Extensive bleaching of the coral Oculina patagonicain the eastern Mediterranean Sea occurs
every summer when the seawater temperature exceeds 25°C. The infection by Vibrio shiloiand subsequent
bleaching occurred only above 25°C because several of the bacterial virulence factors, superoxide dismutase,
adhesion and toxin P, are only expressed above 25°C. Starting in 2002, the corals developed resistance to V.
shiloi. Attempts to explain the resistance led to the Coral Probiotic Hypothesis, which posits that corals can
adapt to their environment, including resistance to pathogens, by changing their symbiotic bacteria. The
hologenome theory considers the holobiont a unit of natural selection. The hologenome is defined as the sum
of the genetic information of the host and its microbiota. The theory is based on four well documented
generalizations: (1) All animals and plants establish symbiotic relationships with microorganisms. (2)
Symbiotic microorganisms are transmitted between generations. (3) The association between host and
symbiont affects the fitness of the holobiont. (4) Under environmental stress, the symbiotic microbial
community can change rapidly. These points taken together suggest that the genetic wealth of diverse
microbial symbionts can play an important role both in adaptation and in evolution of higher organisms