Siderophore-based iron acquisition and pathogen control

Differential Responses of Bacterial and Fungal Communities to Siderophore Supplementation in Soil Affected by Tobacco Bacterial Wilt (Ralstonia solanacearum)

Siderophores secreted by microorganisms can promote ecological efficiency and could be used to regulate the unbalanced microbial community structure. The influence of the siderophore activity of Trichoderma yunnanense strain 2-14F2 and Beauveria pseudobassiana strain (2-8F2) on the physiological/biochemical functions and community structure of soil microbes affected by tobacco bacterial wilt (TBW) was studied. DNS Colorimetry and Biolog-eco plates were used to quantify the impacts of strain siderophores on soil enzyme activities and microbial metabolism. Based on Illumina MiSeq high-throughput sequencing, the soil 16S rDNA and ITS sequences were amplified to dissect the response characteristics of alpha/beta diversity and the structure/composition of a soil microbial community toward siderophores. The KEGG database was used to perform the PICRUSt functional prediction of the microbial community. We found that siderophores of 2-14F2 and 2-8F2, at certain concentrations, significantly increased the activities of sucrase (S-SC) and urease (S-UE) in the TBW soil and enhanced the average well color development (AWCD, carbon source utilization capacity) of the microbial community. The metabolic capacity of the diseased soil to amino acids, carbohydrates, polymers, aromatics, and carboxylic acids also increased significantly. The response of the bacterial community to siderophore active metabolites was more significant in alpha diversity, while the beta diversity of the fungal community responded more positively to siderophores. The relative abundance of Actinobacteria, Chloroflexi, and Acidobacteria increased and was accompanied by reductions in Proteobacteria and Firmicutes. LEfSe analysis showed that Pseudonocardiaceae, Gemmatimonas, Castellaniella, Chloridiumand and Acrophialophora altered the most under different concentrations of siderophore active metabolites. The PICRUSt functional prediction results showed that siderophore increased the abundance of the redox-related enzymes of the microbial community in TBW soil. The BugBase phenotypic prediction results showed that the siderophore activity could decrease the abundance of pathogenic bacteria. The study concludes that siderophore activity could decrease the abundance of pathogenic bacteria and regulate the composition of the microbial community in TBW soil. The activities of sucrase (S-SC) and urease (S-UE) in TBW soil were significantly increased. Overall, the siderophore regulation of community structures is a sustainable management strategy for soil ecosystems.

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.

Acinetobacter baumannii ATCC 17978 encodes a microcin system with antimicrobial properties for contact-independent competition

Acinetobacter baumannii is a multidrug-resistant opportunistic pathogen that persists in the hospital environment and causes various clinical infections, primarily affecting immunocompromised patients. A. baumannii has evolved a wide range of mechanisms to compete with neighbouring bacteria. One such competition strategy depends on small secreted peptides called microcins, which exert antimicrobial effects in a contact-independent manner. Here, we report that A. baumannii ATCC 17978 (AB17978) encodes the class II microcin 17 978 (Mcc17978) with antimicrobial activity against closely related Acinetobacter, and surprisingly, also Escherichia coli strains. We identified the genetic locus encoding the Mcc17978 system in AB17978. Using classical bacterial genetic approaches, we determined that the molecular receptor of Mcc17978 in E. coli is the iron-catecholate transporter Fiu, and in Acinetobacter is Fiu's homolog, PiuA. In bacteria, the Ferric uptake regulator (Fur) positively regulates siderophore systems and microcin systems under iron-deprived environments. We found that the Mcc17978 system is upregulated under low-iron conditions commonly found in the host environment and identified a putative Fur binding site upstream of the mcc17978 gene. When we tested the antimicrobial activity of Mcc17978 under different levels of iron availability, we observed that low iron levels not only triggered transcriptional induction of the microcin, but also led to enhanced microcin activity. Taken together, our findings suggest that A. baumannii may utilize microcins to compete with other microbes for resources during infection.

Leptospiral imelysin (LIC_10713) is secretory, immunogenic and binds to laminin, fibronectin, and collagen IV

Leptospirosis is a widespread zoonotic disease caused by pathogenic Leptospira. Early and accurate diagnosis is the prime step in managing the disease. Secretory proteins of Leptospira remain distinguished for diagnosis due to their availability as soluble proteins in the serum and their interaction with the host immune response due to their extracellular presence. This study presents the cloning, expression, purification, and characterization of imelysin or LruB (LIC_10713), a putative leptospiral protein. We report that the localization of imelysin showed its presence in the inner membrane and in the culture supernatant. The imelysin was upregulated under in vitro physiological conditions of infection. The LIC_10713 interacted significantly with laminin, fibronectin, collagen type I, and collagen type IV in a dose-dependent manner. Phylogenetic analysis showed that LIC_10713 is predominately found in the pathogenic species of Leptospira, and the GxHxxE motif of imelysin-like proteins is represented as the amino acid sequence G_WH_AIE. Also, immunoglobulins in leptospirosis-infected patients recognize recombinant-LIC_10713 with 100% specificity and 90.9% sensitivity. The secretion nature, abundance, upregulation, binding to ECM components, and immunogenicity determine LIC_10713 as an important molecule that can be used as an anti-leptospirosis measure. KEY POINTS: • The imelysin-like protein (LIC_10713) of Leptospira is a secretory protein • The protein LIC_10713 can bind ECM molecules • The LIC_10713 is mainly found in pathogenic leptospires • The anti-LIC_10713 antibody from human serum can detect the r-LIC_10713.

Siderophore conjugation with cleavable linkers boosts the potency of RNA polymerase inhibitors against multidrug-resistant E. coli

The growing antibiotic resistance, foremost in Gram-negative bacteria, requires novel therapeutic approaches. We aimed to enhance the potency of well-established antibiotics targeting the RNA polymerase (RNAP) by utilizing the microbial iron transport machinery to improve drug translocation across their cell membrane. As covalent modifications resulted in moderate-low antibiotic activity, cleavable linkers were designed that permit a release of the antibiotic payload inside the bacteria and unperturbed target binding. A panel of ten cleavable siderophore-ciprofloxacin conjugates with systematic variation at the chelator and the linker moiety was used to identify the quinone trimethyl lock in conjugates 8 and 12 as the superior linker system, displaying minimal inhibitory concentrations (MICs) of ≤1 μM. Then, rifamycins, sorangicin A and corallopyronin A, representatives of three structurally and mechanistically different natural product RNAP inhibitor classes, were conjugated via the quinone linker to hexadentate hydroxamate and catecholate siderophores in 15-19 synthetic steps. MIC assays revealed an up to 32-fold increase in antibiotic activity against multidrug-resistant E. coli for conjugates such as 24 or 29 compared to free rifamycin. Experiments with knockout mutants in the transport system showed that translocation and antibiotic effects were conferred by several outer membrane receptors, whose coupling to the TonB protein was essential for activity. A functional release mechanism was demonstrated analytically by enzyme assays in vitro, and a combination of subcellular fractionation and quantitative mass spectrometry proved cellular uptake of the conjugate, release of the antibiotic, and its increased accumulation in the cytosol of bacteria. The study demonstrates how the potency of existing antibiotics against resistant Gram-negative pathogens can be boosted by adding functions for active transport and intracellular release.

Genomic and Phylogenetic Analysis of a Multidrug-Resistant blaNDM-carrying Klebsiella michiganensis in China

Objective

Klebsiella michiganensis is an emerging hospital-acquired bacterial pathogen. However, there is a dearth of knowledge on the antimicrobial resistance and transmission of K. michiganensis. Here, we characterized the microbiological and genomic features of a carbapenem-resistant K. michiganensis strain harboring the bla_NDM-1 gene in China.

