How many organisms live in the human body

These microbial genes are critical to good health. Those in the gastrointestinal tract, for example, allow humans to digest foods and absorb nutrients that our bodies otherwise could not handle. Microbial genes also produce vitamins and compounds that naturally suppress inflammation in the intestine.

Also, confirming earlier, smaller studies, the new research shows that components of the human microbiome clearly change during an illness. When a patient is sick or takes antibiotics, the species of the microbiome may shift substantially as one bacterial species or another is affected.

Eventually, however, the microbiome settles into a state of equilibrium, even if the previous composition is not completely restored. As part of the Human Microbiome Project, the NIH funded a number of studies to look for links between particular communities of microbes in the body and illness. Results of some of this research, reported in PLoS , underscore the clinical applications of microbiome research to improve human health.

At Washington University, researchers led by Gregory A. Unexplained fever is a common problem in children under age 3, and they are often treated with antibiotics as a precaution, which contributes to antibiotic resistance.

Storch and his colleagues, including Kristine Wylie, a postdoctoral research associate at The Genome Institute, found that specimens from the sick kids contained more species of viruses, some of them novel, than children without fever, who also were included in the study as a comparison. Understanding the difference between viral infections with and without fever will be important in applying microbiome techniques in the clinic, the scientists say. Antibiotics and vaccines have been the weapons unleashed against the likes of smallpox, Mycobacterium tuberculosis or MRSA.

That's been a good thing and has saved large numbers of lives. But some researchers are concerned that our assault on the bad guys has done untold damage to our "good bacteria". Prof Ley told me: "We have over the past 50 years done a terrific job of eliminating infectious disease. The microbiome is also being linked to diseases including inflammatory bowel disease, Parkinson's, whether cancer drugs work and even depression and autism. Obesity is another example. Family history and lifestyle choices clearly play a role, but what about your gut microbes?

A diet of burgers and chocolate will affect both your risk of obesity and the type of microbes that grow in your digestive tract.

So how do you know if it is a bad mix of bacteria metabolising your food in such a way, that contributes to obesity? Prof Knight has performed experiments on mice that were born in the most sanitised world imaginable.

He says: "We were able to show that if you take lean and obese humans and take their faeces and transplant the bacteria into mice you can make the mouse thinner or fatter depending on whose microbiome it got.

Most of them are harmless and keep your mouth healthy. But some cause cavities, gingivitis or bad breath. The skin forms an important barrier against microbes from the outside world. In total, you have about 1.

In some wet places, tens of millions of microbes live on every square centimetre of skin. The majority of them are useful and harmless. They prevent harmful bacteria on your skin from ever getting their feet in the door. With every breath, you take in tidal waves of microbes. Some of them can be harmful to your health. The mucous membranes in your nasal cavities and airways produce mucous, and foreign microbes stick to that mucous.

Your intestines are the undisputed capital of your microbiome. They also crowd out harmful bacteria. At least 30 strains of HPV are sexually transmitted, and the CDC estimates that at least 50 percent of sexually active men and women will be infected with genital HPV at some point.

Of greatest concern are HPV types 16 and 18, which can cause cancers of the cervix, penis, vagina, anus, and rectum. The new vaccine Gardasil protects against the cancers caused by both HPV types.

The flat, wingless insects are tiny between one and two millimeters long—less than a tenth of an inch , suck on human blood, and cement their eggs, or nits, to our hair. The dominant species in this dental plaque are Streptococcus sanguis and S. Even if you brush diligently, these bacteria will still be there: They arrive soon after your teeth do and stay until they fall out. The bacteria ferment sugars and secrete gluey polymers that form the basis of plaque.

Demodex mites—cigar-shaped, stumpy-legged parasites about 0. They are more likely to infect us as we age, so nearly all elderly people carry them.