Bioprospecting for antituberculosis leads from microbial metabolites

Characterization of Streptomyces sp. LS462 with high productivity of echinomycin, a potent antituberculosis and synergistic antifungal antibiotic

A biologically active microbial strain, designated as "LS462," was isolated from a soil sample collected from Yaoli Virgin Forest of Jiangxi Province, China. The strain was able to produce a high yield of echinomycin (172 mg/l) even under nonoptimized culture conditions and is proposed to serve as a promising source of echinomycin. In this study, echinomycin exhibited strong anti-Mycobacterium tuberculosis H37Rv activity and synergistic antifungal effect with a greatly reduced dosage of posaconazole on Candida albicans SC5314. The strain belongs to the genus Streptomyces according to its morphological and 16S rDNA phylogenetic analysis. The 16S rDNA was found to have the highest sequence identity with Streptomyces fuscichromogenes (99.37% similarity). Extensive nuclear magnetic resonance and mass spectroscopic data were used to determine the structure of echinomycin. The strain S. fuscichromogenes has not been previously reported to produce echinomycin. Strain LS462 may be exploited as a new potential source for the commercial production of echinomycin. Also, this work is the first to report the new synergistic antifungal activity of echinomycin and further study of the synergistic mechanism will be helpful to guide the development of antifungal agents.

Computational prediction and validation of specific EmbR binding site on PknH

Tuberculosis drug resistance continues to threaten global health but the underline molecular mechanisms are not clear. Ethambutol (EMB), one of the well-known first - line drugs in tuberculosis treatment is, unfortunately, not free from drug resistance problems. Genomic studies have shown that some genetic mutations in Mycobacterium tuberculosis (Mtb) EmbR, and EmbC/A/B genes cause EMB resistance. EmbR-PknH pair controls embC/A/B operon, which encodes EmbC/A/B genes, and EMB interacts with EmbA/B proteins. However, the EmbR binding site on PknH was unknown. We conducted molecular simulation on the EmbR– peptides binding structures and discovered phosphorylated PknH 273–280 (N′-HEALS^PDPD-C′) makes β strand with the EmbR FHA domain, as β-MoRF (MoRF; molecular recognition feature) does at its binding site. Hydrogen bond number analysis also supported the peptides' β-MoRF forming activity at the EmbR FHA domain. Also, we discovered that previously known phosphorylation residues might have their chronological order according to the phosphorylation status. The discovery validated that Mtb PknH 273–280 (N′-HEALSDPD-C′) has reliable EmbR binding affinity. This approach is revolutionary in the computer-aided drug discovery field, because it is the first trial to discover the protein-protein interaction site, and find binding partner in nature from this site.

Exploring disordered loops in DprE1 provides a functional site to combat drug-resistance in Mycobacterium strains

As an anti-tuberculosis target, DprE1 contains two flexible loops (Loop I and Loop II) which have never been exploited for developing DprE1 inhibitors. Here Leu317 in Loop II was discovered as a new functional site to combat drug-resistance in Mycobacterium strains. Based on TCA1, LZDT1 was designed to optimize the hydrophobic interaction with Leu317. A subsequent biochemical and cellular assay displayed increased potency of LZDT1 in inhibiting DprE1 and killing drug-sensitive/-resistant Mycobacterium strains. The improved activity of LZDT1 and its analogue LZDT2 against multidrug resistant tuberculosis was particularly highlighted. For LZDT1, its enhanced interaction with Leu317 also impaired the drug-insensitivity of DprE1 caused by Cys387 mutation. A new nonbenzothiazole lead (LZDT10) with reduced Cys387-dependence was further produced by optimizing interactions with Leu317, improvement directions for LZDT10 were discussed as well. Our research underscores the value of potential functional sites in disordered loops, and affords a feasible way to develop these functional sites into opportunities for drug-resistance management.

New approaches for antituberculosis leads from Actinobacteria

Bioactive metabolites derived from the phylum Actinobacteria represent many of the existing antimicrobial drugs. Compared with other bacterial pathogens, direct preliminary screening by diffusion assays is a limiting factor against Mycobacterium tuberculosis (Mtb) and different methodologies have been used to improve the search for new molecules. However, the concern remains that most of the previously discovered molecules replicate by conventional procedures. The combination of multidisciplinary approaches with new technologies could advance the discovery of new leads against Mtb like considering the unexplored Actinobacteria jointly with selective and integrative procedures.

