Evaluation of the selective and differential ET medium for detection of Edwardsiella tarda in aquaculture systems

AIMS: Edwardsiella tarda is an important pathogen in aquaculture where it can cause serious losses. A rapid detection of it is vital to minimize the mortalities caused by this disease, and in this work, the effectiveness of the selective differential Edw. tarda medium (ET) was evaluated for the diagnosis of edwardsiellosis as well as for its possible use in epidemiological studies. METHODS AND RESULTS: ET medium was evaluated in parallel with the commercial Salmonella-Shigella agar (SS), which is usually employed for the selective isolation of enteric bacilli. Moreover, two general media (TSA-1 and MA) were employed as a control. The results obtained showed that ET is distinctly selective for the isolation of Edw. tarda, allowing its recovery from mixed cultures and natural samples as a unique species. In contrast, although colonies of Edw. tarda could be clearly distinguishable in SS because of the appearance of a characteristic black centre, other enteric and nonenteric bacterial species were also able to grow on this medium. CONCLUSIONS: We recommend ET agar as an useful medium for the primary isolation of Edw. tarda from aquaculture samples. SIGNIFICANCE AND IMPACT OF THE STUDY: The results obtained support ET medium as the most appropriate to develop epidemiological studies of edwardsiellosis in aquaculture and permits an earlier diagnosis of this important disease.