Identification and quantification of an antibacterial flavonoid from the complex extract prepared from six species of medicinal plants

Bacterial diseases have been severe problems in aquaculture. The wide and frequent use of antibiotics in aquaculture has resulted in the development of antibiotic resistance and problems in food safety [1]. In our on-going research to discover natural products with antimicrobial properties concerning fish bacterial pathogens, we previously found that the complex extract prepared from 6 species of medical plants, seeds of Benincasa hispida C., peels of Citrus unshiu M., flowers of Lonicera japonica T., leaves of Perilla frutescens B., roots of Scutellaria baicalensis G. and Sophora flavescens A. has antibacterial, antiparasitic and antifungal activities against fish pathogens. In this study, we identified an antibacterial compound from the complex extract through bioactivity-guided isolations and quantified it using HPLC/UV analysis. Antibacterial activity-guided fractionation for the complex extract yielded the active methylene chloride (MC) fraction with minimal inhibitory concentrations (MICs) of 1 mg/mL against Edwardsiella tarda and Streptococcus iniae in microdilution method. From further isolations for the MC fraction, an active compound, baicalein was obtained with MICs of 0.125 and 0.25 mg/mL against E. tarda and S. iniae, respectively. The content of baicalein was found to be 9.8% of the active MC fraction. The method developed allowed the limits of detection and quantification of 0.757 and 2.295 µg/mL, respectively. The calibration curve showed good linearity (r > 0.9993) within the test range. Intra- and interday precisions were good with RSD < 6.52%. The average recovery was 105.36%. These results suggested that the active MC fraction of the complex extract may be used as an antimicrobial alternative for fish bacterial diseases and the HPLC method can be used for the quality control.

References:

[1] Defoirdt T, Sorgeloos P, Bossier P. Alternatives to antibiotics for the control of bacterial disease in aquaculture. Curr Opin Microbiol 2011; 14: 251 – 258