The topic of microbiota is fascinating to many of us, and presents new discoveries on a regular basis. I offer this short summary knowing full well the dated nature of it (5 years old). Hard to keep up!

Until more data are available about the molecular and biochemical similarities and differences between beneficial and pathogenic associations, we cannot make generalizations about the evolutionary origin or differential function of convergent features in the two types of symbioses. Because considerably more data are available on pathogenic associations, many authors have posited that beneficial symbioses co-opted these characters. However, if beneficial associations with microbes are the rule rather than the exception, it may be the pathogens that have co-opted or perverted aspects of the dialogue between hosts and their microbial partners. Thus, at this early stage in our knowledge, we should keep an open mind about these mechanisms of host-symbiont interaction, so that we can allow for the discovery of the true meaning of these convergences. 

• Three types of symbiosis are recognized: mutualistic, commensal, and pathogenic, wherein the fitness (genetic contribution to future generations) of the partners is positively affected, unaffected, or negatively affected, respectively. 

• "Commensalism," a term signifying that (genetic contribution to future generations) of the partners have not been determined, such as the association of mammals with their gut microbiota . 

• Symbioses are characterized by their mode of transmission, the complexity of the stable community, and the types of exchange that occurs between the partners. Symbioses of all types emerged early in the evolution of multicellularity and have had an important influence on the adaptive radiation of animals and plants. 

• Microbial pathogens not only threaten the health of the plant or animal host, but can compromise the integrity of the entire community, which is composed of the host and its co-evolved microbiota. To understand the true nature of pathogenesis, biologists and physicians must first understand the dynamics of the normal interactions between the partners of a symbiosis. 

• The development and exploitation of a variety of model systems of host-microbe interaction are critical to creating an in-depth understanding of what factors are shared by all such relationships (i.e., are primitive) and, conversely, what factors have directed the evolution of the vast diversity of associations (i.e., are derived or specific to a given relationship). 

EVOLUTION OF MICROBIAL PATHOGENS
H.S. Seifert & V.J. Dirita, Eds
American Society For Microbiology (2006)

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