Effects of salinity and temperature on long-term survival of the eel pathogen Vibrio vulnificus biotype 2 (serovar E)

Vibrio vulnificus biotype 2 (serovar E) is a primary eel pathogen. In this study, we performed long-term survival experiments to investigate whether t he aquatic ecosystem can be a reservoir for this bacterium. We have used mi crocosms containing water of different salinities (ranging from 0.3 to 3.8% ) maintained at three temperatures (12, 25, and 30 degrees C). Temperature and salinity significantly affected long-term survival: (i) the optimal sal inity for survival was 1.5%; (ii) lower salinities reduced survival, althou gh they were nonlethal; and (ii) the optimal temperature for survival was d ependent on the salinity (25 degrees C for microcosms at 0.3 and 0.5% and 1 2 degrees C for microcosms at 1.5 to 3.8%). In the absence of salts, cultur ability dropped to zero in a few days, without evidence of cellular lysis. Under optimal conditions of salinity and temperature, the bacterium was abl e to survive in the free-living form for at least 3 years. The presence of a capsule on the bacterial cell seemed to confer an advantage, since the lo ng-term survival rate of opaque variants was significantly higher than that of translucent ones. Long-term-starved cells maintained their infectivity for eels (as determined by both intraperitoneal and immersion challenges) a nd mice. Examination under the microscope showed that (i) the capsule was m aintained, (ii) the cell size decreased, (iii) the rod shape changed to coc cuslike along the time of starvation, and (iv) membrane vesicles and extrac ellular material were occasionally produced. In conclusion, V. vulnificus b iotype 2 follows a survival strategy similar to that of biotype 1 of this s pecies in response to starvation conditions in water. Moreover, the aquatic ecosystem is one of its reservoirs.