Preliminary screening of some fractions of Loxostylis alata (Anacardiaceae) for antimicrobial and antioxidant activities

Abstract

An acetone extract of Loxostylis alata was separated into six fractions based on polarity by a solventsolvent fractionation procedure and the different fractions were screened for antimicrobial and antioxidant activities. The number of antimicrobial compounds in the carbon tetrachloride (CCl₄), chloroform (CC), aqueous methanol (AM) and butanol (BT) fractions were determined by bioautography. Each fraction was loaded onto thin layer chromatography (TLC) plates and eluted using two different mobile solvent systems, namely chloroform/ethyl acetate/formic acid (5:4:1): (CEF) and ethyl acetate/methanol/water (40:5.4:5) (EMW). Relative front (R_f) values of active compounds against bacteria ranged from 0.4 - 0.9 in CEF and EMW solvent system. Similarly, the Rf values of compounds active against fungi were in the range of 0.7 - 0.9. Hexane, carbon tetrachloride, chloroform, aqueous methanol and butanol fractions showed areas of inhibition against bacterial organisms, while only hexane and carbon tetrachloride fractions depicted areas of fungal growth inhibition on their chromatograms. The CCl₄ extract was active against six out of the nine microbial strains used and was particularly active against Staphylococcus aureus, Enterococcus faecalis, Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans and Microsporum canis with minimum inhibitory concentration (MIC) of 0.04, 0.04, 0.1, 0.1, 0.06 and 0.03 mg/ml, respectively. M. canis was the most sensitive organism with the lowest average MIC of 0.16 mg/ml. Qualitative antioxidation using diphenylpicryl- hydrazyl (DPPH) and qualitative assay using both 2,2'-azinobis-3-ethylbenzothiazoline-6- sulfonate (ABTS) and DPPH radicals revealed the presence of three antioxidant compounds in the AM and BT fractions, while the water fraction had only one antioxidant compound. However, the concentration of the antioxidant compounds was more in the AM and BT fractions as revealed by their pronounced colour intensity on the TLC plates. The water fraction had more free radical scavenging effect against DPPH with EC₅₀ value of 0.62 ± 0.03 μg/ml. The lower the EC₅₀ value of an extract, the more effective its antioxidant activity. In a similar manner, TEAC value of water fraction was 2.97, which further indicates superior free radical scavenging effect of the fraction against ABTS radical. The greatest reduction of reactive oxygen species (ROS) production induced by diclofenac was also achieved by the water fraction which on overall shows that the water fraction contains more antioxidants. The presence of antimicrobial and antioxidant activities in the fractions of L. alata suggests that this plant may be a source of pharmacologically active substances with beneficial actions towards disease control and treatment in animals and humans.

Key words: Medicinal plant, antibacterial, antifungal, serial microdilution, minimum inhibitory concentration, antioxidants.