HSAF-induced antifungal effects in Candida albicans through ROS-mediated apoptosis

The natural polycyclic tetramate macrolactam HSAF inhibit Fusarium graminearum through altering cell membrane integrity by targeting FgORP1

Fusarium graminearum is a dominant phytopathogenic fungus causing Fusarium head blight (FHB) in cereal crops. Heat-stable antifungal factor (HSAF) is a polycyclic tetramate macrolactam (PoTeM) isolated from Lysobacter enzymogenes that exhibits strong antifungal activity against F. graminearum. HSAF significantly reduces the DON production and virulence of F. graminearum. Importantly, HSAF exhibited no cross-resistance to carbendazim, phenamacril, tebuconazole and pydiflumetofen. However, the target protein of HSAF in F. graminearum is unclear. In this study, the oxysterol-binding protein FgORP1 was identified as the potential target of HSAF using surface plasmon resonance (SPR) combined with RNA-sequence (RNA-seq). The RNA-seq results showed cell membrane and ergosterol biosynthesis were significantly impacted by HSAF in F. graminearum. Molecular docking showed that HSAF binds with arginine 1205 and glutamic acid 1212, which are located in the oxysterol-binding domain of FgORP1. The two amino acids in FgORP1 are responsible for HSAF resistance in F. graminearum though site-directed mutagenesis. Furthermore, deletion of FgORP1 led to significantly decreased sensitivity to HSAF. Additionally, FgORP1 regulates the mycelial growth, conidiation, DON production, ergosterol biosynthesis and virulence in F. graminearum. Overall, our findings revealed the mode of action of HSAF against F. graminearum, indicating that HSAF is a promising fungicide for controlling FHB.

Sacculatane Diterpenoids from the Liverwort Plagiochila nitens Collected in China

Seven new terpenoids, including six sacculatane diterpenoids plagiochilarins A-F (1-6), and one ent-2,3-seco-aromandrane sesquiterpenoid plagiochilarin H (8) with a 6/7/3/5 tetracyclic scaffold, alongside three known compounds, were obtained from the Chinese liverwort Plagiochila nitens Inoue. Plagiochilarin B (2) was unpredictably converted to the more stable artifact 7 under acid catalysis through cyclic ether formation. The reaction mechanism was reasonably deduced and experimentally verified. The structures of these terpenoids were determined by analysis of MS and NMR spectroscopic data and single-crystal X-ray diffraction. The inhibitory effect of all of the isolates was evaluated on the growth of two C. albicans strains, wild strain SC5314 and efflux pump-deficient strain DSY654. However, only plagiochilarin H (8) showed a MIC value of 16 μg/mL against C. albicans DSY654.

Control of citrus blue and green molds by Actinomycin X2 and its possible antifungal mechanism

Citrus blue and green molds caused by Penicillium digitatum, P. italicum, and P. polonicum, are the major postharvest diseases of citrus fruit. In the present study, Actinomycin X2 (Act-X2), a naturally occurring antibiotic produced by Streptomyces species, was found to show excellent antifungal effect against these three pathogens with a minimum inhibitory concentration (MIC) value of 62.5 μg/mL for them all, which was better than the positive control thiophanate-methyl. Act-X2 significantly reduced the percentage of spore germination, and highly inhibited the mycelial growth of P. italicum, P. digitatum, and P. polonicum with EC50 values being 34.34, 13.76, and 37.48 μg/mL, respectively. In addition, Act-X2 greatly decreased the intracellular protein content while increasing the reactive oxygen species (ROS) level and superoxide anion (O2-) content in the mycelia of pathogens. In vivo test indicated that Act-X2 strongly inhibited the infection of navel oranges by these three Penicillium species, with an inhibition percentage of >50% for them all at the concentration of 10 MIC. Transcriptome analysis suggested that Act-X2 might highly influence the ribosomal functions of P. polonicum, which was supported as well by the molecular docking analysis of Act-X2 with some key functional proteins and RNAs of the ribosome. Furthermore, Act-X2 significantly reduced the decay percentage and improved the firmness, color, and sugar-acid ratio of navel oranges spray-inoculated with P. polonicum during the postharvest storage at 4 °C for 60 d.

Antifungal Compound from the Predatory Bacterium Lysobacter enzymogenes Inhibits a Plant Pathogenic Fungus by Targeting the AAA ATPase VpVeb1

Heat-stable antifungal factor (HSAF) isolated from Lysobacter enzymogenes is considered a potential biocontrol agent. However, the target of HSAF in phytopathogenic fungi remains unclear. In this study, we investigated the target of HSAF in Valsa pyri that causes fatal pear Valsa canker. Thirty-one HSAF-binding proteins were captured and identified by surface plasmon resonance (SPR) and high-performance liquid chromatography-mass spectrometry (LC-MS/MS), and 11 deletion mutants were obtained. Among these mutants, only ΔVpVEB1 showed decreased sensitivity to HSAF. Additionally, ΔVpVEB1 exhibited significantly reduced virulence in V. pyri. Molecular docking and SPR results revealed that HSAF bound to threonine 569 and glycine 570 of VpVeb1, which are crucial for AAA ATPase activity. Another study showed that HSAF could decrease the ATPase activity of VpVeb1, leading to the reduced virulence of V. pyri. Taken together, this study first identified the potential target of HSAF in fungi. These findings will help us better understand the model of action of HSAF to fungi.

A novel and high-efficient method for the preparation of heat-stable antifungal factor from Lysobacter enzymogenes by high-speed counter-current chromatography

Heat-stable antifungal factor (HSAF) produced by the biocontrol bacterium Lysobacter enzymogenes shows considerable antifungal activity and has broad application potential in the agricultural and medical fields. There is a great demand for pure HSAF compounds in academic or industrial studies. However, an efficient preparation method that produces a high yield and high purity of HSAF is lacking, limiting the development of HSAF as a new drug. In the present study, high-speed counter-current chromatography (HSCCC) combined with column chromatography was successfully developed for the separation and preparation of HSAF from the crude extract of L. enzymogenes OH11. The crude extract was obtained by macroporous resin adsorption and desorption, and the main impurities were partly removed by ultraviolet light (254 nm) and gel filtration (Sephadex LH-20). In the HSCCC procedure, the selected suitable two-phase solvent system (n-hexane/ethyl acetate/methanol/water = 3:5:4:5, v/v, the lower phase added with 0.1% TFA) with a flow rate of 2.0 mL/min and a sample loading size of 100 mg was optimized for the separation. As a result, a total of 42 mg HSAF with a purity of 97.6% and recovery of 91.7% was yielded in one separation. The structure elucidation based on HR-TOF-MS, 1H and 13C NMR, and antifungal activities revealed that the isolated compound was unambiguously identified as HSAF. These results are helpful for separating and producing HSAF at an industrial scale, and they further demonstrate that HSCCC is a useful tool for isolating bioactive constituents from beneficial microorganisms.

Combating Antimicrobial Resistance in the Post-Genomic Era: Rapid Antibiotic Discovery

Constantly evolving drug-resistant "superbugs" have caused an urgent demand for novel antimicrobial agents. Natural products and their analogs have been a prolific source of antimicrobial agents, even though a high rediscovery rate and less targeted research has made the field challenging in the pre-genomic era. With recent advancements in technology, natural product research is gaining new life. Genome mining has allowed for more targeted excavation of biosynthetic potential from natural sources that was previously overlooked. Researchers use bioinformatic algorithms to rapidly identify and predict antimicrobial candidates by studying the genome before even entering the lab. In addition, synthetic biology and advanced analytical instruments enable the accelerated identification of novel antibiotics with distinct structures. Here, we reviewed the literature for noteworthy examples of novel antimicrobial agents discovered through various methodologies, highlighting the candidates with potent effectiveness against antimicrobial-resistant pathogens.

