Bloodstream Infection due to Piperacillin/Tazobactam Non-Susceptible, Cephalosporin-Susceptible Escherichia coli: A Missed Opportunity for De-Escalation of Therapy.

An increasing number of reports describing Escherichia coli isolates with piperacillin/tazobactam resistance, despite retained cephalosporin susceptibility, suggest further emergence of this phenotypic resistance pattern. In this report, a patient with metastatic breast cancer presented to medical care after two days of chills, nausea, vomiting, reduced oral intake, and generalized weakness. Blood and urine cultures grew E. coli as identified by rapid diagnostics multiplex PCR and MALDI-TOF, respectively. The patient continued to manifest signs of sepsis with hypotension and tachypnea during the first three days of hospitalization despite empirical antimicrobial therapy with intravenous piperacillin/tazobactam. After in vitro antimicrobial susceptibility testing demonstrated a piperacillin/tazobactam minimal inhibitory concentration (MIC) of 64 and a ceftriaxone MIC of ≤1 mcg/mL, antimicrobial therapy was switched from intravenous piperacillin/tazobactam to ceftriaxone. All symptoms and signs of infection resolved within 48 h of starting ceftriaxone therapy. This report describes the clinical failure of piperacillin/tazobactam in the treatment of a bloodstream infection due to E. coli harboring a phenotypic resistance pattern of isolated piperacillin/tazobactam non-susceptibility. The case demonstrates the role of cephalosporins as potential treatment options and highlights the value of early de-escalation of antimicrobial therapy based on rapid diagnostic testing for microbial identification.