When we are born, we get most of our gut microbes from our mothers (see 鈥淲here does your gut microbiome really come from 鈥� and does it matter?鈥�). But as we get older and form other close relationships, including with intimate partners, friends and pets, we start to pick up their microbes too. This could potentially affect our risk of developing conditions like obesity, inflammatory bowel disease, asthma and allergies (see 鈥淲hat is the role of the microbiome in diseases like chronic fatigue?鈥�).
鈥淚 jokingly say that your dating app profile should include your microbiome profile,鈥� says at the University of British Columbia in Canada.
The strongest evidence comes from work published in January by at the University of Trento, Italy, and her colleagues, who conducted the largest study to date of how our gut microbiomes are shaped by the people around us. They analysed DNA in the faeces of more than 7000 people from households around the world, including rural parts of Africa and South America and cities in the US, Europe and China, to find out which bacterial strains were in their guts and what proportion they shared with others.
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Bacterial strains are slight genetic variants of the same bacterial species and are highly individualised, meaning if two people share a strain, it must have been directly transmitted between them. As expected, infants under the age of 1 and their mothers : about 50 per cent. But by the time children reached the age of 3, they only shared 27 per cent of strains with their mothers and an increasing contribution came from other household members, such as their fathers or siblings.
Adults that lived under the same roof 鈥� including partners 鈥� shared 12 per cent of strains with each other, and even those living in the same village shared 8 per cent. In contrast, people who didn鈥檛 live together or in the same village shared none.
It makes sense that people cohabiting or living in the same small communities would share gut microbes, since they can be spread by kissing, shaking hands and eating food prepared by others, says Valles-Colomer. One study, for example, found that couples on average during a 10-second intimate kiss.
Can you catch obesity?
This raises the question of whether we may be able to 鈥渃atch鈥� conditions that are associated with microbial upsets, like obesity and inflammatory bowel disease (IBD), from our nearest and dearest (see 鈥�Does the gut microbiome influence body weight鈥�, below). We know from previous research that , and that .
It is hard to know whether this is simply due to shared lifestyle factors or whether the exchange of gut microbes also plays a role. But evidence for the latter comes from experiments in mice showing that they start to and develop after receiving transplants of gut microbes from people with obesity and inflammatory bowel disease respectively.
How a pet affects your health
Could the same also be true of microbes we pick up from having pets around? In 2017, an analysis of faeces from more than 700 infants aged 3 to 4 months in Canada found that those with a dog or cat at home had in their guts, both of which are associated with reduced incidence of the likes of asthma and obesity.
This microbe transfer may explain why other studies have found that exposure to a dog in infancy is linked with a lower risk of developing , eczema, and , says Finlay, although a causal link is yet to be established. Either way, it is clear that we are sharing a lot more with our housemates than a roof and four walls.
DOES THE GUT MICROBIOME INFLUENCE BODY WEIGHT?
The idea that the microbes in our guts may directly influence our body weight came to the fore in 2006, when at Washington University in St Louis, Missouri, and his colleagues showed that lean mice bred without any gut microbes 鈥� so-called "germ-free" mice 鈥� gained more body fat when they were given gut microbes taken from obese mice than those taken from lean mice.
In 2013, Gordon's team unveiled further proof by transplanting the gut microbes of pairs of identical human twins 鈥� in each pair one was obese and the other lean 鈥� into germ-free mice. They showed that the .
"Microbial activity plays an important role in both energy harvesting and in regulation of energy balance", says at the University of Sydney in Australia. That is because gut microbes are the only way we can get energy from fibre in our diet, and they also influence hormones that help us to feel full.
The findings made others wonder if obesity in people could be treated by recolonising their guts with microbes taken from lean individuals via faecal transplants. However, several clinical trials set up to test this .
One reason why is that it is harder to seed new gut microbes in people with established microbiomes than in germ-free mice, says Holmes. "It's like trying to change a savannah to a rainforest by dumping 20,000 tonnes of rainforest seeds on top 鈥� you might introduce some new species, but you're definitely not going to turn it into a rainforest."
What's more, there are many other factors involved in body weight, including genetics and diet.
In the future, we might be able to identify the most effective weight-loss strategies for individuals with obesity by analysing their microbiomes in combination with genetics and diet.
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Speak to your doctor before seeking new treatments for medical conditions
This article is part of a special series on the microbiome, in which we explore:
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