Reece Courtney and Ian Edwin Cock
Published: 16th December 2021
“Traditional medicines prepared using Terminalia species have been used globally to treat inflammation and pathogenic infections. Recent studies have demonstrated that multiple Asian and African Terminalia spp. inhibit bacterial triggers of some autoimmune inflammatory diseases, including ankylosing spondylitis.
Despite this, the effects of Australian Terminalia spp. on a bacterial trigger of ankylosing spondylitis (K. pneumoniae) remain unexplored.
Fifty-five extracts from five Australian Terminalia spp. were investigated for K. pneumoniae growth inhibitory activity. Methanolic, aqueous and ethyl acetate extracts of most species and plant parts inhibited K. pneumoniae growth, with varying potencies.
Methanolic leaf extracts were generally the most potent bacterial growth inhibitors, with minimum inhibitory concentration (MIC) values of 66 μg/mL (T. ferdinandiana), 128 μg/mL (T. carpenteriae) and 83 μg/mL (T. petiolares).
However, the aqueous leaf extract was the most potent T. grandiflora extract (MIC = 87 μg/mL).
All T. catappa extracts displayed low growth inhibitory activity. The Terminalia spp. methanolic leaf extracts were examined by liquid chromatography-mass spectrometry (LC–MS) and gas chromatography-mass spectrometry (GC–MS). All contained a relative abundance of simple gallotannins (particularly gallic and chebulic acids), the flavonoid luteolin, as well as the monoterpenoids cineole and terpineol.
Notably, all Terminalia spp. were non-toxic or of low toxicity in ALA and HDF toxicity assays, highlighting their potential for preventing the onset of ankylosing spondylitis and treating its symptoms once the disease is established, although this needs to be verified in in vivo systems.”
“The current repertoire of therapies to treat AS is not ideal and only target the symptoms of the disease, whilst allowing irreversible tissue degradation to continue in the vertebral and sacroiliac joints. Therapies targeting the etiological events would prevent the onset of the disease, which would not only alleviate the symptoms of AS but would also prevent tissue damage.
The role of K. pneumoniae infections in the etiology of AS is well established and is supported by a wealth of evidence (Cock and Cheesman 2018; Rashid and Ebringer 2007). Despite this, there are currently no clinical therapeutics for AS that target K. pneumoniae.
Inhibiting the growth of K. pneumoniae using prophylactic antibiotic therapies may be eﬀective in the prevention and treatment of AS, and other autoimmune inﬂammatory diseases. However, prophylactic usage of conventional antibiotics would also induce the development of further antibiotic resistance, rendering the target bacterium (and other microbial species) unsusceptible to the eﬀects of the antibiotic. The use of combinations of antibiotics (or antibiotics and potentiator compounds) may be more eﬀective than antibiotic monotherapies and may also decrease the possibility of the bacteria developing further resistance.
Traditional medicines and herbal therapies are promising targets for antibiotic drug discovery as they have often been used for long periods (in some cases, thousands of years) and are perceived (often erroneously) to be safer than synthetic drugs (Moreira etal 2014)…
Methanolic extracts prepared from all of the Terminalia spp. screened in our study displayed noteworthy K. pneumoniae growth inhibitory activity. Additionally, the aqueous and ethyl acetate extracts of all Terminalia spp. tested (except T. catappa) also inhibited K. pneumoniae growth.
Notably, for all of the Australian Terminalia spp., the leaf extracts had substantially greater potency than the corresponding fruit, bark or seed extracts. Therefore, the methanolic T. ferdinandiana, T. carpentariae, T. grandiﬂora and T. petiolares leaf extracts were further analysed to determine their phytochemical composition using both LC–MS and GC–MS headspace analysis.
Metabolomic ﬁngerprint analyses of the methanolic T. ferdinandiana, T. carpentariae, T. grandiﬂora and T. petiolares leaf extracts conﬁrmed the presence of some noteworthy compounds with therapeutically relevant properties.
In particular, our study targeted the tannins, ﬂavonoids and volatile terpenoids. All of the methanolic leaf extracts evaluated contained a diversity of tannins in relative abundance.
Notably, tannins inhibit the growth of multiple bacterial pathogen growth by disrupting bacterial cell walls (Hogg and Embery 1982; Wu-Yuan etal 1988), binding and inactivating speciﬁc bacterial cell surface proteins, as well as functioning as inhibitors of bacterial enzymes (Wu-Yuan et al 1988; Buzzini etal 2008).
Thus, it is likely that the Australian Terminalia spp. tannins components detected in our study may contribute to the anti-K. pneumoniae activity reported herein. Additionally, the ﬂavonoid luteolin was also identiﬁed in all of the Australian Terminalia spp. methanolic leaf extracts examined in our study.
Interestingly, multiple ﬂavonoids including luteolin have been reported to have potent broad-spectrum antibacterial activity (Cushnie and Lamb 2005; Resende etal 2018) and may, therefore, also contribute to the noteworthy antibacterial activity reported herein…
Our study was limited to examining the inhibitory activity of the Australian Terminalia spp. extracts towards a bacterial trigger of AS. However, these are crude extracts, which contain complex phytochemical mixtures.
It is, therefore, possible that these extracts may also modulate other inﬂammatory processes such as cytokine release and, therefore, eﬀect several aspects of AS disease progression. Indeed, several of the compounds identiﬁed in our study have been reported to have pleuripotent anti-inﬂammatory eﬀects by acting on both the initiator and downstream inﬂammatory stages of the disease.
For example, several of the terpenoids that we have identiﬁed herein can suppress NF-κB signalling, and may, therefore, also directly inhibit inﬂammation (Salminen et al. 2008). Further studies are required to evaluate Australian Terminalia spp. extracts for direct anti-inﬂammatory properties, and for additional combinational eﬀects that may enhance the therapeutic properties of individual components of these extracts…”
“The results of this study demonstrate the potential of T. ferdinandiana, T. carpentariae, T. grandiﬂora and T. petiolares as inhibitors of K. pneumoniae, a bacterial trigger of AS.
The methanolic and ethyl acetate extracts prepared from T. ferdinandiana and T. petiolaris leaves were particularly good inhibitors of K. pneumoniae growth. Additionally T. ferdinandiana, T. grandiﬂora and T. petiolaris fruit extracts, as well several aqueous extracts, also had noteworthy activity.
Whilst the extracts examined in this report are promising as anti-AS agents, caution is needed before these compounds can be applied to medicinal purposes. In particular, further toxicity studies using human cell lines are needed to verify the suitability of these extracts for these purposes. Furthermore, our study focussed on the ability of the extracts to inhibit the growth of a bacterial trigger of AS in vitro.
Whether these eﬀects translate to invitro systems is yet to be determined and further studies are required to test this.
Additionally, bioavailability studies (phramacokinetic and pharmacodynamics studies) are required to determine the eﬃcacy of the extract components invivo. Only then can it be determined whether inhibiting this bacterial trigger of AS has clinically relevant eﬀects in AS suﬀerers.”