Discovered: Cellulose-degrading gut bacteria in the human gut, although at lower levels in industrialized countries

Previously undescribed human gut bacteria that aid in the digestion of plant cellulose are scarce in urban societies but abundant in ancient and hunter-gatherer microbiomes, according to a new study. The findings provide insight into the poorly understood cellulosome-producing bacteria that inhabit the human gut and their response to modern urban diets and lifestyles. Like all mammals, humans rely on the gut microbiome to digest cellulose – the main component of plant fiber and a common element in diets that include plant-based material. Fermentation of dietary fiber via cellulosome-producing bacterial species transforms these indigestible compounds into short-chain fatty acids that play a role in host health, including preventing colon cancer and regulating blood sugar. Dietary fiber is also beneficial to gut microbiome stability and richness. However, modern industrialized diets, which are dominated by processed foods, are severely lacking in plant fiber, and evidence for cellulose degradation and fermentation in the human gut is scarce. Fundamental questions regarding the cellulose-degrading bacteria that inhabit the human gut and their adaptability to host lifestyle and diet remain. Sarah Moraïs and colleagues searched for key cellulosome genes in 92,143 human metagenome-assembled genomes and discovered three distinct, previously undescribed cellulosome-producing ruminococcal bacteria species that inhabit the human gut: Ruminococcus hominiciens, R. primaciens, and R. ruminiciens. Evolutionary analysis suggests that these bacteria may have originated in the ruminant gut and evolved to adapt to the human gut through gene acquisition from other microbial species. The prevalence of these species also exhibited notable variation among human populations. According to Moraïs et al., collectively, these strains were abundant and widespread among ancient humans, hunter-gatherers, and rural populations but extremely rare in industrialized populations. These findings suggest that these strains are disappearing from the guts of individuals in industrialized countries, potentially as a response to changes in diets and lifestyle, which could explain why evidence for cellulose fermentation in the human gut is so rare.