To stay healthy, humans and some other animals rely on a complex community of bacteria in their guts. But research is starting to show that those partnerships might be more the exception than the rule.
Quanta Magazine said:In the summer of 2011, the microbiologist Jon Sanders, then a graduate student, found himself in Peru’s tropical rainforest for the second time in as many years, lugging 60 pounds of lab equipment — a bulky fluorescence microscope and the generator to power it — up the Amazon River. Upon arriving at the remote field site, he quickly set about catching as many different ants as he could, eager to peer at the microbes that populated their guts.
In some of those ant species, he saw “this amazing, dense, packed cloud. It was like a galaxy of microbes,” he said. “They’d explode in your eyes when you looked at them” under the microscope. Which is what you might expect to find, given the extent to which we and so many other animals depend on the trillions of bacterial cells that reside within us — for processing food that we can’t otherwise digest, for providing key nutrients, for training our immune system to act effectively against infections. The microbiome is so critical to our health and survival that some researchers even find it useful to think of animals as the sum of their microbial parts.
But when Sanders turned to the rest of the ants — about two-thirds of the different colonies and species he had collected — he was surprised to find that “you would be hard-pressed to find any cells in the gut that you could readily identify as bacteria,” he said. Food, debris, the cells of the insects’ gut lining — all were present. Microbes that might be engaged in the symbiotic relationships we take for granted — not so much.
As the tools to measure and analyze microbial communities have improved, it’s gradually become clear that the microbiome is nowhere near as ubiquitous and important across the animal kingdom as it’s often portrayed to be. Many animals seem to have more flexible or less stable associations with microbes; some don’t seem to rely on them at all. And ironically, it’s these animals that are now allowing scientists to gain new insights into the mystery of how and why the microbiome evolves — its real importance, and the nuanced balancing act of pros and cons that lies at its core.
In the early 20th century, biologists began to uncover fascinating relationships between complex organisms and their microbes: in tubeworms that had no mouth, anus or gut; in termites that fed on tough, woody plants; in cows whose grassy diet significantly lacked protein. Such observations generated excitement and prompted follow-up experiments. In those years, the absence of microbial helpers in an animal wasn’t considered particularly surprising or interesting, and it often received little more than a passing nod in the literature. Even when it was thought to merit more than that — as in a 1978 report in Science that tiny wood-eating crustaceans, unlike termites, had no stable population of gut bacteria — it ended up flying under the radar.
And so expectations quietly began to shift to a new norm, that every animal had a relationship with bacteria without which it would perish. A few voices protested this oversimplification: As early as 1953, Paul Buchner, one of the founders of symbiosis research, wrote with exasperation about the notion that obligate, fixed and functional symbioses were universal. “Again and again there have been authors who insist that endosymbiosis is an elementary principle of all organisms,” he seethed. But counterexamples drowned in the flood of studies on the importance of host-microbe symbioses, especially those that drew connections between human health and our own microbiome.
“The human microbiome has completely driven a lot of our thinking about how microbes work,” said Tobin Hammer, a postdoctoral researcher in ecology and evolutionary biology at the University of Texas, Austin. “And we often project from ourselves outwards.”
But the human example is not a good model for what’s going on in a diverse range of species, from caterpillars and butterflies to sawflies and shrimp, to some birds and bats (and perhaps even some pandas). In these animals, the microbes are sparser, more transient or unpredictable — and they don’t necessarily contribute much, if anything, to their host. “The story is more complex,” said Sarah Hird, an evolutionary biologist and microbial ecologist at the University of Connecticut, “more fuzzy.”
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