Antibacterial Activity of Different Plant Extracts and Phenolic Phytochemicals Tested on Paenibacillus Larvae Bacteria

Paenibacillus larvae, a Gram-positive and spore-forming bacterium is responsible for American foulbrood disease in bees. The antimicrobial activity of different plant extracts and phenolic phytochemical was evaluated on Paenibacillus larvae bacteria. In addition possible correlation with antioxidant activity of the same plant extracts was studied. Extracts of the following plants were utilized: Achillea millefolium (yarrow), Ocimum basilicum (basil), Thymus vulgaris (thyme) and Urtica dioica (nettle). The extracts that showed antimicrobial activity were later tested to determine the Minimal Inhibitory Concentration (MIC). Although nettle present the lowest polyphenolic content compared with the other plant extracts, exhibit the highest antimicrobial activity, measured as the inhibition zone using Mueller-Hinton agar plates. Basil presented both polyphenolic content and antimicrobial activity at higher levels, while thyme had the lowest antimicrobial activity, even it present high amount of polyphenols.