Characterization of anti-BCG benz[α]anthraquinones and new siderophores from a Xinjiang desert-isolated rare actinomycete Nocardia sp. XJ31

Abstract

The current global demand for novel anti-TB drugs has drawn urgent attention on the discovery of natural product compounds with anti-TB activity. Lots of efforts have emphasized on environmental samples from unexplored or underexplored natural habits and identified numerous rare actinomycete taxa producing structurally diverse bioactive natural products. Herein, we report a survey of the rare actinobacteria diversity in Xinjiang region together with the discovery of anti-TB active natural products from these strains. We have collected 17 soil samples at different sites with different environmental conditions, from which 39 rare actinobacteria were identified by using a selective isolation strategy with 5 media variations. Among those isolated strains, XJ31 was identified as a new Nocardia sp. based on 16S rRNA gene analysis. Through one strain-many compounds (OSMAC) strategy combined with anti-Bacillus Calmette-Guérin bioassay-guided isolation, two groups of compounds were identified. They were twelve siderophores (nocardimicins, 1-12) and two anthraquinones (brasiliquinones, 13 and 14) and ten of them were identified as new compounds. The structures of the purified compounds were elucidated using HR-ESI-MS, 1D NMR, and 2D NMR techniques. The anti-TB bioassays revealed that the two benz[α]anthraquinones have potent activity against BCG (MICs = 25 μM), which can be used as a promising start point for further anti-TB drug development.

Key points

• Ten new natural products were identified from Nocardia sp. XJ31.

• Brasiliquinones13and14showed moderate anti-BCG activity.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10842-2) contains supplementary material, which is available to authorized users.

Genome-Inspired Chemical Exploration of Marine Fungus Aspergillus fumigatus MF071

The marine-derived fungus Aspergillus fumigatus MF071, isolated from sediment collected from the Bohai Sea, China, yielded two new compounds 19S,20-epoxy-18-oxotryprostatin A (1) and 20-hydroxy-18-oxotryprostatin A (2), in addition to 28 known compounds (3-30). The chemical structures were established on the basis of 1D, 2D NMR and HRESIMS spectroscopic data. This is the first report on NMR data of monomethylsulochrin-4-sulphate (4) and pseurotin H (10) as naturally occurring compounds. Compounds 15, 16, 20, 23, and 30 displayed weak antibacterial activity (minimum inhibitory concentration: 100 μg/mL). Compounds 18 and 19 exhibited strong activity against S. aureus (minimum inhibitory concentration: 6.25 and 3.13 μg/mL, respectively) and E. coli (minimum inhibitory concentration: 6.25 and 3.13 μg/mL, respectively). A genomic data analysis revealed the putative biosynthetic gene clusters ftm for fumitremorgins, pso for pseurotins, fga for fumigaclavines, and hel for helvolinic acid. These putative biosynthetic gene clusters fundamentally underpinned the enzymatic and mechanistic function study for the biosynthesis of these compounds. The current study reported two new compounds and biosynthetic gene clusters of fumitremorgins, pseurotins, fumigaclavines and helvolinic acid from Aspergillus fumigatus MF071.

Enniatin A1, A Natural Compound with Bactericidal Activity against Mycobacterium tuberculosis In Vitro

Background: Tuberculosis remains a global disease that poses a serious threat to human health, but there is lack of new and available anti-tuberculosis agents to prevent the emergence of drug-resistant strains. To address this problem natural products are still potential sources for the development of novel drugs. Methods: A whole-cell screening approach was utilized to obtain a natural compound enniatin A1 from a natural products library. The target compound’s antibacterial activity against Mycobacterium tuberculosis (M. tuberculosis) was evaluated by using the resazurin reduction micro-plate assay (REMA) method. The cytotoxicity of the compound against Vero cells was measured to calculate the selectivity index. The intracellular inhibition activity of enniatin A1 was determined. We performed its time-kill kinetic assay against M. tuberculosis. We first tested its synergistic effect in combination with the first and second-line anti-tuberculosis drugs. Finally, we measured the membrane potential and intracellular ATP levels of M. tuberculosis after exposure to enniatin A1. Results: We identified enniatinA1 as a potential antibacterial agent against M. tuberculosis, against which it showed strong selectivity. Enniatin A1 exhibited a time-concentration-dependent bactericidal effect against M. tuberculosis, and it displayed synergy with rifamycin, amikacin, and ethambutol. After exposure to enniatinA1, the membrane potential and intracellular ATP levels of M. tuberculosis was significantly decreased. Conclusions: Enniatin A1 exhibits the positive potential anti-tuberculosis agent characteristics.

