We've long known that hosts malfunction without their microbiome -- whether they are missing key microbial species or are completely microbe free. This malfunctioning is usually explained by the need for microbes to perform unique and beneficial functions, but evolutionary ecologist is questioning that narrative.
We've long known that hosts malfunction without their microbiome -- whether they are missing key microbial species or are completely microbe free. This malfunctioning is usually explained by the need for microbes to perform unique and beneficial functions, but evolutionary ecologist Tobin Hammer of the University of California, Irvine, is questioning that narrative.
In a peer-reviewed opinion article publishing August 29 in the journal Trends in Microbiology, Hammer argues that, in some cases, microbes might not actually be helping their hosts; instead, microbe-free hosts might malfunction because they have evolved an addiction to their microbes. In this case, hosts are dependent on the microbes to function, but the microbes don't actually provide any benefits in return.
Evolutionary addiction is also sometimes called "evolved dependence," and it could occur in any host system -- from the human gut, to plant roots, to microbes that host other microbes. Hammer compares it to his own dependence on coffee.
"I need coffee to perform basic functions, but I do not perform them any better now than before the addiction began; I need coffee just to get back to normal," writes Hammer. "The same process can occur with host-microbe symbioses: a dependence evolves without an improvement in functionality."
Though the concept of evolutionary addiction has been discussed in the context of other symbiotic relationships -- for example, herbivores and plants as well as parasites and their hosts -- it has rarely been considered in the context of the microbiome. Hammer argues that evolutionary addiction should be considered when interpreting microbe-removal experiments because it might have unique implications for the evolution and stability of host-microbe interactions.
"By largely ignoring evolutionary addiction, the microbiome field has missed a plausible and likely common evolutionary explanation for microbially dependent host traits," writes Hammer. "The host organism is a complex, internally interconnected system, and the absence of a microbe that has been integrated into it, like a cog in a machine, will cause components to malfunction."
