PLOS Pathogens: Holobiont–Holobiont Interactions: Redefining Host–Parasite Interactions

The term holobiont (Greek, from holos, whole; bios, life; -ont, to be; whole unit of life) describes a long-term physical association between different living organisms . Theoretically, this definition encompasses all symbiotic associations (along the mutualism–parasitism continuum) spanning all taxa. However, in most cases, the term holobiont is restricted to the host and its associated mutualistic symbionts. The hologenome theory of evolution considers that the holobiont is the unit under natural selection in evolution . I argue that this opens new perspectives on the study of host–parasite interactions. Evidence suggests that all of the diverse microorganisms associated with the host and parasite play a part in the coevolution. This new paradigm has the potential to impact our comprehension of the development and evolution of disease.
It has been established in different model species that immune system maturation requires the presence of mutualistic bacteria. The tsetse fly Glossina moritans carries an obligate mutualist, the bacteria Wigglesworthia glossinidia, which is necessary for maturation of the immune system during development . In vertebrates, species-specific gut bacteria are necessary for the maturation and the maintenance of a healthy immune system . Organisms are associated with a great variety of microorganisms, including viruses and unicellular eukaryotes, and we are starting to realize that they also play an important role in shaping a healthy immune system .

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