Antibacterial constituents of Eremophila alternifolia: An Australian aboriginal traditional medicinal plant
Graphical abstract
Introduction
Geographic isolation has resulted in Australia having a unique and distinct flora. Many Australian plant species have developed specific survival strategies that allow them to inhabit the harsh arid environment found in many parts of Australia. Australian Aboriginal people have used a number of these plant species with distinct curative properties for their well-being (Cock, 2011, Jones et al., 2007). Among over 70 plant species that have been used for medicinal purposes by Aboriginal people living in arid, central regions of the country, species of the genus Eremophila (Scrophulariaceae) are considered as of prime importance in their oral cultural lore (Latz, 1995, Richmond, 1993, Singab et al., 2013).
The unique Australian plant genus Eremophila, comprises 217 described species and several yet undescribed species that are widely distributed in the semi-arid and arid regions of Australia (Chinnock, 2007). A number of desert-adapted Eremophila species have featured in the medicinal, cultural and religious aspects of Australian Aboriginal people’s lives (Ghisalberti, 1994, Latz, 1995). Eremophila species were also used for smoke fumigation (Sadgrove et al., 2014). Some Eremophila species have been investigated for the molecular origins of traditional usages, with serrulatanes and o-naphthoquinones so far identified as antibacterial components (Forster et al., 1986, Ndi et al., 2007a, Ndi et al., 2007b).
In general, plants used in Australian Aboriginal medicine have been prepared as skin washes or drinks (infusions or hot water extracts), ointments, poultices, and steam or smoke treatments. A large number of Aboriginal plant remedies have been prepared to use externally to relieve problems associated with skin disorders, microbial infections, fever, rheumatic pains, wounds and burns (Barr, 1988, Latz, 1995). However, a more limited number, approximately 10% of preparations have been prepared to use internally for conditions such as colds, influenza, sore throat, pain and headaches (Clarke, 2008, Ghisalberti, 1994, Latz, 1995). While a number of Eremophila species are highly valued ingredients in externally used preparations, the species Eremophila alternifolia was and still is highly valued for both internal and external use (Barr et al., 1993). In more recent years small-scale businesses owned by Aboriginal communities selling formulations of this species have emerged providing an opportunity for economic development based on this plant (Morse, 2005).
E. alternifolia grows as a small to medium shrub, usually varying in height from 1−4 m. It is found growing mainly in rocky soils of hills and ranges in Central Australia and different arid zones of Western Australia and South Australia. Plants have alternate leaves and tubular flowers with a variety of colours, usually appearing in early winter to early autumn. Other names include poverty bush, narrow-leaf fuchsia bush and native honeysuckle (Barr, 1988, Chinnock, 2007, Pennacchio et al., 1995, Richmond, 1993).
Ethnobotanical reports have provided much information on the traditional medicinal use of E. alternifolia. In a review of the ethnopharmacology and phytochemistry of Eremophila species (Ghisalberti, 1994) Ghisalberti noted of E. alternifolia that “it has been, and still is, considered ‘number one medicine’” with a variety of uses by Aboriginal people, including the treatment of septic wounds. An ethnobotanical project funded by the Northern Territory Government in Australia (Barr et al., 1993), where Aboriginal communities worked with the project team to document traditional knowledge about plants, recorded the use of this species from five different communities in the Northern Territory. Those recorded uses contain a remarkably high proportion of conditions that Western medicine associates with microbial infections. E. alternifolia has also been used as a tea or rubbed on the skin to induce deep, pleasant sleep and general well-being (Latz, 1995, Low, 1990). In addition, decoctions of leaves were made for skin, eye and body washes (Richmond, 1993). Sometimes leaf-paste with water was prepared and used as ‘rubbing medicine’ for the head. Moreover, this was one of few species whose dried leaves were carried by nomadic Aboriginal groups for use in medicinal and cultural practices in case of need (Barr et al., 1993, Ghisalberti, 1994).
Previous phytochemical investigations of E. alternifolia have shown the occurrence of a moderate amount of essential oils including fenchone, limonene and camphor (Ghisalberti, 1993, Ghisalberti, 1994). Furanosesquiterpene β-ketols (Sutherland and Rodwell, 1989) along with flavonoids, galangin-3-methyl ether and pinobanksin have also been isolated from this species (Ghisalberti, 1994, Jefferies et al., 1962). Another study revealed the significant cardioactive effects of E. alternifolia in rodent hearts with the caffeoyl phenylethanoid glycoside verbascoside isolated as the cardioactive compound, while geniposidic acid was also identified but at a lower concentration (Pennacchio et al., 1995, Pennacchio et al., 1996).
