A Laboratory Activity Demonstrating the Antibacterial Effects of Extracts from Two Plant Species, Moringa oleifera and Allium sativum (Garlic)

A variety of plants synthesize natural products that either kill or inhibit the growth of various microorganisms. These plant products may serve as useful natural alternatives to synthetic antimicrobial pharmaceuticals and can be especially important in regions where commercial drugs are often not available. Despite this, the role of plants as producers of natural antimicrobial agents is often understated or even ignored in undergraduate biology curricula. In this laboratory exercise, students extract water-soluble constituents from two plants, Moringa oleifera (moringa) and Allium sativum (garlic), and determine their activity against both a gram-positive (Bacillus cereus strain 971) and a gram-negative (Escherichia coli strain K12) bacterium using a disk diffusion assay on Mueller-Hinton agar. Disks infused with commercially available antibiotics (e.g., penicillin and tetracycline) serve as controls. Following an incubation period of 24 hours, students obtain quantitative data by measuring zones of growth inhibition that develop as a result of strain sensitivity. To determine the effectiveness of the learning objectives, an unannounced quiz was administered both before and after the activity, and the students showed significant gains in their understanding of key concepts. Because this activity combines aspects of two major branches of biology—plant biology and microbiology—it is suitable for use as a laboratory exercise in courses related to either discipline, or it may be used as a laboratory component of a general biology course.