Exploiting bacterial iron acquisition: siderophore conjugates

Differential effects of inosine monophosphate dehydrogenase (IMPDH/GuaB) inhibition in Acinetobacter baumannii and Escherichia coli

Inosine 5'-monophosphate dehydrogenase (IMPDH), known as GuaB in bacteria, catalyzes the rate-limiting step in de novo guanine biosynthesis and is conserved from humans to bacteria. We developed a series of potent inhibitors that selectively target GuaB over its human homolog. Here, we show that these GuaB inhibitors are bactericidal, generate phenotypic signatures that are distinct from other antibiotics, and elicit different time-kill kinetics and regulatory responses in two important Gram-negative pathogens: Acinetobacter baumannii and Escherichia coli. Specifically, the GuaB inhibitor G6 rapidly kills A. baumannii but only kills E. coli after 24 h. After exposure to G6, the expression of genes involved in purine biosynthesis and stress responses change in opposite directions while siderophore biosynthesis is downregulated in both species. Our results suggest that different species respond to GuaB inhibition using distinct regulatory programs and possibly explain the different bactericidal kinetics upon GuaB inhibition. The comparison highlights opportunities for developing GuaB inhibitors as novel antibiotics.IMPORTANCEA. baumannii is a priority bacterial pathogen for which development of new antibiotics is urgently needed due to the emergence of multidrug resistance. We recently developed a series of specific inhibitors against GuaB, a bacterial inosine 5'-monophosphate dehydrogenase, and achieved sub-micromolar minimum inhibitory concentrations against A. baumannii. GuaB catalyzes the rate-limiting step of de novo guanine biosynthesis and is highly conserved across bacterial pathogens. This study shows that inhibition of GuaB induced a bacterial morphological profile distinct from that of other classes of antibiotics, highlighting a novel mechanism of action. Moreover, our transcriptomic analysis showed that regulation of de novo purine biosynthesis and stress responses of A. baumannii upon GuaB inhibition differed significantly from that of E. coli.

Implementation of Three Gallium-Based Complexes in the "Trojan Horse" Antibacterial Strategy against A. baumannii: A DFT Approach

Microorganisms of the ESKAPE group pose an enormous threat to human well-being, thus requiring a multidisciplinary approach for discovering novel drugs that are not only effective but utilize an innovative mechanism of action in order to decrease fast developing resistance. A promising but still hardly explored implementation in the "Trojan horse" antibacterial strategy has been recognized in gallium, an iron mimicry species with no known function but exerting a bacteriostatic/bactericidal effect against some representatives of the group. The study herewith focuses on the bacterium A. baumannii and its siderophore acinetobactin in its two isomeric forms depending on the acidity of the medium. By applying the powerful tools of the DFT approach, we aim to delineate those physicochemical characteristics that are of great importance for potentiating gallium's ability to compete with the native ferric cation for binding acinetobactin such as pH, solvent exposure (dielectric constant of the environment), different metal/siderophore ratios, and complex composition. Hence, the provided results not only furnish some explanation of the positive effect of three Ga3+-based anti-infectives in terms of metal cation competition but also shed light on reported in vitro and in vivo observations at a molecular level in regard to gallium's antibacterial effect against A. baumannii.

Attaching Metal-Containing Moieties to β-Lactam Antibiotics: The Case of Penicillin and Cephalosporin

Procedures for the preparation of transition metal complexes having intact bicyclic cepham or penam systems as ligands have been developed. Starting from readily available 4-azido-2-azetidinones, a synthetic approach has been tuned using a copper-catalyzed azide-alkyne cycloaddition between 3-azido-2-azetinones and alkynes, followed by methylation and transmetalation to Au(I) and Ir(III) complexes from the mesoionic carbene Ag(I) complexes. This methodology was applied to 6-azido penam and 7-azido cepham derivatives to build 6-(1,2,3-triazolyl)penam and 7-(1,2,3-triazolyl)cepham proligands, which upon methylation and metalation with Au(I) and Ir(III) complexes yielded products derived from the coordination of the metal to the penam C6 and cepham C7 positions, preserving intact the bicyclic structure of the penicillin and cephalosporin scaffolds. The crystal structure of complex 28b, which has an Ir atom directly bonded to the intact penicillin bicycle, was determined by X-ray diffraction. This is the first structural report of a penicillin-transition-metal complex having the bicyclic system of these antibiotics intact. The selectivity of the coordination processes was interpreted using DFT calculations.

Discovery of YFJ-36: Design, Synthesis, and Antibacterial Activities of Catechol-Conjugated β-Lactams against Gram-Negative Bacteria

Cefiderocol is the first approved catechol-conjugated cephalosporin against multidrug-resistant Gram-negative bacteria, while its application was limited by poor chemical stability associated with the pyrrolidinium linker, moderate potency against Klebsiella pneumoniae and Acinetobacter baumannii, intricate procedures for salt preparation, and potential hypersensitivity. To address these issues, a series of novel catechol-conjugated derivatives were designed, synthesized, and evaluated. Extensive structure-activity relationships and structure-metabolism relationships (SMR) were conducted, leading to the discovery of a promising compound 86b (Code no. YFJ-36) with a new thioether linker. 86b exhibited superior and broad-spectrum in vitro antibacterial activity, especially against A. baumannii and K. pneumoniae, compared with cefiderocol. Potent in vivo efficacy was observed in a murine systemic infection model. Furthermore, the physicochemical stability of 86b in fluid medium at pH 6-8 was enhanced. 86b also reduced potential the risk of allergy owing to the quaternary ammonium linker. The improved properties of 86b supported its further research and development.

In Silico Analysis of the Ga3+/Fe3+ Competition for Binding the Iron-Scavenging Siderophores of P. aeruginosa-Implementation of Three Gallium-Based Complexes in the "Trojan Horse" Antibacterial Strategy

The emergence of multidrug-resistant (MDR) microorganisms combined with the ever-draining antibiotic pipeline poses a disturbing and immensely growing public health challenge that requires a multidisciplinary approach and the application of novel therapies aimed at unconventional targets and/or applying innovative drug formulations. Hence, bacterial iron acquisition systems and bacterial Fe2+/3+-containing enzymes have been identified as a plausible target of great potential. The intriguing "Trojan horse" approach deprives microorganisms from the essential iron. Recently, gallium's potential in medicine as an iron mimicry species has attracted vast attention. Different Ga3+ formulations exhibit diverse effects upon entering the cell and thus supposedly have multiple targets. The aim of the current study is to specifically distinguish characteristics of great significance in regard to the initial gallium-based complex, allowing the alien cation to effectively compete with the native ferric ion for binding the siderophores pyochelin and pyoverdine secreted by the bacterium P. aeruginosa. Therefore, three gallium-based formulations were taken into consideration: the first-generation gallium nitrate, Ga(NO3)3, metabolized to Ga3+-hydrated forms, the second-generation gallium maltolate (tris(3-hydroxy-2-methyl-4-pyronato)gallium), and the experimentally proven Ga carrier in the bloodstream-the protein transferrin. We employed a reliable in silico approach based on DFT computations in order to understand the underlying biochemical processes that govern the Ga3+/Fe3+ rivalry for binding the two bacterial siderophores.

