The Secret World of your Elbow

The crook of your elbow is not just a plain patch of skin. It is a piece of coveted real estate, a special ecosystem, a bountiful home to no fewer than six tribes of bacteria. Even after you have washed the skin, there are 1 million bacteria in every square centimeter.

These are not bad bacteria. They are what biologists call commensals, creatures that eat at the same table with people to everyone's mutual benefit. Though they were not invited to enjoy board and lodging in the skin of your inner elbow, they are giving something of value in return.
They are helping to moisturize the skin by processing the raw fats it produces, said Dr. Julia Segre, of the National Human Genome Research Institute.

Segre and colleagues reported their discovery of the six tribes in a paper published online Friday in Genome Research. The research is part of the human microbiome project, "microbiome" meaning the entourage of all microbes that live in people. The project is a government-financed endeavor to catalog the typical bacterial colonies that inhabit each niche in the human ecosystem.

The project, in its early stages, has established that the bacteria in the human microbiome collectively possess at least 100 times as many genes as the mere 20,000 or so in the human genome.

Since humans depend on their microbiome for various essential services, including digestion, a person should be considered a superorganism, microbiologists said, consisting of his or her own cells and those of all the commensal bacteria. The bacterial cells outnumber human cells by 10-1, meaning that if cells could vote, people would be a minority in their own body.

Segre reckons there are at least 20 different niches for bacteria, and maybe many more, on the skin, each with a characteristic set of favored commensals. The types of bacteria she found in the inner elbow are different from those that another researcher identified a few inches away, on the inner forearm. But each of the five people Segre sampled harbored much the same set of bacteria, suggesting this set is specialized for the precise conditions of nutrients and moisture that prevail in the human elbow.

Microbiologists think humans and their commensal bacteria are continually adapting to one another genetically. The precision of this mutual accommodation is indicated by the presence of particular species of bacteria in different niches on the human body, as Segre has found with denizens of the elbow.

Other researchers have found that most gut bacteria belong to just 2 of the 70 known tribes of bacteria. The gut bacteria perform vital services such as breaking down complex sugars in the diet and converting hydrogen, a byproduct of bacterial fermentation, to methane.

The nature of the gut tribes is heavily influenced by diet, according to a research team led by Dr. Ruth Ley and Dr. Jeffrey Gordon of the Washington University School of Medicine in St. Louis.
With the help of colleagues at the San Diego and St. Louis zoos, Ley and Gordon scanned the gut microbes in the feces of people and 59 other species of mammals, including meat eaters, plant eaters and omnivores. Each of these three groups has a distinctive set of bacteria, they reported in Friday's issue of Science, with the gut flora of people grouping with the other omnivores.
Despite the vast changes that people have made to their diet through cooking and agriculture, their gut bacteria "don't dramatically depart in composition from those of other omnivorous primates," Gordon said.

The lifetime of an individual bacterium in the human superorganism may be short, since millions are shed each day from the skin or gut. But the colonies may survive for a long time, cloning themselves briskly to replace members that are sacrificed. Where these colonies come from and how long they last is not known.

Dr. David Relman of Stanford University has tracked the gut flora of infants and found that their first colonists come from their mother. But after a few weeks the babies acquired distinctive individual sets of bacteria, all except a pair of twins who had the same set. Relman said he was trying to ascertain if the first colonists remain with an individual for many years.