The microscopic world within us

"Thousands of species of microbes—bacteria, viruses, fungi, and protozoa—inhabit every internal and external surface of the human body. Collectively, these microbes, known as the microbiome, outnumber the body’s human cells by about 10 to 1 and include more than 1,000 species of microorganisms and several million genes residing in the skin, respiratory system, urogenital, and gastrointestinal tracts. The microbiome’s complicated relationship with its human host is increasingly considered so crucial to health that researchers sometimes call it “the forgotten organ.”

Disturbances to the microbiome can arise from nutritional deficiencies, antibiotic use, and antiseptic modern life. Imbalances in the microbiome’s diverse microbial communities, which interact constantly with cells in the human body, may contribute to chronic health conditions, including diabetes, asthma and allergies, obesity and the metabolic syndrome, digestive disorders including irritable bowel syndrome (IBS), and autoimmune disorders like multiple sclerosis and rheumatoid arthritis, research shows." 

Read at AACC (American Association for Clinical Chemistry)

This article says that 10 to 1 figure is a myth.  If you have 30 trillion cells, you'll only have 39 trillion bacteria.
Nature.

   

Tools.

Of particular interest is fecal transplantation, a procedure in which stool is transferred from a healthy donor to an unhealthy recipient (perhaps the same person, in the case of autologous stool transplant). This procedure has been 90–95% effective for treating Clostridium difficile-associated disease versus only 20–30% efficacy for antibiotics. Understanding which other diseases associated with dysbiosis of the microbiome could be corrected remains a major goal of microbiome research. The current regulatory framework in the United States, regulating stool as a drug and requiring an investigational new drug (IND) application for any application other than C. difficile, however, is a substantial barrier to research. Understanding more generally how various therapies including antibiotics, probiotics, prebiotics (essentially, fertilizer for the microbiome), phage therapy, etc. can reshape the microbiome remains a major technological and theoretical challenge.