Lysobacter enzymogenes prevents Phytophthora infection by inhibiting pathogen growth and eliciting plant immune responses

The Phytophthora pathogen causes enormous damage to important agricultural plants. This group of filamentous pathogens is phylogenetically distant from fungi, making them difficult to control by most chemical fungicides. Lysobacter enzymogenes OH11 (OH11) is a biocontrol bacterium that secretes HSAF (Heat-Stable Antifungal Factor) as a broad-spectrum antifungal weapon. Here, we showed that OH11 could also control a variety of plant Phytophthora diseases caused by three major oomycetes (P. sojae, P. capsici and P. infestans). We provided abundant evidence to prove that OH11 protected host plants from Phytophthora pathogen infection by inhibiting mycelial growth, digesting cysts, suppressing cyst germination, and eliciting plant immune responses. Interestingly, the former two processes required the presence of HSAF, while the latter two did not. This suggested that L. enzymogenes could prevent Phytophthora infection via multiple previously unknown mechanisms. Therefore, this study showed that L. enzymogenes could serve as a promising alternative resource for promoting plant resistance to multiple Phytophthora pathogens.

Antimicrobial peptide AMP-17 exerts anti-Candida albicans effects through ROS-mediated apoptosis and necrosis

There is a need for new anti-Candida albicans (C. albicans) drugs owing to the emergence of drug resistance in recent years. AMP-17, an antimicrobial peptide from Musca domestica (M. domestica), is known to be an effective inhibitor of many fungal pathogens, including C. albicans. In this study, we investigated the potential mechanism underlying the anti-C. albicans effects of AMP-17 using flow cytometry, transmission electron microscopy, fluorescent probes, fluorescence microplate reader, and confocal laser microscopy. Transmission electron microscopy showed that, following AMP-17 treatment, the shape of C. albicans cells became irregular, and vacuoles could be seen in the cytoplasm. Furthermore, AMP-17 treatment resulted in an increase in reactive oxygen species (ROS) levels, depolarization of the mitochondrial membrane potential (MMP), and changes in the cell cycle, leading to the apoptosis and necrosis, which ultimately contributed to the death of C. albicans cells.

Polyene Macrolactams from Marine and Terrestrial Sources: Structure, Production Strategies, Biosynthesis and Bioactivities

Over the past few decades (covering 1972 to 2022), astounding progress has been made in the elucidation of structures, bioactivities and biosynthesis of polyene macrolactams (PMLs), but they have only been partially summarized. PMLs possess a wide range of biological activities, particularly distinctive fungal inhibitory abilities, which render them a promising drug candidate. Moreover, the unique biosynthetic pathways including β-amino acid initiation and pericyclic reactions were presented in PMLs, leading to more attention from inside and outside the natural products community. According to current summation, in this review, the chem- and bio-diversity of PMLs from marine and terrestrial sources are considerably rich. A systematic, critical and comprehensive overview is in great need. This review described the PMLs’ general structural features, production strategies, biosynthetic pathways and the mechanisms of bioactivities. The challenges and opportunities for the research of PMLs are also discussed.

Developing a new treatment for superficial fungal infection using antifungal Collagen‐HSAF dressing

Fungal pathogens are common causes of superficial clinical infection. Their increasing drug resistance gradually makes existing antifungal drugs ineffective. Heat stable antifungal factor (HSAF) is a novel antifungal natural product with a unique structure. However, the application of HSAF has been hampered by very low yield in the current microbial producers and from extremely poor solubility in water and common solvents. In this study, we developed an effective mode of treatment applying HSAF to superficial fungal infections. The marine‐derived Lysobacter enzymogenes YC36 contains the HSAF biosynthetic gene cluster, which we activated by the interspecific signaling molecule indole. An efficient extraction strategy was used to significantly improve the purity to 95.3%. Scanning electron microscopy images revealed that the Type I collagen‐based HSAF (Col‐HSAF) has a transparent appearance and good physical properties, and the in vitro sustained‐release effect of HSAF was maintained for more than 2 weeks. The effective therapeutic concentration of Col‐HSAF against superficial fungal infection was explored, and Col‐HSAF showed good biocompatibility, lower clinical scores, mild histological changes, and antifungal capabilities in animals with Aspergillus fumigatus keratitis and cutaneous candidiasis. In conclusion, Col‐HSAF is an antifungal reagent with significant clinical value in the treatment of superficial fungal infections.

Evaluating the Mode of Antifungal Action of Heat-Stable Antifungal Factor (HSAF) in Neurospora crassa

Heat-stable antifungal factor (HSAF) isolated from Lysobacter enzymogenes has shown a broad-spectrum of antifungal activities. However, little is known about its mode of action. In this study, we used the model filamentous fungus Neurospora crassa to investigate the antifungal mechanism of HSAF. We first used HSAF to treat the N. crassa strain at different time points. Spore germination, growth phenotype and differential gene expression analysis were conducted by utilizing global transcriptional profiling combined with genetic and physiological analyses. Our data showed that HSAF could significantly inhibit the germination and aerial hyphae growth of N. crassa. RNA-seq analysis showed that a group of genes, associated with cell wall formation and remodeling, were highly activated. Screening of N. crassa gene deletion mutants combined with scanning electron microscopic observation revealed that three fungal cell wall integrity-related genes played an important role in the interaction between N. crassa and L. enzymogens. In addition, Weighted Gene Co-Expression Network Analysis (WGCNA), accompanied by confocal microscopy observation revealed that HSAF could trigger autophagy-mediated degradation and eventually result in cell death in N. crassa. The findings of this work provided new insights into the interactions between the predatory Lysobacter and its fungal prey.

D319 induced antifungal effects through ROS-mediated apoptosis and inhibited isocitrate lyase in Candida albicans

BACKGROUND

Candida albicans (C. albicans) is an opportunistic pathogen that can cause superficial and life-threatening systemic infections in immunocompromised patients. However, the available clinically antifungals are limited. Therefore, the development of effective antifungal agents and therapies is urgently needed. Quinoline type of compounds were reported to possess potent anti-fungal effect. A series of quinoline derivatives were synthesized. Moreover their inhibitory activities and mechanisms on C. albicans were evaluated in this study.

METHODS

The structure of D319 was identified by extensive spectroscopic analysis. The antifungal activity of D319 on C. albicans was evaluated using conventional methods, including the inhibition zone diameters with filter paper, Clinical Laboratory Standard Institute (CLSI) broth microdilution method in vitro, and in a murine model in vivo. Flow cytometry, fluorescence microscopy, western blot, knockout mutant and revertant strain techniques, and molecular modeling were applied to explore the mechanism of action of D319 in anti-Candida.

RESULTS

D319 exhibited potent anti-Candida activity with Minimum Inhibitory Concentration value of 2.5 μg/mL in vitro. D319 significantly improved survival rate and reduced fungal burden compared to vehicle control in a murine model in vivo. The treatment of C. albicans with D319 resulted in fungal apoptosis through reactive oxygen species (ROS) accumulation in C. albicans. Furthermore, D319 inhibited the glyoxylate enzyme isocitrate lyase (ICL) of C. albicans, which was also confirmed by docking analysis.

CONCLUSIONS

Quinoline compound D319 exhibited strong anti-Candida activities in vitro and in vivo models through inhibiting ICL activity and ROS accumulation in C. albicans.

GENERAL SIGNIFICANCE

This study showed that compound D319 as a novel isocitrate lyase inhibitor, would be a promising anti-Candida lead compound, which provided a potential application of this type of compounds in fighting clinical fungal infections. Furthermore, this study also supported ICL as a potential target for anti-Candida drug discovery.

Biosynthesis, regulation, and engineering of natural products from Lysobacter

Covering: up to August 2021Lysobacter is a genus of Gram-negative bacteria that was classified in 1987. Several Lysobacter species are emerging as new biocontrol agents for crop protection in agriculture. Lysobacter are prolific producers of new bioactive natural products that are largely underexplored. So far, several classes of structurally interesting and biologically active natural products have been isolated from Lysobacter. This article reviews the progress in Lysobacter natural product research over the past ten years, including molecular mechanisms for biosynthesis, regulation and mode of action, genome mining of cryptic biosynthetic gene clusters, and metabolic engineering using synthetic biology tools.

