Ecology of Vibrio vulnificus and Vibrio parahaemolyticus in brackish environments of the Sada River in Shimane Prefecture, Japan

While there are several studies on the ecology of Vibrio vulnificus and Vibrio parahaemolyticus in estuarine water environments around the world, there is little information on the distribution of both organisms during the cold-weather months. Thus, we conducted a multi-year study on the ecology of both organisms in brackish environments of the Sada River, a drainage canal flowing slowly into the Japan Sea from Lake Shinji in Shimane Prefecture, Japan. Water samples were collected twice a month at five sites from August 2000 to May 2002. Both organisms were enumerated in 10 l water, 100 g sediment and 10 g shellfish by the most probable number (MPN) procedure. Isolates were confirmed as V. vulnificus using hemolysin gene PCR. During the last 7 months (including winter) of the study, water and sediment samples were also analyzed for the presence of both organisms. V. parahaemolyticus was isolated from river mouths and coastal environments of average salinity ≧4.4±2.0 ppt throughout the year at cell concentrations of 10−3 to 101 MPN ml−1. Similar concentrations of V. vulnificus were isolated from coastal environments of average salinity 24.0±5.4 ppt, except for two times when water moved to the upper reaches due to high tide and V. vulnificus was rifted to the upper reaches. These findings suggest that both organisms are continuously distributed in the Sada estuary throughout the year regardless of water temperature.