Inclusion Criteria:

   1. Informed consent obtained from parent(s) or legal guardian(s) (Groups 1-5)

   2. ≤33 weeks gestation at birth (Groups 1-3, 5)

   3. ≥34 weeks gestation at birth (Groups 4 and 5)

   4. PNA <121 days (Groups 1-5)

   5. Sufficient venous access to permit administration of study drug (intravenous [IV])
   (Groups 1-5)

   6. Presenting physical, radiological, and/or bacteriological findings of a complicated
   intra-abdominal infection within 48 hours prior to randomization/first study drug dose
   (Groups 1-4)**. Complicated intra-abdominal infections include secondary peritonitis,
   NEC grade II or higher by Bell's criteria, Hirschsprung's disease with perforation,
   spontaneous intestinal perforation, meconium ileus with perforation, bowel obstruction
   with perforation, gastroschisis with necrosis and/or perforation, omphalocele with
   necrosis and/or perforation, neonatal appendicitis, intestinal pneumatosis or portal
   venous gas, free peritoneal air on abdominal radiographic examination, or abdominal
   abscess.

   7. Suspected or confirmed infection for which the study drug may provide therapeutic
   benefit and planned CSF collection per standard of care (Group 5).

Exclusion Criteria*

   1. History of anaphylaxis in response to study drugs (Groups 1-5)

   2. Serum creatinine >2 mg/dL within 48 hours on measurement prior to and closest to
   randomization /first study drug dose (Groups 1- 5)**

   3. Known ALT >250 U/L or AST >500 U/L on measurement closest to the time of randomization
   or first study drug dose (Groups 1-5)**

   4. Any condition that, in the judgment of the investigator, precludes participation
   because it could affect participant safety (Groups 1-5)

      - Do not apply for Group 5 participants receiving drug per standard of care

         - Criteria must be satisfied by randomization (randomized Groups 1-3) or first
         study drug dose (non-randomized Groups 1-3, Group 4 and Group 5), whichever
         comes first.