TMT-based quantitative proteomic analysis of the effects of novel antimicrobial peptide AMP-17 against Candida albicans

Candida albicans is the most common human fungal pathogen in immunocompromised individuals. With the emergence of clinical fungal resistance, there is an urgent need to develop novel antifungal agents. AMP-17, a novel antimicrobial peptide from Musca domestica, has an antifungal effect against C. albicans, but its mechanism of antifungal action remains unclear. In the current study, we performed a proteomics analysis in C. albicans using TMT technique under the treatment of AMP-17. A total of 3931 proteins were identified, of which 3600 included quantitative information. With a 1.5-fold change threshold and a t-test p-value < 0.05 as standard, 423 differentially expressed proteins (DEPs) were up-regulated and 180 DEPs were down-regulated in the AMP-17/control. Notably, GO enrichment revealed that DEPs associated with the cell wall, RNA and oxidative stress were significantly up-regulated, while DEPs involved in ergosterol metabolism and membrane were significantly down-regulated in the AMP-17/control. KEGG pathway enrichment revealed that DEPs involved seven significant metabolic pathways, mainly involved oxidative phosphorylation, RNA degradation, propanoate metabolism and fatty acid metabolism. These results show that AMP-17 induces a complex organism response in C. albicans, indicating that AMP-17 may inhibit growth by affecting multiple targets in C. albicans cells. SIGNIFICANCE: Antimicrobial peptides (AMPs) are an important part of the innate immune system of organisms and having broad range of activity against fungi, bacteria and viruses. These AMPs are considered as probable candidate for forthcoming drugs, due to their broad range of activity, lesser toxicity and decreased resistance development by target cells. AMP-17, a novel antimicrobial peptide from M. domestica, has significant antifungal activity against C. albicans. It has been confirmed that AMP-17 can play an antifungal effect by destroying the cell wall and cell membrane of C. albicans in previous studies, but its mechanism of action at the protein level is currently unclear. In the current study, using the TMT-based quantitative proteomics method, 603 differentially expressed proteins were identified in the cells of C. albicans treated with AMP-17 for 12 h, and these DEPs were closely related to cell wall, cell membrane, RNA degradation and oxidative stress. The results provide new insights into the potential mechanism of action of AMP- 17 against C. albicans. Meanwhile, it provides certain technical support and theoretical basis for the research and development of novel peptide drugs.

Synthesis and characterization of PCU@C-Ag/AgCl nanoparticles as an antimicrobial material for respiratory tract infection

The pregnant cow urine (PCU) is an active source of antimicrobial agents that is used for fabricating chitosan coated Ag/AgCl nanoparticles (NPs) in the present study. These PCU@C-Ag/AgCl NPs were physicochemically characterized and evaluated for antimicrobial activity against selected respiratory tract infection (RTI) pathogens. The absorption band around 420 nm in UV-Visible spectrum indicated the presence of Ag NPs. The spherical shape of NPs was observed using TEM. Also, the crystalline structure was confirmed using the XRD pattern. The PCU@C-Ag/AgCl NPs showed strong antimicrobial activity against all tested RTI pathogens. In addition, FESEM analysis showed morphological changes in RTI bacterial pathogens. Thereby, PCU@C-Ag/AgCl NPs may be used as an antimicrobial material to treat RTIs in near future at clinical level.

Polycyclic Tetramate Macrolactams-A Group of Natural Bioactive Metallophores

New infectious diseases and increase in drug-resistant microbial pathogens emphasize the need for antibiotics with novel mode-of-action. Tetramates represented by fungi-derived tenuazonic acid and bacterial polycyclic tetramate macrolactams (PTMs) are an important family of natural products with a broad spectrum of antimicrobial activities. Despite their potential application as new antibiotics, it remains unknown how PTMs function. In this study, genomic mining revealed that PTM biosynthetic gene clusters (BGCs) are widespread in both Gram-positive and Gram-negative bacteria, and we investigated a sponge endosymbiont Actinoalloteichus hymeniacidonis harboring a potential PTM-BGC. Xanthobaccin A that previously has only been isolated from a Gram-negative bacterium was obtained after a scale-up fermentation, isolation, and structure elucidation through mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. Xanthobaccin A as well as two previously reported tetramates, equisetin and ikarugamycin, exhibited antibacterial activities against Bacillus subtilis. In addition, these three tetramates were for the first time to be confirmed as metallophores and the stoichiometry of the complexes were shown to be Fe(III)(equisetin)₃/Fe(III)(equisetin)₂ and Fe(III)(ikarugamycin)₂, respectively. Meanwhile, we found that all three tetramates could reduce ferric into ferrous iron, which triggers the Fenton chemistry reaction. Their antibacterial activity was reduced by adding the radical scavenger, vitamin C. Altogether, our work demonstrates that equisetin and PTMs can act as metallophores and their antimicrobial mechanism is possibly mediated through Fenton chemistry.

HECT E3 Ubiquitin Ligase Nedd4 Is Required for Antifungal Innate Immunity

Candida albicans is the most common cause of fungal infections in humans, and disseminated candidiasis has become one of the leading causes of hospital-acquired bloodstream infections with a high mortality rate. However, little is known about the host-pathogen interactions and the mechanisms of antifungal immunity. Here, we report that Nedd4 (neuronal precursor cell-expressed developmentally downregulated 4) is essential for signaling through Dectin-1 and Dectin-2/3. We showed that mice that lack Nedd4 globally or only in the myeloid compartment are highly susceptible to systemic C. albicans infection, which correlates with heightened organ fungal burden, defective inflammatory response, impaired leukocyte recruitment to the kidneys, and defective reactive oxygen species expression by granulocytes. At the molecular level, Nedd4 -/- macrophages displayed impaired activation of TGF-β-activating kinase-1 and NF-κB, but normal activation of spleen tyrosine kinase and protein kinase C-δ on C. albicans yeast and hyphal infections. These data suggest that Nedd4 regulates signaling events downstream of protein kinase C-δ but upstream of or at TGF-β-activating kinase-1.

Antifungal weapons of Lysobacter, a mighty biocontrol agent

Bacteria interact with fungi in a variety of ways to inhibit fungal growth, while the underlying mechanisms remain only partially characterized. The plant-beneficial Bacillus and Pseudomonas species are well-known antifungal biocontrol agents, whereas Lysobacter are far less studied. Members of Lysobacter are easy to grow in fermenters and are safe to humans, animals and plants. These environmentally ubiquitous bacteria use a diverse arsenal of weapons to prey on other microorganisms, including fungi and oomycetes. The small molecular toxins secreted by Lysobacter represent long-range weapons effective against filamentous fungi. The secreted hydrolytic enzymes act as intermediate-range weapons against non-filamentous fungi. The contact-dependent killing devices are proposed to work as short-range weapons. We describe here the structure, biosynthetic pathway, action mode and applications of one of the best-characterized long-range weapons, the heat-stable antifungal factor (HSAF) produced by Lysobacter enzymogenes. We discuss how the flagellar type III secretion system has evolved into an enzyme secretion machine for the intermediate-range antifungal weapons. We highlight an intricate mechanism coordinating the production of the long-range weapon, HSAF and the proposed contact-dependent killing device, type VI secretion system. We also overview the regulatory mechanisms of HSAF production involving specific transcription factors and the bacterial second messenger c-di-GMP.

