Isolation and characterization ofLactococcus garvieae from diseased rainbow trout (Oncorhynchus mykiss, Walbaum) cultured in Iran

Abstract

A total of 200 moribund rainbow trout with clinical signs of a hyperacute haemorrhagic septicemia were collected from rainbow trout farms in Fars, Kohkiloyeh-Boyer Ahmad and Charmohal-Bakhtiari provinces in the south and southwest of Iran during summer 2002 to winter 2008 for detection of Lactococcus garvieae, the causative agent of lactococcosis. Fish kidney samples were cultured aseptically onto brain heart infusion agar plates and incubated at 25°C for 48 h. Using conventional biochemical tests, L. garvieaewas detected from 32 fish (16% of total fish samples). Additionally, isolates were confirmed as L. garvieaeusing a specific PCR assay based on 16S rDNA gene by producing a single band of 1107 bp. Partial analysis of 16S rDNA revealed 100% sequence similarity for all Iranian isolates and there was a close genetic relationship among these isolates and those previously reported from mullet in Taiwan (AF352166) and yellowtail in Japan (AB267897) based on GenBank data. Results of antibiogram tests on L. garvieae isolates showed a high susceptibility to erythromycin, enrofloxacin, chloramphenicol and clarithromycin. In pathogenicity tests, immersion of fish in a bacterial suspension of 6 × 10 5 colony forming unit/ml of L. garvieaein challenge experiments showed 60% mortality during 14 days post-infection. Experimentally, infected fish showed typical blackening of skin haemorrhages, exophthalmia and wide haemorrhages on viscera. The present study provides useful molecular and biochemical information for L. garvieaeisolates in Iran compared with those from different hosts and geographic locations.