The goal of this clinical trial is to test the ability of different bacterial products in restoring natural gut microbiota in C-section born infants. The main question it aims to answer is:

Do maternally derived strains of bacteria perform better than commercially available probiotic strains in restoring the gut microbiota of C-section born infants? Researchers will compare the gut microbiota of treated infants to that of untreated C-section born infants and untreated vaginally born infants to see if the bacterial treatments cause the microbiota to resemble that of vaginally born infants.

Participants will be given a bacterial product orally once daily for either one or four weeks and be asked to collect faecal, urine and saliva samples.

The gut microbiota has long-term effects on host health especially in early life. Infants receive maternal faecal microbes during vaginal birth, which is prevented by C-section. C-section is associated with increased risk of many chronic diseases, likely because of the disturbed gut microbiota. We recently showed the effectiveness of a faecal microbiota transplant from the infant's own mother in restoring normal gut microbiota in C-section born infants. However, FMT contains a large diversity of unknown microbes, some of which may pose a risk of dangerous infection. A safe and widely applicable infant microbiota restoration method is urgently needed. In this project, we develop a selective microbiota transplant from mother to infant and test its efficacy and safety in a clinical trial. The work is conducted in the Faculty of Medicine, University of Helsinki.

Inclusion Criteria:

• Healthy mother and healthy pregnancy
• Singleton pregnancy,
• Mothers who speak Finnish or Swedish
• Mothers who are planning to breastfeed or give breastmilk by bottle to the infant
• Infants who are expected to be healthy and who will not require BCG vaccination.

Exclusion Criteria:

• Participants who lives further than a 2-hour drive from Meilahti, Helsinki, Finland
• Mothers who are not planning to breastfeed or feed breastmilk by bottle to the infant
• Mothers who are diagnosed with gestational diabetes, pre-eclampsia or who have been receiving antibiotics during the pregnancy or delivery
• Premature infants born before the pregnancy week 37
• Infants, who are born by urgent cesarean section or emergency cesarean section
• Infants, who receive antibiotics during the first week of life
• Infants, who are diagnosed with a disease, congenital anomaly or who receive less than 9 Apgar points at 5 minutes.
• Infants, who receive BCG-vaccine