Evaluation of antibacterial efficacy of mangrove leaf extracts on fish bacterial pathogens

Abstract

Bacterial pathogens which affect aquatic animals and humans become resistant to antibiotics due to long term exposure. This study focused on the antibacterial efficacy of six species of mangroves (Avicennia marina, Lumnitzera racemosa, Bruguiera sexangula, Acanthus ilicifolius, Rhizophora mucronata, Excoecaria agallocha) against four pathogenic bacteria, Pseudomonas fluorescens, P. aeruginosa, Shigella flexneri and Listeria monocytogenes. Mangrove leaf extracts were obtained using 95% methanol as a solvent and the final compound was methanol free due to evaporation. All the extracts concentrations were made in to 350 mg/ml. Nutrient agar plates were used to culture bacteria and the diameter of inhibition zones were measured by well diffusion method after 24 hours incubation at 25°C. The results were compared with Tetracycline as the positive control and solvent without extract as a negative control. The highest inhibition zone (18.8 ± 1.04 mm) was observed in R. mucronata leaf extract against L. Monocytogenes while Tetracycline showed 22 mm of inhibition. All the other mangrove extracts were effective against all bacterial pathogens tested at a diameter of 17.4–12.5 mm while Tetracycline exhibited approximately 22 mm of inhibitions for all bacterial pathogens except for S. flexneri. S. flexneri was resistant against Tetracycline and moderately susceptible for all typed of mangrove extracts (7.4–12.3 mm). The tested mangrove leaf extracts were effective against aquatic bacterial pathogens used (p<0.5) in vitro and has a potential to develop as an environmentally friendly antibiotic.