Where is the human microbiota found on the body?
There are numerous microbial types adapted to live on every available surface, and in every enclosed but accessible space of the human body. However, the composition of the microbiota differs considerably in different anatomic sites, since the physical and chemical environments can be quite different. The available surfaces obviously include the outside of the body (skin, nails, hair, etc.), and extend to the adjoining mucus membranes of the nose, respiratory passages, genitals, and urinary tract. Many different microbial communities are found along the digestive tract, adapted to the various surfaces of the mouth (teeth, gums, tongue, etc.), the lining of the esophagus and stomach, and both the lining and the center (or lumen) of the small and large intestines. Because there is both a large mucosal area on the inner surface of the large intestine and a mass of slowly-moving material in the middle (which gives microbes the time and resources to grow), the microbiota of the colon contains a large majority – perhaps about 90% – of all the microbial cells of the body.
So where aren’t there microbes on or in the body? There’s some doubt about the microbiota of the most protected mucosal surfaces. Not long ago it was widely accepted that lining of the bladder or the depths of the lungs, for example, were essentially sterile (without living microbes) unless there was an active infection by a pathogen of some sort. However, these mucus membranes are physically continuous with places where there certainly are native, non-pathogenic microbial populations, so some active process would be needed to keep them sterile. Some evidence has been published that supports the existence of microbes in these protected sites in the absence of disease, but there isn’t yet a consensus whether those microbes are actually native to the sites, or if they simply get carried there from elsewhere and are in the process of being cleared away.
The interior of the body (i.e., places within the barrier of the skin and mucus membranes, like the blood, muscles, bones, brain, liver, etc.) isn’t considered to have a native microbiota, but even here, live microbes might be found. There are always some breaks in skin or mucus membranes where microbes might penetrate, although generally such invaders will quickly be engulfed and then killed by white blood cells. Even without breaks in the barriers, there are specialized immune cells that reach through mucus membranes (especially into the lumen of the gut) to grab samples of what’s on the other side, and the ‘samples’ can include live microbes. This is actually part of a critically important process for training our immune system to tolerate the microbes that are found in the gut, without mounting an inflammatory response.
There are numerous microbial types adapted to live on every available surface, and in every enclosed but accessible space of the human body. However, the composition of the microbiota differs considerably in different anatomic sites, since the physical and chemical environments can be quite different. The available surfaces obviously include the outside of the body (skin, nails, hair, etc.), and extend to the adjoining mucus membranes of the nose, respiratory passages, genitals, and urinary tract. Many different microbial communities are found along the digestive tract, adapted to the various surfaces of the mouth (teeth, gums, tongue, etc.), the lining of the esophagus and stomach, and both the lining and the center (or lumen) of the small and large intestines. Because there is both a large mucosal area on the inner surface of the large intestine and a mass of slowly-moving material in the middle (which gives microbes the time and resources to grow), the microbiota of the colon contains a large majority – perhaps about 90% – of all the microbial cells of the body.
So where aren’t there microbes on or in the body? There’s some doubt about the microbiota of the most protected mucosal surfaces. Not long ago it was widely accepted that lining of the bladder or the depths of the lungs, for example, were essentially sterile (without living microbes) unless there was an active infection by a pathogen of some sort. However, these mucus membranes are physically continuous with places where there certainly are native, non-pathogenic microbial populations, so some active process would be needed to keep them sterile. Some evidence has been published that supports the existence of microbes in these protected sites in the absence of disease, but there isn’t yet a consensus whether those microbes are actually native to the sites, or if they simply get carried there from elsewhere and are in the process of being cleared away.
The interior of the body (i.e., places within the barrier of the skin and mucus membranes, like the blood, muscles, bones, brain, liver, etc.) isn’t considered to have a native microbiota, but even here, live microbes might be found. There are always some breaks in skin or mucus membranes where microbes might penetrate, although generally such invaders will quickly be engulfed and then killed by white blood cells. Even without breaks in the barriers, there are specialized immune cells that reach through mucus membranes (especially into the lumen of the gut) to grab samples of what’s on the other side, and the ‘samples’ can include live microbes. This is actually part of a critically important process for training our immune system to tolerate the microbes that are found in the gut, without mounting an inflammatory response.