Not long ago, researchers thought that infants in the womb were free of bacteria. Infants are inoculated by bacteria during birth, and later by the environment and diet, they said, but not before birth. In 2005, that idea changed when bacteria was discovered inside the umbilical cord. Last spring, the idea was really put to death when researchers discovered that bacteria are present in the placenta. It has now become clear that even before birth, bacteria are helping to shape our health.

Although mode of birth and environmental exposures still play a big role on the community of microbes that colonize an infant, a recent study published in the Proceedings of the National Academy of Sciences journal found that, in premature infants, age also plays an important role on what bacteria are present. Researchers analyzed over 900 stool samples from 58 premature infants ranging from 23 to 33 weeks in gestational age (7 to 17 weeks premature) and weighing 3 pounds, 5 ounces or less.

They found that three major classes of bacteria colonized the infants’ guts sequentially, but in different ratios based on their age. Earliest, Bacillus bacteria dominated, followed by Gammaproteobacterium, and then Clostridium. Environmental factors such as mode of delivery or whether or not antibiotics were given did determine the pace of colonization, but not the order of progression.

Although they do not yet know the significance of these three groups of bacteria, the researchers are interested in Gammaproteobacterium due to its inflammatory properties. In healthy children, Gammaproteobacterium only make up less than one percent of the bacteria in the gut. In many of the premature infants, they made up over 50 percent, and in some infants they represented over 80 percent of total bacteria.

“It is our first glimpse of how these earliest in life bacterial colonizations—events that may have lifelong consequences—occur,” noted Phillip Tarr, MD, lead researcher.

More studies are needed to determine what relationship these bacteria have to health. Because bacteria are present in the placenta and umbilical cord, whether or not bacteria play a role in the early birth of these infants will be an area of interest to researchers.