The aim of the study is to elucidate the relationship between postnatal antibiotic administration on development of gut microbiota and possible protective influence of simultaneously administration of probiotic during antibiotic therapy on development of gut microbiota, functional gastrointestinal disorders of infancy, weight gain and body composition.

The study is double-blind, placebo-controlled randomised trial and is composed from two parts. First part of the study aims to examine any difference in gut microbiota after postnatal antibiotic administration between the neonates who received probiotic Lactobacillus reuteri DSM 17938 or placebo and to compare the gut microbiota of patients to microbiota of healthy newborns. Second part of the study aims to determine whether probiotic L. reuteri DSM 17938 supplementation to antibiotic therapy has any beneficial influence on incidence of functional gastrointestinal disorders of infancy and body composition and bone density 6 weeks and one year after inclusion in the study.

Additionally the associations of the clinical parameters with the composition and developmental characteristics of fecal microbiota during antibiotic treatment in children will be evaluated; relevant clinical data such as mode of delivery, child's nutrition, health problems, growth and developmental attributes which could be found associated with fecal microbial community, will be tracked and statistically evaluated for the investigated population.

Inclusion Criteria:

• term neonates whom antibiotic treatment will be introduced due to clinical suspicion of infection

Exclusion Criteria:

• gestational age under 37 weeks
• birth weight less than 2500 grams
• congenital malformations/syndromes
• perinatal hypoxia
• who had received probiotics before the randomization
• have had cow's milk protein allergy diagnosed during the study
• patient who will be treated with antibiotic for less than 5 days