Marine Polysaccharides in Pharmaceutical Applications: Fucoidan and Chitosan as Key Players in the Drug Delivery Match Field

The use of marine-origin polysaccharides has increased in recent research because they are abundant, cheap, biocompatible, and biodegradable. These features motivate their application in nanotechnology as drug delivery systems; in tissue engineering, cancer therapy, or wound dressing; in biosensors; and even water treatment. Given the physicochemical and bioactive properties of fucoidan and chitosan, a wide range of nanostructures has been developed with these polysaccharides per se and in combination. This review provides an outline of these marine polysaccharides, including their sources, chemical structure, biological properties, and nanomedicine applications; their combination as nanoparticles with descriptions of the most commonly used production methods; and their physicochemical and biological properties applied to the design of nanoparticles to deliver several classes of compounds. A final section gives a brief overview of some biomedical applications of fucoidan and chitosan for tissue engineering and wound healing.

Fractions and isolated compounds from Oxyanthus speciosus subsp. stenocarpus (Rubiaceae) have promising antimycobacterial and intracellular activity

BACKGROUND

Tuberculosis is a deadly disease caused by Mycobacterium species. The use of medicinal plants is an ancient global practice for the treatment and prevention of diverse ailments including tuberculosis. The aim of this study was to isolate and characterize antimycobacterial compounds by bioassay-guided fractionation of the acetone leaf extract of Oxyanthus speciosus.

METHODS

A two-fold serial microdilution method was used to determine the minimum inhibitory concentration (MIC) against mycobacteria. Cytotoxicity and nitric oxide inhibitory activity of the isolated compounds was determined to evaluate in vitro safety and potential anti-inflammatory activity. Intracellular efficacy of the crude extract against Mycobacterium-infected macrophages was also determined.

RESULTS

Two compounds were isolated and identified as lutein (1) and rotundic acid (2). These had good antimycobacterial activity against the four mycobacteria tested with MIC values ranging from 0.013 to 0.1 mg/mL. Rotundic acid had some cytotoxicity against C3A human liver cells. Lutein was not cytotoxic at the highest tested concentration (200 μg/mL) and inhibited nitric oxide production in RAW 264.7 macrophages by 94% at a concentration of 25 μg/mL. The acetone crude extract (120 μg/mL) of O. speciosus had intracellular antimycobacterial activity, reducing colony forming units by more than 90%, displaying bactericidal efficacy in a dose and time-dependent manner.

CONCLUSION

This study provides good proof of the presence of synergism between different compounds in extracts and fractions. It is also the first report of the antimycobacterial activity of lutein and rotundic acid isolated from Oxyanthus speciosus. The promising activity of the crude extract of O. speciosus both in vitro and intracellularly in an in vitro macrophage model suggests its potential for development as an anti- tuberculosis (TB) herbal medicine.

Combinatorial approach for screening and assessment of multiple therapeutic enzymes from marine isolatePseudomonas aeruginosaAR01

A simple and rapid screening of therapeutic enzymes from bacteria was conducted using functional- and sequence-based approach.

In vitro evaluation of dinactin, a potent microbial metabolite against Mycobacterium tuberculosis

Current long duration treatment options and the emergence of drug resistance in tuberculosis (TB) have led to renewed interest in discovery of novel anti-tubercular agents or the scaffolds exhibiting enhanced efficacy with current anti-TB drugs. Herein, dinactin, a potent bioactive macrotetrolide isolated from Streptomyces puniceus AS13, was evaluated against Mycobacterium tuberculosis H37Rv and other susceptible and drug-resistant clinical isolates of M. tuberculosis. In vitro pharmacological assays showed that dinactin is bactericidal against laboratory standard strain M. tuberculosis H37Rv (minimum inhibitory concentration [MIC] 1 µg/mL and minimum bactericidal concentration [MBC] 4 µg/mL). Dinactin also retained its activity against various clinical isolates, including multidrug-resistant strains of M. tuberculosis. Whole cell interaction assays with standard first- and second-line anti-TB drugs showed the synergistic interaction of dinactin with rifampicin or amikacin, reflecting its suitability for use in combination regimens. The killing kinetics studies of dinactin against M. tuberculosis H37Rv revealed that it has strong concentration-dependent anti-TB activity that is also dependent on time. The kill curve also showed dynamic killing capacity of dinactin as it exhibited bactericidal potential at all concentrations tested. Kill curve data demonstrated that dinactin, like isoniazid, exerts its strong tuberculocidal activity within the first two days of exposure. This evidence strongly supports further evaluation of dinactin as a new option in the treatment of TB.

