Antibacterial constituents of Eremophila alternifolia: An Australian aboriginal traditional medicinal plant

Recent Advances in Using Natural Antibacterial Additives in Bioactive Wound Dressings

Wound care is a global health issue with a financial burden of up to US $96.8 billion annually in the USA alone. Chronic non-healing wounds which show delayed and incomplete healing are especially problematic. Although there are more than 3000 dressing types in the wound management market, new developments in more efficient wound dressings will require innovative approaches such as embedding antibacterial additives into wound-dressing materials. The lack of novel antibacterial agents and the misuse of current antibiotics have caused an increase in antimicrobial resistance (AMR) which is estimated to cause 10 million deaths by 2050 worldwide. These ongoing challenges clearly indicate an urgent need for developing new antibacterial additives in wound dressings targeting microbial pathogens. Natural products and their derivatives have long been a significant source of pharmaceuticals against AMR. Scrutinising the data of newly approved drugs has identified plants as one of the biggest and most important sources in the development of novel antibacterial drugs. Some of the plant-based antibacterial additives, such as essential oils and plant extracts, have been previously used in wound dressings; however, there is another source of plant-derived antibacterial additives, i.e., those produced by symbiotic endophytic fungi, that show great potential in wound dressing applications. Endophytes represent a novel, natural, and sustainable source of bioactive compounds for therapeutic applications, including as efficient antibacterial additives for chronic wound dressings. This review examines and appraises recent developments in bioactive wound dressings that incorporate natural products as antibacterial agents as well as advances in endophyte research that show great potential in treating chronic wounds.

Phytochemistry, Medicinal Properties, Bioactive Compounds, and Therapeutic Potential of the Genus Eremophila (Scrophulariaceae)

The genus Eremophila (family Scrophulariaceae) consists of approximately 200 species that are widely distributed in the semi-arid and arid regions of Australia. Multiple Eremophila spp. are used as traditional medicines by the First Australians in the areas in which they grow. They are used for their antibacterial, antifungal, antiviral, antioxidant, anti-diabetic, anti-inflammatory, and cardiac properties. Many species of this genus are beneficial against several diseases and ailments. The antibacterial properties of the genus have been relatively well studied, with several important compounds identified and their mechanisms studied. In particular, Eremophila spp. are rich in terpenoids, and the antimicrobial bioactivities of many of these compounds have already been confirmed. The therapeutic properties of Eremophila spp. preparations and purified compounds have received substantially less attention, and much study is required to validate the traditional uses and to highlight species that warrant further investigation as drug leads. The aim of this study is to review and summarise the research into the medicinal properties, therapeutic mechanisms, and phytochemistry of Eremophila spp., with the aim of focussing future studies into the therapeutic potential of this important genus.

Biodiscoveries within the Australian plant genus Eremophila based on international and interdisciplinary collaboration: results and perspectives on outstanding ethical dilemmas

In a cross-continental research initiative, including researchers working in Australia and Denmark, and based on joint external funding by a 3-year grant from the Novo Nordisk Foundation, we have used DNA sequencing, extensive chemical profiling and molecular networking analyses across the entire Eremophila genus to provide new knowledge on the presence of natural products and their bioactivities using polypharmocological screens. Sesquiterpenoids, diterpenoids and dimers of branched-chain fatty acids with previously unknown chemical structures were identified. The collection of plant material from the Eremophila genus was carried out according to a 'bioprospecting agreement' with the Government of Western Australia. We recognize that several Eremophila species hold immense cultural significance to Australia's First Peoples. In spite of our best intentions to ensure that new knowledge gained about the genus Eremophila and any potential future benefits are shared in an equitable manner, in accordance with the Nagoya Protocol, we encounter serious dilemmas and potential conflicts in making benefit sharing with Australia's First Peoples a reality.

