This study aims to determine the effect of three factors (delivery type, feeding type and the use of two different dietary supplements) on rebiosis after disbiotic delivery. This is a randomized, single-blinded study with two parallel arms. Group 1 will receive L. reuteri (10^8 CFU) once a day, group 2 will receive B. longum and P. Pentosaceus (10^9 CFU) once a day.

Gut colonization during the first days of life represents the start of the infant's own microbiota. This process is influenced by different factors: delivery type, feeding type, antibiotic treatment, etc. On the one hand, vaginally delivered babies are in contact with mother's vaginal and faecal microbiota. This fact will drive a neonatal gut colonization composed of vagina-associated bacteria. In contrast, babies born by C-section are more susceptible to be colonized by microorganisms present in the mother's skin. On the other hand, antibiotic administration during vaginal delivery also produces alterations in the vaginal microbiota of the mother. Described scenarios have been correlated to immunological and metabolic diseases such as asthma, allergies, diabetes or obesity. Moreover, disbiosis has also been associated to functional gastrointestinal disorders (FGID) in babies such as infant colic and functional constipation. After a disbiotic delivery, medical doctors usually recommend the use of probiotics to prevent rebiosis. Since the probiotics.

L. reuteri, and B. longum and P. Pentosaceus have shown efficacy on FGID amelioration in previously published articles, these two probiotics were selected for the present study. All together, this study aims to characterize the role of three different parameters: type of probiotic, delivery type and feeding type.

Inclusion Criteria:

• Healthy infants up to 25 days of life
• Infants with adequate weight according to the gestational age.
• Infants born by vaginal delivery whose mother was given antimicrobial prophylaxis or infants born by C-section.
• Gestational birth equal or greater than 37 weeks.
• Exclusive or almost exclusive breastfeeding (understanding almost exclusive the one in which there is a maximum of one formula feeding per day) or mixed feeding (in which there is more than one formula feeding per day).

Exclusion Criteria:

• Infants with smoker mother during pregnancy and after delivery.
• Infants whose parents cannot follow the study requirements
• Infants who have suffered neonatal admission or have suffered or suffer necrotizing enterocolitis, infectious disease, congenital malformation, short bowel syndrome or any serious illness.
• Infants who have taken probiotics before the start of the trial or who take formula with probiotics.
• Breast-fed infants whose mothers have taken probiotics within the two weeks before study inclusion.
• Infants who ingest special formulas as extensively hydrolysed formulas.
• Breast-fed infants whose mothers have excluded dairy products and egg products in their diet during breastfeeding
• Infants who take specific medication for the treatment of functional digestive disorders: antacids (type IBP or H2 blockers), laxatives (PEG, lactulose, magnesia), lactase and simethicone at the time of inclusion.
• Infants undergoing therapies with acupuncture, homeopathy, medicinal herbs, anti-inflammatories and antispasmodics at the time of inclusion.

Both probiotic products will be provided in their commercial form. Both products will be individually placed inside opaque bags so that the investigator can not recognize the external case.