Bacterial growth inhibition and antioxidant potentials of leaf infusions of (Moringa oleifera), locust beans (Parkia biglobosa) and bitter leaf (Vernonia amygladina)

This study was aimed at assessing the bioactive potential of aqueous leaf infusions of three plant species, moringa (Moringa oleifera), locust beans (Parkia biglobosa) and bitter leaf (Vernonia amygdalina). The antimicrobial studies were carried out using the agar diffusion method against the selected isolates. For growth rate studies, known quantities of the respective crude leaf infusions only or in combination with ascorbic acid were added to sterile nutrient broth before inoculating with the respective test bacterial species and incubated. Growth was then monitored every 1 h, for a 10 h duration and after 24 h. Antimicrobial and growth rate studies were carried out using five bacterial species (Pseudomonas aeruginosa, Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Klebsiella pneumoniae). Results of the agar diffusion procedure revealed bitter leaf infusion showing inhibition against P. aeruginosa, S. aureus, K. pneumoniae and E. coli. The locust bean leaf infusion showed inhibition against E. coli only while the moringa leaf infusion inhibited the growth of P. aeruginosa, S. aureus, K. pneumoniae and E. coli. For the growth rate studies, all test bacterial species showed extended lag period in nutrient broth that contained the leaf infusions or a combination of the leaf infusions and ascorbic acid. None of the infusions was able to completely stop the growth of the test bacterial species for a 24 h growth period. In nutrient broth that contained the locust bean leaf infusion only, longer lag period of the test bacterial species was observed than in medium that contained both the leaf infusion and ascorbic acid. All the leaf infusions showed antioxidant activities when investigated for their ability to scavenge free radicals. The study was able to show the antimicrobial and antioxidant potentials of the infusions, hence their possible application as antimicrobial agents. (C) 2021 The Author(s). Published by Elsevier B.V. on behalf of African Institute of Mathematical Sciences / Next Einstein Initiative.