Abstract
Streptococcosis is one of the most important diseases of a wide range of freshwater and marine fishes. Objectives of the present study were to isolate and identify the pathogens of streptococcosis like infection in silver barb (Barbonymus gonionotus) using phenotypic, molecular, and artificial infection challenge test; investigate the antibiogram profile of the pathogenic isolates; and sequence the whole-genome of a virulent strain. Bacteria were isolated from different organs of diseased fish on Streptococcus selective media. Phenotypic identification of randomly selected isolates was performed through morphological, physiological, and biochemical tests. Molecular identification was done by 16S rRNA gene sequence analyses. Artificial infection challenge test was performed in B. gonionotus through intraperitoneal injection. To investigate the antibiogram profile, 15 commercial antibiotic disks and 24 medicinal plant extracts were screened by disk diffusion-assay. The whole-genome of a selected strain BFPS6 was sequenced by using an Illumina MiSeq platform. In this study, a total of 20 out of 30 randomly selected isolates were phenotypically identified as Enterococcus sp. The 16S rRNA gene sequence homology of 10 randomly selected isolates revealed that these isolates exhibited 99–100% sequence homology with Enterococcus faecalis. The lowest and highest median lethal doses (LD50) of the pathogenic isolates in B. gonionotus were obtained 5.54 × 106 CFU/ml and 8.03 × 108 CFU/ml for the isolates BFPS6 and BFPS13, respectively. The isolates showed resistance to multiple antibiotics but susceptibility to three medicinal plant extracts (Spondias mombin, Allium sativum, and Syzygium aromaticum). The genome size and average G+C content of the strain BFPS6 were 2866,855 bp and 37.5%, respectively. The genome contains 2743 CDS, 55 tRNA, 5 rRNA, 9 repeat regions, and one CRISPR sequence. This is the first report on identification and whole-genome sequence of E. faecalis isolated from B. gonionotus suffering from streptococcosis like infection.
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