This is a multi-center, randomized, double-blind, parallel-group study to evaluate the safety, tolerability, efficacy, and pharmacokinetics of ZTI-01 (IV fosfomycin) compared to piperacillin/tazobactam in the treatment of hospitalized adults with cUTI or AP. Diagnosed and prescreened hospitalized patients will be randomized 1:1 to receive one of two intravenous treatments: 6 g ZTI-01 three times daily (18g total daily dose) or 4.5 g piperacillin/tazobactam three times daily (13.5g total daily dose) for 7 calendar days, with option to extend treatment up to 14 days in patients with positive blood culture at pretreatment. Patients will participate in the study for approximately 26 days. Urine cultures will be obtained and organisms quantified for qualified patients at baseline, during treatment, at end of treatment (EOT), at test of cure (TOC) and late follow up visits (LFU). Blood cultures will be obtained at baseline and repeated if positive throughout the study. Safety and efficacy evaluations will include vital signs, labs, physical exams, ECG and overall response as evaluated by the Investigator. Pharmacokinetic samples will be obtained (sparse sampling) for all patients.