Chlorflavonin Targets Acetohydroxyacid Synthase Catalytic Subunit IlvB1 for Synergistic Killing of Mycobacterium tuberculosis

The flavonoid natural compound chlorflavonin was isolated from the endophytic fungus Mucor irregularis, which was obtained from the Cameroonian medicinal plant Moringa stenopetala. Chlorflavonin exhibited strong growth inhibitory activity in vitro against Mycobacterium tuberculosis (MIC₉₀ 1.56 μM) while exhibiting no cytotoxicity toward the human cell lines MRC-5 and THP-1 up to concentrations of 100 μM. Mapping of resistance-mediating mutations employing whole-genome sequencing, chemical supplementation assays, and molecular docking studies as well as enzymatic characterization revealed that chlorflavonin specifically inhibits the acetohydroxyacid synthase catalytic subunit IlvB1, causing combined auxotrophies to branched-chain amino acids and to pantothenic acid. While exhibiting a bacteriostatic effect in monotreatment, chlorflavonin displayed synergistic effects with the first-line antibiotic isoniazid and particularly with delamanid, leading to a complete sterilization in liquid culture in combination treatment. Using a fluorescent reporter strain, intracellular activity of chlorflavonin against Mycobacterium tuberculosis inside infected macrophages was demonstrated and was superior to streptomycin treatment.

Bovine mastitis: An appraisal of its alternative herbal cure

Bovine mastitis is globally recognized as the most common and costly disease affecting dairy herds. The disease causes huge financial losses to dairy industries by reduced yield and milk quality, deaths and culling of affected cows and also by associated treatment costs. The disease occurs due to invasion of the mammary glands by pathogenic bacteria followed by their multiplication in the milk producing tissues. The most common treatment method available against bovine mastitis is the intra-mammary infusion of antibiotics. However, their use is associated with the problem of antimicrobial resistance. This scenario has made search for alternative treatment approaches necessary. Medicinal plants with their well-established history are an excellent natural product resource used as an alternative therapy. Antibacterial agents from plants can act as important sources of novel antibiotics, efflux pump inhibitors, compounds that target bacterial virulence or can be used in combination with existing drugs. The plants form an essential component of ethno-veterinary medicine used in the treatment of different diseases like bovine mastitis. This review article attempts to provide an overview of the different medicinal plants used in the treatment of bovine mastitis. Antimicrobial studies of these plant species and some of their isolated constituents have been reviewed in detail. It highlights the logic and precedence behind mining this important natural product resource. Our own research findings in this direction and future scope of research are also discussed briefly.

Nutritional control of antibiotic production by Streptomyces platensis MA7327: importance of l-aspartic acid

Streptomyces platensis MA7327 is a bacterium producing interesting antibiotics, which act by the novel mechanism of inhibiting fatty acid biosynthesis. The antibiotics produced by this actinomycete are platensimycin and platencin plus some minor related antibiotics. Platensimycin and platencin have activity against antibiotic-resistant bacteria such as methicillin-resistant Staphylococcus aureus and vancomycin-resistant Enterococcus; they also lack toxicity in animal models. Platensimycin also has activity against diabetes in a mouse model. We have been interested in studying the effects of primary metabolites on production of these antibiotics in our chemically defined production medium. In the present work, we tested 32 primary metabolites for their effect. They included 20 amino acids, 7 vitamins and 5 nucleic acid derivatives. Of these, only l-aspartic acid showed stimulation of antibiotic production. We conclude that the stimulatory effect of aspartic acid is due to its role as a precursor involved in the biosynthesis of aspartate-4-semialdehyde, which is the starting point for the biosynthesis of the 3-amino-2,4-dihydroxy benzoic acid portion of the platensimycin molecule.

Bioprospecting Sponge-Associated Microbes for Antimicrobial Compounds

Sponges are the most prolific marine organisms with respect to their arsenal of bioactive compounds including antimicrobials. However, the majority of these substances are probably not produced by the sponge itself, but rather by bacteria or fungi that are associated with their host. This review for the first time provides a comprehensive overview of antimicrobial compounds that are known to be produced by sponge-associated microbes. We discuss the current state-of-the-art by grouping the bioactive compounds produced by sponge-associated microorganisms in four categories: antiviral, antibacterial, antifungal and antiprotozoal compounds. Based on in vitro activity tests, identified targets of potent antimicrobial substances derived from sponge-associated microbes include: human immunodeficiency virus 1 (HIV-1) (2-undecyl-4-quinolone, sorbicillactone A and chartarutine B); influenza A (H1N1) virus (truncateol M); nosocomial Gram positive bacteria (thiopeptide YM-266183, YM-266184, mayamycin and kocurin); Escherichia coli (sydonic acid), Chlamydia trachomatis (naphthacene glycoside SF2446A2); Plasmodium spp. (manzamine A and quinolone 1); Leishmania donovani (manzamine A and valinomycin); Trypanosoma brucei (valinomycin and staurosporine); Candida albicans and dermatophytic fungi (saadamycin, 5,7-dimethoxy-4-p-methoxylphenylcoumarin and YM-202204). Thirty-five bacterial and 12 fungal genera associated with sponges that produce antimicrobials were identified, with Streptomyces, Pseudovibrio, Bacillus, Aspergillus and Penicillium as the prominent producers of antimicrobial compounds. Furthemore culture-independent approaches to more comprehensively exploit the genetic richness of antimicrobial compound-producing pathways from sponge-associated bacteria are addressed.

