This study was carried out to evaluate the potential antibacterial and immunomodulatory effects of the dietary acidifier propionic acid (PA) when given alone or in combination with oxytetracycline (OTC) on Nile tilapia (Oreochromis niloticus). Apparently healthy O. niloticus (n = 240; 52 ± 3.75 g) were randomly allocated into four equal groups (n = 60/group): control group fed a basal diet alone and the other three groups fed basal diets supplemented with either PA (200 mg /kg of diet, PA group) or OTC (500 mg/kg of diet, OTC group) alone or in combination (PA + OTC group). Each group was subdivided into two subgroups (n = 30/subgroup, each subgroup had triplicate of 10 fish); subgroup (A) was used to evaluate the antibacterial effects with the aforementioned 2 weeks feeding regime, and subgroup (B) was used to evaluate the immunomodulatory effects against Aeromonas hydrophila infection with similar 2 weeks feeding regime. Among the four groups, PA + OTC group showed the highest significant (p < 0.0001) antibacterial activity as indicated by widest inhibition zones against A. hydrophila and lowest total gastrointestinal bacterial counts. Additionally, this group had the best immunomodulatory effect as noticed by a significant (p < 0.05) increase in total serum protein, globulin, IgM, phagocytic activity and index, lysosome activity, and significant (p < 0.05) upregulation in the expression levels of immunity-related genes (MHC I, MHC IIA, MHC IIB, Tlr7, IgM heavy chain, TNFα, and IL1β) in head-kidney. Notably, the combined dietary PA and OTC improved the hematological parameters and reduced the oxidative damage of hepatopancreas and head-kidney induced by OTC. This data suggests dietary PA as potential adjuvant to OTC in O. niloticus diets to get maximal antibacterial and immunomodulatory effects.
Keywords: Immunity; Oreochromis niloticus; Oxytetracycline; Propionic acid.