Ignaz Semmelweis changed our world when, back in 1847, he decided to start washing his hands after he performed an autopsy and before he delivered a baby. When Semmelweis worked in a Viennese obstetric hospital, the germ theory of disease and the concept of “infection” were unknown. Postpartum infections due to “childbed fever” killed a high percentage of women who gave birth in hospitals. Semmelweis knew that there was something in and around dead bodies that had the potential to cause disease, and so he decided to follow the practice of midwives and wash his before delivering a baby. Fewer mothers died, and Semmelweis knew he was onto something. During his lifetime, his innovation was rejected by fellow male physicians, but within decades, evidence from doctors and scientists in other parts of Europe soon provided incontrovertible evidence that he was right. Small organisms like bacteria caused disease, and taking simple precautions like hand washing could lower disease risk.
Thanks to Semmelweis and his intellectual descendants, we follow a range of routines from water boiling and avoiding tropical ice cubes to near-fanatical levels of hand sanitizing, in order to reduce the chances of getting sick because of the nasty bugs in our environment.
We have known for a long time that our bodies harbor lots of “normal flora,” but until about a decade ago, few people studied them. We focused on Semmelweis’s disease-causing bacteria, which we cultured on petri dishes so that we could identify and kill them. The rest of our microbial residents were thought to be pretty much harmless baggage and were ignored.
The introduction of new methods of identifying normal, diverse communities of organisms from DNA alone (including such innovations as high-throughput DNA sequencing) changed all that, and we began to come to realize the magnitude of what we had been missing. The world of critters living in and on us was soon discovered to be vast and complex one, and it mattered.
Since 2008, when the Human Microbiome Project officially started, hundreds of collaborating scientists have started to bring to light the nature and effects of the billions of bacteria that are part of our normal healthy bodies. There isn’t one human microbiome, there are many: There is a microbiome in our hair, one up our nostrils, another in our vaginas, several lavishly differentiated on the vast real estate of our skin, and a veritable treasure trove in our gut, thanks to diligent subcontractors in the esophagus, stomach, and colon.
This great menagerie undergoes changes as we age, so that some of the bacteria that were common and apparently harmless when we were young start to bother us when we’re old, and vice versa. The taxonomic diversity and census of our resident bacteria are more than just subjects of scientific curiosity; they matter greatly to our health. The normal bacteria on our skin, for instance, are essential to maintaining the integrity of the skin’s barrier functions. Many diseases, from psoriasis to obesity, inflammatory bowel disease, some cancers, and even cardiovascular disease, are associated with shifts in our microbiota.
While it’s too early to tell if the changing bacteria are the cause or the result of these problems, the discovery of robust associations between bacterial profiles and disease states opens the door for new treatments and targeted preventive measures. The body’s microbiota also affects and is affected by the body’s epigenome, the chemical factors influencing gene expression. Thus, the bugs on us and in us are controlling the normal action of genes in the cells of our bodies, and changes in the proportions or overall numbers of bacterial affect how our cells work and respond to stress.
Let’s stop thinking about our bodies as temples of sinew and cerebrum, and instead as evolving and sloshing ecosystems full of bacteria, which are regulating our health in more ways than we could ever imagine. As we learn more about our single-celled companions in the coming years, we will take probiotics for curing acute and chronic diseases, we’ll undertake affirmative action to maintain diversity of our gut microflora as we age, and we’ll receive prescriptions for increasingly narrow-spectrum antibiotics to exterminate only the nastiest of the nasties when we have a serious acute infection. Hand sanitizers and colon cleansing will probably be with us for some time, but it’s best just to get used to it now: Bugs R us.