Inclusion Criteria (all of the following):  Children aged 12 months to 12 years Proven or suspected bacterial infection Inpatient children needed antibiotic treatment Oral or intravenous or intramuscular antibiotic administration with at least 2 days of duration Parental informed consent to the participation to the study  Exclusion Criteria:  Age below 12 months or above 12 years Infections with no or only one day of antibiotic treatment Sepsis or severe generalized infection Pancreatitis Known immunodeficiency or chronic gastrointestinal diseases or heart disease Oncologic patients Severe neurological impairment or lack of verbal expression Enteral or parenteral nutrition Gastrointestinal malformations or abdominal surgical interventions Central line catheter Acute or chronic diarrhea when starting antibiotics Current or recent (in the last four weeks) use of any probiotic product Antibiotic treatment started earlier than 24 hours before enrollment Absence of parental consent