Gross though it may sound, microbial cells on and inside the human body outnumber actual human cells by around 10 to one. Somewhat counterintuitively, a good portion of that is actually good for you. Now, researchers have added another layer to the odd tale of the human microbiome: To function properly, the bacteria within us need to compete. In fact, cooperating could de-stabilize their ecosystem and our health.
Researchers have known at least since the 1970s that bacteria have a big influence on people. To paraphrase a 1977 review by Dwayne Savage, there are somewhere around 10 trillion human cells making up a person's body, but another 100 trillion bacteria, yeast, protozoa, and so forth in there too, especially in the stomach and intestines. "These microbes," Savage wrote four decades ago, "profoundly influence some of the physiological processes of their host."
Competition tends to damp out the blips that could otherwise upset a delicate microbiological balance.
Fast forward to 2015, and we now know ... well, not that much more than we did in 1977. Biologists and medical researchers have recently discovered that the microorganisms living inside of us play a role in obesity and depression, among other things, but that's about all anyone knows at this point.
University of Oxford graduate students Katharine Coyte and Jonas Schluter and Kevin Foster, a professor of evolutionary biology, wanted to better understand that role, in particular why individuals' microbiomes seem to be so stable over time. That stability is thought to be essential to our health, but no one's really sure why it's there. Indeed, one standard theory suggests that a microbiome as large and diverse as ours shouldn't be very stable at all, although that theory assumed a mostly zero-sum game in which some species exploited others for their own benefit.
Coyte and her colleagues wanted to know what would happen if microbes could, in a sense, cooperate or compete head to head. In particular, how might cooperation and competition affect the microbiome's stability? Single-celled organisms do not, of course, decide to cooperate or compete, but biological interactions between two species may help both (cooperation), hurt both (competition), or help one and hurt the other (exploitation).
Using computer simulations, the researchers showed that cooperation actually worked against stability. Even though cooperation makes for greater efficiency, the team writes, it also makes for positive feedback, so that even a small blip could rapidly de-stabilize the microbial ecosystem. "[I]f, for example, one species decreases in abundance, it will tend to pull others down with it and destabilize the system," the team writes. Competition, meanwhile, stabilizes the microbiome through negative feedback—that is, competition tends to damp out the blips that could otherwise upset a delicate microbiological balance.
This isn't likely to be the final word, however. "To understand and manipulate the microbiome, we will need to [further] dissect and engineer the interactions within these critical communities," the researchers write.
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