Phenotypic and molecular characterization of Photobacterium damselae, a pathogen of the cultured tongue sole Cynoglossus semilaevis in China

Abstract

Significant numbers of half-smooth tongue sole (Cynoglossus semilaevis) raised on a marine fish farm in the P.R. China presented erratic swimming, loss of appetite and increased ventilation rate. Internally, diseased fish exhibited redness of liver, stomach and intestines, and white materials may be observed in the intestine. Mortality rates reached levels of up to 10% of the total fish on the farm per day. The dominant bacterial colonies were isolated from the liver, kidney and spleen of affected fish, and high pathogenicity of the isolates to Cynoglossus semilaevis, Paralichthys olivaceus and Synechogobius hasta was confirmed. The phenotypic characteristics of the isolates were studied extensively, including morphological, physiological and biochemical characteristics. The 16S rRNA and gyrB genes of a representative strain (ST1) were sequenced, and phylogenetic trees representing genetic relatedness between the isolate and publicly available 16S rRNA and gyrB gene sequence from the GenBank database was constructed as an aid to identification. The sequenced 16S rRNA gene (GenBank accession No. GU228793) and the gyrB gene of strain ST1 (GenBank accession No. GU228794) exhibited high similarity (98-99%) to those of Photobacterium damselae from the GenBank database. In addition, the activities of extracellular enzymes and haemolysin were also studied, the results showed that the isolates produced beta-haemolysis on rabbit blood agar, lecithinase, proteinase and DNase but diastase, lipase and gelatinase were not produced. Antibiotic susceptibility tests showed that the isolates exhibited around 80% of susceptibility to 36 various antibiotics tested. © 2011 The Royal Society of New Zealand.