Urinary tract infections (UTIs) account for 5-14% of pediatric emergency department visits annually. At the moment, up to one third of children suffering of acute (UTI) will have a new infection and there is a lack of effective methods for preventing secondary UTIs in young children. Majority of UTIs in children are caused by intestinal bacteria of the patient, mainly E. coli that colonizes gut of the patient. E. coli Nissle is a probiotic strain that has been used successfully for treating acute gastrointestinal infections in children. The strain has also been proved to be safe for infants and young children. E. coli Nissle could be a potential solution for preventing recurrent urinary tract infections in children as it competes with pathogenic bacteria that usually cause UTIs in children.

The aim of this study is to evaluate efficacy of E. coli Nissle strain in secondary prevention of urinary tract infections in young children.

In order to asses efficacy of E. coli Nissle strain in prevention of recurrent UTIs in children, we aim to conduct randomized, placebo controlled and double blinded clinical trial. We are going to recruit children who are suffering the first urinary tract infection of their life. The UTI will be treated by normal clinical recommendations decided by treating physician. Intervention will start on the day following the last day of the antimicrobial course.

The intervention will last for 15 days and the study participants will receive either intervention drug or placebo. Intervention drug contains 10 E8 CFU/ml of E. coli Nissle strain and will be administered as dose of 1 ml of oral suspension once daily for children aged under one year and twice daily for children aged more than one year.

The study patients will be monitored by monthly electronic surveys for six months after the intervention in order to measure proportion of the children who will have a new UTI during the follow up. We are also going to record possible adverse effects in both study groups. Also use of antimicrobial medication due to any reason will be recorded.

Inclusion Criteria:

• Children who suffer of the first urinary tract infection of their life defined as pyuria, presence of symptoms (fever or focal symptom), no other explanation for infection, bacterial culture positive or culture result pending
• Written informed consent

Exclusion Criteria:

• Preterm birth, gestational age less than 35 weeks
• Signs of life-threatening infection (such as meningitis)
• Detected anomaly in urinary tract
• Primary immunodeficiency
• Detected anomaly of GE tract
• Ongoing antimicrobial prophylaxis
• Hospital-acquired infection

IPD will be shared after reasonable request

The intervention drug or placebo will be given to participants in closed boxes without any labels. Appearance and dosing of the products will be similar. Investigators will not handle the boxes or study products.