PK/PD Analysis of Ceftazidime/Avibactam or Cefiderocol With or Without Fosfomycin for the Treatment of Difficult To-treat Gram-negative Infections

Inclusion Criteria: * Patients with infection due to a difficult-to-treat Gram-negative bacteria treated with CAZ/AVI alone, CEF alone, CAZ/AVI plus FOS, or CEF plus FOS (any age) * Signature of the informed consent (for pediatric patients: parents or guardians able to provide consent) Exclusion Criteria: * Premature newborns * Polymicrobial/mixed infections with the exception of cases with multiple Gram-negative bacteria susceptible to study drugs * Continuous renal replacement therapy (CRRT) applications Inclusion Criteria for Healthy Volunteer Subjects: * Age ≥18 years * Signature of the informed consent Exclusion Criteria for Healthy Volunteer Subjects: \- Any known clinically relevant health problems