Study around very-low birthweight preterm infants at high risk of developing necrotizing enterocolitis (NEC) or late-onset sepsis (LOS). Collection of stool and other biological samples to assess the strain-level stability of gastrointestinal microbiota in these preterm infants who may or may not develop NEC/LOS.

This study aims to collect biological samples (stool, stomach fluid, and blood) from preterm infants at risk of developing NEC or LOS; as well as environmental samples from infants' hospital surroundings (swabs, air), nutritional intake (formula, breastmilk) and samples from their family members (faeces, vaginal swab).

The main focus of the study is to assess strain-level stability of gastrointestinal microbiota in these infants and potentially assess the source of disease-causing microbiota species/strains.

Inclusion Criteria:

-Preterm infants with a gestational age < 32 weeks or a birthweight < 1500g and their family members

Exclusion Criteria:

• Intestinal atresia
• Live born infants referred to from other hospitals later than 3 days after birth

The study population is represented by infants born at gestational age below 32 weeks (very preterm born infants) or with a birthweight below 1500g (very low birthweight infants). It is estimated that approximately 100 infants will be enrolled during the study period. As the aim is to study stability of the premature microbiota within individual patients, the study design precludes the need of control groups.