Microbiology: why modern life is bad for your gut

Biologists have only recently come to appreciate the extent of the human microbiome - the trillions of generally beneficial bacteria that populate our body - and its importance for keeping us healthy. Imbalances in this population, which lives in many of our tissues but is particularly concentrated in the gut, have been linked to diseases from diabetes to dementia.

Two new studies reinforce the growing medical opinion that modern western lifestyles, including diet, medication and sanitation, have had an adverse impact on our bodily bacteria, making people more susceptible to obesity and a range of chronic diseases. Both compared the microbiomes of non-industrialised societies with those in the US - and discovered that Americans host a much less diverse range.

One team discovered the most varied collection of resident bacteria known so far in humans, by analysing the microbiome of an isolated Yanomami tribe in the Amazonian rainforest of southern Venezuela. These hunter-gatherers had no contact with westerners until a medical expedition arrived in 2009; then 34 of the 54 villagers agreed to give faecal, skin and oral samples for microbiological analysis, now published in the journal Science Advances.

"Our results bolster a growing body of data suggesting a link between, on the one hand, decreased bacterial diversity, industrialised diets and modern antibiotics, and on the other, immunological and metabolic diseases such as obesity, asthma, allergies and diabetes, which have dramatically increased since the 1970s," says Gloria Dominguez-Bello of New York University, the project leader.

"We believe there is something environmental occurring in the past 30 years driving these diseases. The microbiome could be involved."

The other study, published in Cell Reports, looked at the gut bacteria of two remote communities practising traditional subsistence farming in Papua New Guinea. Their core human microbiome has about 50 bacterial types missing in US residents.

An intriguing conclusion is that "bacterial dispersal" - the ability of microbes to move from one person to another - is a dominant process shaping how people collect gut bacteria in Papua New Guinea but not in the US. "These findings suggest that lifestyle practices that reduce bacterial dispersal, specifically sanitation and drinking-water treatment, might be an important cause of microbiome alterations," says senior author Jens Walter of the University of Alberta.

Dominguez-Bello said the hypothesis that clean, chlorinated water depleted microbiomes in the industrialised world was "very plausible". While both research groups believe the loss of beneficial microbes has harmed aspects of human health in the west, contributing to chronic diseases, they are cautious about questioning modern lifestyles. After all, healthy lifespans have never been longer than in the industrialised world today.

"However, we can think about how we can reduce the collateral damage of modern lifestyle practices on the gut microbiome without jeopardising the benefits," says Andrew Greenhill, a microbiologist at Federation University Australia and co-author of the Papua New Guinea study. "The findings could be used to develop strategies to prevent and redress the impact of westernisation."

An interesting result from the Yanomami study is that the bacteria in the villagers have genes for antibiotic resistance - not only to natural antibiotics found in the soil but also to synthetic ones.

"In the 1940s and 1950s, the heyday of pharmaceutical antibiotic development, most antibiotics were derived from naturally occurring bacteria in the soil," says co-author Gautam Dantas of Washington University in St Louis. "So we would expect natural resistance to antibiotics to emerge over millions of years. We didn't expect to find resistance to modern synthetic antibiotics."

More research will be needed to explain this. One possibility is that the tribe, despite its apparent isolation, acquired bacteria with antibiotic-resistant genes by indirect contact with people who had been exposed to antibiotics.

Photographs: Andrew Greenhill; iStock

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