Antibacterial Activity of Tannin from Sweet Chestnut Wood Against Aeromonas and Streptococcal Pathogens of Tilapia (Oreochromis niloticus)

The present study determined the inhibitory effect of tannin from sweet chestnut wood against tilapia Oreochromis niloticus bacterial pathogens, Aeromonas hydrophila (20 isolates) and Streptococcus agalactiae (20 isolates). Minimum Inhibitory Concentrations (MICs) corresponding to tannin and oxytetracycline (OTC) for A. hydrophila were 18.75-300 μg/ml and 0.125-128 μg/ml; and for S. agalactiae were 600 μg/ml and 0.5-8 μg/ml, respectively. The antibacterial potency of tannin from sweet chestnut wood was found in the test system of pH: 6.5-7.0, suggesting that the inhibitory effect of tannins is unrelated to their acidity property. The OTC-resistant A. hydrophila and S. agalactiae (MIC of OTC > 4 μg/ml) responded to lower MICs of OTC in the test system with sub-inhibitory concentration of tannins (150 μg/ml). Concerning the trend towards the evolution of antimicrobial resistant strains, the antimicrobial activity of tannins may be a possible additive compound against fish pathogens treated with antibiotics.