Specificity and mechanism of TonB-dependent ferric catecholate uptake by Fiu

We studied the Escherichia coli outer membrane protein Fiu, a presumed transporter of monomeric ferric catecholates, by introducing Cys residues in its surface loops and modifying them with fluorescein maleimide (FM). Fiu-FM bound iron complexes of the tricatecholate siderophore enterobactin (FeEnt) and glucosylated enterobactin (FeGEnt), their dicatecholate degradation product Fe(DHBS)2 (FeEnt*), the monocatecholates dihydroxybenzoic acid (FeDHBA) and dihydroxybenzoyl serine (FeDHBS), and the siderophore antibiotics cefiderocol (FDC) and MB-1. Unlike high-affinity ligand-gated porins (LGPs), Fiu-FM had only micromolar affinity for iron complexes. Its apparent KD values for FeDHBS, FeDHBA, FeEnt*, FeEnt, FeGEnt, FeFDC, and FeMB-1 were 0.1, 0.7, 0.7, 1.0, 0.3, 0.4, and 4 μM, respectively. Despite its broad binding abilities, the transport repertoires of E. coli Fiu, as well as those of Cir and FepA, were less broad. Fiu only transported FeEnt*. Cir transported FeEnt* and FeDHBS (weakly); FepA transported FeEnt, FeEnt*, and FeDHBA. Both Cir and FepA bound FeGEnt, albeit with lower affinity. Related transporters of Acinetobacter baumannii (PiuA, PirA, BauA) had similarly moderate affinity and broad specificity for di- or monomeric ferric catecholates. Both microbiological and radioisotopic experiments showed Fiu's exclusive transport of FeEnt*, rather than ferric monocatecholate compounds. Molecular docking and molecular dynamics simulations predicted three binding sites for FeEnt*in the external vestibule of Fiu, and a fourth site deeper in its interior. Alanine scanning mutagenesis in the outermost sites (1a, 1b, and 2) decreased FeEnt* binding affinity as much as 20-fold and reduced or eliminated FeEnt* uptake. Finally, the molecular dynamics simulations suggested a pathway of FeEnt* movement through Fiu that may generally describe the process of metal transport by TonB-dependent receptors.

Cephalosporin resistance, tolerance, and approaches to improve their activities

Cephalosporins comprise a β-lactam antibiotic class whose first members were discovered in 1945 from the fungus Cephalosporium acremonium. Their clinical use for Gram-negative bacterial infections is widespread due to their ability to traverse outer membranes through porins to gain access to the periplasm and disrupt peptidoglycan synthesis. More recent members of the cephalosporin class are administered as last resort treatments for complicated urinary tract infections, MRSA, and other multi-drug resistant pathogens, such as Neisseria gonorrhoeae. Unfortunately, there has been a global increase in cephalosporin-resistant strains, heteroresistance to this drug class has been a topic of increasing concern, and tolerance and persistence are recognized as potential causes of cephalosporin treatment failure. In this review, we summarize the cephalosporin antibiotic class from discovery to their mechanisms of action, and discuss the causes of cephalosporin treatment failure, which include resistance, tolerance, and phenomena when those qualities are exhibited by only small subpopulations of bacterial cultures (heteroresistance and persistence). Further, we discuss how recent efforts with cephalosporin conjugates and combination treatments aim to reinvigorate this antibiotic class.

Siderophore conjugates to combat antibiotic-resistant bacteria

Antimicrobial resistance (AMR) is a global threat to society due to the increasing emergence of multi-drug resistant bacteria that are not susceptible to our last line of defence antibiotics. Exacerbating this issue is a severe gap in antibiotic development, with no new clinically relevant classes of antibiotics developed in the last two decades. The combination of the rapidly increasing emergence of resistance and scarcity of new antibiotics in the clinical pipeline means there is an urgent need for new efficacious treatment strategies. One promising solution, known as the 'Trojan horse' approach, hijacks the iron transport system of bacteria to deliver antibiotics directly into cells - effectively tricking bacteria into killing themselves. This transport system uses natively produced siderophores, which are small molecules with a high affinity for iron. By linking antibiotics to siderophores, to make siderophore antibiotic conjugates, the activity of existing antibiotics can potentially be reinvigorated. The success of this strategy was recently exemplified with the clinical release of cefiderocol, a cephalosporin-siderophore conjugate with potent antibacterial activity against carbapenem-resistant and multi-drug resistant Gram-negative bacilli. This review discusses the recent advancements in siderophore antibiotic conjugates and the challenges associated with the design of these compounds that need to be overcome to deliver more efficacious therapeutics. Potential strategies have also been suggested for new generations of siderophore-antibiotics with enhanced activity.

Emerging Non-Antibiotic Options Targeting Uropathogenic Mechanisms for Recurrent Uncomplicated Urinary Tract Infection

Urinary tract infections (UTIs) are the most frequent bacterial infections in the clinical setting. Even without underlying anatomic or functional abnormalities, more than 40% of women experience at least one UTI in their lifetime, of which 30% develop recurrent UTIs (rUTIs) within 6 months. Conventional management with antibiotics for rUTIs may eventually lead to the development of multidrug-resistant uropathogens. Targeting of the pathogenicity of rUTIs, the evolution of uropathogenic Escherichia coli (UPEC), and inadequate host defenses by immune responses should be explored to provide non-antibiotic solutions for the management of rUTIs. The adaptive evolution of UPEC has been observed in several aspects, including colonization, attachment, invasion, and intracellular replication to invade the urothelium and survive intracellularly. Focusing on the antivirulence of UPEC and modulating the immunity of susceptible persons, researchers have provided potential alternative solutions in four categories: antiadhesive treatments (i.e., cranberries and D-mannose), immunomodulation therapies, vaccines, and prophylaxis with topical estrogen therapy and probiotics (e.g., Lactobacillus species). Combination therapies targeting multiple pathogenic mechanisms are expected to be a future trend in UTI management, although some of these treatment options have not been well established in terms of their long-term efficacy. Additional clinical trials are warranted to validate the therapeutic efficacy and durability of these techniques.

Siderophores: Chemical Tools for Precise Antibiotic Delivery

The success of precision medicine coupled with the disappointing impact of broad-spectrum antibiotic use on microbiome stability and bacterial resistance, has triggered a shift in antibiotic design strategies toward precision antibiotics. This also includes the implementation of novel vectorization approaches directed to improve the internalization of antibacterial agents into deadly gram-negative pathogens through precise and well-defined mechanisms. The conjugation of antibiotics to siderophores (iron scavengers), which are compounds that are able to afford stable iron-complexes that facilitate the internalization into the cell by using bacterial iron uptake pathways as gateways, is a strategy that has begun to show excellent results with the commercialization of the first antibiotic based on this principle, cefiderocol. This digests review provides an overview of the molecular basis for this antibiotic-siderophore conjugation approach, along with recent successful examples and highlights future challenges facing this booming research area.

All Roads Lead to Rome: Enhancing the Probability of Target Attainment with Different Pharmacokinetic/Pharmacodynamic Modelling Approaches

In light of rising antimicrobial resistance and a decreasing number of antibiotics with novel modes of action, it is of utmost importance to accelerate development of novel treatment options. One aspect of acceleration is to understand pharmacokinetics (PK) and pharmacodynamics (PD) of drugs and to assess the probability of target attainment (PTA). Several in vitro and in vivo methods are deployed to determine these parameters, such as time-kill-curves, hollow-fiber infection models or animal models. However, to date the use of in silico methods to predict PK/PD and PTA is increasing. Since there is not just one way to perform the in silico analysis, we embarked on reviewing for which indications and how PK and PK/PD models as well as PTA analysis has been used to contribute to the understanding of the PK and PD of a drug. Therefore, we examined four recent examples in more detail, namely ceftazidime-avibactam, omadacycline, gepotidacin and zoliflodacin as well as cefiderocol. Whereas the first two compound classes mainly relied on the ‘classical’ development path and PK/PD was only deployed after approval, cefiderocol highly profited from in silico techniques that led to its approval. Finally, this review shall highlight current developments and possibilities to accelerate drug development, especially for anti-infectives.

