a xenosiderophore resulted in (i) the strong induction of the expression of the proteins of the added xenosiderophore uptake pathway, (ii) repression of the PVD- and PCH-dependent iron uptake pathways, and (iii) no effect on the expression levels of the Has, Phu, or Hxu systems, indicating that bacteria use both xenosiderophores and heme to access iron.

New Pqs Quorum Sensing System Inhibitor as an Antibacterial Synergist against Multidrug-Resistant Pseudomonas aeruginosa

Development of new bacterial biofilm inhibitors as antibacterial synergists is an effective strategy to solve the resistance of Pseudomonas aeruginosa. In this paper, a series of 3-hydroxy-pyridin-4(1H)-ones were synthesized and evaluated, and the hit compound (20p) was identified with the effects of inhibiting the production of pyocyanin (IC₅₀ = 8.6 μM) and biofilm formation (IC₅₀ = 4.5 μM). Mechanistic studies confirmed that 20p inhibits the formation of bacterial biofilm by inhibiting the expression of pqsA, blocking pqs quorum sensing system quinolone biosynthesis. Moreover, we systematically investigated the bactericidal effects of combining currently approved antibiotics for CF including tobramycin, ciprofloxacin, and colistin E with 20p, which showed obvious antibacterial synergy to overcome antibiotics resistance in multidrug-resistant P. aeruginosa biofilms. The result indicates that compound 20p may be used in the future as a potentially novel antibacterial synergist candidate for the treatment of P. aeruginosa infections.

Iterative Structure-Based Optimization of Short Peptides Targeting the Bacterial Sliding Clamp

The bacterial DNA sliding clamp (SC), or replication processivity factor, is a promising target for the development of novel antibiotics. We report a structure-activity relationship study of a new series of peptides interacting within the Escherichia coli SC (^EcSC) binding pocket. Various modifications were explored including N-alkylation of the peptide bonds, extension of the N-terminal moiety, and introduction of hydrophobic and constrained residues at the C-terminus. In each category, single modifications were identified that increased affinity to ^EcSC. A combination of such modifications yielded in several cases to a substantially increased affinity compared to the parent peptides with K_d in the range of 30-80 nM. X-ray structure analysis of 11 peptide/^EcSC co-crystals revealed new interactions at the peptide-protein interface (i.e., stacking interactions, hydrogen bonds, and hydrophobic contacts) that can account for the improved binding. Several compounds among the best binders were also found to be more effective in inhibiting SC-dependent DNA synthesis.

Next generation Glucose-1-phosphate thymidylyltransferase (RmlA) inhibitors: An extended SAR study to direct future design

The monosaccharide l-Rhamnose is an important component of bacterial cell walls. The first step in the l-rhamnose biosynthetic pathway is catalysed by glucose-1-phosphate thymidylyltransferase (RmlA), which condenses glucose-1-phosphate (Glu-1-P) with deoxythymidine triphosphate (dTTP) to yield dTDP-d-glucose. In addition to the active site where catalysis of this reaction occurs, RmlA has an allosteric site that is important for its function. Building on previous reports, SAR studies have explored further the allosteric site, leading to the identification of very potent P. aeruginosa RmlA inhibitors. Modification at the C6-NH₂ of the inhibitor's pyrimidinedione core structure was tolerated. X-ray crystallographic analysis of the complexes of P. aeruginosa RmlA with the novel analogues revealed that C6-aminoalkyl substituents can be used to position a modifiable amine just outside the allosteric pocket. This opens up the possibility of linking a siderophore to this class of inhibitor with the goal of enhancing bacterial cell wall permeability.

Galbofloxacin: a xenometal-antibiotic with potent in vitro and in vivo efficacy against S. aureus