Anti-MRSA and anti-TB metabolites from marine-derived Verrucosispora sp. MS100047

Microbes belonging to the genus Verrucosispora possess significant chemical diversity and biological properties. They have attracted the interests of many researchers and are becoming promising resources in the marine natural product research field. A bioassay-guided isolation from the crude extract of Verrucosispora sp. strain MS100047, isolated from sediments collected from the South China Sea, has led to the identification of a new salicylic derivative, glycerol 1-hydroxy-2,5-dimethyl benzoate (1), along with three known compounds, brevianamide F (2), abyssomicin B (3), and proximicin B (4). Compound 1 showed selective activity against methicillin-resistant Staphylococcus aureus (MRSA) with a minimum inhibitory concentration (MIC) value of 12.5 μg/mL. Brevianamide F (2), which was isolated from actinomycete for the first time, showed a good anti-BCG activity with a MIC value of 12.5 μg/mL that has not been reported previously in literatures. Proximicin B (4) showed significant anti-MRSA (MIC = 3.125 μg/mL), anti-BCG (MIC = 6.25 μg/mL), and anti-tuberculosis (TB) (MIC = 25 μg/mL) activities. This is the first report on the anti-tubercular activities of proximicins. In addition, Verrucosispora sp. strain MS100047 was found to harbor 18 putative secondary metabolite gene clusters based on genomic sequence analysis. These include the biosynthetic loci encoding polyketide synthase (PKS) and non-ribosomal peptide synthetase (NRPS) consistent with abyssomicins and proximicins, respectively. The biosynthetic pathways of these isolated compounds have been proposed. These results indicate that MS100047 possesses a great potential as a source of active secondary metabolites.

Synergistic combinations of antifungals and anti-virulence agents to fight against Candida albicans

Candida albicans, one of the pathogenic Candida species, causes high mortality rate in immunocompromised and high-risk surgical patients. In the last decade, only one new class of antifungal drug echinocandin was applied. The increased therapy failures, such as the one caused by multi-drug resistance, demand innovative strategies for new effective antifungal drugs. Synergistic combinations of antifungals and anti-virulence agents highlight the pragmatic strategy to reduce the development of drug resistant and potentially repurpose known antifungals, which bypass the costly and time-consuming pipeline of new drug development. Anti-virulence and synergistic combination provide new options for antifungal drug discovery by counteracting the difficulty or failure of traditional therapy for fungal infections.

Advanced tools in marine natural drug discovery

Marine natural products (MNPs) remain promising drug sources with several marine-derived drugs having been successfully approved. Nevertheless, it is never a smooth sailing to seek bioactive compounds from marine environments, during which many challenges are need to be faced to, for example, discovering unique marine resources, reviving unculturable organisms outside the marine environment, distinguishing novel compounds from the known ones, and disclosing the function of MNPs and optimizing their pharmacological use. Herein we review some advanced techniques and methodologies that can be employed to deal with above challenges with the intent of inspiring the forthcoming efforts in MNPs discovery pipelines.

Dereplication: racing to speed up the natural products discovery process

Covering: 1993-2014 (July)To alleviate the dereplication holdup, which is a major bottleneck in natural products discovery, scientists have been conducting their research efforts to add tools to their "bag of tricks" aiming to achieve faster, more accurate and efficient ways to accelerate the pace of the drug discovery process. Consequently dereplication has become a hot topic presenting a huge publication boom since 2012, blending multidisciplinary fields in new ways that provide important conceptual and/or methodological advances, opening up pioneering research prospects in this field.

