Our Microbiomes: Who Are Our Microscopic Neighbors?

Our bodies are made up of many more microbe cells — bacteria, fungi, and other microscopic critters — than human cells. These microscopic organisms are clearly important for making us who we are. And yet, there’s so much we don’t understand about them. How do our microbiomes help keep us healthy, and why do they sometimes make us sick?

Here we’ll do a survey of what’s known about the various microbiomes than live on, and inside of, our bodies, while next time we’ll dig deeper into how these microscopic communities may be connected to our health.

The mouth microbiome: It’s been found that some 700 species of microbes live in the human mouth (although this number is likely to increase as we find out more about these communities — to find out the latest on this topic, you can check out the Human Oral Microbiome Database). Within the mouth itself, there’s a great deal of variation — for example, the types of microbes found on the tongue are different from the ones living on the roof of the mouth. It’s challenging to study the mouth microbiome because the populations are in constant flux — many microbes are just passing by, primarily on their way to the gut or lungs.

The lung microbiome: There are many fewer microbes living in the lung microbiome compared to some of the busier microbiomes. Specifically, it’s thought to have about 1000 times fewer microbes than the mouth microbiome, and around 1 million to 1 billion less than the gut microbiome. But it still plays a key role in “trading” microbes between the body’s various microbiomes — the lungs are thought to mostly get their microbes from the mouth, where they are aerosolized, and many microbes from the lungs end up in other internal niches of the body.

The gut microbiome: Probably the most microbiome studies have been done on the gut. But while we’ve characterized many of the microbes living there, we still have a long ways to go before we understand the complex interactions taking place, and how this can cause different diseases — tune in next week for much more on this topic.

Genitalia microbiomes: In the last few years we’ve learned a lot about both the penis microbiome and the vagina microbiome. While many more microbes live in the vagina than on (or in) the penis, still distinct communities and differences can be seen in the penis microbiome. It’s been found that the critters living on the tip of the penis vary mostly due to whether the penis has been circumcised or not — an uncircumcised penis is home to a larger number of bacteria, whereas after circumcision (of an adult penis), not only are there fewer bacteria, but within six months of circumcision there are specifically fewer bacteria that are anaerobic (can live without oxygen). The implications of this are unclear. The vagina microbiome has been studied longer, and the presence of certain bacteria (specifically Lactobacillus bacteria species) has been associated with certain vaginal infections (bacterial vaginosis [BV]), but these long-held ideas are starting to be seriously questioned as we better characterize the vagina microbiome and discover the amazing diversity of microbes it encompasses — more on this topic next week!

The skin microbiome: Our skin is home to many different types of microbes, including lots of transients (as we come into contact with new surfaces or air-born microbes all the time). Scientists who study these critters have divided them up into three general groups, which live in three general types of “environments” on the body: (1) oily, or sebaceous locations, such as on the head, neck, and torso; (2) moist creases, such as in the elbow, below toes, or immediately below breasts; (3) dry, large, flat areas, such as on the side of the arm or leg. Certain bacteria commonly dominate these different niches (such as Propionibacterium species in greasy places, Corynebacterium critters in moist creases, and Staphylococcus bacteria in the dry, flat locations). Researchers have also seen big, localized differences depending on our routines — a different microbe community likely develops where, for example, a person applies some lotion daily, or where somebody’s glasses rest on their nose. And this microbiome, too, is suspected to play a large role in our health.

Coming up next week: Next time we’ll revisit these microbiomes and others (including microbiomes associated with wounds, eye infections, and more) as we delve into what scientific studies say about how our microbiomes help keep us healthy, and also sometimes make us sick.

For further reading:

• Tracy Vence’s article “Mining the Mouth’s Many Microbes”
• Rina Shaikh-Lesko’s article “Breathing Life into Lung Microbiome Research”
• Tracy Vence’s article “Parsing the Penis Microbiome”
• David Serwadda, Maria J. Wawer, and Ronald H. Gray’s article “The Effects of Circumcision on the Penis Microbiome”
• Jef Akst’s article “Microbes of the Skin”
• Teisha J. Rowland’s article “What is a ‘Healthy’ Gut Microbiome?”
• Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Animals Both Commonplace and Bizarre