Therapeutic potentials of Excoecaria agallocha against gram-positive and gram-negative fish bacterial pathogens

Background: The present investigation of a mangrove plant, Excoecaria agallocha, which is a popular medicinal substitute for the treatment of microbial ailments, were evaluated for potential antimicrobial activity against pathogenic bacteria Escherichia coli and Streptococcus agalactiae in tilapia, Oreochromis niloticus. Methods: Antibacterial activity was performed using agar diffusion method, disc diffusion method, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and antibiotic susceptibility assays. Experimental fish fed diet containing 0 (control), 5, 25, and 50 mg kg-1 E. agallocha leaf methanol extract for 28 days then challenged individually with E. coli or S. agalactiae and mortalities were recorded over a ten-day post-infection period.  Results: Results indicated that both bacterial species are sensitive to tetracycline, ampicillin, and amoxicillin. E. coli was found to be resistant to neomycin. E. agallocha extract concentration of 50 mg/ml produced a zone of inhibition of 18 mm against E. coli, in contrast to 13 mm against S. agalactiae.  E. agallocha showed bactericidal activity against E. coli and bacteriostatic activity against S. agalactiae. The highest E. agallocha LC50 activity was 83 mg/ml. The highest cumulative mortality was 90.0 ± 10.0% in control as compared to 26.7 ± 11.5% in the group fed with 50 mg kg-1 E. agallocha extract, significant differences (P < 0.05). Conclusion: Hence, E. agallocha showed antibacterial activity against fish pathogens Escherichia coli and Streptococcus agalactiae in tilapia, Oreochromis niloticus; therefore, E. agallocha may be used as an alternative therapeutic agent against fish pathogenic bacteria as an additive to feed at a concentration depend, safe, non-cytotoxic doses