The emergence of multidrug-resistant bacteria has become a global crisis. Accumulating evidence shows that bacteriophages (phages) can rescue animals from a variety of lethal infections and be effective in treating drug-resistant infections in humans. Enterobacteriaceae, producing extended spectrum ß-lactamase enzymes (ESBLs), are resistant to a broad range of ß-lactamase antibiotics. One of the most common ESBL-producing gram-negative bacilli in Enterobacteriaceae is Escherichia coli. Since ESBL-producing E. coli poses a formidable challenge in the management of critically ill patients with bacterial infections, we undertook this study to explore the possible therapeutic utility of phages to control ESBL-producing E. coli infections. The phage Ø9882 used in this study was isolated from our hospital sewage and has lytic activity against a broad range of clinical isolates of ESBL-producing E. coli. ESBL-producing E. coli strains (n=30) were isolated in the clinic, and one of them was used to induce bacteremia in a murine model. Bacteremia was established by intraperitoneal (i.p.) injection of 3x10 7 CFU/ml, the minimum lethal dose (MLD) of bacterium in this animal model. Mice infected with the MLD of this strain alone died within 14 h, whereas a single i.p. inoculation of Ø9882 (MOI ≥10-4) given 40 min after the bacterial challenge led to 100% survival at 24-168 h, compared to 0% survival of salinetreated controls. Protection was obtained even when administration of the phage was delayed up to 60 min after the bacterial infection and the survival rate of infected animals was 60% at 168 h. Furthermore, it was shown that the therapeutic efficacy of Ø9882 in lethal systemic infection in our model is due to the functional capability of the phage and not the nonspecific immune effects. Our data both in vitro and in vivo revealed that: i) the protection of mice from death occurred only in animals infected with selected bacterial strains and the virulent phage specific to them; ii) when the phages were heat-inactivated, survival of the infected mice was strikingly decreased to 0; and iii) the level of antibody against the phage was not substantially elevated when the bacteremic animals were protected by the phage. The present findings indicate that phages can effectively rescue our mouse model from bacteremia and death, and thus provide the rationale and framework to evaluate the therapeutical efficacy of lytic phages against fatal ESBL-producing E. coli infections in humans.