Inclusion Criteria:

• Age ≥ 1 month and < 3 years
• For children younger than 3 months, gestational age > 34 WA
• First episode of urinary tract infection
• AP defined by temperature ≥ 38°C on day of diagnosis AND positive urinalysis (white cell counts ≥ 10^4/mL) with a positive urine culture with one Gram- negative bacillus ≥ 104 UFC/mL. The child temperature will have to be measured with a thermometer according to the French national recommendations [Health Insurance website (AMELI ;see: - https://www.ameli. fr/assure/sante/bons-gestes/soins/prendre-temperature); HAS (see: https://www.has-sante. fr/jcms/c_2674284/fr/prise-en-charge-de-la-fievre-chez-l-enfant)].
• Initial treatment by either ceftriaxone AND/OR amikacin
• Outpatient or hospitalised

Non-inclusion Criteria:

• Urine collected by bag
• Urine culture growing more than one dominant bacterium (cf section 6.2 of the protocol)
• Catheter-associated acute pyelonephritis
• Known congenital anomalies of the kidney and genitourinary tract (other than vesicoureteral reflux and pyelocaliceal dilatation < 10 mm)
• Previous surgery of the genitourinary tract (except circumcision in male children)
• Abnormal renal function for age and weight (defined by a serum creatinine >40µmol/L before 1 year and >75µmol between 1 year et 3 years)
• Known immunocompromising condition (e.g., HIV, primary immunodeficiency, sickle cell disease, use of chronic corticosteroids or other immunosuppressive agents)
• Antibiotic prophylaxis for any reason OR antibiotic treatment in the last 7 days (except treatment administered for the AP)
• Known hypersensitivity to at least one of the active substances /excipients: ceftriaxone (including other cephalosporins and other beta-lactams) and amikacin (including other aminoglycosids).
• Known hypersensitivity to at least one of the active substances /excipients: cotrimoxazole (=sulfamethoxazole/trimethoprim) (including other drugs containing sulfonamides) and cefixime (including other cephalosporins)
• Known blood dyscrasias (megaloblastic haematopoiesis)
• Known severe hepatic insufficiency
• Known G6PD deficiency
• No written consent from holders of parental authority
• Non-affiliation to a social security system (as beneficiary or entitled person)
• Children whose follow-up is not carried out in the centre
• Participation in another interventional or minimal risk trial

Randomization criteria :

• Three days of taking antibiotics (IV or IM) (no interruption or discontinuation)
• Positive urine culture with Gram negative bacillus ≥ 10^4 UFC/mL
• Favorable clinical outcome at day of randomization (D2 or D3) defined by temperature < 38°C at day of randomization and absence of fever measured > 38°C for at least 12 hours AND no abdominal pain AND no feeding problem AND investigator agreement
• No renal abscess AND congenital anomalies of the kidney and genitourinary tract (other than vesicoureteral reflux and pyelocaliceal dilatation < 10 mm) on the renal ultrasound performed between D0 and day of randomization
• No more than 1 type of dominant bacteria on the urine culture
• Sensitivity to the initial antibiotic treatment
• Sensitivity to cefixime OR cotrimoxazole