April 21, 2011|By Carl Zimmer, New York Times

NEW YORK — In the early 1900s, scientists discovered that each person belonged to one of four blood types. Now they have discovered a new way to classify humanity: by bacteria. Each human being is host to thousands of different species of microbes. Yet a group of scientists now report just three distinct ecosystems in the guts of people they have studied.

Blood type, meet bug type.

“It’s an important advance,’’ said Rob Knight, a biologist at the University of Colorado, who was not involved in the research. “It’s the first indication that human gut ecosystems may fall into distinct types.’’

The research team, led by Peer Bork of the European Molecular Biology Laboratory in Heidelberg, Germany, found no link between what they call enterotypes and the ethnic background of the European, American, and Japanese subjects they studied.

Nor could they find a connection to sex, weight, health, or age. They are now exploring other explanations. One possibility is that infants are randomly colonized by different pioneering species of gut microbes. The microbes alter the gut so that only certain species can follow them.

Whatever the cause of the different enterotypes, they may end up having discrete effects on people’s health.

Gut microbes aid in food digestion and synthesize vitamins, using enzymes our own cells cannot make.

Bork and his colleagues have found that each of the types makes a unique balance of these enzymes. Enterotype 1 produces more enzymes for making vitamin B7 (also known as biotin), for example, and Enterotype 2 more enzymes for vitamin B1 (thiamine). The study was reported yesterday in the journal Nature.

The discovery of the blood types A, B, AB, and O had a major effect on how doctors practice medicine. They could limit the chances that a patient’s body would reject a blood transfusion by making sure the donated blood was of a matching type. The discovery of enterotypes could someday lead to medical applications of its own, but they would be far down the road.

“Some things are pretty obvious already,’’ Bork said. Doctors might be able to tailor diets or drug prescriptions to suit people’s enterotypes, for example.

Or, he speculated, doctors might be able to use enterotypes to find alternatives to antibiotics, which are becoming increasingly ineffective.

Bork notes that more testing is necessary.