Enhancement of nisin antibacterial activity by a bearberry (Arctostaphylos uva-ursi) leaf extract

Plant extracts intended for use as antioxidants in foods may also have biological effects on bacteria. The antimicrobial activity of an antioxidant ethanolic extract of bearberry (Arctostaphylos uva-ursi) leaves, alone and in combination with nisin, was determined against 25 food-related bacteria by the spot-on-lawn and the micro-dilution methods. The growth kinetics of four bacteria selected from the 25 were determined in the presence of nisin and bearberry extract by monitoring absorbance at 550 nm, while the degree of sub-lethal injury of these bacteria was determined with bearberry extract alone by plating on agar with and without 4% NaCl. The bearberry extract alone displayed no antimicrobial activity of its own but enhanced the activity of nisin, as determined by minimum inhibitory concentration, against many of the Gram-positive, but none of the Gram-negative, bacteria. The combination of nisin and the bearberry extract slowed the growth of some bacteria, most notably Brochothrix thermosphacta, as compared to nisin alone. Sub-lethal injury was induced in two (Bacillus cereus and Broch. thermosphacta) of the four selected bacteria by the bearberry extract. The extract may have potential as an agent to enhance the antibacterial activity of nisin, but further work is required.