Phenotypic and genotypic characterization of Streptococcus agalactiae isolated from hybrid tilapia (Oreochromis niloticus × O. aureus)

Abstract

Outbreaks of streptococcal infection in cultured hybrid tilapia in Saudi Arabia occurred annually since 1992, causing significant economic losses in the aquaculture industry. Streptococcus agalactiae (Group B Streptococcus, GBS), the etiological agent of streptococcosis in fish, is an important bacterial fish pathogen causing significant morbidity and mortality of cultured and wild fish worldwide. Outbreaks of S. agalactiae in cultured tilapia occurred each year from 2000 through 2015, and mortality rate ranged from 40–80% during the summer months, when water temperatures above 28°C, in combination with high fish stocking density and poor water quality. The most frequent clinical signs of affected fish are anorexia, swimming abnormalities, curved body shape, melanosis, exophthalmia, corneal opacity and petechial hemorrhage in the skin. The phenotypic, biochemical, genotypic and antibiotic susceptibility among S. agalactiae isolates recovered consistently from brain, kidney, liver and spleen of diseased hybrid tilapia (Oreochromis niloticus×Oreochromis aureus) was characterized. Isolates were Gram-positive cocci, catalase-negative, oxidase-negative, non-haemolytic and was serotyped to group antigen. Analysis of all the sixteen isolated strains by the Vitek 2 Compact system, characterized the bacteria as 99% similarity to S. agalactiae. Further, phylogenetic analysis of the 16S rDNA gene sequence revealed that the tilapia isolates were showed highly homologous (ranged from 98.2% to 100%) with S. agalactiae strain CIP 82.43 type (Genbank accession number NR 117503). All of the 16 bacterial isolates have exhibited similar phenotypic, biochemical, genotypic and antibiotic sensitivity profile. In addition, the findings in this study also indicate that S. agalactiae is one of the causative agents of disease outbreaks during the period from 2000 to 2015 in hybrid tilapia, which may change the previous understanding. Therefore, the results of the present study indicated and confirmed that both S. iniae and S. agalactiae are become the major bacterial pathogenic species responsible for infection tilapia in Saudi Arabia. Statement of relevanceStreptococcus agalactiae (GBS) have been associated with disease outbreaks among a variety of wild and cultured freshwater and marine fish species worldwide including tilapia. This is the first report describing the phenotypic and molecular characterization of a non-haemolytic group B Streptococcus agalactiae (AH2) isolated from diseased hybrid tilapia (Oreochromis niloticus×Oreochromis aureus) in Saudi Arabia.