Outbreaks of bacterial-related diseases are one of the main obstacles that impede th e sustainability of t he white-leg Litopeneaus vannmei shrimp culture in many countries. Many studies praised probiotic application in aquaculture owing to their capacity to decrease the density of pathogenic bacteria. P robiotics have mostly been recorded to enhance hematological and immunological parameters. Furthermore, they can sustain barrier function and regulate gene expression pathways. Since aquatic animals are mainly dependent on their innate immune response, this system may contribute resistance against pathogens . The feasibility of continuous and intermittent application of Bacillus clausii on immune, growth, and stress responses in Litopeneaus vannmei has been assessed before and after Vibrio parahaemolyticus (AHPND) bacterial challenge by evaluating changes over time of some immune-related gene expression; prophenoloxidase (ProPO), serine proteinase (SP), transglutaminase (TGase ), lysozyme (LYZ); and superoxide dismutase (SOD).
A total of 315 L. vannamei weighted 0.78 g ± 0.08 (SE) were divided in to three experimental groups; each group consisted of 3 replicates. Frist group (T1) was considered as a control, the second group (T2) fed B. clausii supplemented diet (2×109 CFU/kg feed) daily throughout the experiment. In contrast, the third group (T3) fed B. clausii supplemented diet (2×109 CFU/kg feed) for a week followed by a week of basal diet alternatively.
The findings before the bacterial challenge manifested that proPO of (T2) expressed significantly in 1st, 4th, 7th and 12th weeks, while ProPO of (T3) showed significant expression in 1st, 4th and 12th weeks. In (T2 and T3), SP was expressed highly in the 1st, 2nd and 12th weeks . TGase and LYZ of (T3) presented significant up-regulation in the 1st, 4th, 7th and 12th weeks. TGase of (T2) expressed in 1st, 4th and 7th weeks. LYZ of (T2) expression levels increased significantly in the 2nd, 4th, 7th, and 12th weeks. (T2) SOD was up-regulated in 1stand 4th weeks, while (T3) SOD expression levels significantly increased in 1st and 7th weeks.
Furthermore, unlike the previous study by the researchers on other Bacillus species, there was no significant difference between continuous and intermittent application of Bacillus clausii on immune, growth, and stress responses in Litopeneaus vannmei
The researchers concluded that, a fter induction of Vibrio parahaemolyticus (AHPND) infection, the response of most tested immune genes of two treatment groups with the two application schemes showed no significant differences, suggesting that; unlike others; the continuous use of Bacillus clausii as a probiotic may not exhaust shrimp immune system.