We’ve known for a while that the number of bacteria living inside the body of any healthy adult human are estimated to outnumber human cells 10 to 1 (of course this is an estimation). However, this fact is not very known by general population.
We compulsively wash our hands, spray our countertops and grimace when someone sneezes near us—in fact, we do everything we can to avoid unnecessary encounters with the germ world. But the truth is we are practically walking bacterial colonies from our skin to the deepest recesses of our guts. Actually, changes in these microbial communities may be responsible for digestive disorders, skin diseases, gum disease and even obesity.
Despite their vital importance in human health and disease, these communities residing within us remain largely unstudied. But how important really is the bacteria inside our body?
The infestation begins at birth: Babies ingest mouthfuls of bacteria during birthing and pick up plenty more from their mother’s skin and milk—during breast-feeding, the mammary glands become colonized with bacteria. Then we consume bacteria in our food, water and many other sources we deal with in our everyday lives.
One key process accomplished by bacteria is digestion: it largely depends on bacteria health. Germ-free rodents have to consume nearly a third more calories than normal rodents to maintain their body weight, and when the same animals were later given a dose of bacteria, their body fat levels spiked, even if they didn’t eat any more than they had before. Intestinal bacteria also appear to keep our immune systems healthy. Several studies suggest that microbes regulate the population and density of intestinal immune cells by aiding in the development of gut-associated lymphoid tissues that mediate a variety of immune functions.
The bacteria also appear to influence the function of immune cells like dendritic cells, T cells and B cells, although scientists don’t know the precise mechanisms yet. Further, probiotics—dietary supplements containing potentially beneficial microbes—have been shown to boost immunity.
Of course, they can’t protect against every onslaught, which is why we still have to depend on antibiotics to rid us of some disease-causing infections. But antibiotics don’t just kill off the “bad” microbes, they wipe out the “good” ones, too.
The bacterial body has even made a contribution to our humanity—genes: soon after the Human Genome Project published its preliminary results in 2001, a group of scientists announced that a handful of human genes (the consensus today is around 40) appear to be bacterial in origin.
The question that remains, however, is how exactly they got there. Some scientists argue that the genes must have been transferred to humans from bacteria fairly recently in evolutionary history, because the genes aren’t found in our closest animal ancestors. Others argue that they may be ancient relics from evolutionary events that took place early in our species’s history and, for reasons unknown, the genes were lost in these ancestors. It’s impossible to know for sure at this point.
One thing is for sure: our lives and even our identities are more closely linked to the microbial world than we may think. Bacteria do a lot to keep us healthy, and scientists are just beginning to uncover their valuable secrets.