Cytoplasmic Delivery of an Antibiotic, Trimethoprim, with a Simple Bidentate Catechol Analog as a Siderophore Mimetic

Concerns about antibiotic-resistant Gram-negative pathogens are escalating, and accordingly siderophore-based intracellular antibiotic delivery is attracting more attention as an effective means to overcome these infections. Despite the successful clinical translation of this strategy, the delivery potential of siderophores has been limited to periplasm targeting, and this has appreciably restricted the repertoire of applicable antibiotics. To overcome this shortcoming of the current technology, this study focused on investigating the capability of simple bidentate catechol analogs to function as vehicles for cytoplasmic antibiotic delivery. Specifically, by employing trimethoprim, an inhibitor of dihydrofolate reductase located in the cytoplasm, as a model antibiotic, a chemical library of chelator-antibiotic conjugates featuring four different catechol analogs was prepared. Then, their various pharmacological properties and antimicrobial activities were evaluated. Analysis of these characterization data led to the identification of the active conjugates exhibiting notable iron- and trimethoprim-dependent potency against Escherichia coli. Further characterization of these hit molecules using E. coli mutant strains revealed that 2,3-dihydroxybenzoate could effectively deliver several corresponding conjugates to the cytoplasm by exploiting the siderophore uptake machineries present across the outer and inner membranes, originally designated for the native siderophore of E. coli, enterobactin. Considering the synthetic simplicity, such a catechol analog could have appreciable usage in potentiating cytoplasm-active antibiotics against recalcitrant Gram-negative pathogens.

Design and Synthesis of 3-Hydroxy-pyridin-4(1H)-ones-Ciprofloxacin Conjugates as Dual Antibacterial and Antibiofilm Agents against Pseudomonas aeruginosa

Pseudomonas aeruginosa infections are often complicated by the fact that it can easily form a biofilm that increases its resistance to antibiotics. Consequently, the development of novel antibacterial agents against biofilm-associated drug-resistant P. aeruginosa is urgently needed. Herein, we report a series of 3-hydroxy-pyridin-4(1H)-ones-ciprofloxacin conjugates that were designed and synthesized as dual antibacterial and antibiofilm agents against P. aeruginosa. A potential 2-substituted 3-hydroxy-1,6-dimethylpyridin-4(1H)-one-ciprofloxacin conjugate (5e) was identified and had the best minimum inhibitory concentrations of 0.86 and 0.43 μM against P. aeruginosa 27853 and PAO1 and reduced 78.3% of biofilm formation. In addition, 5e eradicates mature biofilms and kills living bacterial cells that are incorporated into the biofilm. Studies on the antibiofilm mechanism of conjugates showed that 5e interferes with iron uptake by bacteria, inhibits their motility, and reduces the production of virulence. These results demonstrate that 3-hydroxy-pyridin-4(1H)-ones-ciprofloxacin conjugates are potent in the treatment of biofilm-associated drug-resistant P. aeruginosa infections.

Resistance to Some New Drugs and Prevalence of ESBL- and MBL-Producing Enterobacteriaceae Uropathogens Isolated from Diabetic Patients

Diabetes is a leading non-communicable disease and a risk factor for relapsing infections. The current study was aimed at investigating the prevalence and antibiotic susceptibility of carbapenem-resistant (CR) uropathogens of the family Enterobacteriaceae in diabetic patients. The data of 910 bacterial isolates was collected from diagnostic laboratories during January 2018 to December 2018. The bacterial isolates were identified using traditional methods including colonial characteristics, biochemical tests, and API (20E). Antimicrobial susceptibility and phenotypic characterization of ESBL, MBLs, and KPC was determined by utilizing CLSI recommended methods. The phenotypically positive isolates were further analyzed for resistance-encoding genes by manual PCR and Check-MDR CT103XL microarray. Susceptibility to colistin and cefiderocol was tested in accordance with CLSI guidelines. The data revealed that most of the patients were suffering from type 2 diabetes for a duration of more than a year and with uncontrolled blood sugar levels. Escherichia coli and Klebsiella pneumoniae were the most frequently encountered pathogens, followed by Enterobacter cloacae and Proteus mirabilis. More than 50% of the isolates showed resistance to 22 antibiotics, with the highest resistance (>80%) against tetracycline, ampicillin, and cefazolin. The uropathogens showed less resistance to non-β-lactam antibiotics, including amikacin, fosfomycin, and nitrofurantoin. In the phenotypic assays, 495 (54.3%) isolates were found to be ESBL producers, while ESBL-TEM and -PER were the most prevalent ESBL types. The resistance to carbapenems was slightly less (250; 27.5%) than ESBL producers, yet more common amongst E. coli isolates. MBL production was a common feature in carbapenem-resistant isolates (71.2%); genotypic characterization also validated this trend. The isolates were found to be sensitive against the new drugs, cefiderocol and eravacycline. with 7−28% resistance, except for P. mirabilis which had 100% resistance against eravacycline. This study concludes that a few types of ESBL and carbapenemases are common in the uropathogens isolated from the diabetic patients, and antibiotic stewardship programs need to be revisited, particularly to cure UTIs in diabetic patients.

Proteomic Investigation of the Antibacterial Mechanism of Cefiderocol against Escherichia coli

This study aimed to investigate the antibacterial mechanism of cefiderocol (CFDC) using data-independent acquisition quantitative proteomics combined with cellular and molecular biological assays. Numerous differentially expressed proteins related to the production of NADH, reduced cofactor flavin adenine dinucleotide (FADH2), NADPH and reactive oxygen species (ROS), iron-sulfur cluster binding, and iron ion homeostasis were found to be upregulated by CFDC. Furthermore, parallel reaction monitoring analysis validated these results. Meanwhile, we confirmed that the levels of NADH, ROS, H2O2, and iron ions were induced by CFDC, and the sensitivity of Escherichia coli to CFDC was inhibited by the antioxidant vitamin C, N-acetyl-l-cysteine, and deferoxamine. Moreover, deferoxamine also suppressed the H2O2 stress induced by CFDC. In addition, knockout of the NADH-quinone oxidoreductase genes (nuoA, nuoC, nuoE, nuoF, nuoG, nuoJ, nuoL, nuoM) in the respiratory chain attenuated the sensitivity of E. coli to CFDC far beyond the effects of cefepime and ceftazidime; in particular, the E. coli BW25113 ΔnuoJ strain produced 60-fold increases in MIC to CFDC compared to that of the wild-type E. coli BW25113 strain. The present study revealed that CFDC exerts its antibacterial effects by inducing ROS stress by elevating the levels of NADH and iron ions in E. coli. IMPORTANCE CFDC was the first FDA-approved siderophore cephalosporin antibiotic in 2019 and is known for its Trojan horse tactics and broad antimicrobial activity against Gram-negative bacteria. However, its antibacterial mechanism is not fully understood, and whether it has an impact on in vivo iron ion homeostasis remains unknown. To comprehensively reveal the antibacterial mechanisms of CFDC, data-independent acquisition quantitative proteomics combined with cellular and molecular biological assays were performed in this study. The findings will further facilitate our understanding of the antibacterial mechanism of CFDC and may provide a theoretical foundation for controlling CFDC resistance in the future.

