Pig microbiota after weaning differs considerably from the microbiota of nursed piglets (for additional detail see here or here). Intestinal tract of newborn piglets is first colonised by E. coli. However, Clostridium perfringens may increase in microbiota of healthy newborn piglets as well. Microbiota quickly develops and that with composition characteristic for piglets under lactation establishes within a few days. One of the characteristics of microbiota of nursed piglets is the enrichment for different Bacteroides species. The colonisation of piglet intestinal tract by Bacteroides results in a decrease of both E. coli and C. perfringens. However, Bacteroides species are only lowly represented in microbiota of sows, in which Bacteroides is substituted by species or related genus Prevotella. Sows in the first hours and days of piglet life do not represent an ideal source of microbiota required by newborn piglets, what may enable multiplication of microbiota of suboptimal composition. The shorter this period lasts, the better for the whole production. Understanding the principles of microbiota development in newborn piglets thus allows for prediction of strains which can be used as new types of probiotics for piglets under lactation.