Grey mullet (Mugil cephalus), a high value aquacultural species in Taiwan, is frequently affected by Nocardia seriolae resulting serious economic lost. Nocardiosis is characterized by chronic infections with subtle clinical manifestations, rendering prevention and management challenging. Because probiotic intervention has been reported as an eco-friendly way to prevent disease, the present study aimed to enhance the health status of grey mullet and fortify their resistance against N. seriolae through probiotic intervention.
In this study, potential probiotics were selected with criteria including resilience within the gastrointestinal milieu, biological safety, and the capacity to produce digestive enzymes and exhibit antibacterial properties. Then, the selected probiotics were mixed with feed and given to the grey mullet twice a day. After 28 days, the expression of immune gene in spleen was determined using RT-qPCR. Moreover, 15 fish of each group were challenged with Nocardia seriolae, and survival rate was recorded for 35 days.
The results showed that Lactobacillus rhamnosus, L. reuteri, and Bacillus subtilis natto were identified as suitable probiotics. The groups receiving probiotic supplementation exhibited superior growth performance compared to the control group. Remarkable enhancements were particularly evident in the groups supplemented with L. reuteri and B. subtilis natto. Immune gene regulation analyses indicated significant upregulation of pro-inflammatory factors, namely IL-1β and TNF-α, in the groups receiving L. rhamnosus and L. reuteri. Additionally, these probiotics induced anti-Nocardia immune responses (Fig. 1) and elevated survival rates post-infection (Fig. 2). In conclusion, the probiotic strains promise the protection against N. seriolae infections in grey mullet aquaculture