Occurrence of High Levels of Cefiderocol Resistance in Carbapenem-Resistant Escherichia coli before Its Approval in China: a Report from China CRE-Network

Cefiderocol has been approved in the United States and Europe but not in China. We aim to evaluate carbapenem-resistant Enterobacterales (CRE) susceptibility to cefiderocol to provide baseline data and investigate the resistance mechanism. From 2018 to 2019, 1,158 CRE isolates were collected from 23 provinces and municipalities across China. The MICs of antimicrobials were determined via the agar dilution and broth microdilution methods. Whole-genome sequencing was performed for 26 cefiderocol-resistant Escherichia coli isolates to investigate the resistance mechanism. Clone transformations were used to explore the function of cirA, pbp3, and blaNDM-5 in resistance. Among the 21 antimicrobials tested, aztreonam-avibactam had the highest antibacterial activity (98.3%), followed by cefiderocol (97.3%) and colistin (95.3%). A total of 26 E. coli isolates harboring New Delhi metallo-beta-lactamase 5 (NDM-5) showed high levels of cefiderocol resistance, of which sequence type 167 (ST167) accounted for 76.9% (20/26). We found 4 amino-acid insertions (YRIN/YRIK) at position 333 of penicillin-binding protein 3 (PBP3) in the 26 E. coli isolates, and 22 isolates had a siderophore receptor cirA premature stop codon. After obtaining the wild-type cirA supplementation, the MIC of the transformants decreased by 8 to 16 times in two cefiderocol-resistant isolates. A cefiderocol-susceptible isolate harboring NDM-5 has an MIC increased from 1 μg/mL to 64 μg/mL after cirA deletion, and the MIC decreased from 64 μg/mL to 0.5 μg/mL after blaNDM-5 deletion. The MIC of the E. coli DH5α, from which the pbp3 mutant was obtained, increased from 0.064 μg/mL to 0.25 μg/mL. Cefiderocol showed activity against most CRE in China. The resistance of ST167 E. coli to cefiderocol is a combination of the premature stop codon of cirA, pbp3 mutation, and blaNDM-5 existence. IMPORTANCE Cefiderocol, a new siderophore cephalosporin, has been approved in the United States and Europe but not in China. At present, there are almost no antimicrobial susceptibility evaluation data on cefiderocol in China. We evaluated the in vitro susceptibility of 1,158 strains of carbapenem-resistant Enterobacterales to cefiderocol and other antibiotics. We found that a high proportion of Escherichia coli showed high-level resistance to cefiderocol. Whole-genome sequencing (WGS) and molecular cloning experiments confirmed that the synergistic effect of the cirA gene premature stop codon, blaNDM-5 existence, and the pbp3 mutation is associated with high levels of cefiderocol resistance.

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.

Native Heavy Metal-Tolerant Plant Growth Promoting Rhizobacteria Improves Sulla spinosissima (L.) Growth in Post-Mining Contaminated Soils

The potential of rhizobacteria in assisting plants used in the phytostabilization or re-vegetation of soils contaminated by heavy metals is gaining interest all around the world. In this context, six rhizobacterial strains isolated from highly heavy metal-contaminated soils situated in abandoned mining sites around the Oujda region (Morocco) were tested with Sulla spinosissima (L.), a native leguminous plant expanding in this area. The strains used were multi-resistant to heavy metals and possessed multiple plant growth-promoting traits. Potential beneficial effects of the strains were also evaluated in planta by measuring various growth and physiological parameters of inoculated Sulla plants grown in sterilized sand. Inoculation with the Rhodococcus qingshengii strain LMR340 boosted plant biomass (39% to 83% increase compared to uninoculated plants), chlorophyll and carotenoid content (up to 29%), and antioxidant enzyme activities (15% to 80% increase). Based on these interesting findings, selected strains were inoculated into plants growing in a heavy metal, multi-polluted, and poor soil. Under these conditions, non-inoculated plants and those inoculated with the strain LMR250 were unable to grow, while the other five bacterial inoculants restored plant growth. The best performing strain, Pseudarthrobacter oxydans LMR291, could be considered as a good biofertilizer and/or biostimulant candidate to be used for promoting the growth of selected plants in re-vegetation and/or phytostabilization programs of degraded and contaminated soils.

Tracking Uptake and Metabolism of Xenometallomycins Using a Multi-Isotope Tagging Strategy

Synthetic and naturally occurring siderophores and their conjugates provide access to the bacterial cytoplasm via active membrane transport. Previously, we displaced iron with the radioactive isotope ⁶⁷Ga to quantify and track in vitro and in vivo uptake and distribution of siderophore Trojan Horse antibiotic conjugates. Here, we introduce a multi-isotope tagging strategy to individually elucidate the fate of metal cargo and the ligand construct with radioisotopes ⁶⁷Ga and ¹²⁴I. We synthesized gallium(III) model complexes of a ciprofloxacin-functionalized linear desferrichrome (Ga-D6) and deferoxamine (Ga-D7) incorporating an iodo-tyrosine linker to enable radiolabeling using the metal-binding (⁶⁷Ga) and the cargo-conjugation site (¹²⁴I). Radiochemical experiments with Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa wt strains show that ⁶⁷Ga-D6/D7 and Ga-D6-¹²⁴I/D7-¹²⁴I have comparable uptake, indicating intact complex import and siderophore-mediated uptake. In naive mice, ⁶⁷Ga-D6/D7 and Ga-D6-¹²⁴I/D7-¹²⁴I demonstrate predominantly renal clearance; urine metabolite analysis indicates in vivo dissociation of Ga(III) is a likely mechanism of degradation for ⁶⁷Ga-D6/D7 when compared to ligand radiolabeled compounds, Ga-D6-¹²⁴I/D7-¹²⁴I, which remain >60% intact in urine. Cumulatively, this work demonstrates that a multi-isotope tagging strategy effectively elucidates the in vitro uptake, pharmacokinetics, and in vivo stability of xenometallomycins with modular chemical structures.

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

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.

In-Depth Analysis of the Role of the Acinetobactin Cluster in the Virulence of Acinetobacter baumannii

Acinetobacter baumannii is a multidrug-resistant pathogen that represents a serious threat to global health. A. baumannii possesses a wide range of virulence factors that contribute to the bacterial pathogenicity. Among them, the siderophore acinetobactin is one of the most important, being essential for the development of the infection. In this study we performed an in-depth analysis of the acinetobactin cluster in the strain A. baumannii ATCC 17978. For this purpose, nineteen individual isogenic mutant strains were generated, and further phenotypical analysis were performed. Individual mutants lacking the biosynthetic genes entA, basG, basC, basD, and basB showed a significant loss in virulence, due to the disruption in the acinetobactin production. Similarly, the gene bauA, coding for the acinetobactin receptor, was also found to be crucial for the bacterial pathogenesis. In addition, the analysis of the ΔbasJ/ΔfbsB double mutant strain demonstrated the high level of genetic redundancy between siderophores where the role of specific genes of the acinetobactin cluster can be fulfilled by their fimsbactin redundant genes. Overall, this study highlights the essential role of entA, basG, basC, basD, basB and bauA in the pathogenicity of A. baumannii and provides potential therapeutic targets for the design of new antivirulence agents against this microorganism.