Siderophore-antibiotic drug conjugates are considered potent tools to deliver and potentiate the antibacterial activity of antibiotics, but only few have seen preclinical and clinical success. Here, we introduce the gallium(iii) complex of a ciprofloxacin-functionalized linear desferrichrome, Galbofloxacin, with a cleavable serine linker as a potent therapeutic for S. aureus bacterial infections. We employed characterization using in vitro inhibitory assays, radiochemical, tracer-based uptake and pharmacokinetic assessment of our lead compound, culminating in in vivo efficacy studies in a soft tissue model of infection. Galbofloxacin exhibits a minimum inhibitory concentration of (MIC₉₈) 93 nM in wt S. aureus, exceeding the potency of the parent antibiotic ciprofloxacin (0.9 μM). Galbofloxacin is a protease substrate that can release the antibiotic payload in the bacterial cytoplasm. Radiochemical experiments with wt bacterial strains reveal that ⁶⁷Galbofloxacin is taken up efficiently using siderophore mediated, active uptake. Biodistribution of ⁶⁷Galbofloxacin in a mouse model of intramuscular S. aureus infection revealed renal clearance and enhanced uptake in infected muscle when compared to ⁶⁷Ga-citrate, which showed no selectivity. A subsequent in vivo drug therapy study reveals efficient reduction in S. aureus infection burden and sustained survival with Galbofloxacin for 7 days. Ciprofloxacin had no treatment efficacy at identical molecular dose (9.3 μmol kg⁻¹) and resulted in death of all study animals in <24 hours. Taken together, the favorable bacterial growth inhibitory, pharmacokinetic and in vivo efficacy properties qualify Galbofloxacin as the first rationally designed Ga-coordination complex for the management of S. aureus bacterial infections.

Galbofloxacin, a novel theranostic xenosiderophore antibiotic, exhibits unparalleled potency in combating S. aureus infections in vivo.

Emergent antibacterial activity of N-(thiazol-2-yl)benzenesulfonamides in conjunction with cell-penetrating octaarginine

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy. In this work, we have synthesized N-(4-(4-(methylsulfonyl)phenyl)-5-phenylthiazol-2-yl)benzenesulfonamide derivatives that combine thiazole and sulfonamide, groups with known antibacterial activity. These molecules are investigated for their antibacterial activity, in isolation and in complex with the cell-penetrating peptide octaarginine. Several of the synthesized compounds display potent antibacterial activity against both Gram-negative and Gram-positive bacteria. Compounds with 4-tert-butyl and 4-isopropyl substitutions exhibit attractive antibacterial activity against multiple strains. The isopropyl substituted derivative displays low MIC of 3.9 μg mL⁻¹ against S. aureus and A. xylosoxidans. The comparative antibacterial behaviour of drug–peptide complex, drug alone and peptide alone indicates a distinctive mode of action of the drug–peptide complex, that is not the simple sum total of its constituent components. Specificity of the drug–peptide complex is evident from comparison of antibacterial behaviour with a synthetic intermediate–peptide complex. The octaarginine–drug complex displays faster killing-kinetics towards bacterial cells, creates pores in the bacterial cell membranes and shows negligible haemolytic activity towards human RBCs. Our results demonstrate that mere attachment of a hydrophobic moiety to a cell penetrating peptide does not impart antibacterial activity to the resultant complex. Conversely, the work suggests distinctive modes of antibiotic activity of small molecules when used in conjunction with a cell penetrating peptide.

Hybrid antimicrobials that combine the effect of two or more agents represent a promising antibacterial therapeutic strategy.

Iron coordination to pyochelin siderophore influences dynamics of FptA receptor from Pseudomonas aeruginosa: a molecular dynamics simulation study

FptA is a TonB-dependent transporter that permits the high affinity binding and transport of Fe(III)-pyochelin complex across the outer membrane of Pseudomonas aeruginosa. Molecular dynamics simulations were employed to FptA receptor and its complexes with pyochelin, and co-crystallized Fe(III)-pyochelin-ethanediol and Fe(III)-pyochelin-water embedded in dilauroyl phosphatidyl choline bilayer for the evaluation of their structural and dynamical properties. The evaluation of properties of the receptor bound to pyochelin molecule and Fe(III)-pyochelin complexes helped to figure out the iron coordination effect on the receptor properties. Moreover, comparison of these four simulation systems revealed further information on the dynamical changes occurred in extracellular loops, in particular loop-7 corresponding to the missing amino acid residues including the close-by loop-8 that was largely affected by the metal coordination to pyochelin. The binding of iron to pyochelin molecule affected the overall structure of the receptor therefore, evaluation fo the gyration radii and hydrogen bonding were evaluated as well as analysis of the pore size were also carried out to understand the effect of metal coordination on the dynamics of the helices which form a kind of translocation channel to transport the siderophore across the FptA protein into the periplasmic space. The properties of each component of the molecular systems were therefore observed to be perturbed by the incorporation of iron to the pyochelin molecule thus demonstrating that the bacteria use its receptor to abstract and transport iron from extracellular environment for its survival and that was made possible to understand at the molecular level through successful implementation of molecular dynamics simulations.