In general, the enteric microbiota composition is relatively stable due to the ongoing competition of bacterial members for space and nutrients. Newly arriving bacteria hardly find an empty niche and sufficient nutrients to thrive and colonize. Shortly after birth, however, this situation is markedly different. The neonate is born sterile and newly incoming bacteria can easily find a place and nutrients to stay and colonize the neonate's intestinal mucosa. Notably, it is generally thought that this process is mainly driven by exposure to bacteria derived e.g. from the mother of the environment.
But is that really true? If only the environment determines the microbiota composition couldn't that go terribly wrong? Shouldn't we expect that host factors influence the emerging microbiota ensuring a beneficial bacterial composition?