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 autochthonous 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 screened in vitro for haemolytic activity, pathogen antagonism and extracellular enzyme activity. One of the best performing isolates from the in vitro screening, identified as a strain of Bacillus subtilis, was top-dressed onto the basal diet to produce the live probiotic diet (BSpro; equivalent to log 6 CFU/g). Another experimental diet was prepared using heat-inactivated version of B. subtilis (Bspara; equivalent to log 6 CFU/g). Two commercially available inactivated Lactobacillus products (Lactob1 and Lactob2; both equivalent to log 6 CFU/g) were used to prepare two further experimental diets. Sterile PBS was used as the vehicle for top dressing and an equal volume of sterile PBS was top-dressed onto the control diet (CON). A 35-day feeding trial was conducted in the Tropical Unit Aquarium of the University of Plymouth. Using triplicate tanks per treatment, 18 fish per tank (5.59 ± 0.08 g) were distributed randomly into 13 L tanks. Fish were fed at 2-3.5% biomass per day, to assess fish zootechnical performance under suboptimal conditions. At the end of the trial, three fish per tank were humanely euthanised and samples were taken for analysis.
No statistically significant differences of zootechnical performance or intestinal morphometrics were observed between groups (data not shown). However, skin goblet cell coverage and density were significantly higher in carp fed with the paraprobiotics (Table 1). These results indicate a potential for modulation of skin mucus production. Further analyses are being conducted to elucidate the effects of the experimental diets on the molecular microbial ecology, host gene expression and digestive enzyme activity.