Screening for antimicrobial compounds in Gardenia volkensii and Meyna tetraphylla (rubiaceae)

Abstract

In Africa, 80% of the population use traditional medicinal plants to treat common ailments like malaria, headache and pneumonia. This is due to the high cost of conventional drugs from developed countries. Examples of these medicinal plants include Gardenia volkensii and Meyna tetraphylla that are both used by the Pokots in Kenya to treat a variety of ailments including antimicrobial diseases. The aim of this project was to test for the antimicrobial activity of the crude extracts and isolated compounds in the above two plants. The plant samples were collected from Baringo District in Kenya. The air dried and ground plant material of G. volkensii fruit seeds (418 g) was sequentially extracted using hexane, dichloromethane and methanol to give 8.00 g; 14.36 g and 10.85 g of extract. The G. volkensii fruit covers (408.5 g) yielded 3.68 g, 6.04 g and 7.41 g of hexane, dichloromethane and methanol extracts. The sequential extraction of M. tetraphylla leaves (636 g) yielded 7.38 g, 11.84 g and 9.55 g of hexane, dichloromethane and methanol extracts. The G. volkensii stem bark (10.22 g), leaves (9.34 g) and the M. tetraphylla root bark (20.47 g) were extracted using methanol only. The crude extracts for each solvent was screened for antimicrobial activity. Exactly 20-40 μL of 10,000 mg/L solution of extract was spiked and its antimicrobial activities on Bsb, SA, ST, CA and EC studied. The crude dichloromethane fractions from both plants were purified by step gradient isolation (dichloromethane/methanol) followed by repeated column chromatography (ethyl acetate/hexane). This gave GV1 and GV2 from G. volkensii. GV1 (65.10 mg) was a mixture while GV2 (34 80 mg) was pure. M. tetraphylla gave MT1 (164 mg), MT2 (48 mg), MT3 (63 mg), MT4 (72.20 mg) and MT5 (88.80 mg) pure compounds after repeated column chromatography (dichloromethane/methanol). GV2 (monoterpenoid or modified iridoid) was a novel compound. GV1 showed antimicrobial activity on SA and EC while GV2 showed activity on EC. MT1 and MT5 showed activity on EC, MT2 on Bsb and EC, MT3 on Bsb while MT4 showed no activity. There was no activity on CA and ST for all the crude extracts and pure compounds. Methanol was used as the negative control for the pure compounds while some selected antibiotics (Amoxicillin®, Tetracycline®, Doxycycline®, and Septrin®) were used as positive controls. The IC50 for G. volkensii stem bark and M. tetraphylla root bark, the most active crude extracts on Bacillus subtilis were determined using probit analysis (graphpad prism) and compared with the IC50 for Doxycycline® antibiotic which was 0.335 mg/mL. The IC50 for G. volkensii stem bark was 1166.809 mg/mL while for M. tetraphylla root bark was 699.842 mg/ml, 3,483 times and 2,089 times less active as compared to the Doxycycline® antibiotic respectively. This is an indication that, both plants can be used to cure microbial diseases though a higher dose is needed. Further research on their toxicity was recommended.