Outer Membrane Vesicle-Mediated Codelivery of the Antifungal HSAF Metabolites and Lytic Polysaccharide Monooxygenase in the Predatory Lysobacter enzymogenes

Lysobacter are new biocontrol agents known for their prolific production of lytic enzymes and bioactive metabolites. L. enzymogenes is a predator of fungi and produces several structurally distinct antimicrobial compounds, such as the antifungal HSAF (heat stable antifungal factor) and analogs. The mechanism by which L. enzymogenes interacts with fungal prey is not well understood. Here, we found that the production of HSAF and analogs in L. enzymogenes OH11 was significantly induced in media supplemented with ground fungal mycelia or chitin. In the OH11 genome, we identified a gene (LeLPMO10A) that was annotated to encode a chitin-binding protein. The stimulation of HSAF and analogs by chitin was diminished when LeLPMO10A was deleted. We expressed the gene in E. coli and demonstrated that purified LeLPMO10A oxidatively cleaved chitin into oligomeric products, including 1,5 δ-lactones and aldonic acids. The results revealed that LeLPMO10A encodes a lytic polysaccharide monooxygenase, which has not been reported in Lysobacter. The metabolite analysis, antifungal assay, and proteomic analysis showed that the antifungal compounds and the chitin-cleaving LeLPMO10A are colocalized in outer membrane vesicles. The enzymatic products that resulted from in vitro LeLPMO10A-cleaved chitin also significantly induced HSAF and analogs in OH11. Scanning electron microscopic analysis indicated that spherical vesicles were formed outside of OH11 cells, and fewer OH11 cells were observed to attach to fungal hyphae when LeLPMO10A was deleted. Together, the study revealed a previously uncharacterized synergistic strategy utilized by the predatory Lysobacter during interaction with fungal prey.

Characterisation of the Antibiotic Profile of Lysobacter capsici AZ78, an Effective Biological Control Agent of Plant Pathogenic Microorganisms

Determining the mode of action of microbial biocontrol agents plays a key role in their development and registration as commercial biopesticides. The biocontrol rhizobacterium Lysobacter capsici AZ78 (AZ78) is able to inhibit a vast array of plant pathogenic oomycetes and Gram-positive bacteria due to the release of antimicrobial secondary metabolites. A combination of MALDI-qTOF-MSI and UHPLC-HRMS/M was applied to finely dissect the AZ78 metabolome and identify the main secondary metabolites involved in the inhibition of plant pathogenic microorganisms. Under nutritionally limited conditions, MALDI-qTOF-MSI revealed that AZ78 is able to release a relevant number of antimicrobial secondary metabolites belonging to the families of 2,5-diketopiperazines, cyclic lipodepsipeptides, macrolactones and macrolides. In vitro tests confirmed the presence of secondary metabolites toxic against Pythium ultimum and Rhodococcus fascians in AZ78 cell-free extracts. Subsequently, UHPLC-HRMS/MS was used to confirm the results achieved with MALDI-qTOF-MSI and investigate for further putative antimicrobial secondary metabolites known to be produced by Lysobacter spp. This technique confirmed the presence of several 2,5-diketopiperazines in AZ78 cell-free extracts and provided the first evidence of the production of the cyclic depsipeptide WAP-8294A2 in a member of L. capsici species. Moreover, UHPLC-HRMS/MS confirmed the presence of dihydromaltophilin/Heat Stable Antifungal Factor (HSAF) in AZ78 cell-free extracts. Due to the production of HSAF by AZ78, cell-free supernatants were effective in controlling Plasmopara viticola on grapevine leaf disks after exposure to high temperatures. Overall, our work determined the main secondary metabolites involved in the biocontrol activity of AZ78 against plant pathogenic oomycetes and Gram-positive bacteria. These results might be useful for the future development of this bacterial strain as the active ingredient of a microbial biopesticide that might contribute to a reduction in the chemical input in agriculture.

An Antifungal Polycyclic Tetramate Macrolactam, Heat-Stable Antifungal Factor (HSAF), Is a Novel Oxidative Stress Modulator in Lysobacter enzymogenes

Polycyclic tetramate macrolactams (PoTeMs) are a fast-growing family of antibiotic natural products found in phylogenetically diverse microorganisms. Surprisingly, none of the PoTeMs have been investigated for potential physiological functions in their producers. Here, we used heat-stable antifungal factor (HSAF), an antifungal PoTeM from Lysobacter enzymogenes, as a model to show that PoTeMs form complexes with iron ions, with an association constant (K_a ) of 2.71 × 10⁶ M^-1 The in vivo and in vitro data showed formation of 2:1 and 3:1 complexes between HSAF and iron ions, which were confirmed by molecular mechanical and quantum mechanical calculations. HSAF protected DNA from degradation in high concentrations of iron and H₂O₂ or under UV radiation. HSAF mutants of L. enzymogenes barely survived under oxidative stress and exhibited markedly increased production of reactive oxygen species (ROS). Exogenous addition of HSAF into the mutants significantly prevented ROS production and restored normal growth in the mutants under the oxidative stress. The results reveal that the function of HSAF is to protect the producer microorganism from oxidative damage rather than as an iron-acquisition siderophore. The characteristic structure of PoTeMs, a 2,4-pyrrolidinedione-embedded macrolactam, may represent a new iron-chelating scaffold of microbial metabolites. The study demonstrated a previously unrecognized strategy for microorganisms to modulate oxidative damage to the cells.IMPORTANCE PoTeMs are a family of structurally distinct metabolites that have been found in a large number of bacteria. Although PoTeMs exhibit diverse therapeutic properties, the physiological function of PoTeMs in the producer microorganisms had not been investigated. HSAF from Lysobacter enzymogenes is an antifungal PoTeM that has been subjected to extensive studies for mechanisms of biosynthesis, regulation, and antifungal activity. Using HSAF as a model system, we here showed that the characteristic structure of PoTeMs, a 2,4-pyrrolidinedione-embedded macrolactam, may represent a new iron-chelating scaffold of microbial metabolites. In L. enzymogenes, HSAF functions as a small-molecule modulator for oxidative damage caused by iron, H₂O₂, and UV light. Together, the study demonstrated a previously unrecognized strategy for microorganisms to modulate oxidative damage to the cells. HSAF represents the first member of the fast-growing PoTeM family of microbial metabolites whose potential biological function has been studied.

Mechanism of Antifungal Activity by 5-Aminoimidazole-4-Carbohydrazonamide Derivatives against Candida albicans and Candida krusei

Systemic mycoses are one major cause of morbidity/mortality among immunocompromised/debilitated individuals. Studying the mechanism of action is a strategy to develop safer/potent antifungals, warning resistance emergence. The major goal of this study was to elucidate the mechanism of action of three (Z)-5-amino-N’-aryl-1-methyl-1H-imidazole-4-carbohydrazonamides (2h, 2k, 2l) that had previously demonstrated strong antifungal activity against Candida krusei and C. albicans ATCC strains. Activity was confirmed against clinical isolates, susceptible or resistant to fluconazole by broth microdilution assay. Ergosterol content (HPLC-DAD), mitochondrial dehydrogenase activity (MTT), reactive oxygen species (ROS) generation (flow cytometry), germ tube inhibition and drug interaction were evaluated. None of the compounds inhibited ergosterol synthesis. Ascorbic acid reduced the antifungal effect of compounds and significantly decreased ROS production. The metabolic viability of C. krusei was significantly reduced for values of 2MIC. Compounds 2h and 2k caused a significant increase in ROS production for MIC values while for 2l a significant increase was only observed for concentrations above MIC. ROS production seems to be involved in antifungal activity and the higher activity against C. krusei versus C. albicans may be related to their unequal sensitivity to different ROS. No synergism with fluconazole or amphotericin was observed, but the association of 2h with fluconazole might be valuable due to the significant inhibition of the dimorphic transition, a C. albicans virulence mechanism.

