Non-specific immune responses of rainbow trout, Oncorhynchus mykiss, to dietary cottonseed meal and probiotic Saccharomyces cerevisiae (PTCC 5052) inclusion

The aim of this study was to evaluate the effects of adding cottonseed meal and yeast, Saccharomyces cerevisiae (strain PTCC 5052), to diet on humoral and mucosal immunological responses, as well as bactericidal activities in juvenile rainbow trout. Fish (approximately 31 g) were distributed into 12 tanks across three treatments with four replications. Each treatment was fed with either a control diet without cottonseed meal (CTL), a diet containing 150 g/kg cottonseed meal (CSM), or a diet containing 150 g/kg cottonseed meal plus 1 × 108 CFU/g of yeast (CSMY), for 8 weeks. The results indicated no significant effects of dietary cottonseed meal on plasma lysozyme, alternative complement, and bactericidal activity against Yersinia ruckeri and Streptococcus iniae. Dietary cottonseed meal also showed no significant effects on skin mucus lysozyme, immunoglobulin, and bactericidal activity against Aeromonas hydrophila, gut lysozyme, alternative complement activities and immunoglobulin level. However, the CSMY treatment significantly increased the above-mentioned parameters in plasma, skin mucus and gut. Skin mucus bactericidal activity against S. iniae significantly increased in fish fed diets containing cottonseed meal and cottonseed meal plus yeast, compared to the control group. In conclusion, adding 150 g/kg cottonseed meal to the diet has no negative effects on fish immunological parameters. Furthermore, adding 1 × 108 CFU/g of yeast improves various humoral, mucosal, and intestinal immunological parameters, potentially increasing fish resistance against opportunistic pathogens. © 2026 Shanghai Ocean University