Although several ethnobotanical reports have emphasized the pharmacological importance of this species for conditions that would appear to be associated with microbial infections, to our best knowledge, identification of the antimicrobial compounds from this plant has not been studied. However, a few reports of the in vitro antimicrobial activity of crude extracts of E. alternifolia have been published. A study which investigated the antibacterial activity of traditional Australian medicinal plants showed that an ethanolic leaf- extract of E. alternifolia exhibited substantial antibacterial activity against the Gram-positive bacteria, Staphylococcus aureus and Streptococcus pyogenes (Palombo and Semple, 2001). A collaborative project with Aboriginal groups from arid regions of Australia also demonstrated the antibacterial activity of this species (Evans et al., 2010). Another similar study reported the antimycobacterial activity of an extract from this species against Mycobacterium smegmatis and M. fortuitum (Meilak and Palombo, 2008). The reported traditional uses of this plant, the high esteem in which it is held as a medicine in a number of different Aboriginal communities and its previously reported antimicrobial activity led us investigate the antimicrobial activity of this species and to isolate and identify the active antibacterial compounds.
Here we present the activity-guided isolation, structural elucidation and antibacterial activity of four compounds from the leaves of E. alternifolia. These compounds belong to two different classes; flavonoids and serrulatane diterpenoids. They include three known flavanones, pinobanksin (1), pinobanksin-3-acetate (2) and pinobanksin-3-cinnamate (3) and a known serrulatane diterpene, 8-hydroxyserrulat-14-en-19-oic acid (4). Notably, this is the first report of studying antibacterial compounds from E. alternifolia as well as of the occurrence of flavanones 2 and 3 in the genus Eremophila. The identification of these four compounds establishes the scientific basis for the traditional usage of this plant for ailments associated with bacterial infections and provides an interesting initial comparison of structure versus antibacterial activity for pinobanksin type flavanones.
Section snippets
Collection of plant material
Leaves of E. alternifolia R.Br were collected from a private property near Dutton, north of the Barossa Valley Region, South Australia (GPS Coordinates: 34.3297 S, 139.1616 E) in September 2013. The plant was typical of the narrow-leaved, reddish pink flowering form, which is the most common form of this species, growing naturally in the area and over a wide range of inland Australia. A voucher specimen (AD 271534) was deposited at the State Herbarium of South Australia, Adelaide and species
Structural determination
The structures of all compounds (1–4) were identified by analysing their mass and 1D/2D NMR spectra, and putative structures were then compared with published data. Compounds 1−3 were identified as flavanones while compound 4 was identified as a serrulatane diterpene.
All compounds (1–4) could be identified as previously reported compounds. On the basis of the mass and NMR data and comparison with literature data, we arrived at the assignments of pinobanksin (1) previously isolated from E.
Discussion
Eremophila alternifolia has been recorded as being a highly valued medicinal species by a number of Australian Aboriginal groups (Barr et al., 1993, Richmond and Ghisalberti, 1994). It has been used both internally and externally for a variety of symptoms indicative of bacterial infections (Barr et al., 1993, Richmond, 1993). Using a bioassay-guided approach we have identified three compounds with antibacterial activity against staphylococci from this species. The active compounds belong to two
Concluding remarks
Previous findings suggested that the antibacterial activity of the investigated Eremophila species was due to compounds from two different classes, o-naphthoquinones and serrulatanes (Forster et al., 1986, Ndi et al., 2007a, Ndi et al., 2007b). Interestingly, Eremophila alternifolia differs substantially from the (relatively few) other Eremophila species investigated thus far, in that active flavanones contribute to the antibacterial activity of E. alternifolia. In this study, we have
Acknowledgements
The authors would like to acknowledge the support of the Australian Government's Cooperative Research Centres Program (CRC WMI). We thank Dr Bradley Simpson for assistance with mass spectrometry. High resolution mass spectrometry was performed at Flinders Analytical, Flinders University, South Australia.
References (59)
- et al.
The antimicrobial activity of 3, 5, 7-trihydroxyflavone isolated from the shoots of Helichrysum aureonitens
J. Ethnopharmacol.
(1997) - et al.
Antibacterial activity of flavonoids against methicillin-resistant Staphylococcus aureus strains
J. Theor. Biol.
(2000) - et al.
Antiproliferative activity of the Netherlands propolis and its active principles in cancer cell lines
J. Ethnopharmacol.
(2002) - et al.
Antibacterial serrulatane diterpenes from the Australian native plant Eremophila microtheca
Phytochemistry
(2013) - et al.
Assessment of the antibacterial activity of galangin against 4-quinolone resistant strains of Staphylococcus aureus
Phytomedicine
(2006) - et al.
Antimicrobial activity of flavonoids
Int. J. Antimicrob. Agents
(2005) - et al.
Flavonoids and stilbenes from armand pine
Phytochemistry
(1988) - et al.