Diaza-anthracene Antibiotics from a Freshwater-Derived Actinomycete with Selective Antibacterial Activity toward Mycobacterium tuberculosis

Multidrug- and extensively drug-resistant strains of Mycobacterium tuberculosis are resistant to first- and second-line drug regimens and resulted in 210,000 fatalities in 2013. In the current study, we screened a library of aquatic bacterial natural product fractions for their ability to inhibit this pathogen. A fraction from a Lake Michigan bacterium exhibited significant inhibitory activity, from which we characterized novel diazaquinomycins H and J. This antibiotic class displayed an in vitro activity profile similar or superior to clinically used anti-tuberculosis agents and maintained this potency against a panel of drug-resistant M. tuberculosis strains. Importantly, these are among the only freshwater-derived actinomycete bacterial metabolites described to date. Further in vitro profiling against a broad panel of bacteria indicated that this antibiotic class selectively targets M. tuberculosis. Additionally, in the case of this pathogen we present evidence counter to previous reports that claim the diazaquinomycins target thymidylate synthase in Gram-positive bacteria. Thus, we establish freshwater environments as potential sources for novel antibiotic leads and present the diazaquinomycins as potent and selective inhibitors of M. tuberculosis.

Natural antimicrobial peptides against Mycobacterium tuberculosis

TB, caused by Mycobacterium tuberculosis, is one the leading infectious diseases worldwide. There is an urgent need to discover new drugs with unique structures and uncommon mechanisms of action to treat M. tuberculosis and combat antimycobacterial resistance. Naturally occurring compounds contain a wide diversity of chemical structures, displaying a wide range of in vitro potency towards M. tuberculosis. A number of recent studies have shown that natural antimycobacterial peptides can disrupt the function of the mycobacterial cell wall through different modes of action and thereafter interact with intracellular targets, including nucleic acids, enzymes and even organelles. More importantly, the probability of antimycobacterial resistance is low. This review presents several natural antimicrobial peptides isolated from different organism sources, including bacteria, fungi, plants and animals. In addition, the molecular features of these molecules are the subject of much attention. Such peptides have common traits among their chemical features, which may be correlated with their biological activities; hence, different parts of the molecular structures can be modified in order to increase penetration into the target cells. This review also summarizes the available information on the properties of antimycobacterial peptides associated with their biological activities.

A new salicylate synthase AmS is identified for siderophores biosynthesis in Amycolatopsis methanolica 239(T)

Siderophores are important for the growth of bacteria or the applications in treatment of iron overload-associated diseases due to the iron-chelating property. Salicylate synthase played a key role in the biosynthesis of some NRPS-derived siderophores by the providing of an iron coordination moiety as the initial building block. A new salicylate synthase, namely AmS, was identified in the biosynthesis pathway of siderophore amychelin in Amycolatopsis methanolica 239(T), since it shunt chorismate, an integrant precursor, from primary to secondary metabolite flow. The amino acid sequence alignment and phylogenetic analysis showed that AmS grouped into a new cluster. In vitro assays of AmS revealed its wide temperature tolerance ranged from 0 to 40 °C and narrow pH tolerant ranged from 7.0 to 9.0. AmS was resistant to organic solvents and non-ionic detergents. Moreover, AmS converted chorismate to salicylate with K m of 129.05 μM, k cat of 2.20 min(-1) at optimal conditions, indicating its low substrate specificity and comparable velocity to reported counterparts (Irp9 and MbtI). These properties of AmS may improve the iron-seizing ability of A. methanolica to compete with its neighbors growing in natural environments. Most importantly, serine and cysteine residues were found to be important for the catalytic activity of AmS. This study presented AmS as a new cluster of salicylate synthase and the reaction mechanism and potential applications of salicylate synthase were highlighted as well.

Bioactive Potential of Actinomycetes from Less Explored Ecosystems against Mycobacterium tuberculosis and Other Nonmycobacterial Pathogens

Bioactive potential of actinomycetes isolated from certain less explored Indian ecosystems against Mycobacterium tuberculosis and other nonmycobacterial pathogens was investigated. Actinomycetes were isolated from the soil samples collected from desert, coffee plantation, rubber forest, and hill area and their cultural and micromorphological characteristics were studied. Crude extracts were prepared by agar surface fermentation and tested against M. tuberculosis isolates by luciferase reporter phage (LRP) assay at 100 µg/mL. Activity against nonmycobacterial pathogens was studied by agar plug method. Totally 54 purified cultures of actinomycetes including 43 Streptomyces and 11 non-Streptomyces were isolated. While screening for antitubercular activity, extracts of 39 actinomycetes showed activity against one or more M. tuberculosis isolates whereas 27 isolates exhibited antagonistic activity against nonmycobacterial pathogens. In particular crude extracts from sixteen actinomycete isolates inhibited all the three M. tuberculosis isolates tested. Findings of the present study concluded that less explored ecosystems investigated in this study are the potential resource for bioactive actinomycetes. Further purification and characterization of active molecule from the potential extracts will pave the way for determination of MIC, toxicity, and specificity studies.

