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 tissues of all of its host corals, Oculina patagonica, which were bleached. During the winter, when seawater temperatures dropped below 20°C, V. shiloi could not be detected in either healthy or bleached corals, even when the corals were placed in aquaria and the temperature was slowly increased to 28°C. Using specific anti-V. shiloi antibodies to enumerate the bacterium, it was shown that 99 to 99.99% of V. shiloi inside coral tissues were 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 demonstrated that VBNC V. shiloi was infectious, i.e., it adhered to, penetrated into and multiplied inside corals. Laboratory aquaria experiments indicated that when corals were infected with V. shiloi at 28°C and then shifted slowly to their winter in situ temperature (16°C), the bacteria died and lysed. Since the bacteria remain viable under the same temperature shifts outside the coral, either in growth media or seawater, it follows that the coral must have a host-defense mechanism for killing intracellular bacteria. The rapid killing of intracellular V. shiloi at 16°C and 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 terms of the general problem of the coral bleaching disease.