Many probiotics used in aquaculture are derived from terrestrial sources, presumably having limited ability to interact with existing microbial communities within the gut environments of aquatic animals. Host-associated probiotics (HAPs) are autochthonous bacteria inhabiting the mucosal layer of host animals, which produce beneficial substances such as digestive enzymes and physiologically active compounds. Synbiotics, the mixture of probiotics and prebiotics, have been applied in aquaculture due to its better effects than the use of probiotics. Therefore, the current study was conducted to evaluate effects of synbiotics, consisting of a single or mixture of HAPs isolated from olive flounder and fructo-oligosaccharides (FOS) as a prebiotic on growth performance, immunity, and temperature stress responses in juvenile olive flounder.
Four hundred eighty-six juveniles with an initial body weight of 7.26 ± 0.04 g (mean ± SEM) were randomly distributed into 27 rectangular tanks (18 fish per tank; N = 3 tanks per treatment). Three HAPs, including Bacillus sonorensis, Bacillus subtilis, and Bacillus velezensis and one prebiotic, FOS were supplemented in a basal diet to prepare for nine experimental diets as follows: no addition of probiotic and prebiotic as a control diet; prebiotic only; three different synbiotics with single addition of each probiotics with FOS; four different synbiotics with mixed addition of two or three probiotics with FOS. The concentration of each probiotic and FOS was 1 × 107 CFU/g and 5 g/kg basal diet, respectively. Following the 8-week growth trial, the juveniles were subjected to two temperature stress exposures: 1) lethal exposure: gradual increase in water temperature (0.5 °C increment every 30 m) until reaching 30.5 °C and 2) acute exposure: 2-h heat shock at 30 °C followed by 2-h recovery at the ambient water temperature (19.5 °C).
Results of the growth trial showed that there was no significant difference in growth performance, including weight gain, feed efficiency, and survival rate of the juvenile fed the experimental diets (P > 0.05). In addition, no significant difference in morphological changes such as condition factor, hepatosomatic index and viscerosomatic index as well as in plasma metabolites, including glutamic oxaloacetic transaminase, glutamic pyruvic transaminase, glucose, total protein, triglyceride, and total cholesterol was observed among the experimental diets. Indices of immunity such as immunoglobulin M, lysozyme, and antioxidant enzyme activity levels in plasma were not affected by the experimental diets. These results indicated that there was no adverse effect of the synbiotics addition in the diets of the juvenile olive flounder.
The lethal exposure test result demonstrated that the relatively higher survival rate (16.4%) of the juveniles fed the synbiotics diets (FOS+B. sonorensis+B. velezensis, FOS+ B. sonorensis+B. subtilis+B. velezensis) than that (4.2%) of those fed the other diets although no significant difference was detected among the diets. Expression levels of genes involved in temperature stress responses of the juveniles responding to the acute exposure will be discussed later.