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The antimicrobial activities of two flavonoids, namely genistein 7-O-glucoside (1) and (–)-epi-catechin (2), isolated from Azadirachta indica A. Juss (neem) leaves, were evaluated against five fungal species, viz: Alternaria alternata (Fr.) Keissler, Aspergillus fumigatus Fresenius, Aspergillus niger van Tieghem, Macrophomina phaseolina (Tassi) Goid. and Penicillium citrii, and four bacterial species, viz. Lactobacillus sp., Escherichia coli, Azospirillium lipoferum and Bacillus sp. Six concentrations, viz. 100, 300, 500, 700, 900 and 1000 ppm of each of the two flavonoids were employed using malt extract agar medium. All the concentrations of both the test compounds significantly suppressed fungal as well as bacterial growth. The highest concentration (1000 ppm) of both fractions 1 and 2 reduced the growth of the different test fungal species by 83–99 % and 82–95 %, respectively. Compound 1 was highly effective against Lactobacillus sp., against which its various concentrations reduced the bacterial growth by 52–99.8 %. Compound 2 was highly effective against A. lipoferum and Bacillus sp., resulting in 94–100 % and 73–99% reduction in bacterial growth, respectively.

Five flavonoids, viz. (–)-epicatechin-3-O-b-glucopyranoside (1), 5-hydroxy-3-(4-hydroxylphenyl)pyrano[3,2-g]chromene-4(8H)-one (2), 6-(p-hydroxybenzyl)taxifolin-7-O-b-D-glucoside (tricuspid) (3), quercetin-3-O-a-glucopyranosyl-(1®2)-b-glucopyranoside (4) and (–)-epicatechin(2-(3,4-dihydroxyphenyl)-3,4-dihydro-2H-chromene-3,5,7-triol) (5), were isolated from the leaves of mango (Mangifera indica L.). The antibacterial activity of different concentrations of these flavonoids (100, 300, 500, 700, 900 and 1000 ppm) was evaluated against four bacterial species, namely Lactobacillus sp., Escherichia coli, Azospirillium lipoferum and Bacillus sp. All the tested concentrations of the five flavonoids significantly reduced the growth of all the five tested bacterial species. However, differences in the antibacterial activity of the flavonoids were evident. Compound 1 exhibited the lowest antibacterial activity, resulting in a 7–75 % reduction in the growth of the different bacterial species. Compound 5 showed the greatest antibacterial activity and the different concentrations reduced the bacterial growth by 45–99.9 %. A. lipoferum and Bacillus sp. showed the highest susceptibility to this compound. Compounds 2–4 also depicted pronounced antibacterial activity. Different concentrations of these compounds decreased bacterial growth by 52–96 %. From the present study, it can be concluded that compound 5 is the most effective of the tested flavonoids against A. lipoferum and Bacillus sp.