Navigating through chemical space and evolutionary time across the Australian continent in plant genus Eremophila

Eremophila is the largest genus in the plant tribe Myoporeae (Scrophulariaceae) and exhibits incredible morphological diversity across the Australian continent. The Australian Aboriginal Peoples recognize many Eremophila species as important sources of traditional medicine, the most frequently used plant parts being the leaves. Recent phylogenetic studies have revealed complex evolutionary relationships between Eremophila and related genera in the tribe. Unique and structurally diverse metabolites, particularly diterpenoids, are also a feature of plants in this group. To assess the full dimension of the chemical space of the tribe Myoporeae, we investigated the metabolite diversity in a chemo-evolutionary framework applying a combination of molecular phylogenetic and state-of-the-art computational metabolomics tools to build a dataset involving leaf samples from a total of 291 specimens of Eremophila and allied genera. The chemo-evolutionary relationships are expounded into a systematic context by integration of information about leaf morphology (resin and hairiness), environmental factors (pollination and geographical distribution), and medicinal properties (traditional medicinal uses and antibacterial studies), augmenting our understanding of complex interactions in biological systems.

Design, synthesis and screening of a drug discovery library based on an Eremophila-derived serrulatane scaffold

Chemical studies of the aerial parts of the Australian desert plant Eremophila microtheca afforded the targeted and known diterpenoid scaffolds, 3,7,8-trihydroxyserrulat-14-en-19-oic acid and 3-acetoxy-7,8-dihydroxyserrulat-14-en-19-oic acid. The most abundant serrulatane scaffold was converted to the poly-methyl derivatives, 3-hydroxy-7,8-dimethoxyserrulat-14-en-19-oic acid methyl ester and 3,7,8-trimethoxyserrulat-14-en-19-oic acid methyl ester using simple and rapid methylation conditions consisting of DMSO, NaOH and MeI at room temperature. Subsequently a 12-membered amide library was synthesised by reacting the methylated scaffolds with a diverse series of commercial primary amines. The chemical structures of the 12 undescribed semi-synthetic analogues were fully characterised following 1D/2D NMR, MS, UV, ECD and [α]_D data analyses. All compounds were evaluated for their anthelmintic, anti-microbial and anti-viral activities. While none of the compounds significantly inhibited motility or development of the exsheathed third-stage larvae (xL3s) of a pathogenic ruminant parasite, Haemonchus contortus, the tri-methylated analogue induced a skinny phenotype in fourth-stage larvae (L4s) after seven days of treatment (IC₅₀ = 14 μM). Anti-bacterial and anti-fungal activities were not observed at concentrations up to 20 μM. Activity against HIV latency reversal was tested in inducible, chronically-infected cells, with the tri-methylated analogue being the most active (EC₅₀ = 38 μM).

Natural Compounds from the Marine Brown Alga Caulocystis cephalornithos with Potent In Vitro-Activity against the Parasitic Nematode Haemonchus contortus

Eight secondary metabolites (1 to 8) were isolated from a marine sponge, a marine alga and three terrestrial plants collected in Australia and subsequently chemically characterised. Here, these natural product-derived compounds were screened for in vitro-anthelmintic activity against the larvae and adult stages of Haemonchus contortus (barber's pole worm)-a highly pathogenic parasitic nematode of ruminants. Using an optimised, whole-organism screening system, compounds were tested on exsheathed third-stage larvae (xL3s) and fourth-stage larvae (L4s). Anthelmintic activity was initially evaluated on these stages based on the inhibition of motility, development and/or changes in morphology (phenotype). We identified two compounds, 6-undecylsalicylic acid (3) and 6-tridecylsalicylic acid (4) isolated from the marine brown alga, Caulocystis cephalornithos, with inhibitory effects on xL3 and L4 motility and larval development, and the induction of a "skinny-straight" phenotype. Subsequent testing showed that these two compounds had an acute nematocidal effect (within 1-12 h) on adult males and females of H. contortus. Ultrastructural analysis of adult worms treated with compound 4 revealed significant damage to subcuticular musculature and associated tissues and cellular organelles including mitochondria. In conclusion, the present study has discovered two algal compounds possessing acute anthelmintic effects and with potential for hit-to-lead progression. Future work should focus on undertaking a structure-activity relationship study and on elucidating the mode(s) of action of optimised compounds.