Reactive oxygen mediated apoptosis as a therapeutic approach against opportunistic Candida albicans

Candida albicans are polymorphic fungal species commonly occurs in a symbiotic association with the host's usual microflora. Certain specific changes in its usual microenvironment can lead to diseases ranging from external mucosal to severally lethal systemic infections like invasive candidiasis hospital-acquired fatal infection caused by different species of Candida. The patient acquired with this infection has a high mortality and morbidity rate, ranging from 40% to 60%. This is an ill-posed problem by its very nature. Hence, early diagnosis and management is a crucial part. Antifungal drug resistance against the first and second generation of antifungal drugs has made it difficult to treat such fatal diseases. After a few dormant years, recently, there has been a rapid turnover of identifying novel drugs with low toxicity to limit the problem of drug resistance. After an initial overview of related work, we examine specific prior work on how a change in oxidative stress can facilitate apoptosis in C. albicans. Subsequently, it was investigated that Candida spp. suppresses the production of ROS mediated host defense system. Here, we have reviewed possibly all the small molecule inhibitors, natural products, antimicrobial peptide, and some naturally derived semi-synthetic compounds which are known to influence oxidative stress, to generate a proper apoptotic response in C. albicans and thus might be a novel therapeutic approach to augment the current treatment options.

Biocontrol ability and action mechanism of dihydromaltophilin against Colletotrichum fructicola causing anthracnose of pear fruit

BACKGROUND

Anthracnose caused by Colletotrichum fructicola is one of the most important diseases in pear fruit, resulting in huge economic losses. Public awareness of protecting the environment and food safety, together with pathogen resistance to many key fungicides have led to an urgent need to develop alternative strategies for controlling fruit diseases. Here, the antifungal activity of a natural product, dihydromaltophilin [heat-stable antifungal factor (HSAF)], against C. fructicola in vitro and in vivo was investigated to determine its efficacy for anthracnose management.

RESULTS

HSAF exhibited pronounced antifungal activity against in vitro mycelial growth of C. fructicola, with a half-inhibition concentration of 0.43 mg L^-1 . Hyphae treated with HSAF showed defects such as hyperbranching, swelling and depolarized growth. Conidia germination in the pathogen was inhibited by HSAF in a dose-dependent manner. In the presence of 4 mg L^-1 HSAF, conidia germination was significantly delayed, and germ tube growth was inhibited. HSAF at 8 mg L^-1 completely blocked conidia germination in C. fructicola. In addition, HSAF disrupted coordination of cytokinesis with growth and nuclear division, induced reactive oxygen species production in conidia, and damaged the integrity of the conidia cell wall. Moreover, an in vivo test confirmed that 50 mg L^-1 HSAF significantly reduced the development of anthracnose decay in pear fruit caused by C. fructicola.

CONCLUSION

HSAF was highly effective in reducing pear anthracnose caused by C. fructicola and has great potential to become a new type of fruit preservative.

Characterization of Lysobacter spp. strains and their potential use as biocontrol agents against pear anthracnose

Colletotrichum fructicola, is an important fungal pathogen that has been reported to cause pear (Pyrus) anthracnose in China, resulting in substantial economic losses due to severe defoliation and decreased fruit quality and yield. In the search for novel strategies to control pear anthracnose, Lysobacter strains have drawn a great deal of attention due to their high-level production of extracellular enzymes and bioactive metabolites. In the present study, we compared four Lysobacter strains including Lysobacter enzymogenes OH11, Lysobacter antibioticus OH13, Lysobacter gummosus OH17 and Lysobacter brunescens OH23 with respect to their characteristics and activity against pear anthracnose caused by C. fructicola. The results showed that the evaluated Lysobacter species presented various colony morphologies when cultured on different media and were proficient in producing protease, chitinase, cellulase and glucanase, with L. enzymogenes OH11 showing typical twitching motility. L. enzymogenes OH11 and L. gummosus OH17 showed potent activity against the tested fungi and oomycetes. L. gummosus OH17 produced HSAF (heat-stable antifungal factor) which was demonstrated to be a major antifungal factor in L. enzymogenes OH11 and C3. Furthermore, L. antibioticus OH13 and L. brunescens OH23 exhibited strong antibacterial activity, especially against Xanthomonas species. Cultures of L. enzymogenes OH11 protected pear against anthracnose caused by C. fructicola, and the in vivo results indicated that treatment with an L. enzymogenes OH11 culture could decrease the diameter of lesions in pears by 35 % and reduce the severity of rot symptoms compared to that observed in the control. In the present study, we systemically compared four Lysobacter strains and demonstrated that they have strong antagonistic activity against a range of pathogens, demonstrating their promise in the development of biological control agents.

An intrinsic mechanism for coordinated production of the contact-dependent and contact-independent weapon systems in a soil bacterium

Soil bacteria possess multiple weapons to fend off microbial competitors. Currently, we poorly understand the factors guiding bacterial decisions about weapon systems deployment. In this study, we investigated how such decisions are made by the soil bacterium Lysobacter enzymogenes, used in antifungal plant protection. We found that weapons production is guided by environmental cues. In rich media, which likely mimic environments crowded with other microbes, L. enzymogenes produces a contact-dependent weapon, type six secretion system (T6SS). In nutrient-poor media, likely dominated by filamentous oomycetes and fungi, L. enzymogenes synthesizes and secretes a heat-stable antifungal factor (HSAF), a contact-independent weapon. Surprisingly, the T6SS inner tube protein Hcp is accumulated intracellularly even in nutrient-poor media, when the T6SS is not assembled. We found that Hcp interacts with the transcription factor Clp required for activating HSAF biosynthesis operon expression. Hcp protects Clp from binding to c-di-GMP, an intracellular second messenger inhibiting DNA binding. The increased concentration of c-di-GMP-free Clp thus leads to higher gene expression and HSAF production. Therefore, when the contact-dependent weapon, T6SS, is not in use, accumulation of one of its structural components, Hcp, serves as a signal to enhance production of the contact-independent weapon, HSAF. The uncovered environment-dependent and auto-regulatory mechanisms shed light on the processes governing deployment of various weapon systems in environmental bacteria.

Soil bacteria face competition from diverse microbial species. To stay competitive, they deploy a variety of weapons. At present, we know little about factors influencing decisions about which weapons to produce at any given time, and about mechanisms through which these decisions are carried out. In this study, we show that in the soil bacterium, Lysobacter enzymogenes, synthesis of the contact-dependent weapon, known as type six secretion system (T6SS) occurs under different conditions, compared to those conductive to the production of the contact-independent weapon, toxin HSAF. Further, when T6SS is not assembled, one of its structural components, Hcp, coactivates HSAF operon expression and HSAF synthesis. This study reveals that decisions about contact-dependent and contact-independent weapon production in bacteria are governed by both environmental cues and intrinsic coordination mechanisms.

Terpenoids from the Chinese liverwort Heteroscyphus coalitus and their anti-virulence activity against Candida albicans

In this study, 14 previously undescribed terpenoids were isolated from the Chinese liverwort Heteroscyphus coalitus (Hook.) Schiffner, including a rare harziane type diterpenoid, heteroscyphsic acid A; eight ent-clerodane diterpenoids, heteroscyphsic acids B-I; four labdane diterpenoids, heteroscyphins A-D; and one guaiane sesquiterpene, heteroscyphin E; as well as a known ent-junceic acid. Their structures were determined by a combination of MS, NMR spectroscopy, electronic circular dichroism (ECD) and single crystal X-ray diffraction analyses. The anti-virulence activity of the isolated compounds against Candida albicans DSY654 demonstrated that most of them could block hyphal growth at concentrations ranging from 4-32 μg/ml. Further investigation of the most active compound, heteroscyphin D, revealed that it could suppress the ability of C. albicans DSY654 to adhere to A549 cells and form biofilms, and modulate the transcription of related genes in this fungus.

