However, if germ free animals were colonized early in life, these effects could be reversed.

FORBES: Thinking With Your Gut. Literally.

But the cells from the first divisions retain the ability to form entire new embryos themselves. (This is how identical twins form.) Germ-line manipulation would require taking some of these early-embryonic cells apart in order to discover what the genetic lottery had delivered to them (a process that would destroy the cells in question).

ECONOMIST: A survey of the human genome