Nguyen Le Anh Dao^*, Guy Degand, François Brose, Tran Minh Phu, JoëlleQuetin-Leclercq,  Bui ThiBuu Hue, Le Thi Bach, Truong QuynhNhu, Bui ThiBich Hang, Do ThiThanhHuong, Nguyen Thanh Phuong, Patrick Kestemont, Marie-Louise Scippo
*College of Aquaculture & Fisheries, Can Tho University, Can Tho, Vietnam
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The aim of this study was to examine the in vitro antibacterial activities of 20 herbal extract samples and 3 commercial productsto select extracts which could constitute antibiotic alternatives or improve seafood and aquaculture products preservation. The antimicrobial capacity against two strains of Aeromonashydrophila, the most opportunisticpathogenic bacteria in fish, was tested using a colorimetric indicator (resazurin) for the minimum inhibitory concentration (MIC) determination.Both bacterial strains were isolated from red tilapia (Oreochromis sp.)in Vinh Long province (Vietnam). Samples were tested at final concentrations ranging from 5 to 2500 µg/mL.Four plant extracts (PhyllanthusamarusSchum. et Thonn., Piper betle L., Psidiumguajava L. andEuphorbia hirta L.) and one commercial product (Alba) showed moderate antimicrobial activities against the first strain, with MICs values ranging between 156 and 625 µg/mL, while the rest of the samples showed weak or no antibacterial activity (MIC ≥1250 or 2500µg/mL, respectively).The growth of the second strainwas also inhibited by PhyllanthusamarusSchum. etThonn., Piper betle L. and Alba, but at higher concentrations for plant extracts (MIC = 625 µg/mL).Psidiumguajava L. and Euphorbia hirta L., which inhibited the growth of the first strain at a concentration of 312 and 625 µg/mL respectively, needed a concentration of 1250µg/mL to inhibit the second strain of Aeromonashydrophila. Six other plant extractswith a weak or no antibacterial activity on the first strain were also tested on the second strain and showed similar activities (weak or no inhibition). The results showed that both strains of Aeromonashydrophila display the same pattern of sensitivity towards tested plant extracts, but the first strain is more sensitive than the second one. This shows also that all strains of the same bacteria do not display the same level of sensitivity. These findings of a significant antibacterial activity of two plant extracts (PhyllanthusamarusSchum. et Thonn.,Piper betle L.) against two strains of the pathogenic bacteria Aeromonashydrophilacould be useful for the selection of natural alternative to antibiotics as well as to prevent bacterial growth in fish products during storage.

Key words:  Aeromonashydrophila, antibacterial activity, plant extracts, minimum inhibitory concentration