Methods

K. michiganensis strain 2563 was recovered from the sputum sample of a hospitalized patient with pulmonary infection. Whole-genome sequencing of K. michiganensis strain 2563 was conducted using both the short-read Illumina and long-read MinION platforms to thoroughly characterize the genetic context of bla_NDM-carrying plasmid in K. michiganensis 2563. Furthermore, BacWGSTdb server was utilized to perform in silico multilocus sequence typing (MLST), identify antimicrobial resistance genes, and conduct genomic epidemiological analyses of the closely related isolates deposited in the public database.

Results

K. michiganensis 2563 was resistant to piperacillin, aztreonam, meropenem, imipenem, amoxicillin-clavulanic acid, ampicillin, cefotaxime, cefazolin, ampicillin/sulbactam, cefepime, piperacillin-tazobactam, and ceftazidime. It belonged to sequence type (ST) 43, and the bla_NDM-1 gene was found to be located on the plasmid p2563_NDM (54,035 bp). This plasmid showed remarkable similarity to other bla_NDM-1-encoding plasmids found in various Enterobacterium species in the public database. The occurrence of global ST43 K. michiganensis was primarily sporadic, and the closest relative of K. michiganensis 2563 was another ST43 isolate 12,084 recovered from China in 2013, which differed by 171 SNPs.

Conclusion

Our study reports the genome characteristics of a carbapenem-resistant K. michiganensis strain carrying the bla_NDM-1 gene in China, highlighting the need for ongoing surveillance of this pathogen in clinical settings.

Identification and Biosynthetic Study of the Siderophore Lysochelin in the Biocontrol Agent Lysobacter enzymogenes

Lysobacter is a genus of bacteria emerging as new biocontrol agents in agriculture. Although iron acquisition is essential for the bacteria, no siderophore has been identified from any Lysobacter. Here, we report the identification of the first siderophore, N1,N8-bis(2,3-dihydroxybenzoyl)spermidine (lysochelin), and its biosynthetic gene cluster from Lysobacter enzymogenes. Intriguingly, the deletion of the spermidine biosynthetic gene encoding arginine decarboxylase or SAM decarboxylase eliminated lysochelin and the antifungals, HSAF and its analogues, which are key to the disease control activity and to the survival of Lysobacter under oxidative stresses caused by excess iron. The production of lysochelin and the antifungals is greatly affected by iron concentration. Together, the results revealed a previously unrecognized system, in which L. enzymogenes produces a group of small molecules, lysochelin, spermidine, and HSAF and its analogues, that are affected by iron concentration and critical to the growth and survival of the biocontrol agent.

Ferritinophagy-mediated iron competition in RUTIs: Tug-of-war between UPEC and host

Uropathogenic Escherichia coli (UPEC) is the main pathogen of recurrent urinary tract infections (RUTIs). Urinary tract infection is a complicated interaction between UPEC and the host. During infection, UPEC can evade the host's immune response and retain in bladder epithelial cells, which requires adequate nutritional support. Iron is the first necessary trace element in life and a key nutritional factor, making it an important part of the competition between UPEC and the host. On the one hand, UPEC grabs iron to satisfy its reproduction, on the other hand, the host relies on iron to build nutritional immunity defenses against UPEC. Ferritinophagy is a selective autophagy of ferritin mediated by nuclear receptor coactivator 4, which is not only a way for the host to regulate iron metabolism to maintain iron homeostasis, but also a key point of competition between the host and UPEC. Although recent studies have confirmed the role of ferritinophagy in the progression of many diseases, the mechanism of potential interactions between ferritinophagy in UPEC and the host is poorly understood. In this paper, we reviewed the potential mechanisms of ferritinophagy-mediated iron competition in the UPEC-host interactions. This competitive relationship, like a tug-of-war, is a confrontation between the capability of UPEC to capture iron and the host's nutritional immunity defense, which could be the trigger for RUTIs. Therefore, understanding ferritinophagy-mediated iron competition may provide new strategies for exploring effective antibiotic alternative therapies to prevent and treat RUTIs.

Pulcherriminic acid modulates iron availability and protects against oxidative stress during microbial interactions

Siderophores are soluble or membrane-embedded molecules that bind the oxidized form of iron, Fe(III), and play roles in iron acquisition by microorganisms. Fe(III)-bound siderophores bind to specific receptors that allow microbes to acquire iron. However, certain soil microbes release a compound (pulcherriminic acid, PA) that, upon binding to Fe(III), forms a precipitate (pulcherrimin) that apparently functions by reducing iron availability rather than contributing to iron acquisition. Here, we use Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model to show that PA is involved in a peculiar iron-managing system. The presence of the competitor induces PA production, leading to precipitation of Fe(III) as pulcherrimin, which prevents oxidative stress in B. subtilis by restricting the Fenton reaction and deleterious ROS formation. In addition, B. subtilis uses its known siderophore bacillibactin to retrieve Fe(III) from pulcherrimin. Our findings indicate that PA plays multiple roles by modulating iron availability and conferring protection against oxidative stress during inter-species competition.

Polyfluorinated salicylic acid analogs do not interfere with siderophore biosynthesis

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) is a leading cause of infectious disease mortality. The salicylic acid derived small molecule siderophores known as mycobactins are essential in vivo for iron acquisition of Mtb where iron is restricted in the host. Herein, we synthesize and explore the mechanism of action of polyfluorinated salicylic acid derivates, which were previously reported to possess potent antimycobacterial activity. We hypothesized fluorinated salicylic acid derivates may inhibit mycobactin biosynthesis through initial bioactivation and conversion to downstream metabolites that block late steps in assembly of the mycobactins. Enzymatic studies demonstrated that some of the fluorinated salicylic acid derivatives compounds were readily activated by the bifunctional adenylating enzyme MbtA, responsible for incorporation of salicylic acid into the mycobactin biosynthetic pathway; however, they did not inhibit mycobactin biosynthesis as confirmed by LS-MS/MS using an authentic synthetic mycobactin standard. Further mechanistic analysis of the most active derivative (Sal-4) using an MbtA-overexpressing Mtb strain as well as complementation studies with iron and salicylic acid revealed Sal-4 cannot be antagonized by overexpression of MbtA or through supplementation with iron or salicylic acid. Taken together, our results indicate the observed antimycobacterial activity of polyfluorinated salicylic acid derivative is independent of mycobactin biosynthesis.

Links between Diet, Intestinal Anaerobes, Microbial Metabolites and Health

A dense microbial community resides in the human colon, with considerable inter-individual variability in composition, although some species are relatively dominant and widespread in healthy individuals. In disease conditions, there is often a reduction in microbial diversity and perturbations in the composition of the microbiota. Dietary complex carbohydrates that reach the large intestine are important modulators of the composition of the microbiota and their primary metabolic outputs. Specialist gut bacteria may also transform plant phenolics to form a spectrum of products possessing antioxidant and anti-inflammatory activities. Consumption of diets high in animal protein and fat may lead to the formation of potentially deleterious microbial products, including nitroso compounds, hydrogen sulphide, and trimethylamine. Gut anaerobes also form a range of secondary metabolites, including polyketides that may possess antimicrobial activity and thus contribute to microbe-microbe interactions within the colon. The overall metabolic outputs of colonic microbes are derived from an intricate network of microbial metabolic pathways and interactions; however, much still needs to be learnt about the subtleties of these complex networks. In this review we consider the multi-faceted relationships between inter-individual microbiota variation, diet, and health.

