| Summary: | Results from the present study found for the first time that the use of bacterial strains of Lactobacillus acidophilus, Lactobacillus casei, Bifidobacterium thermophilum, and Enterococcus faecium from the commercial product PrimaLac® had the potential to act as a possible probiotic for juvenile red drum. The addition of PrimaLac® probiotics [whether as a water-soluble probiotic (WSP) or in a probiotic enhanced starter feed (PESF)] reduced mortality (%M), enhanced growth rates (MW, LT, SGR, and DGR), improved feed conversion efficiencies (FCEs), and physical condition factors in the juvenile red drum. Improvement was quantified using external morphological condition indices (MCIs) and blood physiological condition indices (PCIs). Results showed a strong positive relationship between MCIs and PCIs, which suggested that probiotics treated fish were in better health (lower MCIs) with lower fasting blood glucose and lactate levels than control fish. Addition of probiotics also resulted in improved water quality (lower nitrate, nitrite, and ammonia) in the treatment tanks compared to controls. Application of PrimaLac® probiotics on the morphology and histology of three different regions of the intestine (proximal, mid-and distal) improved intestinal length (Li), mass (Mi), and digesta mass (Md). In 5μm histological sections examined for differences among treatments (probiotics vs. controls), five variables within each intestinal region were also quantified: (i) perimeter ratio (PR) between the internal perimeter (IP) of the intestinal lumen and the external perimeter (EP) of the intestine where, PR = IP/EP; (ii) width of lamina propria (wLP); (iii) thickness of the muscle layer (tM); (iv) the number of goblet cells per segments (GC); and (v) height of enterocytes (hE). Overall, Li and Mi significantly increased by 13.4 ± 1.2% and 11.8 ± 0.9%, respectively, and Md decreased by 33.2 ± 1.21% compared to controls. Significant increases in microvilli heights, PR (by 21.3 ± 1.2%), tM (by 18.4 ± 0.9%), and increased numbers of GC (by 35.1 ± 2.8%) provided evidence that bacteria species from PrimaLac® (whether in WSP or PESF) affected the intestinal histomorphology of juvenile red drum. Collectively, these changes were partially responsible for increased feed assimilation (supported by reduced digesta) and improved overall juvenile red drum growth and condition. These results will add significantly to the limited database on how probiotics, as possible standard practice at the fish farm, may offer significant benefits for fish production and fish health. Addition of probiotics will also be of benefit to most aquaculture species and not only for red drum, but also for other high-value carnivorous species, such as Atlantic salmon, without threat to natural populations or damaging fisheries-based ecosystems that are already on the edge of collapse from over fishing and pollution.
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