Dietary administration of probiotics can modulate the gut microbiota and improve host fish intestinal morphology, digestion, and immune response. Inactivated probiotics, or paraprobiotics, are gaining interest due to their better stability during feed production and longer product shelf life. This study aimed to investigate the effects of potential host-derived probiotics, in live and paraprobiotic forms, on the growth, feed utilisation, and intestinal health of mirror carp.
Bacterial isolates (n = 150) derived from the intestinal mucosa and digesta of carp (n = 5) were tested in vitro for haemolytic activity, pathogen antagonism and extracellular enzyme activity. A basal diet was formulated to meet the known nutritional requirements of mirror carp. One of the best performing candidate probiotic from the in vitro screening, identified as Bacillus subtilis, was top-dressed onto the basal diet to produce the live probiotic diet (BSpro). Another experimental diet was prepared using heat-inactivated version of B. subtilis (BSpara). Two commercially available inactivated Lactobacillus products (Lpara1 and Lpara2) were used to prepare two further experimental diets. The control diet (CON) was top-dressed with sterile PBS. A 35-day feeding trial was conducted in the Tropical Unit Aquarium of the University of Plymouth within a recirculating aquaculture system (RAS). Mirror carp juveniles (5.59 ± 0.08 g) were distributed randomly into 13 L experimental tanks at 18 fish per tank with three replicate tanks per treatment. Fish were fed at a low feeding regime (2-3.5% biomass per day) to ascertain the effect of the experimental diets on fish reared under suboptimal conditions. At the end of the trial, three fish per tank were humanely euthanised and samples were taken for analysis.
No differences on zootechnical performance were observed between groups (Table 1). Further analyses are being conducted to elucidate the effects of the experimental diets on the molecular microbial ecology, phenotyping intestinal morphology, host gene expression and digestive enzyme activity of carp.