AIM: The present study is an observational clinical study aimed to study the intestinal microbiota composition of critically ill children in paediatric intensive care unit and its relationship with clinical complications and inflammatory biomarkers.

DESIGN: This study is a multicentre, observational and prospective study including 100 critically ill children admitted in three paediatric intensive care units (Hospital Materno-Infantil Torrecrdenas de Almera, y en el Hospital Materno-Infantil Carlos Haya de Mlaga and Complejo Hospitalario Universitario de Santiago) This study is a multicentre, observational and prospective study conducted by members of the Department of Biochemistry and Molecular Biology II, Nutrition and Food Technology Institute "Jos Mataix" of the University of Granada research group in collaboration with the Hospital Materno-Infantil Torrecrdenas from Almera, the Hospital Materno-Infantil Carlos Haya from Mlaga and Complejo Hospitalario Universitario from Santiago de Compostela pediatric intensive care units and primary care unit of the University Hospital Virgen de las Nieves from Granada as recruitment centres.

This trial will include 100 critically ill children admitted from the three different pediatric intensive care units with an expected hospitalization time of more than 5 days. In addition, 50 healthy children will be recruited from a primary care centre as controls. Faecal and respiratory system (oropharynx) samples will be collected from ill and healthy children, while blood samples will be harvested only from critically ill children at the time of admission to the pediatric intensive care units as well as at the day of discharge. Sample aliquots will be sent to the Department of Biochemistry and Molecular Biology II, University of Granada for further analysis.

Study population The study population will comprise 50 healthy children aged between 1 month and 16 years, recruited in a paediatric primary care center from Granada, and 100 critically ill children admitted to the pediatric intensive care units aged between 1 month and 16 years. 50 patients will be recruited at the pediatric intensive care units of Hospital Materno-Infantil Torrecrdenas from Almera, the Hospital Materno-Infantil Carlos Haya from Mlaga and Complejo Hospitalario Universitario from Santiago de Compostela. Only those children who satisfy all inclusion and none of exclusion criteria will be included in the study.