Long-term Starvation Survival of Yersinia Ruckeri at Different Salinities Studied by Microscopical and Flow Cytometric Methods

Cultures of three strains of the fish pathogenic bacterium Yersinia ruckeri survived starvation in unsupplemented water for at least 4 months. At salinities of 0 to 20/1000 there were no detectable changes in CFU during the first 3 days of starvation and only a small decrease during the following 4 months, whereas at 35/1000 salinity, the survival potential of the cultures was markedly reduced. These results suggest that Y. ruckeri may survive for long periods in freshwater and brackish environments after an outbreak of enteric redmouth disease. Survival was also examined by use of the direct viable count method, and we show that this method can be combined with flow cytometry for automatic counting of viable bacteria. By flow cytometry, it was shown that genome replication initiated before the onset of starvation was completed, during the initial phase of starvation, and that starved cells could contain up to six genomes per cell.