This study will use a noninferiority design to examine whether the administration of L rhamnosus ATCC 53103 & L reuteri DSM 29063 (Flostrum Baby) is no worse than (or as good as) the administration of a recommended probiotic L rhamnosus ATCC 53103 (commercially available as Dicoflor, hereafter a reference product) for the prevention of antibiotic-associated diarrhea in children,

The recruitment will take place in hospitalized patients at the Pediatric Department of St. Hedwig of Silesia Hospital, Trzebnica, Poland. Additionally, patients treated at the out-patient clinics collaborating with the St. Hedwig of Silesia Hospital will be recruited. The involvement of other recruiting wards and/or sites are under consideration provided that the personnel are adequately trained and competent in conducting clinical trials. The start of the recruitment is planned in March 2020 and should be completed within the following 2 years.

The test product will be Flostrum Baby, which is a food supplement consisting of two bacterial strains: Lactobacillus rhamnosus ATCC 53103 and Lactobacillus reuteri DSM 29063, 5x10^9 CFU and 1x10^8 CFU, respectively, per seven drops.

The control product will be Dicoflor, which is a food supplement containing L rhamnosus ATCC 53103, 5x10^9 CFU, per five drops.

Caregivers will receive oral and written information regarding the study. Written informed consent will be obtained by the physicians involved in the study. Participants will be randomized within 24 hours after the initiation of the antibiotic administration.

Throughout the study period, healthcare providers and/or caregivers will record the number and consistency of stools in a standard stool diary. To record stool consistency, in children younger than 1 year, the Amsterdam Infant Stool Scale (AISS)9 will be used, and loose or watery stools will correspond to A-consistency. In children older than 1 year, the Bristol Stool Form (BSF) scale10 will be used, and loose or watery stools will correspond to scores of 5- 7. In the case of missing or incomplete data, data from hospital charts will be obtained. At any time, caregivers will have the right to withdraw the participating child from the study; they will be not obliged to give reasons for this decision, and there will be no effect on subsequent physician and/or institutional medical care.

In the event of loose or watery stools, the presence of viral or bacterial pathogens in the stool samples will be investigated. The presence of viral pathogens will be checked by using a standard rapid, qualitative, chromatographic immunoassay that simultaneously detects rotaviruses, adenoviruses, and noroviruses. Standard microbiological techniques will be used to isolate and identify bacterial pathogens (Salmonella spp, Shigella spp, Campylobacter spp, and Yersinia spp). C. difficile toxins A and B will be identified by standard enzyme immunoassay.

All study participants will be followed up for the duration of the intervention (antibiotic treatment) and then for up to 1 week after the intervention.

Inclusion criteria:

1. age younger than 18 years;
2. oral or intravenous antibiotic therapy which started within 24 hours of enrolment;
3. signed informed consent.

Exclusion criteria:

1. pre-existing acute or chronic diarrhea, history of chronic gastrointestinal disease (e.g. inflammatory bowel disease, cystic fibrosis, coeliac disease, food allergy) or other severe chronic diseases (eg, neoplastic diseases),
2. major medical problems (eg. immunocompromised, major developmental or genetic abnormality);
3. taking any of the study products (or probiotic strains included in the study products) at the time of study commencement;
4. use of antibiotics within 4 weeks prior to enrolment,
5. prematurity;
6. exclusive breastfeeding.