The primary objectives of this study are to assess the safety, tolerability, and pharmacokinetics (PK) of cefiderocol after single-dose administration in hospitalized pediatric participants 3 months to < 12 years of age with suspected or confirmed aerobic Gram-negative bacterial infections and after multiple-dose administration in hospitalized pediatric participants 3 months to < 18 years of age with suspected or confirmed complicated urinary tract infection (cUTI), hospital-acquired bacterial pneumonia (HABP), or ventilator-associated bacterial pneumonia (VABP).

This study consists of a nonrandomized single-dose phase in children aged 3 months to less than 12 years with suspected or confirmed aerobic Gram-negative bacterial infections and a randomized multiple-dose, active-comparator standard of care (SOC) phase in children aged 3 months to less than 18 years with cUTI, HABP, or VABP to assess the PK, safety, and tolerability of cefiderocol in hospitalized participants requiring systemic antibiotics for an expected 5 to 14 days.

Inclusion Criteria:

1. Participant's parent(s) or legally authorized representative(s) (LAR) provides written informed consent in accordance with regional- and country-specific laws and regulations

2. Participant provides written informed assent, when feasible (age of assent to be determined by institutional review board/independent ethics committee [IRBs/IECs] or be consistent with local legal requirements)

3. Hospitalized participant is 3 months to < 18 years of age at the time written informed consent/assent is obtained for the multiple-dose phase. Hospitalized participant is 3 months to < 12 years of age at the time written informed consent/assent is obtained for the single-dose phase.

4. Single-dose phase: Participant has a suspected or confirmed infection type (including but not limited to cUTI, complicated intra-abdominal infections [cIAI], pneumonia, HABP/VABP, and sepsis or bloodstream infections [BSI]) that requires hospitalization for treatment with IV antibiotics.

   Multiple-dose phase: Participant has a suspected or confirmed cUTI, HABP, or VABP that requires hospitalization for treatment with IV antibiotics

5. If participant is a sexually active female of childbearing potential and has reached menarche or Tanner stage 3, participant agrees to use barrier contraception (including condom, diaphragm, or cervical cap) with spermicide or agrees to use a highly effective method of contraception (including contraceptive implant, injectable contraceptive, combination oral contraceptive, or an intrauterine [IUD] contraceptive device) from Screening up to 28 days after administration of the last dose of cefiderocol.

Exclusion Criteria:

1. Participant has a documented history of any hypersensitivity or allergic reaction to any β-lactam antibiotic (Note: for β-lactams, a history of a mild rash followed by uneventful re-exposure is not a contraindication to enrollment.)

2. Multiple-dose only: Participant has an infection caused only by a confirmed Gram-positive pathogen.

3. Participant has a suspected or confirmed central nervous system (CNS) infection (for example, meningitis, brain abscess, shunt infection) or osteomyelitis (which would require prolonged antibiotic therapy).

4. Participant has cystic fibrosis.

5. Single-dose phase: Participant has moderate or severe renal impairment based on estimated glomerular filtration rate (eGFR) (based on the Schwartz equation if ≥ 3 months to < 1 year of age and modified Bedside Schwartz equation if ≥ 1 to < 18 years of age) of < 60 milliliter (mL)/ minute (min)/1.73 square meters (m^2)² at Screening .

   Multiple-dose phase: Participant has an eGFR (based on the Schwartz equation if ≥ 3 months to < 1 year of age and modified Bedside Schwartz equation if ≥ 1 to < 18 years of age) of < 15 mL/min/1.73 m² at Screening.

6. Participant has end-stage renal disease (ESRD), is on hemodialysis (HD), or receiving continuous venovenous hemofiltration (CVVH).

7. Participant has experienced shock in the prior month or is in shock at the time of Screening.

8. Participant has severe neutropenia or is severely immunocompromised.

9. Participant has multiorgan failure .

10. Participant with a life expectancy of < 30 days due to severity of a concurrent illness.

11. Participant is a female who has a positive pregnancy test at Screening.

12. Participant is a female who is breastfeeding.

13. Participant has received any other investigational medicinal product (IMP) within 30 days.

14. Participant has any condition or circumstance that, in the opinion of the investigator, would compromise the safety of the participant or the quality of the study data, including acute trauma to the pelvis or urinary tract.

15. Participant is receiving vasopressor therapy at Screening.