Isolation, Identification and Biological Characteristics Analysis of Enterococcus faecalis from the Intestinal Tract of Trachinotus ovatus

【Objective】In recent years, probiotics have been widely used in aquaculture, but the probiotics derived from Trachinotus ovatus been rarely reported. In the study, a strain was isolated from the intestine of Trachinotus ovatus and its biological characteristics were analyzed, which provided a theoretical basis its application in T. ovatus culture.【Method】The bacteria were isolated and identified through the sterile collection of intestinal contents from the healthy T. ovatus. And the morphological observation, gram staining, biochemical identification, 16S rRNA gene sequence analysis, drug sensitivity test, salt tolerance test, acid production ability, growth curve determination, and virulence gene identification of the typical bacterial colonies were performed.【Result】Based on the morphological characteristics, gram staining results biochemical characteristics, and 16S rRNA gene sequence alignment of the isolated bacterium, the results showed that it shared 100% similarities with Enterococcus faecalis, therefore, the isolated bacterium was identified as E. faecalis. The colony of the isolated strain was light grayish white, and Gram-positive. The biochemical indicators including esculin, cellobios, maltose, mannose and salicylic acid were tested positive. The results of the drug sensitivity test showed that the isolated bacterium exhibited varying degrees of resistance to drugs such as piperacillin, polymyxin B, penicillin, trimethoprim-sulfamethoxazole and oxacillin, exhibited intermediate sensitivity to ciprofloxacin, medemycin and ceftriaxone and sensitive to 11 drugs such as vancomycin, cefoperazone, ofloxacin and etc. The results of the salt tolerance test showed that the isolated strain could grow at 0%-8% NaCl concentrations, showing salt tolerance to some extent. The results of acid production capacity and growth curve measurements indicated that the pH of the culture media rapidly decreased and the acid production capacity was enhanced as the proliferation of the strain. The identification results of virulence genes showed that the isolated strains possessed four virulence genes including efaA, agg, as and fsrA.【Conclusion】A potential probiotic strain was successfully isolated from the intestine of T. ovatus and identified as E. faecalis. The isolated strain has good salt tolerance and acid production capability, and it contains functional genes associated with intestinal adhesion, which will provide a theoretical basis for further application of the strain.