Welcome back to the Nature Briefing podcast. The Friday show, we talk about a couple of stories we've read about in The Nature Briefing, which is, of course, nature's daily roundup of the latest science stories. And joining me once again is Marant Hunsburger. Maron, how you doing? I'm doing great. Thanks for having me, Ben.

Well, listen, why don't you go first this week? What have you got? I've got a story that I read about in nature that provides new evidence that the gut microbiome is linked to cognitive signs of aging, brackets in mice. Double underlined, as we always do. Right, so you and I have both been microbiologists in a former life, and there is a growing body of correlative evidence in many cases

that there is this link between the microbes that live in the gut and effects on other parts of the

body, particularly the brain, for example. Yes, and this is a new way into this investigation. So

the researchers housed old mice and young mice together. And the old mice had started to indicate

signs of cognitive decline in relation to their memory. And the old mice had started to indicate signs of cognitive

decline in relation to their memory. And I have to ask then, how does one go about testing

what a mouse can remember? This is a great question. Often, it's on timed maze trials. So if you

put a mouse through a maze and they can do it faster, you know, after the first time, it means

that they've remembered something about the maze. You can also introduce them to a variety of objects and then introduce new objects

every time. If they spend the same amount of time investigating old objects and new objects,

it means they probably can't really remember the old objects. So once these mice had been

cohabitating for a while, the young mice and the old mice, what the researchers say if we

were to translate over into human years would be someone in their teens and someone in their later

50s. The young mice started to indicate the same symptoms of cognitive decline in memory.

And we've set this up as a microbiome-based story, so I'm guessing then that the researchers

think that this has something to do with that. Yes, so what had shifted for those young mice was their gut microbiome, which through close habitation with the older mice had shifted to more closely resemble that older gut microbiome. Okay. And I think we've set up that this is in mice, double underlined. Does this mean that the same thing might happen in humans? Not so much, Ben, because humans don't tend to eat each other's

feces. Mice do. So that's how that gut microbiome shift takes place under close cohabitation.

Now, as you and I both know, Ben, most microbiome studies are typically correlative. So we see an

association between two factors. These researchers really wanted to start to explore causality. Was the shift in

microbiome causing the shift in cognitive function? And to do this, they looked at it from a couple

of different ways. One, they implanted a specific bacterium that they had honed in on. It's called

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