The postnatal period of a new human being is characterized, from the microbiological point of view, by the formation of a new ecosystem: the microflora of the human gut. In adulthood a number of barriers exert a potent selective action on bacteria arriving from the mouth, but in the very first stage of our life, these barriers are kept at a very low level, temporarily allowing penetration into the gut of bacteria that are not really believed to be “gut related.” Moreover, type of delivery (natural vs. cesarean) and feeding (breast vs. bottle feeding) play dramatic roles in determining the microflora composition. In the last decade a number of articles have reported results on neonates' microflora obtained by means of culture-independent analysis. Data obtained by means of these techniques are in agreement with those produced by selective media, but they also provide some new insights about the presence of anaerobic bacteria. The focus of this article is to update knowledge on infants' microflora during the first 6 mo of life.
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