Multi Dynamic Extraction: An Innovative Method to Obtain a Standardized Chemically and Biologically Reproducible Polyphenol Extract from Poplar-Type Propolis to Be Used for Its Anti-Infective Properties

Antimicrobial activity is a well-known property of propolis, making it a candidate for antimicrobial surfaces in biomedical devices. Nevertheless, large-scale use of propolis as an anti-infective agent is limited by the heterogeneity of its chemical composition and consequent variation in antimicrobial activity. The aim of this study was to demonstrate that the multi dynamic extraction (M.E.D.) method produces standardized polyphenolic mixtures from poplar-type propolis, with reproducible chemical composition and anti-microbial activity, independently from the chemical composition of the starting raw propolis. Three raw propolis samples, from Europe, America, and Asia, were analyzed for their polyphenol chemical composition by means of HPLC-UV and then combined to obtain three mixtures of propolis, which werme submitted to the M.E.D. extraction method. The chemical composition and the antimicrobial activity of M.E.D. propolis against bacteria and fungi were determined. The three M.E.D. propolis showed similar chemical compositions and antimicrobial activities, exhibiting no relevant differences against antibiotic-susceptible and antibiotic-resistant strains. The batch-to-batch reproducibility of propolis extracts obtained with the M.E.D. method encourages the design of drugs alternative to traditional antibiotics and the development of anti-infective surface-modified biomaterials.

Antibacterial Performance of Terpenoids from the Australian Plant Eremophila lucida

Plants in the Australian genus Eremophila (Scrophulariaceae) have attracted considerable recent attention for their antimicrobial compounds, which possess a wide range of chemical structures. As they are typically associated with the oily-waxy resin layer covering leaves and green branchlets, and Eremophila lucida is prominent among the species containing a pronounced sticky resin layer, this species was considered of interest for assessing its antibacterial constituents. The n-hexane fraction of the crude acetone extract of the leaves exhibited antibacterial activity against Staphylococcus aureus. Isolation led to the known compounds cembratriene, (3Z, 7E, 11Z)-15-hydroxycembra-3,7,11-trien-19-oic acid (1), the sesquiterpenoid, farnesal (2) and the viscidane diterpenoid, 5α-hydroxyviscida-3,14-dien-20-oic acid (3). The purified compounds were tested for antibacterial activity with 2 and 3 showing moderate antibacterial activity against Gram-positive bacteria.

Antifungal Activity in Compounds from the Australian Desert Plant Eremophila alternifolia with Potency Against Cryptococcus spp

Plant metabolites that have shown activity against bacteria and/or environmental fungi represent valuable leads for the identification and development of novel drugs against clinically important human pathogenic fungi. Plants from the genus Eremophila were highly valued in traditional Australian Aboriginal medicinal practices, and E. alternifolia was the most prized among them. As antibacterial activity of extracts from E. alternifolia has been documented, this study addresses the question whether there is also activity against infectious fungal human pathogens. Compounds from leaf-extracts were purified and identified by 1- and 2-D NMR. These were then tested by disk diffusion and broth microdilution assays against ten clinically and environmentally relevant yeast and mould species. The most potent activity was observed with the diterpene compound, 8,19-dihydroxyserrulat-14-ene against Cryptococcus gattii and Cryptococcus neoformans, with minimum inhibition concentrations (MIC) comparable to those of Amphotericin B. This compound also exhibited activity against six Candida species. Combined with previous studies showing an antibacterial effect, this finding could explain a broad antimicrobial effect from Eremophila extracts in their traditional medicinal usage. The discovery of potent antifungal compounds from Eremophila extracts is a promising development in the search for desperately needed antifungal compounds particularly for Cryptococcus infections.

Antibacterial activity of flavonoids and their structure-activity relationship: An update review

Based on World Health Organization reports, resistance of bacteria to well-known antibiotics is a major global health challenge now and in the future. Different strategies have been proposed to tackle this problem including inhibition of multidrug resistance pumps and biofilm formation in bacteria and development of new antibiotics with novel mechanism of action. Flavonoids are a large class of natural compounds, have been extensively studied for their antibacterial activity, and more than 150 articles have been published on this topic since 2005. Over the past decade, some promising results were obtained with the antibacterial activity of flavonoids. In some cases, flavonoids (especially chalcones) showed up to sixfold stronger antibacterial activities than standard drugs in the market. Some synthetic derivatives of flavonoids also exhibited remarkable antibacterial activities with 20- to 80-fold more potent activity than the standard drug against multidrug-resistant Gram-negative and Gram-positive bacteria (including Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus). This review summarizes the ever changing information on antibacterial activity of flavonoids since 2005, with a special focus on the structure-activity relationship and mechanisms of actions of this broad class of natural compounds.

