The Influence of Commercial Feed Supplemented with Carnobacterium maltaromaticum Environmental Probiotic Bacteria on the Rearing Parameters and Microbial Safety of Juvenile Rainbow Trout

SIMPLE SUMMARY: Rainbow trout meat is characterized by a high nutritional value and microbial safety. The growing demand for trout meat has promoted the rapid development of modern aquaculture methods. Fish feeds are supplemented with various strains of probiotic bacteria to conform to the modern production requirements in aquaculture. Probiotic bacteria, including Carnobacterium maltaromaticum, are defined as live microorganisms that confer health benefits. The beneficial influence exerted by probiotics on fish growth and welfare is determined by the microbial species, its metabolic activity, and origin. However, commercial cultures of probiotic bacteria generally do not deliver the anticipated effects on the farming of fish and other aquatic organisms. In this study, fish feed was supplemented with a probiotic strain of C. maltaromaticum that naturally colonizes cold water in deep lake strata. Juvenile rainbow trout were fed commercial feed supplemented with the analyzed bacterial isolate. Feed supplementation significantly increased the fish biomass, improved the apparent digestibility of feed and nutrients, and contributed to a several–fold decrease in the counts of potentially pathogenic bacteria in the feed, digestive tract contents, and the skin of fish. The results of this study indicate that the C. maltaromaticum environmental strain is a promising probiotic for rearing juvenile rainbow trout in aquaculture. ABSTRACT: The aim of this study was to determine the effect of commercial feed (CF) supplemented with 0.1% of the Carnobacterium maltaromaticum environmental probiotic strain on the rearing parameters, apparent nutrient digestibility, and microbial safety of juvenile rainbow trout (Oncorhynchus mykiss). The fish were fed CF (control group, CG) and experimental feed (EF) supplemented with 0.1% of C. maltaromaticum (experimental group, EG) for 56 days. The final body weight and total body length of the fish were measured. The growth rate, condition factor, feed conversion ratio, viscerosomatic and hepatosomatic indices, and apparent digestibility coefficients of protein (PAD), lipids (LAD), ash (AAD), and nitrogen-free extract (NFEAD) were calculated. The total viable counts of C. maltaromaticum bacteria, mesophilic bacteria, hemolytic mesophilic bacteria, Pseudomonas fluorescens, Aeromonas hydrophila, Staphylococcus sp., and sulfite-reducing anaerobic spore-forming Clostridium sp. were determined in digestive tract contents and the skin of fish. Feed supplementation with C. maltaromaticum significantly affected most rearing parameters, as well as the PAD, LAD, AAD and NFE values, and bacterial counts. The principal component analysis (PCA) revealed significant positive correlations (p < 0.05) between fish growth rates, PAD and LAD values vs. C. maltaromaticum counts in the EF and in the digestive tract contents of the fish.