 Methods: the investigators aim to conduct an open-label, unique-centered, randomized controlled trial using cefepime in prolonged infusions in patients being treated for urinary or respiratory tract infections. Patients developing these infections after 72 hours of hospital admission, requiring the use of broader spectrum antibiotics after clinical failure or isolating gram-negative bacteria from adequate sample sensible to cefepime will be enrolled. The use of a second antibiotic such as clindamycin, vancomycin or metronidazole will be allowed. 