Testing for microbiologically active compounds extracted from members of the family laminaceae and other indigenous plants.

Abstract

The Labiatae is a large family that occurs worldwide and have species that are adapted to almost all habitats and altitudes. Plectranthus is in this family. Plectranthus species are beautiful South African shrubs. The genus Plectranthus belongs to subfamily Nepetoideae of tribe Ocimeae. The test microorganisms were chosen carefully as each one belonged to a different taxonomic group of fungi and bacteria. Biologically active mono- and sesquiterpenoids are frequently found in many species of Plectranthus but there are little published data that directly link the presence of specific compounds in a species with the traditional uses of that species. Various Plectranthus spp. were collected and dried, followed by chemical extraction using various solvents. Dichloromethane extracts of P. fruticosus and P. ecklonii were screened for antibacterial and antifungal activities using the agar well and trench diffusion methods. It was found that both methods produced inconsistent results. The trench method required a bigger volume of plant extract to be filled into the well, hence, better biological activity was observed with that method. The well method required a smaller volume therefore poor activity was noted with this assay. The size of inhibition zones are dosage dependent. Overall, both plant extracts exhibited antibacterial but no antifungal properties. The pure compound (1), 11-Hydroxy-2-(4-hydroxybenzoyl)-5,7,9(11),13-abietatetraen-12-one, isolated from P. ecklonii was found to be the same as compound (10) which was isolated from P. lucidus. P. hadiensis was extracted using dichloromethane and hexane. The dichloromethane extract proved to contain much higher biological activity than the hexane extract. Three pure compounds, identified as diterpenes, were isolated from the crude dichloromethane extract of P. hadiensis. 6,7-Dihydroxyroyleanone-6,7,12-trihydroxy-8,12-abietadiene-ll,14-dione (2) and 7(alpha)-formoxy-6(beta)-hydroxyroyleanone (3) exhibited good antibacterial and antifungal activity but not against all the test organisms. The remaining pure compound, 7(alpha)-acetoxy-6(beta)hydroxyroyleanone (4), exerted good antifungal activity. This was measured by the inhibition zone which measured up to 14mm when compound 4 was tested against S. sclerotiorum. When testing the hexane extract against the Bacillus formulations, the pellets that were suspended once in Ringer's solution produced bigger inhibition zones than the pellets that were suspended twice. This could be due to bacterial cells washing out of the suspension. The dichloromethane extract of P. praetermissus proved to be very active against X campestris, producing an inhibition zone of 8 - 20mm. Two pure compounds were isolated from the crude extract and identified as diterpenes. Compound 5, 20(10--> 5)-abeo1( 10),6,8,11,13-abietapentaene-11,12,16-triol, and compound 6, 11,12,15-trihydroxy-20( 10-->5)-abeo-abieta-1-(10),6,8,11,13-pentaene are both known compounds which have previously been isolated from Salvia apiana. P. cilatus was extracted with chloroform and tested against various microorganisms for antifungal and antibacterial activities. It showed poor biological activity overall, except against S. sclerotiorum. The crude dichloromethane extract of P. zuluensis exhibited good antibacterial activity, which was limited to the Gram negative test organism. The extract produced an inhibition zone of 10-12mm when tested against X campestris. Pure compound 7, 2-hydroxy-4,6dimethoxyacetophenone, exerted excellent inhibition against B. subtilis and S. sclerotiorum. Neither compound 8, 1,2,4-trimethoxy-5-(2-propenyl)-benzene, nor compound 7, inhibited Candida spp., F. oxysporum and R. solani. Two diterpenes were isolated from the aerial plant parts of P. lucidus with dichloromethane and their structures elucidated by spectroscopic means. The pure compound 9, 11-hydroxy19-( 3-methyl-2-butenoyl)-5,7,9(11), 13-abietatetraen-12-one, showed moderate antifungal activity whereas compound 10, 11-hydroxy-2-(4-hroxybenzoyl)-5,7,9(11),13-abietatetraen12- one, showed high antifungal activity against R. solani, S. sclerotiorum and F. oxysporum. The crude and the pure compounds (formerly isolated from P. parviflorus) showed inhibition against X campestris. The dichloromethane extracts of P. purpuratus subsp. purpuratus and P. purpuratus subsp. tongaensis exhibit similar levels of biological activity when tested against the same test organisms. Poor antibacterial activity was noted with both extracts. However, excellent antifungal activity was depicted when both plant extracts were tested against F. oxysporum, R. solani and S. sclerotiorum. However, the highest biological activity was noted by R. solani which was totally inhibited by both dichloromethane extracts. The pure compound (11) isolated from P. purpuratus subsp. purpuratus was found to have the same chemical structure as compound (9) previously isolated from P. lucidus. The bioautography assay was used to detect and activity-guide the fractionation of antimicrobial compounds from all the Plectranthus spp. tested. The TLC fingerprint showed a zone of clearing around the lower bands of P. fruticosus and P. ecklonii when the plate was sprayed with a suspension of B. subtilis. This result is consistent with the agar well diffusion method. Clear zones were also noted on some bands of the extracts of P. zuluensis, P. ciliatus, P. hadiensis and P. praetermussis. Clear zones indicate inhibition of growth. Other plant extracts tested for biological activity were from the family Lamiaceae, however, not of the genus Plectranthus. Persicaria senegalensis, Pycnostachys reticulata and Ficus sur possessed moderate biological activity overall. It is interesting to note that P. senegalensis and F. sur exert high biological activity against Candida spp. This could be useful as herbal remedies for yeast infections.