Serrulatane diterpenes from Eremophila spp
Phytochemistry
(1986) The phytochemistry of the Myoporaceae
Phytochemistry
(1993)The ethnopharmacology and phytochemistry of Eremophila species (Myoporaceae)
J. Ethnopharmacol.
(1994)
(2R, 3S)-Pinobanksin-3-cinnamate improves cognition and reduces oxidative stress in rats with vascular dementia
J. Nat. Med.
Bactericidal and cyclooxygenase inhibitory diterpenes from Eremophila sturtii
Phytochemistry
Antibacterial activity and mode of action of plant flavonoids against proteus vulgaris and Staphylococcus aureus
Phytochemistry
Esters, amides and substituted derivatives of cinnamic acid: synthesis, antimicrobial activity and QSAR investigations
Eur. J. Med. Chem.
Antimicrobial compounds from Eremophila serrulata
Phytochemistry
Antibacterial activity of traditional Australian medicinal plants
J. Ethnopharmacol.
Cardioactive effects of Eremophila alternifolia extracts
J. Ethnopharmacol.
Cardioactive compounds from Eremophila species
J. Ethnopharmacol.
Cytogeography of essential oil chemotypes of Eremophila longifolia F. Muell (Scrophulariaceae)
Phytochemistry
Phenolic compounds and their anti-oxidative properties and protein kinase inhibition from the Chinese mangrove plant Laguncularia racemosa
Phytochemistry
Circular dichroism, a powerful tool for the assessment of absolute configuration of flavonoids
Phytochemistry
Trypanocidal activity of Lychnophora staavioides mart.(Vernonieae, Asteraceae)
Phytomedicine
Comparative study on the antibacterial activity of phytochemical flavanones against methicillin-resistant Staphylococcus aureus
J. Ethnopharmacol.
Antibacterial spectrum and cytotoxic activities of serrulatane compounds from the Australian medicinal plant Eremophila neglecta
J. Appl. Microbiol.
Traditional Bush Medicines: An Aboriginal Pharmacopoeia
Flavonoids from the seeds of Alpinia galanga Willd
Yao xue xue bao=Acta Pharm. Sin.
Pinocembrin chalcone: an antibacterial compound from Helichrysum trilineatum
Planta Med.
Natural flavonoids as antimicrobial agents
Jana
Cited by (16)
Design, synthesis and screening of a drug discovery library based on an Eremophila-derived serrulatane scaffold
2021, PhytochemistryCitation Excerpt :Compound 3 reduced the mid-body width of treated L4s to 8.8 ± 1.5 μm as compared with 16.3 ± 1.5 μm for untreated WT H. contortus, as shown in Fig. 3. Due to the previously reported anti-microbial activity for some Eremophila-derived diterpenoid serrulatanes (Barnes et al., 2013; Biva et al., 2016; Ndi et al., 2007; Smith et al., 2007), compounds 1–16 were tested by the Community for Open Antimicrobial Drug Discovery (CO-ADD) (Blaskovich et al., 2015; Zuegg et al., 2020) against the “ESKAPE” pathogens: Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Staphylococcus aureus (MRSA), and the yeasts Candida albicans and Cryptococcus neoformans. None of the compounds showed growth inhibitory activity at 32 μg/mL (or ~20 μM).
Flavonoids isolated from the fresh sweet fruit of Averrhoa carambola, commonly known as star fruit
2018, PhytochemistryCitation Excerpt :The 1H and 13C NMR spectra demonstrated signals of five aromatic protons assignable for a mono-substituted phenyl (H-2′–H-6′) and two ortho-coupled aromatic protons (H-6 and H-8) for the other phenyl, in addition to signals of two oxygenated methines (OCH-2 and OCH-3), a carbonyl carbon (C-4), and a β-glucosyl moiety. The HMBC correlations from H-2 to C-3, C-4, C-9, C-1′, and C-2'/6′, H-3 to C-2, C-4, C-10, and C-1′, as well as from H-1″ to C-3 and H-3′ to C-1″ clarified the presence of 3,5,7-trihydroxydihydroflavone aglycone (Biva et al., 2016) and the connection of glucosyl moiety to C-3. The coupling constant of 9.8 Hz between H-2 and H-3 clarified their trans relative configurations.
Antibacterial compounds from the Australian native plant Eremophila glabra
2018, FitoterapiaCitation Excerpt :While some species were used for treatment of rheumatism, diarrhoea, and to encourage deep sleep [3]. From a western perspective, these species have the potential to assist in the treatment of illnesses that mainly come from bacterial origin, and as such there have been reports of Eremophila demonstrating antimicrobial activities [3,4]. A broad antimicrobial screening program of 72 Eremophila species, which included E. glabra, showed that many of these species have antimicrobial activity against Gram-positive organisms related to some important human diseases [6].
Synthesis of antimalarial amide analogues based on the plant serrulatane diterpenoid 3,7,8-trihydroxyserrulat-14-en-19-oic acid
2017, Bioorganic and Medicinal Chemistry Letters