β-glucan alleviates the immunosuppressive effects of oxytetracycline on the non-specific immune responses and resistance against Vibrio alginolyticus infection in Epinephelus fuscoguttatus x Epinephelus lanceolatus hybrids

This study was conducted to examine the combinatory effects of beta-glucan and oxytetracycline (OTC) on hybrid giant tiger groupers (Epinephelus fuscoguttatus x Epinephelus lanceolatus). In vitro tests, OTC significantly reduced superoxide anion production and phagocytic activity in primary head kidney leukocytes. However, this suppressive effect was alleviated by co-treatment with beta-glucan. Subsequently, feeding trials were performed to investigate the potential immunomodulatory effects of dietary beta-glucan alone or in combination with OTC on groupers. A total of 210 healthy groupers (368.00 +/- 51.03 g) were divided into six groups. Group 1 was the control group, group 2 (BG) received 5 g beta-glucan per kg feed weight, groups 3-5 received 5 g/kg beta-glucan in combination with 10, 30, or 50 mg OTC/kg fish weight/day (groups M1, M2, and M3, respectively), and group 6 (O) received 50 mg OTC/kg fish weight/day. Fish were sampled to determine the innate immunity parameters and residual OTC levels in the muscle tissue during a 28-day feeding regimen. Residual OTC levels were considerably higher in groups M3 and O compared with the other groups, and peaked on day 14. This was followed by a slight decrease on day 28, despite a continuous supply of OTC. Notably, fish fed with OTC alone had significantly decreased phagocytic rates and superoxide anion production observed in head kidney leukocytes, as well as poorer protection against Vibrio alginolyticus infection. These immunosuppressive effects were not observed in the fish fed with beta-glucan in combination with a lower dose of OTC (group M2). Thus, these data suggest that the combination of dietary beta-glucan and OTC exerts synergistic immunostimulating effects that protect groupers from bacterial infection.