LeTetR Positively Regulates 3-Hydroxylation of the Antifungal HSAF and Its Analogs in Lysobacter enzymogenes OH11

The biocontrol agent Lysobacter enzymogenes OH11 produces several structurally distinct antibiotic compounds, including the antifungal HSAF (Heat Stable Antifungal Factor) and alteramides, along with their 3-dehydroxyl precursors (3-deOH). We previously showed that the 3-hydroxylation is the final step of the biosynthesis and is also a key structural moiety for the antifungal activity. However, the procedure through which OH11 regulates the 3-hydroxylation is still not clear. In OH11, the gene orf3232 was predicted to encode a TetR regulator (LeTetR) with unknown function. Here, we deleted orf3232 and found that the LeTetR mutant produced very little HSAF and alteramides, while the 3-deOH compounds were not significantly affected. The production of HSAF and alteramides was restored in orf3232-complemented mutant. qRT-PCR showed that the deletion of orf3232 impaired the transcription of a putative fatty acid hydroxylase gene, orf2195, but did not directly affect the expression of the HSAF biosynthetic gene cluster (hsaf). When an enzyme extract from E. coli expressing the fatty acid hydroxylase gene, hsaf-orf7, was added to the LeTetR mutant, the production of HSAF and alteramides increased by 13-14 fold. This study revealed a rare function of the TetR family regulator, which positively controls the final step of the antifungal biosynthesis and thus controls the antifungal activity of the biocontrol agent.

A semisynthetic borrelidin analogue BN-3b exerts potent antifungal activity against Candida albicans through ROS-mediated oxidative damage

In the process of investigating the antifungal structure-activity relationships (SAR) of borrelidin and discovering antifungal leads, a semisynthetic borrelidin analogue, BN-3b with antifungal activity against Candida albicans, was achieved. In this study, we found that oxidative damage induced by endogenous reactive oxygen species (ROS) plays an important role in the antifungal activity of BN-3b. Further investigation indicated that BN-3b stimulated ROS accumulation, increased malondialdehyde (MDA) levels, and decreased reduced/oxidized glutathione (GSH/GSSG) ratio. Moreover, BN-3b decreased mitochondrial membrane potential (MMP) and ATP generation. Ultrastructure analysis revealed that BN-3b severely damaged the cell membrane of C. albicans. Quantitative PCR (RT-qPCR) analysis revealed that virulence factors of C. albicans SAPs, PLB1, PLB2, HWP1, ALSs, and LIPs were all down-regulated after BN-3b exposure. We also found that BN-3b markedly inhibited the hyphal formation of C. albicans. In addition, in vivo studies revealed that BN-3b significantly prolonged survival and decreased fungal burden in mouse model of disseminated candidiasis.

The rhizosphere signature on the cell motility, biofilm formation and secondary metabolite production of a plant-associated Lysobacter strain

Lysobacter spp. are common bacterial inhabitants of the rhizosphere of diverse plant species. However, the impact of the rhizosphere conditions on their physiology is still relatively understudied. To provide clues on the behaviour of Lysobacter spp. in this ecological niche, we investigated the physiology of L. capsici AZ78 (AZ78), a biocontrol strain isolated from tobacco rhizosphere, on a common synthetic growth medium (LBA) and on a growth medium containing components of the plant rhizosphere (RMA). The presence of a halo surrounding the AZ78 colony on RMA was a first visible effect related to differences in growth medium composition and it corresponded to the formation of a large outer ring. The lower quantity of nutrients available in RMA as compared with LBA was associated to a higher expression of a gene encoding cAMP-receptor-like protein (Clp), responsible for cell motility and biofilm formation regulation. AZ78 cells on RMA were motile, equipped with cell surface appendages and organised in small groups embedded in a dense layer of fibrils. Metabolic profiling by mass spectrometry imaging revealed increased diversity of analytes produced by AZ78 on RMA as compared with LBA. In particular, putative cyclic lipodepsipeptides, polycyclic tetramate macrolactams, cyclic macrolactams and other putative secondary metabolites with antibiotic activity were identified. Overall, the results obtained in this study shed a light on AZ78 potential to thrive in the rhizosphere by its ability to move, form biofilm and release secondary metabolites.

Knockout of Diguanylate Cyclase Genes in Lysobacter enzymogenes to Improve Production of Antifungal Factor and Increase Its Application in Seed Coating

Heat-stable antifungal factor (HSAF) is a broad-spectrum antifungal antibiotic produced by the biological control agent, Lysobacter enzymogenes. In our earlier works, we have applied HSAF to effectively control wheat and pear fungal disease. However, a major bottleneck in its practical application is the low HSAF production level; therefore, boosting its production is essential for its wide application. In the past, we find that c-di-GMP, a universal bacterial second messenger, is inhibitory to HSAF production. In this work, we further identified eight active diguanylate cyclases (DGCs) responsible for c-di-GMP synthesis in Lysobacter enzymogenes via both bioinformatics and genetic analyses. We generated a strain lacking seven active DGC genes and found that this DGC-modified strain, OH11LC, produced a higher HSAF amount in a c-di-GMP concentration-dependent manner. Subsequently, by employing OH11LC as the host fermentation strain, we could even produce a much higher HSAF amount (> 200-fold). After improving the HSAF production, we further developed a technique of seed coating method with HSAF, which turned out to be effective in fighting against the maize seed-borne filamentous pathogen, Pythium gramineacola. Overall, via combining strain modification and fermentation optimization, we demonstrated a good example of translating fundamental knowledge of bacterial c-di-GMP signaling into biological control application in which we relieved the inhibitory effect of c-di-GMP on HSAF biosynthesis by deleting a bunch of potentially active L. enzymogenes DGC genes to improve HSAF yield and to expand its usage in antifungal seed coating.

Role of membrane sterol and redox system in the anti-candida activity reported for Mo-CBP2, a protein from Moringa oleifera seeds

Plant proteins are emerging as an alternative to conventional treatments against candidiasis. The aim of this study was to better understand the mechanism of action of Mo-CBP₂ against Candida spp, evaluating redox system activity, lipid peroxidation, DNA degradation, cytochrome c release, medium acidification, and membrane interaction. Anti-candida activity of Mo-CBP₂ decreased in the presence of ergosterol, which was not observed with antioxidant agents. C. albicans treated with Mo-CBP₂ also had catalase and peroxidase activities inhibited, while superoxide dismutase was increased. Mo-CBP₂ increased the lipid peroxidation, but it did not alter the ergosterol profile in live cells. External medium acidification was strongly inhibited, and cytochrome c release and DNA degradation were detected. Mo-CBP₂ interacts with cell membrane constituents, changes redox system enzymes in C. albicans and causes lipid peroxidation by ROS overproduction. DNA degradation and cytochrome c release suggest apoptotic or DNAse activity. Lipid peroxidation and H⁺-ATPases inhibition may induce the process of apoptosis. Finally, Mo-CBP₂ did not have a cytotoxic effect in mammalian Vero cells. This study highlights the biotechnological potential of Mo-CBP₂ as a promising molecule with low toxicity and potent activity. Further studies should be performed to better understand its mode of action and toxicity.

The hidden enzymology of bacterial natural product biosynthesis

Bacterial natural products display astounding structural diversity, which, in turn, endows them with a remarkable range of biological activities that are of significant value to modern society. Such structural features are generated by biosynthetic enzymes that construct core scaffolds or perform peripheral modifications, and can thus define natural product families, introduce pharmacophores and permit metabolic diversification. Modern genomics approaches have greatly enhanced our ability to access and characterize natural product pathways via sequence-similarity-based bioinformatics discovery strategies. However, many biosynthetic enzymes catalyse exceptional, unprecedented transformations that continue to defy functional prediction and remain hidden from us in bacterial (meta)genomic sequence data. In this Review, we highlight exciting examples of unusual enzymology that have been uncovered recently in the context of natural product biosynthesis. These suggest that much of the natural product diversity, including entire substance classes, awaits discovery. New approaches to lift the veil on the cryptic chemistries of the natural product universe are also discussed.