Enhancing Stevia rebaudiana growth and yield through exploring beneficial plant-microbe interactions and their impact on the underlying mechanisms and crop sustainability

Stevia rebaudiana is an important medicinal plant which represents the most important sugar substitute in many countries. Poor seed germination of this plant is a critical problem that affects the final yield and the availability of the products in the market. Continuous cropping without supplying soil nutrients is also a serious issue as it results in declining soil fertility. This review highlights the important use of beneficial bacteria for the enhancement of Stevia rebaudiana growth and its dynamic interactions in the phyllosphere, rhizosphere, and endosphere. Fertilizers can increase crop yield and preserve and improve soil fertility. There is a rising concern that prolonged usage of chemical fertilizers may have negative impacts on the ecosystem of the soil. On the other hand, soil health and fertility are improved by plant growth-promoting bacteria which could eventually increase plant growth and productivity. Accordingly, a biocompatible strategy involving beneficial microorganisms inoculation is applied to boost plant growth and reduce the negative effects of chemical fertilizers. Plants benefit extensively from endophytic bacteria, which promote growth and induce resistance to pathogens and stresses. Additionally, several plant growth-promoting bacteria are able to produce amino acids, polyamines, and hormones that can be used as alternatives to chemicals. Therefore, understanding the dynamic interactions between bacteria and Stevia can help make the favorable bacterial bio-formulations, use them more effectively, and apply them to Stevia to improve yield and quality.

Research progress on iron uptake pathways and mechanisms of foodborne microorganisms and their application in the food sector

Iron is one of the essential nutrients for almost all microorganisms. Under iron-limited conditions, bacteria can secrete siderophores to the outside world to absorb iron for survival. This process requires the coordinated action of energy-transducing proteins, transporters, and receptors. The spoilage factors of some spoilage bacteria and the pathogenic mechanism of pathogenic bacteria are also closely related to siderophores. Meanwhile, some siderophores have also gradually evolved toward beneficial aspects. First, a variety of siderophores are classified into three aspects. In addition, representative iron uptake systems of Gram-negative and Gram-positive bacteria are described in detail to understand the common and specific pathways of iron uptake by various bacteria. In particular, the causes of siderophore-induced bacterial pathogenicity and the methods and mechanisms of inhibiting bacterial iron absorption under the involvement of siderophores are presented. Then, the application of siderophores in the food sector is mainly discussed, such as improving the food quality of dairy products and meat, inhibiting the attack of pathogenic bacteria on food, improving the plant growth environment, and promoting plant growth. Finally, this review highlights the unresolved fate of siderophores in the iron uptake system and emphasizes further development of siderophore-based substitutes for traditional drugs, new antibiotic-resistance drugs, and vaccines in the food and health sectors.

Iron Uptake Controls Trypanosoma cruzi Metabolic Shift and Cell Proliferation

(1) Background: Ionic transport in Trypanosoma cruzi is the object of intense studies. T. cruzi expresses a Fe-reductase (TcFR) and a Fe transporter (TcIT). We investigated the effect of Fe depletion and Fe supplementation on different structures and functions of T. cruzi epimastigotes in culture. (2) Methods: We investigated growth and metacyclogenesis, variations of intracellular Fe, endocytosis of transferrin, hemoglobin, and albumin by cell cytometry, structural changes of organelles by transmission electron microscopy, O₂ consumption by oximetry, mitochondrial membrane potential measuring JC-1 fluorescence at different wavelengths, intracellular ATP by bioluminescence, succinate-cytochrome c oxidoreductase following reduction of ferricytochrome c, production of H₂O₂ following oxidation of the Amplex^® red probe, superoxide dismutase (SOD) activity following the reduction of nitroblue tetrazolium, expression of SOD, elements of the protein kinase A (PKA) signaling, TcFR and TcIT by quantitative PCR, PKA activity by luminescence, glyceraldehyde-3-phosphate dehydrogenase abundance and activity by Western blotting and NAD⁺ reduction, and glucokinase activity recording NADP⁺ reduction. (3) Results: Fe depletion increased oxidative stress, inhibited mitochondrial function and ATP formation, increased lipid accumulation in the reservosomes, and inhibited differentiation toward trypomastigotes, with the simultaneous metabolic shift from respiration to glycolysis. (4) Conclusion: The processes modulated for ionic Fe provide energy for the T. cruzi life cycle and the propagation of Chagas disease.

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.

Virulence analysis and antibiotic resistance of Klebsiella pneumoniae isolates from hospitalised patients in Poland

Klebsiella pneumoniae (KP) is a nosocomial pathogen causing difficult-to-treat infections. The presence of virulence genes and antibiotic resistance of 109 KP isolates from hospitalized patients were investigated. Among them, 68.8% were multi-drug resistant (MDR) and 59.6% produced extended-spectrum beta-lactamases (ESBLs). Metallo-β-lactamases (MBLs) were produced by 22% of isolates (mainly from anus), including 16.5% of isolates producing New Delhi metallo-β-lactamase (NDM-1). The genes encoding adhesins (fimH-91.7%, mrkD-96.3%), enterobactin (entB-100%) and yersiniabactin (irp-1-88%) were frequently identified. The genes encoding salmochelin (iroD-9.2%, iroN-7.3%) and colibactin (clbA, clbB-0.9%) were identified rarely. Iron acquisition system-related kfu gene and wcaG gene involved in capsule production were identified in 6.4% and 11% of isolates, respectively. The rmpA gene associated with hypermucoviscosity was present in 6.4% of isolates. In 19.2% of isolates magA gene was detected, specific for K1 capsule serotype, while 22.9% of isolates showed K2 capsule serotype. The rmpA, iroD or iroN genes being diagnostic biomarkers for hypervirulent KP (hvKP) were detected in 16.5% of isolates. We found that 55.5% of hvKP were MDR and produced ESBLs, thus hospital KP isolates pose a serious threat to the healthcare system.

Emergence of Carbapenemase-Producing Hypervirulent Klebsiella pneumoniae in Switzerland

Increasing occurrence of multidrug-resistant (MDR) and hypervirulent (hv) Klebsiella pneumoniae (MDR-hvKp) convergent clones is being observed. Those strains have the potential of causing difficult-to-treat infections in healthy adults with an increased capacity for mortality. It is therefore crucial to track their dissemination to prevent their further spread. The aim of our study was to investigate the occurrence of carbapenemase-producing hvKp isolates in Switzerland and to determine their genetic profile. A total of 279 MDR carbapenemase-producing K. pneumoniae from patients hospitalized all over Switzerland was investigated, and a rate of 9.0% K. pneumoniae presenting a virulence genotype was identified. Those isolates produced either KPC, NDM, or OXA-48 and had been either recovered from rectal swabs, urine, and blood. A series of previously reported K. pneumoniae clones such as ST23-K1, ST395-K2, and ST147-K20 or ST147-K64 were identified. All the isolates defined as MDR-hvKp (4.7%) possessed the aerobactin and the yersiniabactin clusters. The ST23-K1s were the only isolates presenting the colibactin cluster and achieved higher virulence scores. This study highlights the occurrence and circulation of worrisome MDR-hvKp and MDR nonhypervirulent K. pneumoniae (MDR-nhv-Kp) isolates in Switzerland. Our findings raise an alert regarding the need for active surveillance networks to track and monitor the spread of such successful hybrid clones representing a public health threat worldwide.

Metabolomics of bacterial-fungal pairwise interactions reveal conserved molecular mechanisms

Bacterial-fungal interactions (BFIs) can shape the structure of microbial communities, but the small molecules mediating these BFIs are often understudied. We explored various optimization steps for our microbial culture and chemical extraction protocols for bacterial-fungal co-cultures, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) revealed that metabolomic profiles are mainly comprised of fungi derived features, indicating that fungi are the key contributors to small molecule mediated BFIs. LC-inductively coupled plasma MS (LC-ICP-MS) and MS/MS based dereplication using database searching revealed the presence of several known fungal specialized metabolites and structurally related analogues in these extracts, including siderophores such as desferrichrome, desferricoprogen, and palmitoylcoprogen. Among these analogues, a novel putative coprogen analogue possessing a terminal carboxylic acid motif was identified from Scopulariopsis spp. JB370, a common cheese rind fungus, and its structure was elucidated via MS/MS fragmentation. Based on these findings, filamentous fungal species appear to be capable of producing multiple siderophores with potentially different biological roles (i.e. various affinities for different forms of iron). These findings highlight that fungal species are important contributors to microbiomes via their production of abundant specialized metabolites and their role in complex communities should continue to be a priority.

