It was a dirty job: A study done by Stanford scientists, Dr. Patrick Brown and Dr. David Relman, sampled a year's worth of diapers in 14 different babies to confirm that babies are born with no bacteria in their digestive systems. Adults, however, have about 100 trillion bacteria in their digestive systems. While babies are born with no bacteria in their digestive system at the time of birth, it only takes a few days post-birth for bacteria to begin to grow.
The study also found that even over the first year the bacteria found in the stool samples varied widely from baby to baby. Scientists were intrigued by this finding since adults typically have the same bacterial families colonizing in their digestive system with little variance over time. However, at the end of the first year, the scientists found the bacteria in a baby's digestive system does begin to resemble that of an adult.
More interesting facts about babies:
Bacteria is everywhere. Both good and bad. There would be no life without it. If there were no bacteria, we would not be able to digest food to get the nutrients from it. That is why it is so important for a baby to have its first bowel movement so you know its digestive system is working.