Vibrio parahaemolyticus is an important etiological agent of gastroenteritis associated with seafood consumption in Japan and many parts of the world. Thermostable direct hemolysin (TDH) and TDH-related hemolysin (TRH), encoded by the tdh and trh genes respectively, are the most recognized pathogenic factors of this bacterium. During this study a high percentage of V. parahaemolyticus isolated from an area was found to be positive for the tdh and/or trh genes by PCR, although almost all of the isolates lacked the ability to produce active hemolysin. Such tdh and/or trh positive strains were isolated from a specific coastal area, but not isolated from four other areas of the Seto-Inland Sea. This particular area, Kojima Bay, receives freshwater from several adjacent rivers, and such influx of water may have special effects on the growth V. parahaemolyticus, as evidenced by the high density of this bacterium in water samples (2300 to 4600 per 100 mi). V. parahaemolyticus of various sero-groups were isolated from each area
however, Kojima Bay samples were dominated by O1, O3, and O4 sero-groups.