Physiological State of Vibrio anguillarum, a Fish Pathogen, under Starved and Low-Osmotic Environments.

Abstract

Vibriosis caused by Vibrio anguillarum serotype J-O-1 seriously affects the freshwater fish ayu (Plecoglossus altivelis) in Lake Biwa, Japan. Survival patterns of V. anguillarum were investigated in aged lake water (ALW) supplemented with or without 0.75% NaCl. It was found that 0.1-1.0% of V. anguillarum cells maintained the ability to form colonies even after 6 weeks in 0.75% NaCl-ALW. Under the same starved conditions, MPN counts with liquid medium were 100 times higher than CFU counts. When exposed to sterilized aged lake water without NaCl, V. anguillarum entered a non-culturable state within half a day. As the mineralization activity of non-culturable cells was still 0.1-5.4% of that in the culturable phase, this physiological state can be described as viable but non-culturable. However, all attempts to return to a culturable state including re-infection were un-successful. The non-culturable cells in ALW lost all pathogenicity in fish. Since a longer exposure to ALW resulted in less mineralization activity, the non-culturable state of V. anguillarum cells in freshwater seems to reflect a phase of decay leading to cell death. However, certain environmental factors such as coldness and microaerobiosis seem to help the pathogen to survive longer in freshwater without a loss of culturability.