Exploring the potential of endophytes from medicinal plants as sources of antimycobacterial compounds

Natural product drug discovery has regained interest due to low production costs, structural diversity, and multiple uses of active compounds to treat various diseases. Attention has been directed towards medicinal plants as these plants have been traditionally used for generations to treat symptoms of numerous diseases.

It is established that plants harbour microorganisms, collectively known as endophytes. Exploring the as-yet untapped natural products from the endophytes increases the chances of finding novel compounds. The concept of natural products targeting microbial pathogens has been applied to isolate novel antimycobacterial compounds, and the rapid development of drug-resistant Mycobacterium tuberculosis has significantly increased the need for new treatments against this pathogen. It remains important to continuously screen for novel compounds from natural sources, particularly from rarely encountered microorganisms, such as the endophytes.

This review focuses on bioprospecting for polyketides and small peptides exhibiting antituberculosis activity, although current treatments against tuberculosis are described. It is established that natural products from these structure classes are often biosynthesised by microorganisms. Therefore it is hypothesised that some bioactive polyketides and peptides originally isolated from plants are in fact produced by their endophytes. This is of interest for further endophyte natural product investigations.

Salinibacillus xinjiangensis sp. nov., a halophilic bacterium from a hypersaline lake

A Gram-positive, endospore-forming, rod-shaped bacterium, designated isolate J4T, was isolated from a neutral saline lake sample from Xinjiang Uyghur Autonomous Region, China, and subjected to a polyphasic taxonomic investigation. Phylogenetic analysis based on 16S rRNA gene sequence revealed that strain J4T is most closely related to Salinibacillus aidingensis 25-7T (with 96.7 % similarity), Salinibacillus kushneri 8-2T (96.5 %), Ornithinibacillus scapharcae TW25T (96.4 %), Salirhabdus euzebyi CVS-14T (96.4 %) and Ornithinibacillus californiensis MB-9T (96.2 %). Chemotaxonomic analysis showed menaquinone-7 (MK-7) to be the major isoprenoid quinone of strain J4T; diphosphatidylglycerol and phosphatidylglycerol were the major cellular polar lipids and the cell wall contained meso-diaminopimelic acid as the diagnostic diamino acid. The major cellular fatty acids were iso-C15 : 0 and anteiso-C15 : 0. The genomic DNA G+C content of strain J4T was determined to be 36.2 mol%. Strain J4T was positive for catalase activity and negative for oxidase activity. Strain J4T was observed to grow at 25–50 °C (optimal 35–42 °C), pH 6.5–8.0 (optimal 7.0–7.5) and in media containing 1–21 % (w/v) NaCl (optimal 9–12 %). Based on these data, strain J4T represents a novel species of the genus Salinibacillus and the name Salinibacillus xinjiangensis sp. nov. is proposed. The type strain is J4T ( = CGMCC 1.12331T = JCM 18732T).

Endophytic Streptomyces sp. Y3111 from traditional Chinese medicine produced antitubercular pluramycins

As part of a search for antitubercular substances from natural sources, we screened a library of endophytic microbes (50 strains and 300 crude extracts in total) isolated from traditional Chinese medicines (TCMs) for growth inhibitory activity against Bacillus Calmette-Guérin (BCG). The crude extract of Streptomyces sp. strain Y3111, which was associated with the stems of Heracleum souliei, showed good anti-BCG activity with an MIC value of 12.5 μg/mL. Bioassay-guided isolation led to four new pluramycin-type compounds, heraclemycins A-D (1-4). Their structures were determined by different spectroscopic techniques including HRMSESI, 1D NMR, and 2D NMR. This is the first report of pluramycin analogues produced by TCM endophytic microbes as well as the first example of BCG-selective pluramycins. Heraclemycin C (3) showed selective antitubercular activity against BCG with a MIC value of 6.25 μg/mL and a potential new mode of action.

