Unravelling the Antibacterial Activity of Terminalia sericea Root Bark through a Metabolomic Approach

Abstract

Terminalia sericea Burch. ex. DC. (Combretaceae) is a popular remedy for the treatment of infectious diseases. It is widely prescribed by traditional healers and sold at informal markets and may be a good candidate for commercialisation. For this to be realised, a thorough phytochemical and bioactivity profile is required to identify constituents that may be associated with the antibacterial activity and hence the quality of raw materials and consumer products. The aim of this study was to explore the phytochemistry and identify the antibacterial constituents of T. sericea root bark, using a metabolomic approach. The chemical profiles and antibacterial activities of 42 root bark samples collected from three districts in the Limpopo Province, South Africa, were evaluated. Dichloromethane:methanol (1:1) extracts were analysed using ultraperformance liquid chromatography (UPLC)-mass spectrometry (MS), and chemometric models were constructed from the aligned data. The extracts were tested against Bacillus cereus (ATCC 11778), Staphylococcus epidermidis (ATCC 12223), Staphylococcus aureus (ATCC 25923), Escherichia coli (ATCC 8739), Klebsiella pneumoniae (ATCC 13883), Pseudomonas aeruginosa (ATCC 27853), Shigella sonnei (ATCC 9292) and Salmonella typhimurium (ATCC 14028), using the minimum inhibition microdilution assay. Nine compounds; sericic acid, sericoside, resveratrol-3-O-β-rutinoside, ellagic acid, flavogallonic acid dilactone, methyl-flavogallonate, quercetin-3-(2′′-galloylrhamnoside), resveratrol-3-(6′′-galloyl)-O-β-d-glucopyranoside and arjunetin, were isolated from the root bark. All the compounds, with the exception of sericic acid, sericoside and resveratrol-3-O-β-rutinoside, were isolated for the first time from the root bark of T. sericea. Chemometric analysis revealed clustering that was not population specific, and the presence of three groupings within the samples, characterised by sericic acid, sericoside and an unidentified compound (m/z 682/4.66 min), respectively. The crude extracts from different populations displayed varied antibacterial activities against S. typhimurium (minimum inhibitory concentrations (MICs) 0.25–1.0 mg/mL), but similar activity towards Bacillus cereus (1.0 mg/mL). Several compounds present in the root bark were highly active towards all or most of the pathogens tested, but this activity was not reflected by the chemical profiles of extracts prepared from the individual samples. Among the pure compounds tested, only flavogallonic acid dilactone and methyl-flavogallonate exhibited broad-spectrum activity. A biochemometric analysis indicated that there was no consistent association between the levels of phytochemicals and the activity of the active or non-active extracts. Although it was deduced that the major constituents of T. sericea root bark contributed to the chemotypic variation, further investigation of the interactions of compounds present in the root bark may provide antibacterial efficacies not evident when examining compounds singularly. The data reported herein will provide information that is fundamentally important for the development of quality control protocols.

Phytochemical Profiling and Quality Control of Terminalia sericea Burch. ex DC. Using HPTLC Metabolomics

“…Considering the ranges indicated for each of the three major compounds, the loadings plot revealed that sericic acid and sericoside contributed to the clustering ( Figure 4 A). This trend was similar to the observation of the previously published UPLC-MS data [ 20 ] ( Figure S1B ).…”

“…Differences in the intensities of the bands representing the major compounds are evident within samples from different localities (for example, Kruger and Maila samples), as well as within samples from the same locality, for example Mookgophong samples P3 and P4 (Tracks 30 and 31). Inspection of the HPTLC plates revealed that resveratrol-3- O - β -rutinoside (R1), sericic acid (R2) and sericoside (R3) are major constituents of the root bark extracts, confirming previous reports [ 15 , 20 ]. Differences in their concentrations are evident from the different band intensities.…”

“…It can therefore be concluded that a characteristic T. sericea root bark fingerprint for species authentication should contain clear bands representing resveratrol-3- O - β -rutinoside, sericic acid and sericoside. The commercial potential of Terminalia sericea as an anti-infective agent has not been fully exploited and this has been attributed to a lack of standardisation and quality control [ 20 ]. The reported Rf values and colours of the bands can be used for easy identification of the compounds in extracts of T. sericea root bark.…”

“…This visual observation motivated the use of chemometric models to establish if samples from the Mopani (Giyani, Tzaneen), Waterberg (Bela-Bela, Mookgophong) and Vhembe (Maila, Vuwani, Mavhambe, Kruger, Tshandama, Tshitavha) districts representing distinctly different geographical localities in the Limpopo Province, could be chemically distinguished. The chemometric models obtained from the rTLC were compared with previous models obtained from UPLC-MS data [ 20 ].…”

“…Phytochemical studies have revealed the presence of hydrolysable tannins (ellagic acid, flavogallonic acid dilactone, methyl-flavogallonate) lignans (anolignan b, termilignan b), stilbenes (resveratrol-3- O -β-rutinoside, resveratrol-3-(6′’-galloyl)- O -β- d -glucopyranoside), flavonoids (quercetin-3-(2′′- galloylrhamnoside) and triterpenoids (arjunic acid, sericic acid, arjunglucoside I, sericoside, arjunetin) in the roots of T. sericea [ 15 , 16 , 17 , 18 , 19 , 20 ]. In a recent study [ 20 ], the chemical variability within T. sericea root bark samples representing several populations in Limpopo Province, South Africa, was attributed mainly to sericic acid, sericoside and resveratrol-3- O - β -rutinoside. The current study was aimed at establishing a characteristic HPTLC chemical fingerprint for T. sericea root bark for quality control purposes.…”