Most new parents very aware today that one’s characteristics depend on pieces of information called genes that are made from DNA whose information-carrying ability depends on the ability to combine different genetic letters into long words, and that genes are passed from parents to children. Many people also know that genetic information is preserved in DNA in all types of life the same way that it is preserved in humans, and that there is a large, government-funded project, called the Human Genome Project of the US National Institutes of Health (NIH) that is helping scientists to understand the human genome, the collection of genes in our human cells. But less commonly known is that over the past few decades scientists and health professionals have become increasingly aware of that each of us carries a large community of non-human organisms –mostly microscopic organisms—whose total number of cells outnumbers the human cells by ten fold, at minimum. These organisms inhabit different regions of the body, including the skin, the mouth, the vagina, and the innards of the gastrointestinal tract. Your intestines alone contain many trillions of one-celled organisms, mostly bacteria, and importantly many more types of organisms that we knew were in their prior to the age of DNA analysis, when the only way to identify microorganisms was to culture them, to grown them in the laboratory. But only a small fraction of the microbes of the microbiome of your gut can be cultured and DNA analysis has revealed an enormous diversity of organisms living in an on you that previously was unknown. By ten years ago, this growing realization led to the establishment of what is called the Human Microbiome Project, also run by NIH.
The idea that our bodies act as homes for smaller life forms is something that humans had to grasp little by little, over the course of centuries. This began with the realization that microorganisms located on the outside of people –on the hands, mouth, or nose– could cause disease. The presence of germs is something that you take for granted today, whether you are a mother reminding her children to wash hands before dinner, or a surgeon scrubbing up, gloving and gowning up, and being masked in preparation to perform an operation. But you would be given a blank look, if you tried to explain the matter to somebody in the early 19th century, when it was customary for doctors to wash themselves only after performing surgery, basically to remove the blood and other fluids from patients before going home. This is not to say that microorganisms were unknown; in fact, two scientists, Robert Hooke and Antoni van Leeuwenhoek, discovered microorganisms in the late 17th century using early microscopes. But realizing that microorganisms could cause disease, and that they could be transferred from one person to another, was another matter. The first person to suspect such a phenomenon was the Hungarian physician, Ignaz Philipp Semmelweis, who discovered transferable agents on the body with human disease while in a hospital in Vienna in during the 1840s. At this time in the history of medicine, astonishingly high numbers of women were dying of an illness featuring high fever, following childbirth or spontaneous abortion (miscarriage). After learning that the febrile disease –called childbirth fever—occurred five times as frequently in obstetric clinics that were staffed by physicians than in clinics staffed by midwives, Semmelweis started looking for differences between the birthing methods. It turned out that the women who died under physician care were being studied with autopsy in which a pathologist examined the deceased body, assisted by medical students, who subsequently made contact with other patients and doctors in the clinic. In contrast to medical students, midwifery students did not participate in, nor even observe, autopsies. From this, Semmelweis deduced that the disease was caused by something that transferred between people and their hands. He proposed that doctors might solve the problem by washing their hands in between treating different patients, and when he put this into practice in his own clinic, the percentage of women dying of childbirth fever dropped dramatically to below 1 percent. Given his success, he proposed that all doctors, and midwives, should wash hands, and for this the medical community decided that he was a madman, and committed him to an asylum where guards soon beat him and he died of complications at age 47.
Later in the 19th century, groundbreaking research by scientists Louis Pasteur and Joseph Lister led to the germ theory of disease, which explained perfectly Semmelweis’ observations regarding childbirth fever, autopsies, and the benefits of hand washing. From the time of Pasteur to the mid 20th century, the field of microbiology advanced at lightning speed, with the study particularly of bacteria that cause disease leading the way.
Today we know that bacteria are just one large category of microorganisms, that they are not the only type of microorganism that can cause human disease, and also that most microorganisms, including most bacteria, are not harmful to humans. Microorganisms –as well as small organisms that consist of more than one cell– can live on or inside us, with a range of consequences. Some organisms, including various species of bacteria, can cause disease, while many other types can live in an on us and help us while we help them by supplying a home, whereas still others can benefit by living in or on us without affecting us either negatively or positively. In some cases, a type of microorganism can be helpful to us when it lives in a particular location in the body, while it causes problems if it is transferred to another part of the body.
A major story that has emerged in the past several years is that human health and disease are often affected by changes in the organisms of the microbiome, resulting from changes in the environment and diet, and that disease and health, in turn, can cause changes in one’s microbiome. Evidence has accumulated, for instance, suggesting that obesity is strongly linked to the populations of bacteria living in a part of your gastrointestinal tract. Specifically, what seems to matter is the ratio inside your gut of a group of bacteria called Bacteroidetes versus another group called Firmicutes. Obesity and type 2 diabetes are associated with the presence of much greater numbers of Firmicutes, whereas thinner people have a higher ration of Bacteroidetes to Firmicutes. There also is evidence of gut microbime populations affecting the immune system and various diseases apart from obesity.
Extremely relevant to pregnancy and childbirth, it is known that the populations of the gut microbiome of a child depends greatly on the mother. In children born vaginally, the population of the gut microbiome tends to be similar to the vaginal microbiome, and similar to the mothers microbiome over all. On the other hand, in children born by cesarean section, the population of gut microbes tends to resemble the skin microbiome, and is not as related to the microbiome of the mother.
The population of your microbiome changes very quickly and very easily in response to travel and your changes in diet, and scientists are only beginning to understand how this all relates to various health conditions. One thing that is becoming very clear though based on all of the new insights is that the probiotic supplements that are being sold in “health food” stores and marketed aggressively to cure and prevent numerous health conditions, or to boost your immune system, are probably something that you should avoid. That’s because, these supplements typically provide just one, or at most a few species of bacteria that have been identified in the human gut. That’s one to a few species out of thousands, or more, which means that, if the supplements are having any effect at all (which in some cases they may not as the organisms contained within them could easily be dead by the time that you purchase them or consume them), then very likely the effect is to throw the balance of species of your gut microbiome in favor of the species that’s inside the supplement.