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.

Thallium(III) exposure alters diversity and co-occurrence networks of bacterial and fungal communities and intestinal immune response along the digestive tract in mice

The gut microbiota, which includes fungi and bacteria, plays an important role in maintaining gut health. Our previous studies have shown that monovalent thallium [Tl(I)] exposure is associated with disturbances in intestinal flora. However, research on acute Tl(III) poisoning through drinking water and the related changes in the gut microbiota is insufficient. In this study, we showed that Tl(III) exposure (10 ppm for 2 weeks) reduced the alpha diversity of bacteria in the ileum, colon, and feces of mice, as well as the alpha diversity of fecal fungi. In addition, principal coordinate analysis showed that Tl(III) exposure had little effect on the bacterial and fungal beta diversity. LEfSe analyses revealed that Tl(III) exposure altered the abundance of intestinal bacteria in the digestive tract and feces. Moreover, Tl(III) exposure had little effect on fungal abundance in the ileum, cecum, and colon, but had a considerable effect on fungal abundance in feces. After Tl(III) exposure, the fungal composition was more disrupted in feces than in the intestinal tract, suggesting that feces can serve as a representative of the gut mycobiota in Tl(III) exposure studies. Intra-kingdom network analyses showed that Tl(III) exposure affected the complexity of bacterial-bacterial and fungal-fungal co-occurrence networks along the digestive tract. The bacterial-fungal interkingdom co-occurrence networks exhibited increased complexity after Tl(III) exposure, except for those in the colon. Additionally, Tl(III) exposure altered the intestinal immune response. These results reveal the perturbation in gut bacterial and fungal diversity, abundance, and co-occurrence network complexity, as well as the gut immune response, caused by Tl(III) exposure.

Epidemicity and clonal replacement of hypervirulent carbapenem-resistant Klebsiella pneumoniae with diverse pathotypes and resistance profiles in a hospital

OBJECTIVES

The emergence of carbapenem-resistant and hypervirulent Klebsiella pneumoniae (CR-hvKP) poses a great threat to public health. There is a paramount need to increase awareness of the epidemiology, evolution, and pathogenesis of CR-hvKP.

METHODS

We collected strains of K. pneumoniae for over two years in a hospital. CR-hvKP strains were screened by polymerase chain reaction (PCR) with primers targeting the virulence genes. Genome sequencing was used to determine phylogenetic relationships and genetic characterization of virulence elements. The population dynamics within these strains were analyzed through epidemiological data. The string test, siderophore secretion, and murine infection experiments were performed to investigate virulence potential of different clones.

RESULTS

A total of 1172 K. pneumoniae strains were isolated from 817 patients, and 125 isolates were identified as CR-hvKP. In all, 102 CR-hvKP strains belonged to sequence type (ST) 11. Genomic analysis demonstrated that three clones of ST11 successively replaced each other in the hospital. Among them, the strains of clade A and clade B acquired virulence plasmids and the strains of clade C acquired a new integrating conjugative element (ICE). Phenotypic experiments revealed enhanced virulence potential of the recent epidemic clone from clade B. Sequence type 11 strains were favorable hosts for the convergence of virulence and resistance, indicated by clonal replacement and acquisition patterns of virulence elements.

CONCLUSION

The emergence of the enhanced virulence potential of ST11 CR-hvKP suggests that coevolution between hosts and exogenous factors can produce super-virulent CR-hvKP strains, highlighting the need to closely monitor changes in the virulence characteristics of CR-hvKP.

Advances in Ferritin Physiology and Possible Implications in Bacterial Infection

Due to its advantageous redox properties, iron plays an important role in the metabolism of nearly all life. However, these properties are not only a boon but also the bane of such life forms. Since labile iron results in the generation of reactive oxygen species by Fenton chemistry, iron is stored in a relatively safe form inside of ferritin. Despite the fact that the iron storage protein ferritin has been extensively researched, many of its physiological functions are hitherto unresolved. However, research regarding ferritin's functions is gaining momentum. For example, recent major discoveries on its secretion and distribution mechanisms have been made as well as the paradigm-changing finding of intracellular compartmentalization of ferritin via interaction with nuclear receptor coactivator 4 (NCOA4). In this review, we discuss established knowledge as well as these new findings and the implications they may have for host-pathogen interaction during bacterial infection.

Synthesis and Characterization of DOTAM-Based Sideromycins for Bacterial Imaging and Antimicrobial Therapy

The rise of antimicrobial resistance, especially in Gram-negative bacteria, calls for novel diagnostics and antibiotics. To efficiently penetrate their double-layered cell membrane, we conjugated the potent antibiotics daptomycin, vancomycin, and sorangicin A to catechol siderophores, which are actively internalized by the bacterial iron uptake machinery. LC-MS/MS uptake measurements of sorangicin derivatives verified that the conjugation led to a 100- to 525-fold enhanced uptake into bacteria compared to the free drug. However, the transfer to the cytosol was insufficient, which explains their lack of antibiotic efficacy. Potent antimicrobial effects were observed for the daptomycin conjugate 7 (∼1 μM) against multidrug-resistant Acinetobacter baumannii. A cyanin-7 label aside the daptomycin warhead furnished the theranostic 13 that retained its antibiotic activity and was also able to label ESKAPE bacteria, as demonstrated by microscopy and fluorescence assays. 13 and the cyanin-7 imaging conjugate 14 were stable in human plasma and had low plasma protein binding and cytotoxicity.

Siderophore-Linked Ruthenium Catalysts for Targeted Allyl Ester Prodrug Activation within Bacterial Cells

Due to rising resistance, new antibacterial strategies are needed, including methods for targeted antibiotic release. As targeting vectors, chelating molecules called siderophores that are released by bacteria to acquire iron have been investigated for conjugation to antibacterials, leading to the clinically approved drug cefiderocol. The use of small-molecule catalysts for prodrug activation within cells has shown promise in recent years, and here we investigate siderophore-linked ruthenium catalysts for the activation of antibacterial prodrugs within cells. Moxifloxacin-based prodrugs were synthesised, and their catalyst-mediated activation was demonstrated under anaerobic, biologically relevant conditions. In the absence of catalyst, decreased antibacterial activities were observed compared to moxifloxacin versus Escherichia coli K12 (BW25113). A series of siderophore-linked ruthenium catalysts were investigated for prodrug activation, all of which displayed a combinative antibacterial effect with the prodrug, whereas a representative example displayed little toxicity against mammalian cell lines. By employing complementary bacterial growth assays, conjugates containing siderophore units based on catechol and azotochelin were found to be most promising for intracellular prodrug activation.

