Anti-bacterial activity and characterisation of compounds of leaf extracts of Erlangea tomentosa S. Moore

Abstract

Bacterial infections such as pneumonia, lower respiratory tract infections, gastrointestinal disorders and skin infections are health burden to mankind. In Uganda, several medicinal plants are locally used to treat various bacterial infections because of affordability and accessibility of herbal medicine. Erlangea tomentosa (ET) is one of the plants used to treat various bacterial infections such as gastrointestinal disorders, skin infections, diarrhea, syphilis and cough. However, compounds responsible for its activity are not known and this limits its wider use and development into drugs. Therefore, the purpose of the study was to characterise the compounds from leaf extract of ET that could be responsible for its antibacterial properties. The leaves were collected, ground into a course powder and extracted sequentially using n-hexane, dichloromethane, ethyl acetate and methanol. Antibacterial activity of the extracts against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Klebsiella pneumoniae (K. pneumoniae) and Salmonella typhi (S. typhi) was done to identify the more active extract. Isolation of compounds from ethyl acetate extract (ETE) and dichloromethane extract (ETD) was done by column chromatography and compounds characterised by spectroscopic techniques. ETE showed a highest zone of inhibition (17.3 ±0.6, 16±1.0, 11.7±0.6 and 13.7±0.6 mm) followed by methanol extract (ETM) (10.3±0.6, 15±1.0, 14.7±0.6 and 10±0.0 mm) for E. coli, S. aureus, K. pneumoniae and S. typhi, respectively and ETD (17.3±0.6 and 10±0.0 mm) for E. coli and S. typhi, respectively. Hexane extract showed no zone of inhibition. Eriodictyol-7-O-β-glucoside (1) and 5, 7, 3’, 4’-tetrahydroxy-7-O-[6"-O-(acetyl)-β-D-glucopyranosyloxy]-flavanone (2) were isolated from ETE while di-(2-ethylhexyl) phthalate (3) was isolated from ETD. This was the first time to identify these compounds from the plant species of genus Erlangea. Compound 3 has been reported to have a moderate activity against Bacillus subtilis, Staphylococcus aureus, Streptococcus equosemens, Escherichia coli and Pseudomonas aeruginosa. Therefore, the bioactivity of the leaf extracts and the presence of compound 3 that is reported to be very active against various bacterial strains support the use of the plant. Compounds from ETM should be isolated and characterised to obtain more bioactive compounds from ET.