Bacterial pathogens such as Edwardsiella ictaluri, Flavobacterium covae, and virulent Aeromonas hydrophila significantly impact farm-raised catfish in the United States, leading to substantial economic losses. The emergence of antimicrobial resistant strains and the limited availability of licensed vaccines highlights the urgent need to find alternative therapeutics. Garlic (Allium sativum) exhibits strong antibacterial activity against a wide range of Gram-positive and Gram-negative pathogens. This study aims to investigate the in vitro antibacterial activities of Allium sativum and its effects on the cell wall and cell membranes of common fish pathogens. We will also evaluate its potential to protect catfish against enteric septicemia of catfish (ESC) caused by E. ictaluri. The in vitro antibacterial activities of Allium sativum were investigated against E. ictaluri 93-146, A. hydrophila ML09-119, F. covae 94-081, E. ictaluri MS-17-156, E. piscicida MS-18-199, and Plesiomonas shigelloides MS-17-188 using disc diffusion method and broth microdilutions. Secondly, transmission electron microscopy (TEM) was used to examine the effects of Allium sativum on major fish pathogens. Furthermore, the ability of Allium sativum to protect catfish against E. ictaluri 93–146 were assessed using immersion challenge. Four groups were established (minced Allium sativum (30g per kg of diet), minced Allium sativum combined with trans-cinnamaldehyde (TC) (15 ml/kg), and Allium sativum powder combined with TC, and Allium sativum water (30g per kg of diet).
The disc diffusion test results demonstrated that Allium sativum inhibited bacterial growth, with varying degrees (Fig. 1). The broth microdilution tests further confirmed that Allium sativum significantly inhibited bacterial growth at the concentration of 10 mg/ml. TEM of the fish pathogens incubated with Allium sativum revealed notable effects, including cell wall lysis, leakage of cytoplasmic contents, pleomorphism, and rupture of the outer membranes (Fig. 2). A significant reduction in mortality rates was observed in the catfish received minced Allium sativum with TC (31.7%), Allium sativum powder with TC (37.1%), minced Allium sativum alone (37.6%), and Allium sativum water alone (55.5%) compared to the control group, which had a mortality rate of 90%.
Overall, these results indicated that Allium sativum has a strong antibacterial activity against E. ictaluri, F. covae, A. hydrophila, E. piscicida, and P. shigelloides. Using Allium sativum for treatment of ESC, alone and in combination with TC have showed significant decrease in the mortality rates. These findings collectively highlight the potent antibacterial effects of Allium sativum and its potential application in aquaculture for managing fish pathogens.