Evolutionary microbial “addictions” can unlock secrets of biodiversity loss, expert flags

31 Aug 2023 --- Scientific study about the microbiome has potentially been overlooking the key component to unlocking a wealth of knowledge about microbial biodiversity and needs to delve into the “evolutionary addictions” of microbes to open the path, posits a researcher from the University of California in his latest paper.

Sometimes called evolved dependence, the research theorizes that microbes might not always be helping a host but could be the reason for a bodily malfunction because a host’s microbes have evolved an addictive trait.

Evolutionary ecologist Tobin Hammer presented this theory in a peer-reviewed opinion article in Trends in Microbiology. According to Hammer, evolutionary addiction can occur in any host system, from the human gut to plant roots, even microbes that host other microbes.

Host organisms often exhibit defects when their microbiome is disrupted. The phenomenon, known for decades, supports the generally accepted notion that microbes are fundamentally important to host biology.

The malfunctioning of microbe-free hosts – those deprived of specific symbionts or the entire microbiome – encompasses a wide range of host diversity, including key processes such as development, metabolism, nutrition, physiology, immunity and behavior.

In the current research paradigm, detailed molecular mechanisms have emerged for many microbial effects and efforts are underway to leverage this knowledge for microbiome engineering. Hammer argues that “evolutionary addiction” should be taken into account.

Reliance with no added value
What Hammer is questioning in his paper is the current perception in the field that a host malfunctioning without their microbiome is being explained by the need for microbes to perform unique and beneficial functions.

However, according to the researcher, 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,” Hammer notes. 

“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.”Not all microbes provide benefits to the microbiome because evolutionary addiction could be the cause of malfunction while feigning beneficial traits.

He argues that microbes might not be helping their hosts in some cases but that the host malfunctions because they have evolved an addiction to their microbes. That would mean that hosts depend on the microbes to function, but the microbes don’t provide any benefits in return.

A dependence on host-microbe symbioses can evolve without an improvement in functionality. The concept of evolutionary addiction has been discussed in the context of herbivores, plants and parasites but has yet to be considered in the context of the microbiome. 

Meanwhile, research into the gut microbiome’s role in health continues to advance. US-based scientists at Harvard Medical School found a link between suppressing emotions and gut health among women but found no association with diets. Meanwhile, a China-based study found a link between Western diets and Crohn’s disease. 

Pathways to dependence
One way hosts have become evolutionarily addicted to microbes is through immune system calibration, where the system malfunctions when the microbes are absent. There are many variants of evolved dependence. Essentially, hosts adapt to, accommodate and function in the presence of microbes and, in the process, become dependent on them. 

The immune pathway explains how mammalian immune systems became dependent on gut microbes. When a host experiences inflammation during the early stages of a symbiotic relationship, it might be selected to have a less sensitive immune response. 

“One process may engender the other. A microbe providing an adaptive function can be expected to spread among hosts, facilitating the subsequent evolution of dependence,” Hammer explains.

Knowing whether hosts benefit from their microbes or have an “evolutionary addiction” to them can help researchers predict the consequences of microbial biodiversity loss. Evolutionary addiction might be reversible in some cases if hosts can adapt to regain the lost function through genetic variation or new mutations. 

The latest findings in gut health were unveiled earlier this year at the Microbiome & Probiotic R&D and Business Collaboration Forum Europe in Rotterdam, the Netherlands. The forum explored several studies on gut health and its connection to anxiety, depression, bipolar disorder and other brain-related health conditions. 

By Inga de Jong