A new study reports that the superabundance of microbial life lining our GI tracts has coevolved with us. These internal bacteria, which are essential for a healthy immune system, are ultimately our evolutionary partners. In other words, humans may have co-evolved with gut bacteria unique to humans, which are not immunologically functional in other mammals. This study, the first to demonstrate that microbes are specific to their host species, also sheds light on what’s called ‘the hygiene hypothesis.’ According to this idea, living in increasingly hyper-hygienic environments might contribute to recent spikes in childhood allergies, as these beneficial host-specific microbes are hindered by the plethora of antibacterial home products and cleaning chemicals. “For every cell in your body that is you, that contains your specific genetic information, there are approximately nine foreign bacterial cells, primarily in your digestive tract and even on your skin,” said Dennis Kasper, HMS professor of microbiology and immunobiology and senior author on the paper. “From the viewpoint of cell count, every human being is ninety percent microbial. Now we’ve found that these bacteria, which we need for optimal health, are species specific.”
That 500 to 1,000 microbial species inhabit mammals has long been documented. Researchers have suggested that when it comes to digestion and other metabolic activities, the particular species of bacteria may not be significant provided the bacteria contain specific, helpful genes. In other words, a bacterium that breaks down food in the mouse gut can probably do the same in the human.
"If the bacteria within us are specific to us and necessary for normal immune system function, then it’s important to know if we are in fact losing these vital bacteria. Are we losing the bacteria we have coevolved with? If that is the case, then this is yet further evidence supporting the idea that the loss of good bacteria is partly to blame for the increased rates of autoimmunity that we are now seeing."