Marine pharmacology in 2009-2011: marine compounds with antibacterial, antidiabetic, antifungal, anti-inflammatory, antiprotozoal, antituberculosis, and antiviral activities; affecting the immune and nervous systems, and other miscellaneous mechanisms of action

The peer-reviewed marine pharmacology literature from 2009 to 2011 is presented in this review, following the format used in the 1998–2008 reviews of this series. The pharmacology of structurally-characterized compounds isolated from marine animals, algae, fungi and bacteria is discussed in a comprehensive manner. Antibacterial, antifungal, antiprotozoal, antituberculosis, and antiviral pharmacological activities were reported for 102 marine natural products. Additionally, 60 marine compounds were observed to affect the immune and nervous system as well as possess antidiabetic and anti-inflammatory effects. Finally, 68 marine metabolites were shown to interact with a variety of receptors and molecular targets, and thus will probably contribute to multiple pharmacological classes upon further mechanism of action studies. Marine pharmacology during 2009–2011 remained a global enterprise, with researchers from 35 countries, and the United States, contributing to the preclinical pharmacology of 262 marine compounds which are part of the preclinical pharmaceutical pipeline. Continued pharmacological research with marine natural products will contribute to enhance the marine pharmaceutical clinical pipeline, which in 2013 consisted of 17 marine natural products, analogs or derivatives targeting a limited number of disease categories.

Antimycobacterials from lovage root (Ligusticum officinale Koch)

The n-hexane extract of Lovage root was found to significantly inhibit the growth of both Mycobacterium smegmatis mc²155 and Mycobacterium bovis BCG, and therefore a bioassay-guided isolation strategy was undertaken. (Z)-Ligustilide, (Z)-3-butylidenephthalide, (E)-3-butylidenephthalide, 3-butylphthalide, α-prethapsenol, falcarindiol, levistolide A, psoralen and bergapten were isolated by chromatographic techniques, characterized by NMR spectroscopy and MS, and evaluated for their growth inhibition activity against Mycobacterium tuberculosis H₃₇Rv using the whole-cell phenotypic spot culture growth inhibition assay (SPOTi). Cytotoxicity against RAW 264.7 murine macrophage cells was employed for assessing their degree of selectivity. Falcarindiol was the most potent compound with a minimum inhibitory concentration (MIC) value of 20 mg/L against the virulent H₃₇Rv strain; however, it was found to be cytotoxic with a half-growth inhibitory concentration (GIC₅₀) in the same order of magnitude (SI < 1). Interestingly the sesquiterpene alcohol α-prethapsenol was found to inhibit the growth of the pathogenic mycobacteria with an MIC value of 60 mg/L, being more specific towards mycobacteria than mammalian cells (SI ~ 2). Colony forming unit analysis at different concentrations of this phytochemical showed mycobacteriostatic mode of action.

A marine-derived Streptomyces sp. MS449 produces high yield of actinomycin X2 and actinomycin D with potent anti-tuberculosis activity

In the course of our screening program for anti-Mycobacterium bovis bacillus Calmette-Guérin (BCG) and anti-Mycobacterium tuberculosis H37Rv (MTB H37Rv) agents from our marine natural product library, a newly isolated actinomycete strain, designated as MS449, was picked out for further investigation. The strain MS449, isolated from a sediment sample collected from South China Sea, produced actinomycin X(2) and actinomycin D in substantial quantities, which showed strong inhibition of BCG and MTB H37Rv. The structures of actinomycins were elucidated by nuclear magnetic resonance and mass spectrometric analysis. The strain MS449 was taxonomically characterized on the basis of morphological and phenotypic characteristics, genotypic data, and phylogenetic analysis. The 16S rRNA gene sequence of the strain was determined and a database search indicated that the strain was closely associated with the type strain of Streptomyces avermitilis (99.7 % 16S rRNA gene similarity). S. avermitilis has not been previously reported to produce actinomycins. The marine-derived strain of Streptomyces sp. MS449 produced notably higher quantities of actinomycin X(2) (1.92 mg/ml) and actinomycin D (1.77 mg/ml) than previously reported actinomycins producing strains. Thus, MS449 was considered of great potential as a new industrial producing strain of actinomycin X(2) and actinomycin D.

Systematics-guided bioprospecting for bioactive microbial natural products

Advances in the taxonomic characterization of microorganisms have accelerated the rate at which new producers of natural products can be understood in relation to known organisms. Yet for many reasons, chemical efforts to characterize new compounds from new microbes have not kept pace with taxonomic advances. That there exists an ever-widening gap between the biological versus chemical characterization of new microorganisms creates tremendous opportunity for the discovery of novel natural products through the calculated selection and study of organisms from unique, untapped, ecological niches. A systematics-guided bioprospecting, including the construction of high quality libraries of marine microbes and their crude extracts, investigation of bioactive compounds, and increasing the active compounds by precision engineering, has become an efficient approach to drive drug leads discovery. This review outlines the recent advances in these issues and shares our experiences on anti-infectious drug discovery and improvement of avermectins production as well.