Biomimetic enterobactin analogue mediates iron-uptake and cargo transport into E. coli and P. aeruginosa

The design, synthesis and biological evaluation of the artificial enterobactin analogue EntKL and several fluorophore-conjugates thereof are described. EntKL provides an attachment point for cargos such as fluorophores or antimicrobial payloads. Corresponding conjugates are recognized by outer membrane siderophore receptors of Gram-negative pathogens and retain the natural hydrolyzability of the tris-lactone backbone. Initial density-functional theory (DFT) calculations of the free energies of solvation (ΔG(sol)) and relaxed Fe-O force constants of the corresponding [Fe-EntKL]3- complexes indicated a similar iron binding constant compared to natural enterobactin (Ent). The synthesis of EntKL was achieved via an iterative assembly based on a 3-hydroxylysine building block over 14 steps with an overall yield of 3%. A series of growth recovery assays under iron-limiting conditions with Escherichia coli and Pseudomonas aeruginosa mutant strains that are defective in natural siderophore synthesis revealed a potent concentration-dependent growth promoting effect of EntKL similar to natural Ent. Additionally, four cargo-conjugates differing in molecular size were able to restore growth of E. coli indicating an uptake into the cytosol. P. aeruginosa displayed a stronger uptake promiscuity as six different cargo-conjugates were found to restore growth under iron-limiting conditions. Imaging studies utilizing BODIPYFL-conjugates, demonstrated the ability of EntKL to overcome the Gram-negative outer membrane permeability barrier and thus deliver molecular cargos via the bacterial iron transport machinery of E. coli and P. aeruginosa.

Chemical Synthesis of Cell Wall Constituents of Mycobacterium tuberculosis

The pathogen Mycobacterium tuberculosis (Mtb), causing tuberculosis disease, features an extraordinary thick cell envelope, rich in Mtb-specific lipids, glycolipids, and glycans. These cell wall components are often directly involved in host–pathogen interaction and recognition, intracellular survival, and virulence. For decades, these mycobacterial natural products have been of great interest for immunology and synthetic chemistry alike, due to their complex molecular structure and the biological functions arising from it. The synthesis of many of these constituents has been achieved and aided the elucidation of their function by utilizing the synthetic material to study Mtb immunology. This review summarizes the synthetic efforts of a quarter century of total synthesis and highlights how the synthesis layed the foundation for immunological studies as well as drove the field of organic synthesis and catalysis to efficiently access these complex natural products.

Function of Fimsbactin B as an Acinetobacter-Selective Antibiotic Delivery Vehicle

The ability of fimsbactin B, a natural siderophore of Acinetobacter baumannii, to function as an antibiotic delivery vehicle was investigated by synthesizing three structurally diversified fimsbactin B-cefaclor conjugates. Their antimicrobial activities were Acinetobacter-selective and up to 128-fold more potent than that of cefaclor alone. This activity enhancement originated from the fimsbactin-B-dependent active uptake of cefaclor. Thus, fimsbactin-B-based antibiotic delivery can be an effective approach in combating antibiotic-resistant Acinetobacter infections.

Coordination Complexes to Combat Bacterial Infections: Recent Developments, Current Directions and Future Opportunities

Drug discovery aimed at the efficient eradication of life-threatening bacterial infections, especially in light of the emergence of multi-drug resistance of pathogenic bacteria, has remained a challenge for medicinal chemists over the past several decades. As nutrient acquisition and metabolism at the host-pathogen interface become better elucidated, new drug targets continue to emerge. Metal homeostasis is among these processes, and thus provides opportunities for medicinal inorganic chemists to alter or disrupt these processes selectively to impart bacteriostatic or bacteriotoxic effects. In this minireview, we showcase some of the recent work from the field of metal-based antibacterial agents and highlight divergent strategies and mechanisms of action.

Macrocycle-Antibiotic Hybrids: A Path to Clinical Candidates

The tale of abate in antibiotics continued defense mechanisms that chaperone the rise of drug-defying superbugs—on the other hand, the astray in antibacterial drug discovery and development. Our salvation lies in circumventing the genesis of resistance. Considering the competitive advantages of antibacterial chemotherapeutic agents equipped with multiple warheads against resistance, the development of hybrids has rejuvenated. The adoption of antibiotic hybrid paradigm to macrocycles has advanced novel chemical entities to clinical trials. The multi-targeted TD-1792, for instance, retained potent antibacterial activities against multiple strains that are resistant to its constituent, vancomycin. Moreover, the antibiotic conjugation of rifamycins has provided hybrid clinical candidates with desirable efficacy and safety profiles. In 2020, the U.S. FDA has granted an orphan drug designation to TNP-2092, a conjugate of rifamycin and fluoroquinolone, for the treatment of prosthetic joint infections. DSTA4637S is a pioneer antibacterial agent under clinical development and represents a novel class of bacterial therapy, that is, antibody–antibiotic conjugates. DSTA4637S is effective against the notorious persistent S. aureus bacteremia, a revelation of the abracadabra potential of antibiotic hybrid approaches.

Design and Syntheses of New Antibiotics Inspired by Nature's Quest for Iron in an Oxidative Climate

This Account describes fundamental chemistry that promoted the discovery of new antibiotics. Specifically, the NH acidity of simple hydroxamic acid derivatives facilitated the syntheses of novel β-lactams (oxamazins and monobactams), siderophore mimics that limit bacterial iron uptake and bacterially targeted sideromycins (siderophore-antibiotic conjugates). The development of resistance to our current limited set of antibiotic scaffolds has created a dire medical situation. As recently stated, "if you weren't taking antibiotic resistance seriously before, now would be a good time to start." A project commissioned by the British government (https://amr-review.org/) has released estimates of the near-future global toll of antibiotic resistance that are jaw-dropping in their seriousness and scale: 10 million deaths per year and at least $100 trillion in sacrificed gross national product. The 2020 COVID pandemic confirmed that infectious disease problems are no longer localized but worldwide. Many classical antibiotics, especially β-lactams, previously provided economical cures, but the evolution of antibiotic destructive enzymes (i.e., β-lactamases), efflux pumps, and bacterial cell wall permeability barriers has made many types of bacteria, especially Gram-negative strains, resistant. Still, and in contrast to other therapies, the public expectation is that any new antibiotic must be inexpensive. This creates market limitations that have caused most major pharmaceutical companies to abandon antibiotic research. Much needs to be done to address this significant problem.The critical need for bacteria to sequester essential iron provides an Achilles' heel for new antibiotic development. Although ferric iron is extremely insoluble, bacteria need micromolar intracellular concentrations for growth and virulence. To this end, they biosynthesize siderophores (Gr. iron bearer) and excrete them into their environment, where they bind iron with high affinity. The iron complexes are recognized by specific outer-membrane transporters, and once actively internalized, the iron is released for essential processes. To conserve biosynthetic energy, some bacteria recognize and utilize siderophores made by competing strains. As a counter-revolution in the never-ending fight for survival, bacteria have also evolved sideromycins, which are siderophores conjugated to warheads that are lethal to rogue bacteria. While none are now used therapeutically, natural sideromycins called albomycins have been used clinically, and others have been shown to be well tolerated and active in animal infection models. Herein we describe practical methods to synthesize new antibiotics and artificial sideromycins with the generalized structure shown above (siderophore-linker drug). Utilizing the molecular-recognition-based siderophore/sideromycin bacterial assimilation processes, it is possible to design both broad spectrum and exquisitely narrow spectrum (targeted) sideromycins and even repurpose older or more classical antibiotics. Relevant microbiological assays, in vivo animal infection studies, and the recent FDA approval of cefiderocol demonstrate their effectiveness.

