Antibacterial properties of extracts from selected planktonic freshwater cyanobacteria – a comparative study of bacterial bioassays

Aqueous and methanol extracts from five selected cyanobacteria were examined for antibacterial properties in six different bacterial bioassays. All five cyanobacteria revealed antibacterial properties. Methanol extracts made from Tychonema bourrellyi, Aphanizomenon flos‐aquae and Cylindrospermopsis raciborskii showed the most pronounced inhibitory effects. Aqueous extracts made from Microcystis aeruginosa and T. bourrellyi possessed evident antibacterial properties. The bacterial bioassays were based on agar diffusion tests and included pour‐plate methods commonly used to detect residues of antibacterial substances in food. In addition, a pour‐plate bioassay with Aeromonas hydrophila was developed and described. Antibacterial effects were observed in five of the six bacterial bioassays. No antibacterial effect was observed in the Micrococcus luteus bioassay. Bioassays based on Aer. hydrophila, Bacillus cereus and B. subtilis grown in Antibiotic Medium 8, pH 5·85, seemed to be sensitive and suitable. The MIC value of diluted MeOH extracts made from C. raciborskii and T. bourrellyi against Aer. hydrophila corresponded to 38 mg freeze‐dried cyanobacteria. Bacillus subtilis was more sensitive when grown in a culture medium with pH 5·85 than 7·9. The antibacterial properties of extracts from the cyanobacteria examined differed from defined cyanotoxins and antibacterial substances. The pattern of inhibition in the bacterial bioassays indicated that various antibacterial substances are involved.