Activity of Bulgarian propolis against 94 Helicobacter pylori strains in vitro by agar-well diffusion, agar dilution and disc diffusion methods

Propolis exhibits antimicrobial, anti-inflammatory and other biological effects. The aim of this study was to evaluate the activity of 30 % ethanolic extract of Bulgarian propolis against 94 Helicobacter pylori strains by three methods. By the agar-well diffusion method, only 13.8 % of the strains exhibited no inhibition by 30 μl propolis extract (containing 9 mg propolis) and all isolates were inhibited to some extent by 90 μl of the extract (27 mg propolis) per well. The mean diameters of growth inhibition by 30, 60 or 90 μl propolis extract or 30 μl 96 % ethanol per well were 16.8, 19.2, 27.5 and 8.3 mm, respectively. The propolis extract was more active than the ethanol ( P < 0.001). With 90 μl propolis extract per well, 69.4 % of the strains exhibited large diameters of growth inhibition (⩾20 mm) versus 26.6 % with 30 μl per well ( P < 0.001). With moist propolis discs, inhibition was detected in more strains (92.1 %) than with dried discs (78.2 %, P < 0.05), with mean inhibitory diameters of 18.7 and 13.8 mm, respectively. By the agar dilution method, 100 and 300 μg propolis ml −1 inhibited the growth of 57.1 % and 76.2 %, respectively, of the 21 strains tested. In conclusion, Bulgarian propolis had a strong and dose-dependent activity against most of the H. pylori strains tested. Although the effect of propolis on H. pylori in vitro is promising, further microbiological, pharmacological and clinical trials are required.