Antimicrobial activity of commercial organic honeys against clinical isolates of human pathogenic bacteria

Extracts of five organic and one conventional honey sources, available in Finnish markets, were tested for antimicrobial activity and inhibitory concentrations against Escherichia coli, Salmonella Typhi, Pseudomonas aeruginosa, Klebsiella pneumoniae, Bacillus cereus, and Staphylococcus epidermidis, obtained from human specimens. Five (honeys A, B, D, E, F) of the six studied honeys were organic. All the studied honeys had inhibitory activity (zone of inhibition (ZI) > 9.4 +/- 0.5 mm) compared to control artificial honey (ZI < 8 mm) against all the studied pathogens. Four organic honeys (B, D, E, F) showed inhibitory activity with ZI> 13.3 mm against all the studied bacteria with minimal inhibitory concentration (MIC) values of 12.5-50%. Against E. coli, the organic honeys E and F had activity index (AI) of 0.64 and 0.73, respectively, compared to the antibiotic AI of 1.0. Against S. Typhi, the organic honeys D and F had AI of 0.59 and 0.64, respectively. Against P. aeruginosa, the organic honeys D, E, and F had the highest AI of 0.71-0.80, and against S. epidermidis the honeys B, D, E, and F possessed relatively high AI of 0.60, 0.67, 0.73, and 0.78, respectively. Against K. pneumoniae and B. cereus, the detected AI of the organic honeys B, D, E, and F varied between AI of 0.48 and 0.58. The organic honey A and conventionally produced honey C possessed only minor activity with MIC values of 80%. Here, we show that commercially available culinary organic honeys possess remarkable antimicrobial activity against several important human bacterial pathogens.