Synthesis of temporin L hydroxamate-based peptides and evaluation of their coordination properties with iron(III )

Ferric iron is an essential nutrient for bacterial growth. Pathogenic bacteria synthesize iron-chelating entities known as siderophores to sequestrate ferric iron from host organisms in order to colonize and replicate. The development of antimicrobial peptides (AMPs) conjugated to iron chelators represents a promising strategy for reducing the iron availability, inducing bacterial death, and enhancing simultaneously the efficacy of AMPs. Here we designed, synthesized, and characterized three hydroxamate-based peptides Pep-cyc1, Pep-cyc2, and Pep-cyc3, derived from a cyclic temporin L peptide (Pep-cyc) developed previously by some of us. The Fe³⁺ complex formation of each ligand was characterized by UV-visible spectroscopy, mass spectrometry, and IR and NMR spectroscopies. In addition, the effect of Fe³⁺ on the stabilization of the α-helix conformation of hydroxamate-based peptides and the cotton effect were examined by CD spectroscopy. Moreover, the antimicrobial results obtained in vitro on some Gram-negative strains (K. pneumoniae and E. coli) showed the ability of each peptide to chelate efficaciously Fe³⁺ obtaining a reduction of MIC values in comparison to their parent peptide Pep-cyc. Our results demonstrated that siderophore conjugation could increase the efficacy and selectivity of AMPs used for the treatment of infectious diseases caused by Gram-negative pathogens.

Biofortification: A long-term solution to improve global health- a review

Biofortification is a revolutionary technique for improving plant nutrition and alleviating human micronutrient deficiency. Fertilizers can help increase crop yield and growth, but applying too much fertilizer can be a problem because it leads to the release of greenhouse gases and eutrophication. One of the major global hazards that affects more than two million people globally is the decreased availability of micronutrients in food crops, which results in micronutrient deficiencies or "hidden hunger" in people. Micronutrients, like macronutrients, perform a variety of roles in plant and human nutrition. This review has highlighted the importance of micronutrients as well as their advantages. The uneven distribution of micronutrients in geological areas is not the only factor responsible for micronutrient deficiencies, other parameters including soil moisture, temperature, texture of the soil, and soil pH significantly affects the micronutrient concentration and their availability in the soil. To overcome this, different biofortification approaches are assessed in the review in which microbes mediated, Agronomic approaches, Plant breeding, and transgenic approaches are discussed. Hidden hunger can result in risky health conditions and diseases such as cancer, cardiovascular disease, osteoporosis, neurological disorders, and many more. Microbes-mediated biofortification is a novel and promising solution for the bioavailability of nutrients to plants in order to address these problems. Biofortification is cost effective, feasible, and environmentally sustainable. Bio-fortified crops boost our immunity, which helps us to combat these deadly viruses. The studies we discussed in this review have demonstrated that they can aid in the alleviation of hidden hunger.

Draft genome of Raoultella planticola, a high lead resistance bacterium from industrial wastewater

Isolation of heavy metals-resistant bacteria from their original habitat is a crucial step in bioremediation. Six lead (Pb) resistant bacterial strains were isolated and identified utilizing 16S rRNA to be Enterobacter ludwigii FACU 4, Shigella flexneri FACU, Microbacterium paraoxydans FACU, Klebsiella pneumoniae subsp. pneumonia FACU, Raoultella planticola FACU 3 and Staphylococcus xylosus FACU. It was determined that all these strains had their Minimum inhibitory concentration (MIC) to be 2500 ppm except R. planticola FACU 3 has a higher maximum tolerance concentration (MTC) up to 2700 ppm. We evaluated the survival of all six strains on lead stress, the efficiency of biosorption and lead uptake. It was found that R. planticola FACU 3 is the highest MTC and S. xylosus FACU was the lowest MTC in this evaluation. Therefore, transmission electron microscopy (TEM) confirmed the difference between the morphological responses of these two strains to lead stress. These findings led to explore more about the genome of R. planticola FACU 3 using illumine Miseq technology. Draft genome sequence analysis revealed the genome size of 5,648,460 bp and G + C content 55.8% and identified 5526 CDS, 75 tRNA and 4 rRNA. Sequencing technology facilitated the identification of about 47 genes related to resistance to many heavy metals including lead, arsenic, zinc, mercury, nickel, silver and chromium of R. planticola FACU 3 strain. Moreover, genome sequencing identified plant growth-promoting genes (PGPGs) including indole acetic acid (IAA) production, phosphate solubilization, phenazine production, trehalose metabolism and 4-hydroxybenzoate production genes and a lot of antibiotic-resistant genes.

Evaluation of immunogenicity and efficacy of the enterobactin conjugate vaccine in protecting chickens from colibacillosis

Colibacillosis is one of the most common and economically devastating infectious diseases in poultry production worldwide. Innovative universal vaccines are urgently needed to protect chickens from the infections caused by genetically diverse avian pathogenic Escherichia coli (APEC). Enterobactin (Ent) is a highly conserved siderophore required for E. coli iron acquisition and pathogenesis. The Ent-specific antibodies induced by a novel Ent conjugate vaccine significantly inhibited the in vitro growth of diverse APEC strains. In this study, White Leghorn chickens were immunized with the Ent conjugate vaccine using a crossed design with two variables, vaccination (with or without) and APEC challenge (O1, O78, or PBS control), resulting in six study groups (9 to 10 birds/group). The chickens were subcutaneously injected with the vaccine (100 μg per bird) at 7 days of age, followed by booster immunization at 21 days of age. The chickens were intratracheally challenged with an APEC strain (10⁸ CFU/bird) or PBS at 28 days of age. At 5 days post infection, all chickens were euthanized to examine lesions and APEC colonization of the major organs. Immunization of chickens with the Ent vaccine elicited a strong immune response with a 64-fold increase in the level of Ent-specific IgY in serum. The hypervirulent strain O78 caused extensive lesions in lung, air sac, heart, liver, and spleen with significantly reduced lesion scores observed in the vaccinated chickens. Interestingly, the vaccination did not significantly reduce APEC levels in the examined organs. The APEC O1 with low virulence only caused sporadic lesions in the organs in both vaccination and control groups. The Ent conjugate vaccine altered the bacterial community of the ileum and cecum. Taken together, the findings from this study showed the Ent conjugate vaccine could trigger a strong specific immune response and was promising to confer protection against APEC infection.

A Four-Step Platform to Optimize Growth Conditions for High-Yield Production of Siderophores in Cyanobacteria

In response to Iron deprivation and in specific environmental conditions, the cyanobacteria Anabaena flos aquae produce siderophores, iron-chelating molecules that in virtue of their interesting environmental and clinical applications, are recently gaining the interest of the pharmaceutical industry. Yields of siderophore recovery from in vitro producing cyanobacterial cultures are, unfortunately, very low and reach most of the times only analytical quantities. We here propose a four-step experimental pipeline for a rapid and inexpensive identification and optimization of growth parameters influencing, at the transcriptional level, siderophore production in Anabaena flos aquae. The four-steps pipeline consists of: (1) identification of the promoter region of the operon of interest in the genome of Anabaena flos aquae; (2) cloning of the promoter in a recombinant DNA vector, upstream the cDNA coding for the Green Fluorescent Protein (GFP) followed by its stable transformation in Escherichia Coli; (3) identification of the environmental parameters affecting expression of the gene in Escherichia coli and their application to the cultivation of the Anabaena strain; (4) identification of siderophores by the combined use of high-resolution tandem mass spectrometry and molecular networking. This multidisciplinary, sustainable, and green pipeline is amenable to automation and is virtually applicable to any cyanobacteria, or more in general, to any microorganisms.

Diversity of Fast-Growth Spore-Forming Microbes and Their Activity as Plant Partners

Biofertilizers are agricultural materials capable of reducing the usage amounts of chemical fertilizers. Spore-forming microorganisms (SFM) could be used for plant growth promotion or to improve plant health. Until now, biofertilizers based on SFM have been applied for rice and other crops. In this study, we isolated and characterized SFM, which were colonized on the Oryza sativa L. roots. SFM were analyzed regarding the short-term effects of biofertilization on the nursery growths. Analysis was performed without nitrogen or any inorganic fertilizer and was divided into two groups, including bacteria and fungi. SF-bacteria were dominated by the Firmicutes group, including species from Viridibacillus, Lysinibacillus, Solibacillus, Paenibacillus, Priestia, and mainly Bacillus (50%). The fungi group was classified as Mucoromycota, Basidiomycota, and mainly Ascomycota (80%), with a predominance of Penicillium and Trichoderma species. In plant performance in comparison with B. pumilus TUAT1, some bacteria and fungus isolates significantly improved the early growth of rice, based on 48 h inoculum with 10⁷ CFU mL^-1. Furthermore, several SFM showed positive physiological responses under abiotic stress or with limited nutrients such as phosphorous (P). Moreover, the metabolic fingerprint was obtained. The biofertilizer based on SFM could significantly reduce the application of the inorganic fertilizer and improve the lodging resistances of rice, interactively enhancing better plant health and crop production.

