City Dwellers Possess Less Gut Bacteria That Break Down Cellulose, Affecting Their Ability To Digest Plants

Humans and other mammals rely on the microbial community in their gut to break down the components of plant cell wall, such as cellulose and associated polysaccharides. While cellulose-containing dietary fiber is beneficial for gut-microbiome health and overall human well-being, there is limited evidence for cellulose fermentation in the human gut.

Challenge in Digesting Cellulose

Cellulose refers to a complex carbohydrate which serves as a structural protein in plants and algae. As the primary component of a plant cell wall, it is responsible for the properties of materials such as cotton and wood. This linear polysaccharide polymer is also the primary component of dietary fiber.

Cellulose is composed of a group of sugar molecules linked together by glycosidic bonds. Its toughness makes it very challenging to use as a food source. Animals capable of extracting calories from cellulose do so by using specialized digestive tracts which provide a home for symbiotic bacteria.

A 2003 study revealed that humans also act as host to bacteria that break down cellulose. The research indicates that our gut contains a mix of cellulose-eating bacteria acquired either through our primate ancestry or herbivore domestication.

Search for Cellulose-Degrading Community

A new research discovered that the number of cellulose-eating bacteria in the human gut has dramatically decreased due to urban living. Only a single species from this group has been identified since they were discovered in 2003.

In the recent study, scientists focused on acquiring a more complete picture of the microbial community by obtaining gut samples from humans and ruminants. These were used to investigate the DNA from the cellulose-eating bacteria.

After conducting computer analysis of bacterial DNA, the researchers identified 25 genomes from ruminants and 22 genomes from humans. An evolutionary analysis suggested the presence of four distinct groups of cellulose-digesting species in the human gut.

To better understand the microbial community, the research team obtained more gut samples, along with those from other primates. They also studied some vintage fecal samples left by ancient humans more than 1,000 years ago.

The team discovered that the prevalence of cellulose-digesting bacteria is changing with changes in the human diet. For non-human primates, the frequency of these strains of bacteria was in the 30% to 40% range. This is similar to the frequencies observed in the old samples of human feces, but higher than that seen in modern humans.

Meanwhile, present-day hunter-gatherers and those living in a rural environment still had about 20% prevalence of these cellulose-digesting bacteria. It should be noted that both of these groups of people eat very high fiber diets. By contrast, those living in industrialized countries had a prevalence of under 5%.

This means that the more fiber in the diet of a culture, the more diverse their cellulose-digesting bacteria were. The researchers conclude that the diversity of gut microbiomes in humans has been going down as more of our population has shifted into urban living.

While humans possess bacteria strains passed on by our primate ancestors, there were also additional strains which did noy match up quite as well. Analysis of their metabolic genes suggests that these strains were likely obtained from the guts of animals, possibly during domestication. In other words, horizontal gene transfer between species could be a key feature of the adaptation of cellulose-digester to their hosts.