Dr. Kevin Foster, from the University of Oxford, visited the Center for Evolution and Medicine at Arizona State University last week to talk about competition and sociability among a variety of bacteria, some of which call our guts home. Using humorous descriptions of psychedelic broccoli, tiger and lion fights, and breathing on hornet’s nests, he walked us through the complexity of sociality found in microbes, which ranges from competition among specific bacterial cells to between-species cooperation. Foster used to study social insects, but now he applies his expertise of social behavior (and kin selection) to microbes. While kin selection provides an evolutionary explanation for many complex social behaviors in eukaryotic organisms, it may also be a good model to use in understanding the behavior of genetically similar microbes and how such behavior may affect human health.
News of the microbiome seems to be everywhere these days. Researchers are learning how it can relate to childhood problems with allergies and asthma, how specific bacteria may be associated with breast cancer, and how personal hygiene may or may not alter the microbiome. Work like this, while very important, can sometimes support a view of the microbiome as a long list of different bacterial types—a simple collective of individuals; the chances of a disease or disorder may depend on whether a specific bacteria is on the list or not. However, this view doesn’t lend itself to understanding the network of ecological interactions that are occurring among the bacteria within each of us.
The microbiome is a heaving bacterial community that is in constant flux. As we learn about the competitive and (yes, even) cooperative interactions between bacterial species, we are starting to gain a new respect for how social these organisms can be. A lot of these interactions can be thought of as a sort of kin selection—helping those that are genetically similar to you.
For example, if you have two types of bacteria living in a community—one that secretes an enzyme beneficial to bacterial growth and one that doesn’t secrete that enzyme—you will likely find that the secretor phenotype only persists when it is grouped with other similar bacteria. If it is mixed in among non-secretors, those non-secretors can gain a large benefit from the secretions. This cheating gives them a reproductive edge, and, as free riders, they eventually outcompete the secretors. However, if the secretors are grouped, they benefit each other, and are able to outcompete the non-secretors and promote the secreting phenotype.
Competition for benefits such as this in animals often selects for the ability to discriminate between kin and non-kin. Foster found that bacteria need to solve the same competition dilemmas and at least some bacteria have evolved recognition systems that enable them to discriminate between similar and different genotypes. In fact, bacteria can not only distinguish between genotypes of neighboring bacteria, but they can also determine if cellular damage is occurring due to bacteria of a different genotype, or due to environmental influences like dehydration or heat stress.
These abilities also apply to what is arguably the most social state of bacteria: the biofilm. Biofilms were previously thought to be a product of cooperation between multiple strains and species of microbes. But biofilms are more discerning than that. Foster and colleagues showed that when two biofilms of the same genotype are mixed, the biofilm grows and cells from each group survive in about equal measure. However, when biofilms that have different genotypes are mixed, the biofilm also grows, but only one of the biofilm types remains—the other type is destroyed by whichever biofilm can dominate, using cellular hole-punching toxins called pyocins.
OK, so bacteria are social insomuch as they promote survival of their own genotype, at various levels of social interaction. But how does this relate to the microbiome living inside of you? In your exceedingly diverse microbiome, bacteria are often mixed with other populations, or bordered by various other microbe types. Does this lead to a constant knock-down, drag-out competition between each of these species as they fight to promote their own genes? Or do these bacteria ever work together in ways that can benefit multiple species?
We see examples of mutualisms throughout nature—animals pollinate or protect plants that provide food, different species help keep each other clean and fed, and animals farm plant or fungus species and proliferate their genes in return for food. One of Foster’s newest studies shows that different species of bacteria can also cooperate in a sort of mutualism that creates a benefit for both species.
Bacteroides species are involved in the digestion of various polysaccharides, including inulin. Some of these bacteria prefer to grow on undigested inulin, while others benefit from inulin that has been broken down extracellularly. However, one bacterial species that does better on undigested inulin nevertheless releases enzymes that break inulin down—an odd product to make if it won’t benefit them directly. Through a trail of well-designed experiments, Foster and colleagues discovered that the bacteria were breaking down inulin so that other bacterial species could benefit. As those other bacteria thrived and digested inulin further, they produced a digestion product that benefitted the first bacteria. However, neither of these species was outcompeting the other, meaning that bacteria can form evolutionarily stable cooperative relationships. Thus, the bacteria that have made a lovely home out of your gut may be developing important relationships with each other—yet one more reason why promoting good intestinal flora may help the whole community inside of you flourish.
As we seek to understand the vast ways that microbiomes affect human health, it’s important to learn more about how microbes may be interacting and shaping the ecological outcomes. Foster’s work continues to shed light into the dark interiors of bacterial interactions outside and inside of the human gut.
The video of Kevin Foster’s talk is now available in the Videos section of ASU’s Center for Evolution & Medicine.