Complex intra-abdominal abscess involving retroperitoneal and extraperitoneal compartments with multiple <i>E. coli</i> strains

We report the case of a man in his 70s who presented with constitutional symptoms and intermittent confusion, ultimately diagnosed with a large, complex retroperitoneal and extraperitoneal abscess. He had a background of recurrent urinary tract infections over the preceding year. Initial imaging revealed a multi-loculated collection. Surgical incision and drainage yielded 1000 mL of pus, and cultures identified multiple strains of <i>Escherichia coli</i> required tailored antibiotic therapy. Interval imaging revealed a residual collection requiring a second surgical intervention. Further inpatient investigations also raised suspicion for underlying prostate carcinoma. The patient made a full recovery and demonstrated complete resolution of inflammatory markers.This case illustrates the diagnostic complexity of deep-seated polymicrobial infections presenting with non-specific constitutional symptoms and underscores the need to consider multistrain pathogens and underlying urological pathology in patients with a history of recurrent urinary tract infections. It also underscores the value of holistic assessment and management during and after hospital admission.