Growth inhibition of pathogens by bacteria in corals.

Corals have a symbiotic relationship with zooxanthellae, which live in the tissues of corals. Coral bleaching is the expulsion of zooxanthellae which occurs when they are put under any external environmental physical and/or toxic stresses. Another way by which coral bleaching can occur is from bacterial infections. Nissimov et al. (2009) conducted a study to observe the antimicrobial effects of bacteria in the coral mucus from Oculina patagonica, on various pathogens of this coral. The pathogens observed in this study were Vibrio shiloi (which is no longer a pathogen to this coral), Vibrio coralliilyticus, Thallassomonas loyana, and 9 others.

 In terms of the V. Shiloi coral pathogen, it is temperature stimulated, at temperatures of 25°C and over. It is attracted to the coral mucus layer, which is supports a diverse range of micro organisms such as bacteria, archaea, fungi and viruses. The pathogen adheres to a β-galactoside receptor which is on the surface of the coral. However, this only occurs when the temperatures are over 25°C, and only on corals that have photosynthetically active zooxanthellae. When the V. Shiloi has successfully entered the coral tissue, it multiplies and produces toxins which block photosynthesis, and bleach and lyse the zooxanthellae.

 The Coral Probiotic Hypothesis proposes that a corals' associated microbial community changes with the surrounding conditions, which allows the coral to be tolerant of these changes. This would then apply to seasonal alterations in microbial community to withstand the changes in temperature, for example. This hypothesis could explain the occurrence of V. Shiloi adhering to coral tissues, and then the population declining, and eventually disappearing.

 Two healthy fragments of O. patagonica were collected, and the bacteria from the mucus was extracted, isolated and identified. From the 156 colonies which were tested, nine were shown to inhibit the growth of the coral pathogen V. Shiloi (JNM1, 3, 8, 11, 12, 14, 106, 119 & 149). These 9 colonies were then tested against other known coral pathogens. In particular, strain JNM12 showed the highest activity against coral pathogens V. shiloi, V. coralliilyticus, and T. loyana. Strain JNM14 inhibited the growth of all target species, showing a broad range of activity. Strains JNM12 & JNM14, were therefore selected for further studies on growth inhibition in mixed cultures, inhibition by cells and supernatants of mucus bacteria cultures, and interactions between V. shiloi and biofilms of coral mucus isolates.

The inhibition of V. shiloi growth in mixed cultures depended largely on the density of the initial coral isolate cultures. V. shiloi viable counts decreased from between 10²-fold to 10⁵-fold when using supernatants of mucus bacteria cultures (strains JNM12 & JNM14). The V. shiloi viable counts also decreased in the presence of biofilms composed of the coral isolates (strains JNM12 & JNM14).

 The inhibitive properties of the strains extracted from the coral mucus show their importance in regulating a microbial community which remains beneficial to the coral all year round. The different bacteria in the coral may contribute towards various modes of protecting the coral, rather than just one or two contributing to disease immunity.

 Further studies focusing on the other isolates in this study would be beneficial to see how they contribute towards immunity against pathogens in other corals. This could also benefit human health eventually by gaining broader knowledge on pathogens in general, especially in terms of waterborne pathogens which now infect humans, eg Vibrio cholerae.


Joseph Nissimov, Eugene Rosenberg & Colin B. Munn. (2009) Antimicrobial properties of resident coral mucus bacteria of Oculina patagonica. Federation of European Microbiological Societies Microbiology Letters. 292 (2): 210-215.