STARVATION-SURVIVAL PROCESSES OF THE BACTERIAL FISH PATHOGEN YERSINIA-RUCKERI

The fish pathogen Yersinia ruckeri survived for more than three months in the environments studied (river, lake and estuary). The three strains showed similar survival dynamics, regardless of their origin or serotype. The number of culturable cells increased in the first 15 days from 1 (water microcosms) to 3-log-unit (sediment microcosms), and then declined during a 100-day period. Persistence of culturable cells was greater in sediments than in waters, as well as at 6 degrees C than at 18 degrees C. Therefore, while a situation of long term survival could be stated in all sediments at both temperatures and in river water at 6 degrees C, in estuary and lake waters situations of non-culturability were observed. In addition, measurement of the cellular metabolic activity showed decreases in the respiratory rates to 60-70% of the original values in the cases of sediments and river water, and to 10-15% when the cells became non-culturable. However, in all the microcosms, the acridine orange direct counts (AODC) remained nearly constant during the experimental period at values of about 10(6) cell/ml in water and 10(8) cell/ml in sediments. These findings demonstrated that Y. ruckeri may undergo a dormant state under certain starvation conditions. Non-culturable cells showed marked changes in morphology and size. Slight changes in LPS patterns of dormant cells were also detected, but not in membrane proteins or plasmid profiles. Moreover, maintenance of virulence during the non-culturability state was demonstrated. Dormant cells were easily resuscitated by addition of fresh medium to the microcosms, showing the resuscitated cells levels of metabolic activity and plate counts similar to those seen prior the start of the experiment.