Even strict vegetarians need the help of microbes to digest plants.Although enzymes in the human mouth, stomach and intestines can break down simple carbohydrates in candy and potatoes, they cannot dissolve cellulose, a key component of the plant's cell wall, on their own.Instead, people rely on bacteria in their gut to do the job.
Recently, a research team reported in Science that thousands of years ago, humans may have obtained these beneficial microorganisms from cows or other animals for the first time.However, the team concluded that these microbes are now becoming increasingly rarer in some populations, especially in industrialized countries.
The culprit may be humanity's growing obsession with fast food and packaged foods.Samuel Forster, a microbiologist at the Hudson Institute of Medicine who was not involved in the study, said eating highly processed foods "seems to alter these bacterial communities, and these changes can have adverse effects on our health."
A few years ago, Itzik Mizrahi and Sarah Morais, microbial ecologists at Ben Gurion University in Israel, discovered that a gene called cttA is found not only in cow gut bacteria but also in human gut microbes.People often rely on gut bacteria to break down dietary fiber into byproducts, such as short-chain fatty acids, to keep the body's immune and cardiovascular systems functioning and to aid metabolism.Microbiologists delving deeper into the human gut microbiome, a mixture of bacteria and other bacteria in tissue, have discovered a bacterium that processes cellulose, but it lacks the cttA gene.
Morais, Mizrahi and colleagues wondered if bacterial genes from other cows could be found in the human gut microbiome.They scanned nearly 5,000 samples of bovine gut microbiome DNA and 92,143 samples of human gut microbiome DNA, ultimately identifying 62 complete human and cow microbiome genomes for analysis.
The team reported that when comparing the cellulose body coding genes in these genomes, they found that the cellulose bodies of three human gut microbial species were very similar to those of cow bacteria.Mizrahi believes that early farmers probably first acquired the bacteria when they treated cow droppings in the early stages of domestication about 10,000 years ago.Once in the human body, these ruminant microbes adapt to their new environment and become diverse.
To better understand this history, researchers looked for cellulose-degrading bacteria in human feces dating from 1,000 to 2,000 years ago in Mexico and the southwestern United States, as well as in the feces of modern hunter-gatherers and rural residents.They compared all of this data with human microbiome data from industrial countries.
Studies have shown that specific strains of cellulose-degrading bacteria have declined or even disappeared in many people in industrialized societies over the centuries, most likely because diets there tend to contain less fiber, which these microbes need to thrive.More than 40 percent of ancient human samples contain these bacteria, as do a fifth of modern hunter-gatherers and rural residents.But the team reports that fewer than one in 20 people in Denmark, Sweden, the United States and China have the bacteria.Without these bacteria, the researchers say, digesting cellulose would be very difficult.
Tom Van de Wiele, a microbial ecologist at the University of Ghent in Belgium, said: "Ultimately, industrialization seems to have deprived our gut of enormous microbial diversity, depriving us of dietary fiber and helping us get better gut health.”
However, biochemist Bernard Paul Henrissat of the Technical University of Denmark says more work needs to be done in the future to determine whether cellulose is actually broken down by the human gut and, if so, which bacteria.Eric Martens, a microbiologist at the University of Michigan School of Medicine, also questioned whether the bacteria worked the same way as cows.He pointed out that ruminants take a long time to break down raw cellulose, and in humans, these bacteria may simply process more digestible cellulose.