Updated August 09, 2015.
Written or reviewed by a board-certified physician. See About.com's Medical Review Board.
We humans are made up of a lot of cells. A lot. Trillions, in fact. 1,000,000,000,000's. It's hard to count how many cells exactly. But it's safe to say, most of these cells are microorganisms, like bacteria, not mammal. We carry probably 10 times more bacteria than human cells in our bodies.
These bacteria live all over and inside of our bodies. Our guts are full of bacteria that help break down our food.
Our skin is our largest organ and supports different bacterial niches from our toes to our armpits to our scalps. Our lungs and noses support even additional bacterial communities.These various bacterial species living together with cells counting in the trillions collectively are called the Microbiome. (Microbiota is often used to refer just to the microorganisms).
These bacteria don't take up much space. They're smaller - and weigh less than our human cells. Usually all the bacteria in someone weighs only about 3 pounds - or about 1-3% of our body weight.
The microbiome was first coined "to signify the ecological community of commensal, symbiotic, and pathogenic microorganisms that literally share our body space and have been all but ignored as determinants of health and disease" by Joshua Lederberg, Nobel Prize Winner.
Trying to make sense of the Microbiome
It's hard to sort through dense bacterial communities and their interactions. Complex bacteria populations cannot easily be cultured through traditional lab techniques.
These populations are being studied through genetic techniques. New approaches are trying to sort this resulting avalanche of data. Research groups and the NIH funded Human Microbiome Project are identifying and cataloging our microbial populations and interactions.
What we do know about the Microbiome
What we do know is that we are born without bacteria and our mothers give us our first bacteria at birth; manner of birth (cesarean section or not) even determines which bacteria we receive. Our early diet affects this too. These bacterial populations can wax and wane - when we travel, when we get an infection, when we change our diet.
We also know that these bacteria variations may impact our health in ways we don't understand. From psoriasis to obesity to cardiovascular disease, there are correlations between our microbiota and disease. However, causation (knowing a bacteria's presence leads to disease) is not clear. These bacterial changes may reflect, rather than cause, changes in our health.
We know that taking antibiotics when we are not sick may flush away "good" bacteria. Targeted probiotics (microorganisms ingested to try to change the microbiome) would seem like they would help, but we don't yet know how to do this effectively.
We do not know yet how to always manipulate these bacteria to ensure the healthy outcomes we want. What may seem like "good" bacteria may also be "bad". Some bacteria may correlate with increased and decreased disease simultaneously; H pylori, a bacteria sometimes in the stomach, increases the risk for ulcers and gastric cancer, but is associated with less esophageal reflux and asthma. There have even been cases of infections caused by the bacteria in probiotics, though generally these are safe. Fecal transplants are gaining momentum for the replacement of dangerous bacterial overgrowth in our guts - such as chronic C diff (Clostridium difficle) infections.
What else we know:
Microbiome and Weight gain
We also do know that antibiotics can lead to weight gain. For over 60 years we've known that farm animals fed antibiotics grow bigger; the same seems true in children. We now know transplanting gut bacteria can make mice gain weight. It may even be that no-calorie sweeteners can affect our microbiome, making blood sugars rise without any calories.
Less exposure to microorganisms may mean more autoimmune diseases. Our immune systems can attack ourselves - and some believe increased hygiene is partly to blame.
Childhood exposure to a wide range of microorganisms - such as on a farm or through not using antibiotics - correlates with (and may lead to) protection against asthma. Bacteria in the lung may play a role in diseases such as Asthma. It may be that viral and bacterial exposures affect the development of Type 1 Diabetes.
There is a hope probiotics may prevent side-effects of necessary treatments, such as diarrhea after life-saving antibiotics, but this has not been easy to accomplish. We also hope to identify who will have wound infections or Clostridium difficile intestinal infections by observing bacteria at the wound site or intestines beforehand. This could open the door for targeted antibiotics or probiotics, especially as drug resistance continues to spread.
Take home points
It is important for patients who are ill (and need antibiotics) to take antibiotics. It is important, as well, that patients who are not ill (and do not need antibiotics) to not take antibiotics. Hopefully through continued careful research, we will understand the implications of the bacteria we carry around with us every day.