Dormancy as a survival strategy of the fish pathogen Streptococcus parauberis in the marine environment

The fate of Streptococcus parauberis in seawater and sediment microcosms at different temperatures (6 and 22 degrees C) was investigated by comparing the survival dynamics of 2 strains of this bacterial species, isolated respectively from diseased turbot and cattle. The turbot and the bovine isolate showed similar survival kinetics, remaining culturable for approximately 1 mo in water and 6 mo in sediment. A slight influence of temperature on the stability of the cells was observed, in that the number of culturable cells was about 1 log10 unit higher at 6 than at 22 degrees C. During the starvation period, the metabolic activity of the cells, after suffering a strong reduction during the first 12 d, stabilized at levels ranging from 20 to 40% of the initial values. However, in all the microcosms, the acridine orange (AO) and 4',6-diamidino-2-phenilindole (DAPI) counts remained at about 10(5) cells ml(-1) throughout the experimental period, even when cells became undetectable by standard plate count methods. The addition of fresh medium to microcosms containing nonculturable cells induced the return to culturability of S. parauberis strains. On the basis of these results, it seems that S. parauberis has the ability to enter into a viable but nonculturable (VBNC) state. Dormant cells of the turbot isolate maintained their infectivity and pathogenic potential for fish.