Antibacterial activities of trans-cinnamaldehyde, caprylic acid, and β-resorcylic acid against catfish pathogens

Bacterial pathogens such as Edwardsiella ictaluri, Flavobacterium columnare, and virulent Aeromonas hydrophila have a significant economic impact on farm-raised catfish in the United States. Due to the limited number of licensed vaccines, and regulations with antimicrobials usage, there is need to find alternative therapeutics for sustainable aquaculture. Therefore, the antibacterial activities of trans-cinnamaldehyde (TC), caprylic acid (CA), and beta-resorcylic acid (BA) were assessed against E. ictaluri 93-146, F. columnare 94-081, and A. hydrophila ML09-119 using broth microdilutions, and the capacity of TC to prevent E. ictaluri infection was evaluated during a challenge trial. Results demonstrated that TC inhibited growth of E. ictaluri, F. columnare, and A. hydrophila at 40, 20, and 80 mu g/ml. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of E. ictaluri, F. columnare, and A. hydrophila after incubation with TC revealed cell wall lysis, leakage of cytoplasmic contents, and pleomorphism in the bacterial shape with rupture of outer membranes. By the histological analysis, no abnormal changes were observed in the catfish intestines as a result of supplementation of TC. Moreover, an increasing number of goblet cells was observed in catfish received TC compared to control. In the E. ictaluri challenge trial, significantly higher survival was found in catfish that received TC at the levels of 15 and 20 mg/kg compared to control group (49.12% and 65.52% survival vs 11.11% survival). Bacterial concentrations in spleen and anterior kidney were significantly (P < .05) lower in fish fed with TC (20 mg/kg diet) compared to control at 5-day post-infection. Results indicate that supplementation of catfish feed with TC reduces E. ictaluri infection. A safe and efficacious alternative to antimicrobials will reduce the emergence and spread of antimicrobial resistant strains.