Sodium propionate as a dietary acidifier for European seabass (Dicentrarchus labrax) fry: immune competence, gut microbiome, and intestinal histology benefits

This study was undertaken to assess the effects of dietary sodium propionate (SP) supplementation on the growth, haemato-immunological parameters, gut microbiota, and intestinal histology of European seabass (Dicentrarchus labrax) fry. In total, four diets were evaluated during a 90-day growth trial: a control (CTRL) diet without SP supplementation and three diets including 0.1%, 0.2%, or 0.3% SP, which were called SP1, SP2, and SP3, respectively. Diets were assigned to seabass (initial body weight: 0.45 ± 0.23 g), and the results revealed a 34%, 19%, and 15.6% increase in weight gain in SP3-, SP2-, and SP1-fed fish, respectively, concomitant with a lower feed conversion ratio in these fish than in fish not receiving the supplement (CTRL), which exhibited the lowest performance among all fish groups. Administration of 0.3% or 0.2% SP elevated the carcass protein content, major blood constituents, and immune function indicators compared to the CTRL. In addition, dietary inclusion of SP positively modified the bacterial community composition in the distal intestine (DI) of fish, causing an approximately 95% and 87% reduction in total microbial count in SP3- and SP2-fed fish, respectively, compared to CTRL fish. Vibrio spp. were non-existent (zero or negligible) in fish fed the SP2 and SP3 diets, and the number of faecal coliform bacteria gradually decreased to zero in SP3-fed fish. Additionally, an improvement in fish intestinal features, particularly in the DI, was evident from measurements of histological sections from the proximal, mid, and distal intestine. The greater villus area (height and width) and goblet cell count in fish fed the SP3 or SP2 diet than in fish not receiving the supplement suggested stimulation of digestive secretions with enhanced mucus production. In conclusion, enrichment of the diet with either 0.3% or 0.2% SP is capable of improving seabass fry growth performance, general health status, immune response efficiency, and intestinal function.