Bacterial single cell protein from methanotrophic bacteria as an alternative ingredient for gilthead seabream (Sparus aurata) aquafeeds: Effects on digestibility, growth performance, gut microbiota, and disease resistance

Bacterial single cell protein (BSCP) from methanotrophic bacteria was evaluated in gilthead sea bream ( Sparus aurata ) as an alternative protein source to fish meal (FM). For this purpose, five isoproteic (45% crude protein, CP), isolipidic (17% crude lipid; CL), and isoenergetic (17.1 MJ kg −1 ) diets containing graded levels of BSCP (0, 44, 85, 126 and 168 g kg −1 ) were tested in quadruplicate for 96 days. The replacement of FM by BSCP ranged from 0 to 100%. From a nutritional point of view, diets containing graded levels of BSCP (44, 85 and 126 g kg −1 ) exhibited similar protein digestibility values (ADC CP = 81.3% to 83%) comparable to FM (ADC CP = 80.7%), whereas full FM replacement by BSCP (168 g kg −1 ) showed a tendency in reduction of protein digestibility (ADC CP = 76.0%; P = 0.064). There were no differences in ADC values for dietary crude lipids and dry matter when gilthead seabream were fed BSCP-containing diets (ADC CL = 85.9–90.3%, ADC DM = 66.1–70.2%) with regard to the CTRL group (ADC CL = 89.0%, ADC DM = 64.2%). Essential and non-essential AA digestibility values remained stable across different BSCP inclusion levels ( P > 0.05). Results in terms of key performance indicators associated to growth performance and feed efficiency indicated that the optimal replacement threshold of FM replacement by BSCP was at 50% (85 g kg −1 ), which maintained growth performance, preserved physiological homeostasis, enhanced gut microbiota composition, and sustained disease resistance. In contrast, higher replacement levels of 75% (126 g kg −1 BSCP) and 100% (168 g kg −1 BSCP) resulted in a reduction in body weight of 9.1% and 20.1%, respectively ( P < 0.05). These reduction in growth performance was attributed to decreased feed intake ( P < 0.05), associated with palatability concerns at high inclusion levels, and to suboptimal dietary levels of essential fatty acids, particularly DHA and n-3 PUFAs in the diet with full FM replacement. The incorporation of BSCP modulated the intestinal microbiota of gilthead seabream ( P < 0.05); notable changes included increased phylogenetic diversity, substantial enrichment of beneficial taxa such as Candidatus Arthromitus, and reduced relative abundance of potentially pathogenic genera like Photobacterium . These microbiological changes suggested enhanced gut health and improved competitive exclusion of opportunistic pathogens. Disease resistance evaluation revealed that BSCP inclusion up to 126 g kg −1 did not compromise gilthead survival following a bacterial challenge with Vibrio harveyi . The full replacement of FM by BSCP (168 g kg −1 ) resulted in reduced disease resistance ( P < 0.05), which may be attributable to deficiencies in dietary essential fatty acids. This holistic evaluation integrating diet digestibility, growth performance, feed utilization, physiological responses, gut microbiota modulation, and disease resistance demonstrated that BSCP from methanotrophic bacteria represents a viable alternative protein source for gilthead seabream feeds.