Emerging Target-Directed Approaches for the Treatment and Diagnosis of Microbial Infections

With the rising levels of drug resistance, developing efficient antimicrobial therapies has become a priority. A promising strategy is the conjugation of antibiotics with relevant moieties that can potentiate their activity by target-directing. The conjugation of siderophores with antibiotics allows them to act as Trojan horses by hijacking the microorganisms' highly developed iron transport systems and using them to carry the antibiotic into the cell. Through the analysis of relevant examples of the past decade, this Perspective aims to reveal the potential of siderophore-antibiotic Trojan horses for the treatment of infections and the role of siderophores in diagnostic techniques. Other conjugated molecules will be the subject of discussion, namely those involving vitamin B12, carbohydrates, and amino acids, as well as conjugated compounds targeting protein degradation and β-lactamase activated prodrugs.

Catechol siderophores framed on 2,3-dihydroxybenzoyl-L-serine from Streptomyces varsoviensis

Enterobactin is an archetypical catecholate siderophore that plays a key role in the acquisition of ferric iron by microorganisms. Catechol moieties have been shown to be promising siderophore cores. Variants of the conserved 2,3-dihydroxybenzoate (DHB) moiety with structural modifications expand the bioactivity. Streptomyces are characterized by metabolites with diverse structures. The genomic sequence of Streptomyces varsoviensis indicated that it possessed a biosynthetic gene cluster for DHB containing siderophores and metabolic profiling revealed metabolites correlated with catechol-type natural products. Here, we report the discovery of a series of catecholate siderophores produced by S. varsoviensis and a scale-up fermentation was performed to purify these compounds for structural elucidation. A biosynthetic route for the catecholate siderophores is also proposed. These new structural features enrich the structural diversity of the enterobactin family compounds. One of the new linear enterobactin congeners shows moderate activity against a food-borne pathogen Listeria monocytogenes. This work demonstrated that changing culture conditions is still a promising approach to explore unexplored chemical diversity. The availability of the biosynthetic machinery will enrich the genetic toolbox of catechol siderophores and facilitate such engineering efforts.

Structural Studies of Modular Nonribosomal Peptide Synthetases

The non-ribosomal peptide synthetases (NRPSs) are a family of modular enzymes involved in the production of peptide natural products. Not restricted by the constraints of ribosomal peptide and protein production, the NRPSs are able to incorporate unusual amino acids and other suitable building blocks into the final product. The NRPSs operate with an assembly line strategy in which peptide intermediates are covalently tethered to a peptidyl carrier protein and transported to different catalytic domains for the multiple steps in the biosynthesis. Often the carrier and catalytic domains are joined into a single large multidomain protein. This chapter serves to introduce the NRPS enzymes, using the nocardicin NRPS system as an example that highlights many common features to NRPS biochemistry. We then describe recent advances in the structural biology of NRPSs focusing on large multidomain structures that have been determined.

Chaetomadramines A-E, a class of siderophores with potent neuroprotective activity from the fungus Chaetomium madrasense cib-1

Five hydroxamate siderophores, chaetomadramines A-E (1-5), along with seven known compounds were isolated from the fermented rice culture of the fungus Chaetomium madrasense cib-1. Compounds 1-5 were structurally elucidated on the basis of spectroscopic data, which were a group of unusual hydroxamate siderophores, bearing a long fatty acyl on the α-NH₂ of the N^δ-hydroxylated ornithine. Compounds 2-5 were new. The structural elucidation and spectroscopic data of 1 were reported for the first time. Compounds 2-4 significantly improved the survival rates of PC12 cells in the neuroprotective activity assay at the concentration of 40 μM.

Elucidating the effect of iron acquisition systems in Klebsiella pneumoniae on susceptibility to the novel siderophore-cephalosporin cefiderocol

BACKGROUND

Cefiderocol (CFDC) is a novel siderophore-cephalosporin, effective against multidrug-resistant Gram-negative bacteria. As it has a siderophore side chain, it can utilize iron acquisition systems for penetration of the bacterial outer membrane. We aimed to elucidate the role of siderophores and iron uptake receptors in defining Klebsiella pneumoniae susceptibility to CFDC.

METHODS

Initially, 103 K. pneumoniae strains were characterized for susceptibility to different antibiotics including CFDC. CFDC minimum inhibitory concentrations (MIC) were determined in iron-depleted and iron-enriched conditions. Iron uptake genes including siderophores, their receptors, ferric citrate (fecA) and iron uptake (kfu) receptors were detected by PCR in all the strains. For 10 selected strains, gene expression was tested in iron-depleted media with or without CFDC treatment and compared to expression in iron-enriched conditions.

RESULTS

CFDC exhibited 96.1% susceptibility, being superior to all the other antibiotics (MIC50: 0.5 and MIC90: 4 μg/ml). Only three strains (2.9%) were intermediately susceptible and a pandrug resistant strain (0.97%) was resistant to CFDC (MIC: 8 and 256 μg/ml, respectively). The presence of kfu and fecA had a significant impact on CFDC MIC, especially when co-produced, and if coupled with yersiniabactin receptor (fyuA). CFDC MICs were negatively correlated with enterobactin receptor (fepA) expression and positively correlated with expression of kfu and fecA. Thus, fepA was associated with increased susceptibility to CFDC, while kfu and fecA were associated with reduced susceptibility to CFDC. CFDC MICs increased significantly in iron-enriched media, with reduced expression of siderophore receptors, hence, causing less drug uptake.

CONCLUSION

Iron acquisition systems have a significant impact on CFDC activity, and their altered expression is a factor leading to reduced susceptibility. Iron concentration is also a major player affecting CFDC susceptibility; therefore, it is essential to explore possible ways to improve the drug activity to facilitate its use to treat infections in iron-rich sites.

Signaling and Detoxification Strategies in Plant-Microbes Symbiosis under Heavy Metal Stress: A Mechanistic Understanding

Plants typically interact with a variety of microorganisms, including bacteria, mycorrhizal fungi, and other organisms, in their above- and below-ground parts. In the biosphere, the interactions of plants with diverse microbes enable them to acquire a wide range of symbiotic advantages, resulting in enhanced plant growth and development and stress tolerance to toxic metals (TMs). Recent studies have shown that certain microorganisms can reduce the accumulation of TMs in plants through various mechanisms and can reduce the bioavailability of TMs in soil. However, relevant progress is lacking in summarization. This review mechanistically summarizes the common mediating pathways, detoxification strategies, and homeostatic mechanisms based on the research progress of the joint prevention and control of TMs by arbuscular mycorrhizal fungi (AMF)-plant and Rhizobium-plant interactions. Given the importance of tripartite mutualism in the plant-microbe system, it is necessary to further explore key signaling molecules to understand the role of plant-microbe mutualism in improving plant tolerance under heavy metal stress in the contaminated soil environments. It is hoped that our findings will be useful in studying plant stress tolerance under a broad range of environmental conditions and will help in developing new technologies for ensuring crop health and performance in future.

