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 Much of it is behind a paywall, but this article at Nature is interesting:

A species of gut bacteria that proliferates as mice get older plays a part in the animals’ cognitive decline, a study finds1. Researchers determined that the bacterium interferes with signalling along sensory nerves connecting the gut to the brain.

Although the experiments were conducted in mice, the gut–brain circuit that the team identified “is likely conserved in humans”, says David Vauzour, a biochemist at the University of East Anglia in Norwich, UK. That would need to be confirmed, but if the circuit is present in humans, then this research could reveal a mechanism that explains why people’s memory and ability to learn naturally decreases with age — and offer hope that gut-targeted therapies could reverse the decline.

The effects of the bacteria, which dampens the gut–brain circuit in mice, seem similar to other consequences of ageing. “When we get older, we need things like glasses and hearing aids”, says co-author Christoph Thaiss, an immunologist at Stanford University in California. The study — published today in Nature1 — shows that, just as ageing causes a decline in sensory perception of the external world, it might also be causing a loss of perception of internal signals, too, he says. 

The experiment was done by putting young mice and old mice together: 

After a month of living in the same cage, the young mice began performing in a similar way to the old mice on a maze task and another memory test. In the latter test, mice usually remember objects that they’ve seen before and therefore spend more time exploring new objects. But the younger animals that had been housed with the old ones instead spent equal time investigating both familiar and new objects — just as their older counterparts did. This indicated that they had lost their short-term memory.

“Their deficit was so profound, they were basically undistinguishable from the old mice,” says co-author Timothy Cox, a neuroscientist at the University of Pennsylvania in Philadelphia. “It was almost unbelievable.”

What had changed was that the young animals’ microbiomes had become similar to those of the older mice.

The next line is the reason for the title to this post:

By living near and eating each other’s faeces, mice can easily transmit their gut microbes to one another. The researchers suspected that one or more species of bacteria found in the gut of the old mice might be causing cognitive decline.