Single-strain probiotics enhance growth, anti-pathogen immunity, and resistance to Nocardia seriolae in grey mullet (Mugil cephalus) via gut microbiota modulation.

Grey mullet (Mugil cephalus) aquaculture is economically vital due to the high value of its roe. However, it faces significant risks from disease outbreaks, particularly from Nocardia seriolae. Current reliance on antibiotics has drawbacks, highlighting the potential of probiotics as a promising alternative. Despite this, no studies have focused on the effects and mechanisms of probiotics in disease prevention and treatment in grey mullet. This study, therefore, investigates the efficacy of probiotics in enhancing disease resistance and promoting growth in grey mullet. Three strains of probiotics, Lacticaseibacillus rhamnosus FS3051, Limosilactobacillus reuteri FS3052, and Bacillus subtilis natto NTU-18, were selected to evaluate their anti-N. seriolae activity and hydrolytic enzyme secretion in vitro. Then, 144 grey mullet were randomly divided into four groups: control, L. rhamnosus FS3051, L. reuteri FS3052, and B. subtilis natto NTU-18. After being fed the corresponding diet for 28 days, fish were measured for immune gene expression and short-term growth followed by challenge of N. seriolae. Survival rates were recorded for 35 days post challenge. Additionally, the gut microbiota of the control and probiotic groups with effects on both growth and protection against N. seriolae were analyzed to investigate the potential role of gut microbiota. Results demonstrated that L. rhamnosus FS3051 and L. reuteri FS3052 inhibited N. seriolae, while B. subtilis natto NTU-18 did not inhibited N. seriolae. Probiotics also had the ability to secrete hydrolytic enzymes. Probiotic-fed grey mullet showed significant improvements in weight gain ratio, feed efficiency, and specific growth rate, particularly in the B. subtilis natto NTU-18 group. Immune gene expression was enhanced by probiotics, especially L. rhamnosus, FS3051, which induced IL-8, IL-1β, TNF-α, IFN-γ, and MHCI. Survival rates post-N. seriolae challenge improved significantly for L. rhamnosus FS3051-fed fish. L. rhamnosus FS3051 also altered the gut microbiota, enriching beneficial genera like Lactobacillus, which correlated positively with immune responses and growth, while reducing Mycoplasma and Rhodobacter, which were negatively correlated with immune responses. This study underscores the potential of probiotics in enhancing disease resistance and growth via regulating gut microbiota in grey mullet.