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

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

Catechol-Based Antimicrobial Polymers

Catechol is a key constituent in mussel adhesive proteins and is responsible for strong adhesive property and crosslinking formation. Plant-based polyphenols are also capable of chemical interactions similar to those of catechol and are inherently antimicrobial. This review reports a series of catechol-based antimicrobial polymers classified according to their antimicrobial mechanisms. Catechol is utilized as a surface anchoring group for adhering monomers and polymers of known antimicrobial properties onto various types of surfaces. Additionally, catechol's ability to form strong complexes with metal ions and nanoparticles was utilized to sequester these antimicrobial agents into coatings and polymer matrices. During catechol oxidation, reactive oxygen species (ROS) is generated as a byproduct, and the use of the generated ROS for antimicrobial applications was also introduced. Finally, polymers that utilized the innate antimicrobial property of halogenated catechols and polyphenols were reviewed.

Enterobactin- and salmochelin-β-lactam conjugates induce cell morphologies consistent with inhibition of penicillin-binding proteins in uropathogenic Escherichia coli CFT073

The design and synthesis of narrow-spectrum antibiotics that target a specific bacterial strain, species, or group of species is a promising strategy for treating bacterial infections when the causative agent is known. In this work, we report the synthesis and evaluation of four new siderophore-β-lactam conjugates where the broad-spectrum β-lactam antibiotics cephalexin (Lex) and meropenem (Mem) are covalently attached to either enterobactin (Ent) or diglucosylated Ent (DGE) via a stable polyethylene glycol (PEG3) linker. These siderophore-β-lactam conjugates showed enhanced minimum inhibitory concentrations against Escherichia coli compared to the parent antibiotics. Uptake studies with uropathogenic E. coli CFT073 demonstrated that the DGE-β-lactams target the pathogen-associated catecholate siderophore receptor IroN. A comparative analysis of siderophore-β-lactams harboring ampicillin (Amp), Lex and Mem indicated that the DGE-Mem conjugate is advantageous because it targets IroN and exhibits low minimum inhibitory concentrations, fast time-kill kinetics, and enhanced stability to serine β-lactamases. Phase-contrast and fluorescence imaging of E. coli treated with the siderophore-β-lactam conjugates revealed cellular morphologies consistent with the inhibition of penicillin-binding proteins PBP3 (Ent/DGE-Amp/Lex) and PBP2 (Ent/DGE-Mem). Overall, this work illuminates the uptake and cell-killing activity of Ent- and DGE-β-lactam conjugates against E. coli and supports that native siderophore scaffolds provide the opportunity for narrowing the activity spectrum of antibiotics in clinical use and targeting pathogenicity.

Design, synthesis, molecular docking, anti-quorum sensing, and anti-biofilm activity of pyochelin-zingerone conjugate

In this article, we report the chemical synthesis of pyochelin-zingerone conjugate via a hydrolysable ester linkage for drug delivery as a "Trojan Horse Strategy." It is a new therapeutic approach to combat microbial infection and to address the issue of multi drug resistance in Gram-negative, nosocomial pathogen Pseudomonas aeruginosa. Pyochelin (Pch) is a catecholate type of phenolate siderophore produced and utilized by the pathogen P. aeruginosa to assimilate iron when colonizing the vertebrate host. Zingerone, is active component present in ginger, a dietary herb known for its anti-virulent approach against P. aeruginosa. In the present study, zingerone was exploited to act as a good substitute for existing antibiotics, known to have developed resistance by most pathogens. Encouraging results were obtained by docking analysis of pyochelin-zingerone conjugate with FptA, the outer membrane receptor of pyochelin. Conjugate also showed anti-quorum sensing activity and also inhibited swimming, swarming, and twitching motilities as well as biofilm formation in vitro.

Metallotherapeutics development in the age of iron-clad bacteria

Drug-resistant infections pose a significant risk to global health as pathogenic bacteria become increasingly difficult to treat. The rapid selection of resistant strains through poor antibiotic stewardship has reduced the number of viable treatments and increased morbidity of infections, especially among the immunocompromised. To circumvent such challenges, new strategies are required to stay ahead of emerging resistance trends, yet research and funding for antibiotic development lags other classes of therapeutics. Though the use of metals in therapeutics has been around for centuries, recent strategies have devoted a great deal of effort into the pathways through which bacteria acquire and utilize iron, which is critical for the establishment of infection. To target iron uptake systems, siderophore-drug conjugates have been developed that hijack siderophore-based iron uptake for delivery of antibiotics. While this strategy has produced several potential leads, the use of siderophores in infection is diminished over time when bacteria adapt to utilize heme as an iron source, leading to a need for the development of porphyrin mimetics as therapeutics. The use of such strategies as well as the inclusion of gallium, a redox-inert iron mimic, are herein reviewed.

Platform to Discover Protease-Activated Antibiotics and Application to Siderophore-Antibiotic Conjugates

Here we present a platform for discovery of protease-activated prodrugs and apply it to antibiotics that target Gram-negative bacteria. Because cleavable linkers for prodrugs had not been developed for bacterial proteases, we used substrate phage to discover substrates for proteases found in the bacterial periplasm. Rather than focusing on a single protease, we used a periplasmic extract of E. coli to find sequences with the greatest susceptibility to the endogenous mixture of periplasmic proteases. Using a fluorescence assay, candidate sequences were evaluated to identify substrates that release native amine-containing payloads. We next designed conjugates consisting of (1) an N-terminal siderophore to facilitate uptake, (2) a protease-cleavable linker, and (3) an amine-containing antibiotic. Using this strategy, we converted daptomycin-which by itself is active only against Gram-positive bacteria-into an antibiotic capable of targeting Gram-negative Acinetobacter species. We similarly demonstrated siderophore-facilitated delivery of oxazolidinone and macrolide antibiotics into a number of Gram-negative species. These results illustrate this platform's utility for development of protease-activated prodrugs, including Trojan horse antibiotics.

Modified Galacto- or Fuco-Clusters Exploiting the Siderophore Pathway to Inhibit the LecA- or LecB-Associated Virulence of Pseudomonas aeruginosa

Galacto- and fuco-clusters conjugated with one to three catechol or hydroxamate motifs were synthesised to target LecA and LecB lectins of Pseudomonas aeruginosa (PA) localised in the outer membrane and inside the bacterium. The resulting glycocluster-pseudosiderophore conjugates were evaluated as Trojan horses to cross the outer membrane of PA by iron transport. The data suggest that glycoclusters with catechol moieties are able to hijack the iron transport, whereas those with hydroxamates showed strong nonspecific interactions. Mono- and tricatechol galactoclusters (G1C and G3C) were evaluated as inhibitors of infection by PA in comparison with the free galactocluster (G0). All of them exhibited an inhibitory effect between 46 to 75 % at 100 μM, with a higher potency than G0. This result shows that LecA localised in the outer membrane of PA is involved in the infection mechanism.

Cefiderocol: A Siderophore Cephalosporin

OBJECTIVE

This article reviews the available data on the chemistry, spectrum of activity, pharmacokinetic and pharmacodynamic properties, clinical efficacy, and potential place in therapy of cefiderocol.

DATA SOURCES

A literature search through PubMed, Google Scholar, and ClinicalTrials.gov was conducted (2009 to March 2020) using the search terms cefiderocol and S-649266. Abstracts presented at recent conferences, prescribing information, and information from the US Food and Drug Administration (FDA) and the manufacturer's website were reviewed.

