The effects of Lactobacillus plantarum and oxolinic acid on growth, immune-related genes, and gut bacterial community structures in juvenile Japanese eel (Anguilla Japonica)

Abstract

This research aims to understand the distinct effects of Lactobacillus plantarum and Oxolinic Acid on the growth, immune-related genes, and gut bacterial community structures of juvenile Japanese eel.In this study, the Lactobacillus plantarum strain MH079448 was added to the diet for Japanese eel juveniles as test group (LP), with no bacteria administrated as control group (CK). Meanwhile, the antibiotic Oxolinic Acid immersion treatment was administrated in an extra group as a comparison for alternative evaluation (OA). The mRNA levels of toll-like receptors 2 (TLR-2) and tumor necrosis factor-alpha (TNF-α) in livers and gills of Japanese eel were analyzed by qRT-PCR technology. The intestinal microbiota was analyzed by High-throughput Sequencing (HTS) analysis.The results showed that at the end of experiment, the specific growth rates (SGR) and weight-gain rate (WGR) of Japanese eels in both LP and OA were improved to an approximately equal level, viz., 38.38 ± 15.2%, 1.18 ± 0.39%/d and 43.67 ± 16.52%, 1.29 ± 0.39%/d, respectively, which were significantly higher than the CK (8.73 ± 4.81%, 0.3 ± 0.16%/d) (p < 0.05). At the beginning of the experiment, there were no significant differences in the expression levels of TLR-2 and TNF-α in the gill and liver (p > 0.05). However, on day 28, compared with CK, both LP and OA showed significant upregulation in the expression levels of TLR-2 and TNF-α in the gill. Specifically, there was no significant difference in the expression levels of TLR-2 between LP and OA in the gill (5.97 ± 0.33 and 5.73 ± 0.38 respectively) (p > 0.05), while TNF-α exhibited significant differences in the gill (3.40 ± 0.24 and 4.96 ± 0.17 respectively) (p < 0.05). In the liver, on day 28, LP showed significant upregulation in the expression levels of TLR-2 and TNF-α (0.079 ± 0.006 and 0.116 ± 0.018, respectively) (p < 0.05). Concerning the TLR-2 in the liver of OA, it showed significant upregulation to 0.132 ± 0.009 (p < 0.05), which was significantly higher than that of LP (0.079 ± 0.006) in the liver(p < 0.05). However, on day 28, compared with CK and LP (0.049 ± 0.04 and 0.116 ± 0.018, respectively), the expression level of TNF-α in OA of liver was significantly downregulated (0.039 ± 0.002) (p < 0.05).Further, at the end of the experiment, Gammaproteobacteria was still the dominant class in CK and OA with the relative abundance of 95.09% and 92.49%, correspondingly, whereas in the LP, its value dropped to 82.79%. Compared to CK, the relative abundance of total probiotics in LP were increased by 1.619%, but it decreased by 0.464% in OA. Their total (opportunistic) pathogens in LP and OA were decreased by 4.562% and 16.803%, respectively. Functionally, compared to CK, the predicted functions of intestinal microbiota, such as energy metabolism, nutrient absorption and secondary metabolites were significantly enhanced in both LP and OA(p < 0.05). The results of this study firstly demonstrated that compared to Oxolinic Acid, Lactobacillus plantarum MH079448 approximately equally improved the growth performance, and approximately equally enhanced immune response. However, concerning the optimized community structures of intestinal microbiota in juvenile Japanese eels, Lactobacillus plantarum MH079448 was better than Oxolinic Acid, especially in relative abundance of total probiotics improvement.