Flavonoids isolated from the fresh sweet fruit of Averrhoa carambola, commonly known as star fruit

Thirteen flavonoids were isolated from the fresh sweet fruit of Averrhoa carambola L. (Oxalidaceae), commonly known as star fruit, and their structures were determined by spectroscopic and chemical methods. 8-Carboxymethyl-(+)-epicatechin methyl ester, pinobanksin 3-O-β-d-glucoside, and carambolasides M-Q were undescribed structures. (+)-Epicatechin, aromadendrin 3-O-β-d-glucoside, helicioside A, taxifolin 3'-O-β-d-glucoside, galangin 3-O-rutinoside, and isorhamnetin 3-O-rutinoside were reported from this species for the first time. Pinobanksin 3-O-β-d-glucoside and carambolasides M-Q showed more potent 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS) radical cation scavenging activity (IC₅₀ = 5.3-2.3 μM) than l-ascorbic acid (10.5 μM). Further, (+)-epicatechin, pinobanksin 3-O-β-d-glucoside, isorhamnetin 3-O-rutinoside, and carambolasides O-Q exhibited weak porcine pancreatic lipase inhibitory activity.

Antibacterial compounds from the Australian native plant Eremophila glabra

Recent reports of Eremophila glabra (R.Br.) Ostenf. (Scrophulariaceae) displaying antibacterial activity has led us to investigate the bioactive secondary metabolites responsible for this activity. Bioassay-directed fractionation of solvent extracts prepared from the leaves of E. glabra led to the isolation of seven serrulatane diterpenes, three flavonoids and the caffeoyl ester disaccharide verbascoside. Among these, four serrulatanes, namely 18-acetoxy-8, 20-dihydroxyserrulat-14-en-19-oic acid (14), 18,20-diacetoxy-8-hydroxyserrulat-14-en-19-oic acid (16), 8,18,20-triacetoxyserrulat-14-en-19-oic acid (17) and 18-acetoxy-8-hydroxyserrulat-14-en-19-oic acid (18) are described for the first time, while 8,20-diacetoxyserrulat-14-en-19-oic acid (3), 8,18,20-trihydroxyserrulat-14-en-19-oic acid (5) and 20-acetoxy-8-hydroxyserrulat-14-en-19-oic acid (19) were previously reported. All three flavonoids hispidulin (12), jaceosidin (13) and cirsimaritin (15) are known but reported for the first time in E. glabra. All compounds were tested in an agar diffusion antimicrobial assay against Staphylococcus aureus (NCTC 10442) and Staphylococcus epidermidis (ATCC 14990). Compounds 12, 13, 17, 18 and 19 exhibited moderate activity, with minimum inhibitory concentrations (MICs) ranging from 32 to 512μg/mL. Compound 19 demonstrated the highest activity against S. epidermidis ATCC 14990 with MIC of 32μg/mL, while 13 demonstrated the highest activity against S. aureus NCTC 10442 with MIC of 128μg/mL.

Antibacterial Nerol Cinnamates from the Australian Plant Eremophila longifolia

Two new antimicrobial agents, neryl ferulate (1) and neryl p-coumarate (2), were identified using bioassay-guided isolation from the leaves of Eremophila longifolia, which is a medicinal plant used by some Australian Aboriginal communities. Although gradual autoxidation of the nerol subunit hindered the initial attempts to purify and characterize 1 and 2, it was found that the autoxidation could be stopped through storage under argon at -20 °C. Biological evaluation showed that neryl ferulate (1) had moderate activity against various Gram-positive bacteria, while neryl p-coumarate (2) was active only against Enterococcus faecium.