Iron acquisition strategies in pseudomonads: mechanisms, ecology, and evolution

Iron is important for bacterial growth and survival, as it is a common co-factor in essential enzymes. Although iron is very abundant in the earth crust, its bioavailability is low in most habitats because ferric iron is largely insoluble under aerobic conditions and at neutral pH. Consequently, bacteria have evolved a plethora of mechanisms to solubilize and acquire iron from environmental and host stocks. In this review, I focus on Pseudomonas spp. and first present the main iron uptake mechanisms of this taxa, which involve the direct uptake of ferrous iron via importers, the production of iron-chelating siderophores, the exploitation of siderophores produced by other microbial species, and the use of iron-chelating compounds produced by plants and animals. In the second part of this review, I elaborate on how these mechanisms affect interactions between bacteria in microbial communities, and between bacteria and their hosts. This is important because Pseudomonas spp. live in diverse communities and certain iron-uptake strategies might have evolved not only to acquire this essential nutrient, but also to gain relative advantages over competitors in the race for iron. Thus, an integrative understanding of the mechanisms of iron acquisition and the eco-evolutionary dynamics they drive at the community level might prove most useful to understand why Pseudomonas spp., in particular, and many other bacterial species, in general, have evolved such diverse iron uptake repertoires.

The making of hypervirulent Klebsiella pneumoniae

Klebsiella pneumoniae is a notorious bacterium in clinical practice. Virulence, carbapenem-resistance and their convergence among K. pneumoniae are extensively discussed in this article. Hypervirulent K. pneumoniae (HvKP) has spread from the Asian Pacific Rim to the world, inducing various invasive infections, such as pyogenic liver abscess, endophthalmitis, and meningitis. Furthermore, HvKP has acquired more and more drug resistance. Among multidrug-resistant HvKP, hypervirulent carbapenem-resistant K. pneumoniae (Hv-CRKP), and carbapenem-resistant hypervirulent K. pneumoniae (CR-HvKP) are both devastating for their extreme drug resistance and virulence. The hypervirulence of HvKP is primarily attributed to hypercapsule, macromolecular exopolysaccharides, or excessive siderophores, although it has many other factors, for example, lipopolysaccharides, fimbriae, and porins. In contrast with classical determination of HvKP, that is, animal lethality test, molecular determination could be an optional and practical method after improvement. HvKP, including Hv-CRKP and CR-HvKP, has been progressing. R-M and CRISPR-Cas systems may play pivotal roles in such evolutions. Hv-CRKP and CR-HvKP, in particular the former, should be of severe concern due to their being more and more prevalent.

Exploration of Dual Ionic Cross-Linked Alginate Hydrogels Via Cations of Varying Valences towards Wound Healing

This study explored the synergistic effects of simultaneously using calcium and gallium cations in the cross-linking of alginate, detailing its effects on the characteristics of alginate compared to its single cation counterparts. The primary goal is to determine if there are any synergistic effects associated with the utilisation of multiple multivalent cations in polymer cross-linking and whether or not it could therefore be used in pharmaceutical applications such as wound healing. Given the fact divalent and trivalent cations have never been utilised together for cross-linking, an explanation for the mode of binding that occurs between the alginate and the cations during the cross-linking process and how it may affect the future applications of the polymer has been investigated. The calcium gallium alginate polymers were able to retain the antibacterial effects of gallium within the confines of the polymer matrix, possessing superior rheological properties, 6 times that of pure calcium and pure gallium, coupled with an improved swelling capacity that is 4 times higher than that of gallium alginate.

A Review of Pseudomonas aeruginosa Metallophores: Pyoverdine, Pyochelin and Pseudopaline

P. aeruginosa is a common Gram-negative bacterium found in nature that causes severe infections in humans. As a result of its natural resistance to antibiotics and the ability of biofilm formation, the infection with this pathogen can be therapeutic challenging. During infection, P. aeruginosa produces secondary metabolites such as metallophores that play an important role in their virulence. Metallophores are metal ions chelating molecules secreted by bacteria, thus allowing them to survive in the host under metal scarce conditions. Pyoverdine, pyochelin and pseudopaline are the three metallophores secreted by P. aeruginosa. Pyoverdines are the primary siderophores that acquire iron from the surrounding medium. These molecules scavenge and transport iron to the bacterium intracellular compartment. Pyochelin is another siderophore produced by this bacterium, but in lower quantities and its affinity for iron is less than that of pyoverdine. The third metallophore, pseudopaline, is an opine narrow spectrum ion chelator that enables P. aeruginosa to uptake zinc in particular but can transport nickel and cobalt as well. This review describes all the aspects related to these three metallophore, including their main features, biosynthesis process, secretion and uptake when loaded by metals, in addition to the genetic regulation responsible for their synthesis and secretion.

Deciphering the role of endophytic microbiome in postharvest diseases management of fruits: Opportunity areas in commercial up-scale production

As endophytes are widely distributed in the plant's internal compartments and despite having enormous potential as a biocontrol agent against postharvest diseases of fruits, the fruit-endophyte-pathogen interactions have not been studied detail. Therefore, this review aims to briefly discuss the colonization patterns of endophytes and pathogens in the host tissue, the diversity and distribution patterns of endophytes in the carposphere of fruits, and host-endophyte-pathogen interactions and the molecular mechanism of the endophytic microbiome in postharvest disease management in fruits. Postharvest loss management is one of the major concerns of the current century. It is considered a critical challenge to food security for the rising global population. However, to manage the postharvest loss, still, a large population relies on chemical fungicides, which affect food quality and are hazardous to health and the surrounding environment. However, the scientific community has searched for alternatives for the last two decades. In this context, endophytic microorganisms have emerged as an economical, sustainable, and viable option to manage postharvest pathogens with integral colonization properties and eliciting a defense response against pathogens. This review extensively summarizes recent developments in endophytic interactions with harvested fruits and pathogens-the multiple biocontrol traits of endophytes and colonization and diversity patterns of endophytes. In addition, the upscale commercial production of endophytes for postharvest disease treatment is discussed.

Bacterial Siderophores: Classification, Biosynthesis, Perspectives of Use in Agriculture

Siderophores are synthesized and secreted by many bacteria, yeasts, fungi, and plants for Fe (III) chelation. A variety of plant-growth-promoting bacteria (PGPB) colonize the rhizosphere and contribute to iron assimilation by plants. These microorganisms possess mechanisms to produce Fe ions under iron-deficient conditions. Under appropriate conditions, they synthesize and release siderophores, thereby increasing and regulating iron bioavailability. This review focuses on various bacterial strains that positively affect plant growth and development through synthesizing siderophores. Here we discuss the diverse chemical nature of siderophores produced by plant root bacteria; the life cycle of siderophores, from their biosynthesis to the Fe-siderophore complex degradation; three mechanisms of siderophore biosynthesis in bacteria; the methods for analyzing siderophores and the siderophore-producing activity of bacteria and the methods for screening the siderophore-producing activity of bacterial colonies. Further analysis of biochemical, molecular-biological, and physiological features of siderophore synthesis by bacteria and their use by plants will allow one to create effective microbiological preparations for improving soil fertility and increasing plant biomass, which is highly relevant for sustainable agriculture.

Nutritional immunity: the battle for nutrient metals at the host-pathogen interface

Trace metals are essential micronutrients required for survival across all kingdoms of life. From bacteria to animals, metals have critical roles as both structural and catalytic cofactors for an estimated third of the proteome, representing a major contributor to the maintenance of cellular homeostasis. The reactivity of metal ions engenders them with the ability to promote enzyme catalysis and stabilize reaction intermediates. However, these properties render metals toxic at high concentrations and, therefore, metal levels must be tightly regulated. Having evolved in close association with bacteria, vertebrate hosts have developed numerous strategies of metal limitation and intoxication that prevent bacterial proliferation, a process termed nutritional immunity. In turn, bacterial pathogens have evolved adaptive mechanisms to survive in conditions of metal depletion or excess. In this Review, we discuss mechanisms by which nutrient metals shape the interactions between bacterial pathogens and animal hosts. We explore the cell-specific and tissue-specific roles of distinct trace metals in shaping bacterial infections, as well as implications for future research and new therapeutic development.

