Effective Microorganisms (EM) technology uses microorganisms to work in the surrounding environment. Complex microbiological preparations show synergistic effects, combining the effects of probiotics, prebiotics and synbiotics. EMTM technology is well-known and used in 120 countries worldwide, mainly in agriculture, including aquaculture, and in environmental protection. Fish farmers use these preparations to enhance fish growth, condition and immunity. The aim of the study was to evaluate the effect of EM-Probiotic (Greenland) on the development of cellular non-specific resistance in pikeperch (Sander lucioperca) at the initial stage of rearing in a recirculating aquaculture system (RAS). The experimental diet was administered for 28 days with EM supplementation at 0% (control group), 2% and 4% of the feed. Water temperature in the RAS was maintained at 22.0ºC. Hematopoietic organs (spleen and kidney) were collected twice, after 28 days of EM supplementation (first term) and 28 days after cessation of supplementation (second term). The respiratory burst activity (RBA) and potential killing activity (PKA) of macrophages, as well as the proliferative activity of T- and B-lymphocytes (MTT assay) were assessed. The results of the study show that EM at the initial stage of pikeperch development (body weight of 2 to 14 g) suppress cellular defence mechanisms, decreasing the activity of immunocompetent cells in RBA, PKA and MTT tests. The results from the first organ harvesting showed differentiated EM activity at a statistically insignificant level (P > 0.05). In samples collected 28 days later, the inhibitory effect was demonstrated at a statistically significant level (P < 0.05) in all parameters in both experimental groups. Non-specific cellular immunity in fish plays a key role in defence against damaging factors, including pathogenic ones. Since fish in an RAS are also susceptible to potentially pathogenic agents, immunosuppression of these mechanisms may aggravate the disease.
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