Serological and molecular characterization of Streptococcus iniae in cultured Nile tilapia (Oreochromis niloticus) in Thailand

Abstract

Streptococcus iniae infections developed in on-growing tilapia farms located in the northeastern and central regions of Thailand during 2003-2011. In the present study, S. iniae isolates recovered from diseased tilapia were investigated using biochemical, serological and molecular techniques. Biochemical characterization showed different profiles. The arginine dihydrolase (ADH) activity was distinctive between S. iniae, isolates obtained from the outbreaks in central region were ADH-positive (ADH+ve) while northeastern isolates were ADH-negative (ADH-ve). The serological assay demonstrated corresponding results to the ADH activity. Serotype I and serotype II were ascribed to the ADH+ve and ADH-ve isolates, respectively. Despite the serotypic differences, phylogenetic analysis of 16S rRNA and superoxide dismutase A (sodA) genes demonstrated high genetic similarity with ≥96%identity amongst S. iniae isolates in Thailand, and isolates from aquatic animals elsewhere.