Control of Wheat Fusarium Head Blight by Heat-Stable Antifungal Factor (HSAF) from Lysobacter enzymogenes

Heat-stable antifungal factor (HSAF), which belongs to the polycyclic tetramate macrolactam family, was isolated from Lysobacter enzymogenes fermentations and exhibited inhibitory activities against a wide range of fungal pathogens. In this study, the antifungal activity of HSAF against Fusarium graminearum in vitro and in vivo was investigated. A total of 50% of mycelial growth of F. graminearum was suppressed with 4.1 μg/ml of HSAF (EC₅₀ value). HSAF treatment resulted in abnormal morphology of the hyphae, such as curling, apical swelling, and depolarized growth. Furthermore, HSAF adequately inhibited conidial germination and conidiation of F. graminearum with an inhibition rate of 100% when 1 and 6 μg/ml of HSAF were applied, respectively. HSAF caused ultrastructural changes of F. graminearum, including cell wall thickening and plasmolysis. Moreover, the application of HSAF significantly controlled Fusarium head blight in wheat caused by F. graminearum in the field. Overall, these results indicate that HSAF has potential for development as a fungicide against F. graminearum.

OX4 Is an NADPH-Dependent Dehydrogenase Catalyzing an Extended Michael Addition Reaction To Form the Six-Membered Ring in the Antifungal HSAF

The polycyclic tetramate macrolactam HSAF is an antifungal natural product isolated from Lysobacter enzymogenes. HSAF and its analogues have a distinct chemical structure and new mode of antifungal action. The mechanism by which the 5/5/6 tricycle of HSAF is formed from the polyene precursor is not totally clear. Here, we used purified OX4, a homologous enzyme of alcohol dehydrogenase/Zn-binding proteins, to show the enzymatic mechanism for six-membered ring formation. The results from the deuterium isotope incorporation demonstrated that OX4 selectively transfers the pro-R hydride of NADPH to C21 and one proton from water to C10 of 3-deOH alteramide C (1), resulting in 3-deOH HSAF (2) through a reductive cyclization of the polyene precursor by a mechanism consistent with an extended 1,6-Michael addition reaction. The regioselective incorporation of the NADPH hydride into C21 of 1 is also stereoselective, leading to the 21S configuration of 2. This work represents the first characterization of the activity and selectivity of the enzyme for six-membered ring formation in a group of distinct antifungal polycyclic tetramate macrolactams.

ClCPI, a cysteine protease inhibitor purified from Cassia leiandra seeds has antifungal activity against Candida tropicalis by inducing disruption of the cell surface

Infections caused by Candida tropicalis have increased significantly worldwide in parallel with resistance to antifungal drugs. To overcome resistance novel drugs have to be discovered. The objective of this work was to purify and characterize a cysteine protease inhibitor from the seeds of the Amazon rainforest tree Cassia leiandra and test its inhibitory effect against C. tropicalis growth. The inhibitor, named ClCPI, was purified after ion exchange and affinity chromatography followed by ultrafiltration. ClCPI is composed of a single polypeptide chain and is not a glycoprotein. The molecular mass determined by SDS-PAGE in the absence or presence of β-mercaptoethanol and ESI-MS were 16.63 kDa and 18.362 kDa, respectively. ClCPI was stable in the pH range of 7.0-9.0 and thermostable up to 60 °C for 20 min. ClCPI inhibited cysteine proteases, but not trypsin, chymotrypsin neither alpha-amylase. Inhibition of papain was uncompetitive with a Ki of 4.1 × 10^-7 M and IC₅₀ of 8.5 × 10^-7 M. ClCPI at 2.6 × 10^-6 M reduced 50% C. tropicalis growth. ClCPI induced damages and morphological alterations in C. tropicalis cell surface, which led to death. These results suggest that ClCPI have great potential for the development of an antifungal drug against C. tropicalis.

Construction of a hybrid gene cluster to reveal coupled ring formation-hydroxylation in the biosynthesis of HSAF and analogues from Lysobacter enzymogenes

HSAF and analogues are polycyclic tetramate macrolactams (PoTeMs) isolated from Lysobacter enzymogenes. Due to their antifungal activity, distinct chemical structure and new mode of action, PoTeMs have been the subject of several studies for their biosynthetic mechanism. However, polycycle formation is still not well understood. HSAF and several analogues (alteramides) carry a C20-hydroxyl, which is absent in most known PoTeMs such as combamides and pactamides. Previous studies indicated that two genes encoding NAD(P)H-dependent flavin enzymes (OX1/OX2) are responsible for the second five-membered ring formation in HSAF and alteramides. Intriguingly, the products of OX1/OX2 always carry the C20-OH. To test the hypothesis that the formation of the second five-membered ring is coupled with the C20-hydroxylation, we constructed a hybrid PoTeM gene cluster through removing OX1/OX2 in the HSAF cluster and functional complementation by CbmB, which also catalyzes the second five-membered ring formation in combamides but lacking the C20-OH. Two heterologous hosts carrying the hybrid cluster generated the same three PoTeMs, including lysobacterene B (3, the one-ring precursor of HSAF) and combamide D (4, a two-ring product lacking the C20-OH). The third product was not related to either of the clusters and was identified to be pactamide A (5) using mass spectrometry, 1D- and 2D-NMR, and ECD spectroscopy. The results demonstrate the feasibility of producing new PoTeM compounds through combinatorial biosynthesis. More importantly, this study provides the first experimental evidence to support that the second ring formation is coupled with the C20-hydroxylation in the biosynthesis of HSAF and analogues.

Spermidine-Regulated Biosynthesis of Heat-Stable Antifungal Factor (HSAF) in Lysobacter enzymogenes OH11

Heat-Stable Antifungal Factor (HSAF) and its analogs are antifungal natural products produced by the biocontrol agent Lysobacter enzymogenes. The production of HSAF is greatly influenced by environmental stimuli and nutrients, but the underlying molecular mechanism is mostly unclear. Here, we found that HSAF production in L. enzymogenes OH11 is strictly controlled by spermidine, which is the most prevalent triamine in bacteria. When added into OH11 cultures, spermidine regulated the production of HSAF and analogs in a concentration-dependent manner. To verify the role of spermidine, we deleted LeSDC and LeADC genes, encoding S-adenosylmethionine decarboxylase and arginine decarboxylase, respectively, that are the key enzymes for spermidine biosynthesis. Both deletion mutants produced barely detectable spermidine and HSAF including its analogs, whereas the antifungals production was restored by exogenous spermidine. The results showed that the OH11 cells must maintain a proper spermidine homeostasis for the antifungals production. Indeed, the expression level of the key HSAF biosynthetic genes was significantly impaired in LeSDC and LeADC mutants, and exogenous spermidine restored the gene expression level in the mutants. Ornithine is a key substrate for HSAF biosynthesis, and OH11 genome contains arg1 and arg2 genes, encoding arginases that convert arginine to ornithine. While the expression of arg1 and arg2 was affected slightly upon mutation of LeSDC and LeADC, exogenous spermidine significantly increased the arginase gene expression in LeSDC and LeADC mutants. Together, the data revealed a previously unrecognized mechanism, in which spermidine controls antibiotic production through controlling both the biosynthetic genes and the substrate-production genes.

Interactions of Aspergillus fumigatus and Stenotrophomonas maltophilia in an in vitro Mixed Biofilm Model: Does the Strain Matter?

Introduction:Aspergillus fumigatus (Af) and Stenotrophomonas maltophilia (Sm) are pathogenic microorganisms, which coexist in the respiratory tract of cystic fibrosis (CF) patients. We recently developed an in vitro model of mixed biofilm associating Af ATCC 13073-GFP (Af13073) and Sm ATCC 13637 (Sm13637) and described an antibiosis effect. The present study aim was to assess the antibiosis of Sm on Af using different strains and to analyze the potential synergistic virulence of these strains in an in vivo Galleria mellonella model.

Methods: The effect of Sm13637 was evaluated on eight Af strains and the effect of nine Sm strains was evaluated on Af13073. The strains originated from clinical cases (human and animal) and from environment. Fungal and bacterial inocula were simultaneously inoculated to initiate mixed biofilm formation. Fungal growth inhibition was analyzed by qPCR and CLSM and the fungal cell wall modifications by TEM analysis. The virulence of different Sm strains was assessed in association with Af in G. mellonella larvae.

