During recent years there has been an increasing interest on the development of strategies for modulating the process of microbiota establishment in preterm infants. For successfully developing of such strategies, a detailed knowledge of the microbiota establishment process in these infants is needed. In a previous study we evidenced clear alterations in the process of microbiota establishment in preterm newborns when compared with a control group of full-term breast-fed infants. Here we have analyzed these data more in depth, corroborating a reduced proportion of strict anaerobes with respect to facultatives in the fecal microbiota of preterm infants. The potential benefits, as well as the side effects, of strategies aimed at counterbalancing this alteration in the facultative to strict anaerobes ratio are discussed in this addendum.