Siderophore Immunization Restricted Colonization of Adherent-Invasive Escherichia coli and Ameliorated Experimental Colitis

Inflammatory bowel diseases (IBD) are characterized by chronic inflammation of the gastrointestinal tract and profound alterations to the gut microbiome. Adherent-invasive Escherichia coli (AIEC) is a mucosa-associated pathobiont that colonizes the gut of patients with Crohn's disease, a form of IBD. Because AIEC exacerbates gut inflammation, strategies to reduce the AIEC bloom during colitis are highly desirable. To thrive in the inflamed gut, Enterobacteriaceae acquire the essential metal nutrient iron by producing and releasing siderophores. Here, we implemented an immunization-based strategy to target the siderophores enterobactin and its glucosylated derivative salmochelin to reduce the AIEC bloom in the inflamed gut. Using chemical (dextran sulfate sodium) and genetic (Il10-/- mice) IBD mouse models, we showed that immunization with enterobactin conjugated to the mucosal adjuvant cholera toxin subunit B potently elicited mucosal and serum antibodies against these siderophores. Siderophore-immunized mice exhibited lower AIEC gut colonization, diminished AIEC association with the gut mucosa, and reduced colitis severity. Moreover, Peyer's patches and the colonic lamina propria harbored enterobactin-specific B cells that could be identified by flow cytometry. The beneficial effect of siderophore immunization was primarily B cell-dependent because immunized muMT-/- mice, which lack mature B lymphocytes, were not protected during AIEC infection. Collectively, our study identified siderophores as a potential therapeutic target to reduce AIEC colonization and its association with the gut mucosa, which ultimately may reduce colitis exacerbation. Moreover, this work provides the foundation for developing monoclonal antibodies against siderophores, which could provide a narrow-spectrum strategy to target the AIEC bloom in Crohn's disease patients. IMPORTANCE Adherent-invasive Escherichia coli (AIEC) is abnormally prevalent in patients with ileal Crohn's disease and exacerbates intestinal inflammation, but treatment strategies that selectively target AIEC are unavailable. Iron is an essential micronutrient for most living organisms, and bacterial pathogens have evolved sophisticated strategies to capture iron from the host environment. AIEC produces siderophores, small, secreted molecules with a high affinity for iron. Here, we showed that immunization to elicit antibodies against siderophores promoted a reduction of the AIEC bloom, interfered with AIEC association with the mucosa, and mitigated colitis in experimental mouse models. We also established a flow cytometry-based approach to visualize and isolate siderophore-specific B cells, a prerequisite for engineering monoclonal antibodies against these molecules. Together, this work could lead to a more selective and antibiotic-sparing strategy to target AIEC in Crohn's disease patients.

In Vitro Reconstitution of Fimsbactin Biosynthesis from Acinetobacter baumannii

Siderophores produced via nonribosomal peptide synthetase (NRPS) pathways serve as critical virulence factors for many pathogenic bacteria. Improved knowledge of siderophore biosynthesis guides the development of inhibitors, vaccines, and other therapeutic strategies. Fimsbactin A is a mixed ligand siderophore derived from human pathogenic Acinetobacter baumannii that contains phenolate-oxazoline, catechol, and hydroxamate metal chelating groups branching from a central l-Ser tetrahedral unit via amide and ester linkages. Fimsbactin A is derived from two molecules of l-Ser, two molecules of 2,3-dihydroxybenzoic acid (DHB), and one molecule of l-Orn and is a product of the fbs biosynthetic operon. Here, we report the complete in vitro reconstitution of fimsbactin A biosynthesis in a cell-free system using purified enzymes. We demonstrate the conversion of l-Orn to N₁-acetyl-N₁-hydroxy-putrescine (ahPutr) via ordered action of FbsJ (decarboxylase), FbsI (flavin N-monooxygenase), and FbsK (N-acetyltransferase). We achieve conversion of l-Ser, DHB, and l-Orn to fimsbactin A using FbsIJK in combination with the NRPS modules FbsEFGH. We also demonstrate chemoenzymatic conversion of synthetic ahPutr to fimsbactin A using FbsEFGH and establish the substrate selectivity for the NRPS adenylation domains in FbsH (DHB) and FbsF (l-Ser). We assign a role for the type II thioesterase FbsM in producing the shunt metabolite 2-(2,3-dihydroxyphenyl)-4,5-dihydrooxazole-4-carboxylic acid (DHB-oxa) via cleavage of the corresponding thioester intermediate that is tethered to NRPS peptidyl carrier domains during biosynthetic assembly. We propose a mechanism for branching NRPS-derived peptides via amide and ester linkages via the dynamic equilibration of N-DHB-Ser and O-DHB-Ser thioester intermediates via hydrolysis of DHB-oxa thioester intermediates. We also propose a genetic signature for NRPS "branching" in the presence of a terminating C-T-C motif (FbsG).

The Key Element Role of Metallophores in the Pathogenicity and Virulence of Staphylococcus aureus: A Review

The ubiquitous bacterium Staphylococcus aureus causes many diseases that sometimes can be fatal due to its high pathogenicity. The latter is caused by the ability of this pathogen to secrete secondary metabolites, enabling it to colonize inside the host causing infection through various processes. Metallophores are secondary metabolites that enable bacteria to sequester metal ions from the surrounding environment since the availability of metal ions is crucial for bacterial metabolism and virulence. The uptake of iron and other metal ions such as nickel and zinc is one of these essential mechanisms that gives this germ its virulence properties and allow it to overcome the host immune system. Additionally, extensive interactions occur between this pathogen and other bacteria as they compete for resources. Staphylococcus aureus has high-affinity metal import pathways including metal ions acquisition, recruitment and metal-chelate complex import. These characteristics give this bacterium the ability to intake metallophores synthesized by other bacteria, thus enabling it to compete with other microorganisms for the limited nutrients. In scarce host conditions, free metal ions are extremely low because they are confined to storage and metabolic molecules, so metal ions are sequestered by metallophores produced by this bacterium. Both siderophores (iron chelating molecules) and staphylopine (wide- spectrum metallophore) are secreted by Staphylococcus aureus giving it infectious properties. The genetic regulation of the synthesis and export together with the import of metal loaded metallophores are well established and are all covered in this review.

Monoclonal antibody-based indirect competitive ELISA for quantitative detection of Enterobacteriaceae siderophore enterobactin

Enterobactin (Ent) is a promising indicator to monitor intestinal level of Enterobacteriaceae for assessment of gut inflammation. In this study, we developed a monoclonal antibody (mAb)-based ELISA for Ent quantification. We immunized mice with an Ent conjugate vaccine. An mAb named 2E4, with the highest anti-Ent antibody titer, was selected for developing indirect competitive ELISA (ic-ELISA). The purified mAb 2E4 showed high affinity (3.1 × 10^-10 M) and specificity to Ent. The limit of detection of ic-ELISA was 0.39 μg/mL. The intra- and inter-assay recovery rates of standard curve were up to 94.6% with the coefficients of variation between 4.0% and 12.3%, indicating high accuracy, repeatability, and reproducibility of the ic-ELISA. In addition, the ic-ELISA was able to quantitatively detect Ent produced in different bacterial cultures. Collectively, this study developed an ic-ELISA with excellent performance in Ent quantification, laying a solid foundation for Ent-based diagnostics of gut health.