Growth, differentiation and death of Vibrio shiloi in coral tissue as a function of seawater temperature

The coral pathogen Vibrio shiloi was present during the summer in the tissu es of all of its host corals, Oculina patagonica, which were bleached. Duri ng the winter, when seawater temperatures dropped below 20 degreesC, V shil oi could not be detected in either healthy or bleached corals, even when th e corals were placed in aquaria and the temperature was slowly increased to 28 degreesC. Using specific anti-V shiloi antibodies to enumerate the bact erium, it was shown that 99 to 99.99% of V; shiloi inside coral tissues wer e in a viable-but-not-culturable (VBNC) state. All attempts to culture VBNC V.shiloi in liquid or on solid media were unsuccessful. However, it was de monstrated that VBNC V. shiloi was infectious, i.e., it adhered to, penetra ted into and multiplied inside corals. Laboratory aquaria experiments indic ated that when corals were infected with V. shiloi at 28 degreesC and then shifted slowly to their winter in situ temperature (16 degreesC), the bacte ria died and lysed. Since the bacteria remain viable under the same tempera ture shifts outside the coral, either in growth media or seawater, it follo ws that the coral must have a host-defense mechanism for killing intracellu lar bacteria. The rapid killing of intracellular V shiloi at 16 degreesC an d their absence from corals during the winter suggest that bleaching of O. patagonica in the Mediterranean Sea requires a fresh infection each spring, rather than the activation of dormant intracellular bacteria. The lessons learned from the V shiloi/ O. patagonica model systems are discussed in ter ms of the general problem of the coral bleaching disease.