Antimicrobial, Antioxidant, Anti-Inflammatory and Acute Toxicity Screening of Leaf Extracts of Morinda lucida (Rubiaceae)
Authors: SK Okwute , IO Ochi 
Published: April 2023

Introduction: Morinda lucida is a medicinal plant popular for its traditional uses in the treatment of several illnesses such as malaria, inflammation, diabetes, jaundice, hypertension and dysentery. In this study, the leaf extract of the plant which is the most commonly used in ethnomedicine was subjected to biological screenings for anti-microbial, anti-inflammatory and anti-oxidant activities, and oral acute toxicity test to confirm its claimed potency in traditional healthcare. Methods: The powdered dried leaf of Morinda lucida was successively extracted with petroleum ether, ethyl acetate and methanol to obtain the corresponding extracts. The extracts obtained were screened for phytochemicals, antimicrobial, antioxidant and anti-inflammatory activities as well as determined its acute toxicity concentration and total phenolic content. The inhibitory activity (sensitivity test) of the extracts was carried out using agar well diffusion method, while the minimum inhibitory concentration determination was done using tube dilution method with the mueller hinton broth used as a diluent. For sensitivity tests the diameters of the zones were measured using a transparent ruler calibrated in millimeter and the results were recorded. The free radical scavenging activity of the extracts of the leaves of Morinda lucida, based on the scavenging activity of the stable 2,2-Diphenyl-1-Picryl Hydrazyl (DPPH) free radical, was determined using the measurement of the absorbance at 517 nm of the reduction of violet to yellow color in the presence of antioxidants. The oral acute toxicity study was carried out in vivo using albino mice in two phases. Each phase had groups of animals and each group received 1200, 1600, 2900 or 5000 mg/kg of the extracts of dried leaves of M. lucida except the control which received normal saline. All the animals were subjected to four hours of fasting prior to treatment and their respective body weights taken. The mice were then carefully monitored for clinical signs of toxicity such as weakness or drowsiness, aggressiveness, loss of weight, diarrhea, discharge from eyes and ears, noisy breathing and the number of deaths in each treated group and the control. The observation was carefully recorded and result documented. For anti-inflammatory activity, Carrageenan induced rat paw oedema method was used and the animals were wistar rats (17-31 g) of either sex. The animals received the crude extract in three different doses (250, 500 and 1000 mg/kg) body weight with piroxicam (10 mg/kg) as the reference drug. Measurement of right hind paw in circumference was taken using digital caliper. Results: Phytochemical analysis showed the presence of alkaloids, phenols, terpenoids, flavonoids, reducing sugars, steroids, saponins, carbohydrates, lignans, xanthones and peptides but no anthraquinines. The phytochemicals were substantially in the methanol and ethylacetate extracts. The extracts exhibited characteristic strong Concentration-Dependent Activity (CDA) against the test organisms with zones of inhibition ranging from 10-23 mm at various concentrations. The petroleum ether extract was completely inactive against E. coli and K. pneumonia bacteria while ethyl acetate extract also showed no activity against K. pneumonia. However, the methanol extract demonstrated high activity against S. aureus, B. subtilis, E. coli, S. typhi, K. pneumonia and P. aeroginosa. All the extracts had no activity against the fungi C. albicans and A. niger. The extracts exhibited free radical scavenging activity of 66.2%, 55.3% and 45.0% at the concentration of 1.0 mg/ml, respectively for methanol, ethyl acetate and petroleum ether, compared to that of the standard, ascorbic acid, which recorded 81.7% at the same concentration. Using DPPH free radical and spectrophotometry, the extracts displayed reducing antioxidant power of 0.594, 0.408 and 0.396 nm, respectively for methanol, ethyl acetate and petroleum ether compared to the standard, ascorbic acid with 0.826 nm at 1.0 mg/ml. The extracts gave total phenolic content of 62.2, 106.0 and 170.7 mg/g for petroleum spirit, ethyl acetate and methanol, respectively. The results of the acute toxicity profile showed that all the animals of different body weights survived at all the concentrations ranging from the lowest concentration of 10 mg/kg to the highest concentration of 5000 mg/kg per oral test dose. Physical and behavioral observations of the experimental mice revealed no visible sign of acute toxicity. The extracts showed significant inhibitory effect on oedema formation at the doses of 250, 500 and 1000 mg/kg body weight when compared to the standard drug, piroxicam, at the dose of 10 mg/kg body weight. The oedema inhibition was found to be concentration dependent only with petroleum ether extract unlike the ethyl acetate and methanol extracts. Conclusion: The results of this study demonstrated that the various extracts of Morinda lucida have great potentials as antimicrobial, antioxidant, anti-inflammatory agents and have low toxicity. The leaf extracts can therefore safely be used to manage human pathogenic infections. These biological activities and the phyto-constituents validate its acclaimed ethnomedicinal uses.