STUDY SELECTION AND DATA EXTRACTION

All relevant published articles, package inserts, and unpublished meeting abstracts on cefiderocol were reviewed.

DATA SYNTHESIS

Cefiderocol is the first siderophore antibiotic to be approved by the FDA. It was shown to be active against a wide range of resistant Gram-negative pathogens, including multidrug-resistant (MDR) Pseudomonas aeruginosa, Acinetobacter baumannii, Enterobacteriaceae, and Stenotrophomonas maltophilia. Cefiderocol was studied in the treatment of adult patients with complicated urinary tract infections (cUTIs) and nosocomial pneumonia and was well tolerated. In a recently completed prospective study, higher mortality was observed with cefiderocol in the treatment of serious infections caused by carbapenem-resistant (CR) Gram-negative pathogens.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

The approval of cefiderocol provides a new option in the treatment of cUTIs and potentially treatment of nosocomial pneumonia caused by resistant Gram-negative pathogens. Given the higher mortality observed with cefiderocol, its use in the treatment of CR Gram-negative infections should be carefully considered.

CONCLUSION

Cefiderocol shows promising activity against MDR Gram-negative pathogens. Its use in the treatment of serious infections caused by CR Gram-negative bacteria needs further evaluation in phase III clinical studies.

Metallocene QACs: The Incorporation of Ferrocene Moieties into monoQAC and bisQAC Structures

Inspired by the incorporation of metallocene functionalities into a variety of bioactive structures, particularly antimicrobial peptides, we endeavored to broaden the structural variety of quaternary ammonium compounds (QACs) by the incorporation of the ferrocene moiety. Accordingly, 23 ferrocene-containing mono- and bisQACs were prepared in high yields and tested for activity against a variety of bacteria, including Gram-negative strains and a panel of clinically isolated MRSA strains. Ferrocene QACs were shown to be effective antiseptics with some displaying single-digit micromolar activity against all bacteria tested, demonstrating yet another step in the expansion of structural variety of antiseptic QACs.

Siderophore and indolic acid production by Paenibacillus triticisoli BJ-18 and their plant growth-promoting and antimicrobe abilities

Paenibacillus triticisoli BJ-18, a N2-fixing bacterium, is able to promote plant growth, but the secondary metabolites that may play a role in promoting plant growth have never been characterized. In this study, untargeted metabolomics profiling of P. triticisoli BJ-18 indicated the existence of 101 known compounds, including N2-acetyl ornithine, which is the precursor of siderophores, plant growth regulators such as trehalose 6-phosphate, betaine and trigonelline, and other bioactive molecules such as oxymatrine, diosmetin, luotonin A, (-)-caryophyllene oxide and tetrahydrocurcumin. In addition, six compounds were also isolated from P. triticisoli BJ-18 using a combination of silica gel chromatography, sephadex LH-20, octadecyl silane (ODS), and high-performance liquid chromatography (HPLC). The compound structures were further analyzed by Nuclear Magnetic Resonance (NMR), Mass Spectrometry (MS), and Electronic Circular Dichroism (ECD). The six compounds included three classical siderophore fusarinines identified as deshydroxylferritriacetylfusigen, desferritriacetylfusigen, and triacetylfusigen, and three indolic acids identified as paenibacillic acid A, 3-indoleacetic acid (IAA), and 3-indolepropionic acid (IPA). Both deshydroxylferritriacetylfusigen and paenibacillic acid A have new structures. Fusarinines, which normally occur in fungi, were isolated from bacterium for the first time in this study. Both siderophores (compounds 1 and 2) showed antimicrobial activity against Escherichia coli, Staphylococcus aureus and Bacillus subtilis, but did not show obvious inhibitory activity against yeast Candida albicans, whereas triacetylfusigen (compound 3) showed no antibiosis activity against these test microorganisms. Paenibacillic acid A, IAA, and IPA were shown to promote the growth of plant shoots and roots, and paenibacillic acid A also showed antimicrobial activity against S. aureus. Our study demonstrates that siderophores and indolic acids may play an important role in plant growth promotion by P. triticisoli BJ-18.

Siderophore-Based Molecular Imaging of Fungal and Bacterial Infections-Current Status and Future Perspectives

Invasive fungal infections such as aspergillosis are life-threatening diseases mainly affecting immuno-compromised patients. The diagnosis of fungal infections is difficult, lacking specificity and sensitivity. This review covers findings on the preclinical use of siderophores for the molecular imaging of infections. Siderophores are low molecular mass chelators produced by bacteria and fungi to scavenge the essential metal iron. Replacing iron in siderophores by radionuclides such as gallium-68 allowed the targeted imaging of infection by positron emission tomography (PET). The proof of principle was the imaging of pulmonary Aspergillus fumigatus infection using [68Ga]Ga-triacetylfusarinine C. Recently, this approach was expanded to imaging of bacterial infections, i.e., with Pseudomonas aeruginosa. Moreover, the conjugation of siderophores and fluorescent dyes enabled the generation of hybrid imaging compounds, allowing the combination of PET and optical imaging. Nevertheless, the high potential of these imaging probes still awaits translation into clinics.

Cefiderocol: Discovery, Chemistry, and In Vivo Profiles of a Novel Siderophore Cephalosporin

The emergence of antimicrobial resistance is a significant public health issue worldwide, particularly for healthcare-associated infections caused by carbapenem-resistant gram-negative pathogens. Cefiderocol is a novel siderophore cephalosporin targeting gram-negative bacteria, including strains with carbapenem resistance. The structural characteristics of cefiderocol show similarity to both ceftazidime and cefepime, which enable cefiderocol to withstand hydrolysis by β-lactamases. The unique chemical component is the addition of a catechol moiety on the C-3 side chain, which chelates iron and mimics naturally occurring siderophore molecules. Following the chelation of iron, cefiderocol is actively transported across the outer membrane of the bacterial cell to the periplasmic space via specialized iron transporter channels. Furthermore, cefiderocol has demonstrated structural stability against hydrolysis by both serine- and metallo-β-lactamases, including clinically relevant carbapenemases such as Klebsiella pneumoniae carbapenemase, oxacillin carbapenemase-48, and New Delhi metallo-β-lactamase. Cefiderocol has demonstrated promising in vitro antibacterial and bactericidal activity, which correlates with its in vivo efficacy in several animal models. This article reviews the discovery and chemistry of cefiderocol, as well as some of the key microbiological and in vivo findings on cefiderocol from recently conducted investigations.

Monobactams: A Unique Natural Scaffold of Four-Membered Ring Skeleton, Recent Development to Clinically Overcome Infections by Multidrug- Resistant Microbes

Background:

Monobactam antibiotics have been testified to demonstrate significant antibacterial activity especially the treatment of infections by superbug microbes. Recently, research has been focused on the structural modifications, and new generation of this privileged natural scaffold.

Objective:

Efforts have been made to discover the structure-antibacterial relationship of monbactams in order to avoid the aimless work involving the ongoing generated analogues. This review aims to summarize the current knowledge and development of monobactams as a broad-spectrum antibacterial scaffolds. The recent structural modifications that expand the activity, especially in the infections by resistant-strains, combinational therapies and dosing, as well as the possibility of crosshypersensitivity/ reactivity/tolerability with penicillins and cephalosporins will also be summarized and inferred. Different approaches will be covered with emphasis on chemical methods and Structure- Activity Relationship (SAR), in addition to the proposed mechanisms of action. Clinical investigation of monobactams tackling various aspects will not be missed in this review.

