Antibacterial activity of medicinal herbs against the fish pathogen Aeromonas hydrophila

Antibiotics play an important role in the disease management of aquaculture practices; however antibiotic resistance develops readily in pathogens following antibiotic treatment. Medicinal herbs play an alternative role to tackle this problem. In the present study, 15 selected plant species belonging to Lamiaceae and Apocynaceae were collected from different localities of Tirunelveli district of Tamilnadu. The antibacterial activity and Minimum Inhibitory Concentration (MIC) of the plant extracts were evaluated against the fish pathogen, Aeromonas hydrophila. The antagonistic effect of the methanolic extracts of Coleus aromaticus of Lamiaceae and Tabernaemontana divaricata of Apocynaceae were found to be most effective against A. hydrophila. Regarding Lamiaceae, C. aromaticus, Mentha arvensis and Leucas aspera exhibited lower MIC value of 12.5 mg/ml with inhibition zones of 10.33 mm, 9.67 mm, and 9.33 mm respectively. T. divaricata of Apocynaceae showed MIC of 12.5 mg/ml with an inhibition zone of 7.33 mm whereas Catharanthus roseus at 25 mg/ml exhibited an inhibition zone of 9.67 mm and Rauvolfia tetraphylla at 50 mg/ml showed an inhibition zone of 9.33 mm. Considering overall performance, C. aromaticus was found to be the most effective antagonistic agent against A. hydrophila.