Results: All strains of Af and Sm were able to produce single and mixed biofilms. The antibiosis effect of Sm13637 was similar whatever the Af strain tested. On the other hand, the antibiosis effect of Sm strains was bacterial-fitness and strain dependent. One strain (1/9) originated from animal clinical case was never able to induce an antibiosis, even with high bacterial concentration. In the G. mellonella model, co-inoculation with Sm13637 and Af13073 showed synergism since the mortality was 50%, i.e., more than the summed virulence of both.

Conclusion: Human clinical strains of Sm yielded in higher antibiosis effect on Af and in a thinner mixed biofilm, probably due to an adaptive effect of these strains. Further research covering Af increased wall thickness in the presence of Sm strains, and its correlation with modified antifungal susceptibility is encouraged in patients with chronic respiratory infections by these 2 microorganisms.

Transcriptomics Analysis of the Chinese Pear Pathotype of Alternaria alternata Gives Insights into Novel Mechanisms of HSAF Antifungal Activities

Alternaria alternata (Fries) Keissler is a lethal pear pathogen that causes leaf black spot disease of pear in Southern China. Heat-stable activity factor (HSAF) is a polycyclic tetramate macrolactam (PTM) produced by Lysobacter enzymogenes and many other microbes with a broad-spectrum antifungal activity against many filamentous fungi. In this study, we evaluated the antifungal effect of HSAF against A. alternata and proposed its antifungal mechanism in A. alternata. We report that HSAF inhibited the mycelial growth of A. alternata in a dose-dependent manner. Transcriptomics analysis revealed that HSAF treatment resulted in an expression alteration of a wide range of genes, with 3729 genes being up-regulated, and 3640 genes being down-regulated. Furthermore, we observed that HSAF treatment disrupted multiple signaling networks and essential cellular metabolisms in A. alternata, including the AMPK signaling pathway, sphingolipid metabolism and signaling pathway, carbon metabolism and the TCA (tricarboxylic acid) cycle, cell cycle, nitrogen metabolism, cell wall synthesis and a key hub protein phosphatase 2A (PP2A). These observations suggest that HSAF breaches metabolism networks and ultimately induces increased thickness of the cell wall and apoptosis in A. alternata. The improved understanding of the antifungal mechanism of HSAF against filamentous fungi will aid in the future identification of the direct interaction target of HSAF and development of HSAF as a novel bio-fungicide.

Biosynthesis of the Polycyclic System in the Antifungal HSAF and Analogues from Lysobacter enzymogenes

The biocontrol agent Lysobacter enzymogenes produces polycyclic tetramate macrolactams (PoTeMs), including the antifungal HSAF. To elucidate the biosynthesis of the cyclic systems, we identified eleven HSAF precursors/analogues with zero, one, two, or three rings through heterologous expression of the HSAF gene cluster. A series of combinatorial gene expression and deletion experiments showed that OX3 is the "gatekeeper" responsible for the formation of the first 5-membered ring from lysobacterene A, OX1 and OX2 are responsible for formation of the second ring but with different selectivity, and OX4 is responsible for formation of the 6-membered ring. In vitro experiments showed that OX4 is an NADPH-dependent enzyme that catalyzes the reductive cyclization of 3-dehydroxy alteramide C to form 3-dehydroxy HSAF. Thus, the multiplicity of OX genes is the basis for the structural diversity of the HSAF family, which is the only characterized PoTeM cluster that involves four redox enzymes in the formation of the cyclic system.

Native and Engineered Clifednamide Biosynthesis in Multiple Streptomyces spp

Polycyclic tetramate macrolactam (PTM) natural products are produced by actinomycetes and other bacteria. PTMs are often bioactive, and the simplicity of their biosynthetic clusters make them attractive for bioengineering. Clifednamide-type PTMs from Streptomyces sp. strain JV178 contain a distinctive ketone group, suggesting the existence of a novel PTM oxidizing enzyme. Here, we report the new cytochrome P450 enzyme (CftA) is required for clifednamide production. Genome mining was used to identify several new clifednamide producers, some having improved clifednamide yields. Using a parallel synthetic biology approach, CftA isozymes were used to engineer the ikarugamycin pathway of Streptomyces sp. strain NRRL F-2890 to yield clifednamides. Further, we observed that strong CftA expression leads to the production of a new PTM, clifednamide C. We demonstrate the utility of both genome mining and synthetic biology to rapidly increase clifednamide production.

The impact of the omics era on the knowledge and use of Lysobacter species to control phytopathogenic micro-organisms

Omics technologies have had a tremendous impact on underinvestigated genera of plant disease biocontrol agents such as Lysobacter. Strong evidence of the association between Lysobacter spp. and the rhizosphere has been obtained through culture-independent methods, which has also contributed towards highlighting the relationship between Lysobacter abundance and soil suppressiveness. It is conceivable that the role played by Lysobacter spp. in soil suppressiveness is related to their ability to produce an impressive array of lytic enzymes and antibiotics. Indeed, genomics has revealed that biocontrol Lysobacter strains share a vast number of genes involved in antagonism activities, and the molecular pathways underlying how Lysobacter spp. interact with the environment and other micro-organisms have been depicted through transcriptomic analysis. Furthermore, omics technologies shed light on the regulatory pathways governing cell motility and the biosynthesis of antibiotics. Overall, the results achieved so far through omics technologies confirm that the genus Lysobacter is a valuable source of novel biocontrol agents, paving the way for studies aimed at making their application in field conditions more reliable.

Indole-Induced Reversion of Intrinsic Multiantibiotic Resistance in Lysobacter enzymogenes

Lysobacter species are a group of environmental bacteria that are emerging as a new source of antibiotics. One characteristic of Lysobacter is intrinsic resistance to multiple antibiotics, which had not been studied. To understand the resistance mechanism, we tested the effect of blocking two-component regulatory systems (TCSs) on the antibiotic resistance of Lysobacter enzymogenes, a prolific producer of antibiotics. Upon treatment with LED209, an inhibitor of the widespread TCS QseC/QseB, L. enzymogenes produced a large amount of an unknown metabolite that was barely detectable in the untreated culture. Subsequent structural elucidation by nuclear magnetic resonance (NMR) unexpectedly revealed that the metabolite was indole. Indole production was also markedly induced by adrenaline, a known modulator of QseC/QseB. Next, we identified two TCS genes, L. enzymogenesqseC (Le-qseC) and Le-qseB, in L. enzymogenes and found that mutations of Le-qseC and Le-qseB also led to a dramatic increase in indole production. We then chemically synthesized a fluorescent indole probe that could label the cells. While the Le-qseB (cytoplasmic response regulator) mutant was clearly labeled by the probe, the Le-qseC (membrane sensor) mutant was not labeled. It was reported previously that indole can enhance antibiotic resistance in bacteria. Therefore, we tested if the dramatic increase in the level of indole production in L. enzymogenes upon blocking of Le-qseC and Le-qseB would lead to enhanced antibiotic resistance. Surprisingly, we found that indole caused the intrinsically multiantibiotic-resistant bacterium L. enzymogenes to become susceptible. Point mutations at conserved amino acids in Le-QseC also led to antibiotic susceptibility. Because indole is known as an interspecies signal, these findings may have implications.IMPORTANCE The environmental bacterium Lysobacter is a new source of antibiotic compounds and exhibits intrinsic antibiotic resistance. Here, we found that the inactivation of a two-component regulatory system (TCS) by an inhibitor or by gene deletion led to a remarkable increase in the level of production of a metabolite in L. enzymogenes, and this metabolite was identified to be indole. We chemically synthesized a fluorescent indole probe and found that it could label the wild type and a mutant of the TCS cytoplasmic response regulator but not a mutant of the TCS membrane sensor. Indole treatment caused the intrinsically multidrug-resistant bacterium L. enzymogenes to be susceptible to antibiotics. Mutations of the TCS sensor also led to antibiotic susceptibility. Because indole is known as an interspecies signal between gut microbiota and mammalian hosts, the observation that indole could render intrinsically resistant L. enzymogenes susceptible to common antibiotics may have implications.