Conclusion:

The conclusion includes the novels approaches, that could be followed to design new research projects and reduce the pitfalls in the future development of monobactams.

Current biochemical understanding regarding the metabolism of acinetobactin, the major siderophore of the human pathogen Acinetobacter baumannii, and outlook for discovery of novel anti-infectious agents based thereon

Covering: 1994 to 2019Owing to the rapid increase in nosocomial infections by antibiotic-resistant Acinetobacter baumannii and the paucity of effective treatment options for such infections, interest in the virulence factors involved in its successful dissemination and propagation in the human host have escalated in recent years. Acinetobacin, a siderophore of A. baumannii, is responsible for iron acquisition under nutritional depravation and has been shown to be one of the key virulence factors for this bacterium. In this Highlight, recent findings regarding various chemical and biological aspects of acinetobactin metabolism closely related to the fitness of A. baumannii at the infection sites have been described. In addition, several notable efforts for identifying novel anti-infectious agents based thereon have been discussed.

Siderophore-Antibiotic Conjugate Design: New Drugs for Bad Bugs?

Antibiotic resistance is a global health concern and a current threat to modern medicine and society. New strategies for antibiotic drug design and delivery offer a glimmer of hope in a currently limited pipeline of new antibiotics. One strategy involves conjugating iron-chelating microbial siderophores to an antibiotic or antimicrobial agent to enhance uptake and antibacterial potency. Cefiderocol (S-649266) is a promising cephalosporin–catechol conjugate currently in phase III clinical trials that utilizes iron-mediated active transport and demonstrates enhanced potency against multi-drug resistant (MDR) Gram-negative pathogens. Such molecules demonstrate that siderophore–antibiotic conjugates could be important future medicines to add to our antibiotic arsenal. This review is written in the context of the chemical design of siderophore–antibiotic conjugates focusing on the differing siderophore, linker, and antibiotic components that make up conjugates. We selected chemically distinct siderophore–antibiotic conjugates as exemplary conjugates, rather than multiple analogues, to highlight findings to date. The review should offer a general guide to the uninitiated in the molecular design of siderophore–antibiotic conjugates.

An overview of recent progress in siderophore-antibiotic conjugates

Multi-drug resistant infections caused by Gram-negative bacteria have become one of the most important reasons for the failure of clinical anti-infective treatment. Siderophore-antibiotic conjugates, which were designed based on a "Trojan horse" strategy wherein features enabled active uptake to bypass the Gram-negative cell wall, have been expected to be a weapon for anti-infective treatment in the clinic. Herein, we review antibiotic drug design strategies based on mimics of nature siderophores reported in recent years, we also focus our attention on the relationship between the type of linker and the corresponding antibacterial activity.

Targeting Mobilization of Ferrous Iron in Pseudomonas aeruginosa Infection with an Iron(II)-Caged LpxC Inhibitor

Iron is essential to all life, and competition for this vital nutrient is central to host-pathogen interactions during infection. The opportunistic Gram-negative pathogen Pseudomonas aeruginosa utilizes a diverse array of iron-acquisition strategies, including those enabling import of extracellular ferrous iron. We hypothesize that soluble and redox-active ferrous iron can be employed to activate caged antibiotics at sites of infection in vivo. Here we describe new chemistry that expands the application of our laboratory's Fe²⁺-activated-prodrug chemistry to cage hydroxamic acids, a class of drugs that present manifold development challenges. We synthesize the caged form of a known LpxC inhibitor and show that it is efficacious in an acute P. aeruginosa mouse-lung infection model, despite showing little activity in cell-culture experiments. Overall, our results are consistent with the Fe²⁺-promoted uncaging of an antibacterial payload at sites of infection in an animal and lend support to recent reports indicating that extracellular pools of ferrous iron can be utilized by bacterial pathogens like P. aeruginosa during infection.

Recent trends in the structural revision of natural products

Covering: 2012 to 2017 This article reviews recent reports on the structural revision of natural products. Through a critical assessment of the original and revised published structures, the article addresses why each structure was targeted for revision, discusses the techniques and key discrepancies that led to the proposal of the revised structure, and offers measures that may have been taken during the original structure determination to prevent error. With the revised structures in hand, weaknesses of original proposals are assessed, providing a better understanding on the logic behind structure determination.

Drug delivery systems designed to overcome antimicrobial resistance

Antimicrobial resistance has emerged as a huge challenge to the effective treatment of infectious diseases. Aside from a modest number of novel anti-infective agents, very few new classes of antibiotics have been successfully developed for therapeutic use. Despite the research efforts of numerous scientists, the fight against antimicrobial (ATB) resistance has been a longstanding continued effort, as pathogens rapidly adapt and evolve through various strategies, to escape the action of ATBs. Among other mechanisms of resistance to antibiotics, the sophisticated envelopes surrounding microbes especially form a major barrier for almost all anti-infective agents. In addition, the mammalian cell membrane presents another obstacle to the ATBs that target intracellular pathogens. To negotiate these biological membranes, scientists have developed drug delivery systems to help drugs traverse the cell wall; these are called "Trojan horse" strategies. Within these delivery systems, ATB molecules can be conjugated with one of many different types of carriers. These carriers could include any of the following: siderophores, antimicrobial peptides, cell-penetrating peptides, antibodies, or even nanoparticles. In recent years, the Trojan horse-inspired delivery systems have been increasingly reported as efficient strategies to expand the arsenal of therapeutic solutions and/or reinforce the effectiveness of conventional ATBs against drug-resistant microbes, while also minimizing the side effects of these drugs. In this paper, we aim to review and report on the recent progress made in these newly prevalent ATB delivery strategies, within the current context of increasing ATB resistance.

Comparative Extracellular Proteomics of Aeromonas hydrophila Reveals Iron-Regulated Secreted Proteins as Potential Vaccine Candidates

In our previous study, several iron-related outer membrane proteins in Aeromonas hydrophila, a serious pathogen of farmed fish, conferred high immunoprotectivity to fish, and were proposed as potential vaccine candidates. However, the protective efficacy of these extracellular proteins against A. hydrophila remains largely unknown. Here, we identified secreted proteins that were differentially expressed in A. hydrophila LP-2 in response to iron starvation using an iTRAQ-based quantitative proteomics method. We identified 341 proteins, of which 9 were upregulated in response to iron starvation and 24 were downregulated. Many of the differently expressed proteins were associated with protease activity. We confirmed our proteomics results with Western blotting and qPCR. We constructed three mutants by knocking out three genes encoding differentially expressed proteins (Δorf01830, Δorf01609, and Δorf03641). The physiological characteristics of these mutants were investigated. In all these mutant strains, protease activity decreased, and Δorf01609, and Δorf01830 were less virulent in zebrafish. This indicated that the proteins encoded by these genes may play important roles in bacterial infection. We next evaluated the immune response provoked by the six iron-related recombinant proteins (ORF01609, ORF01830, ORF01839, ORF02943, ORF03355, and ORF03641) in zebrafish as well as the immunization efficacy of these proteins. Immunization with these proteins significantly increased the zebrafish immune response. In addition, the relative percent survival (RPS) of the immunized zebrafish was 50-80% when challenged with three virulent A. hydrophila strains, respectively. Thus, these extracellular secreted proteins might be effective vaccine candidates against A. hydrophila infection in fish.